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An experimental study of the performance, combustion and emission characteristics of a CI engine under dual fuel mode using CNG and oxygenated pilot fuel blends
Energy xxx (2015) 1e14
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An experimental study of the performance, combustion and emission characteristics of a CI engine under dual fuel mode using CNG and oxygenated pilot fuel blends Abhishek Paul a, *, Raj Sekhar Panua a, Durbadal Debroy a, Probir Kumar Bose b a b
Department of Mechanical Engineering, National Institute of Technology, Agartala 799055, India Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2014 Received in revised form 1 April 2015 Accepted 14 April 2015 Available online xxx
During the past few decades, many researchers have proposed a partial replacement of Diesel in the CI engine by substituting it with natural gas in order to reduce the exhaust emission without altering the performance characteristics of the engine too much. Most of these researches have focused on the combustion of natural gas in a dual fuel mode by using Diesel as the pilot fuel. However, the dual fuel operation of natural gas with pilot Diesel reduces the brake thermal efficiency and increases the hydrocarbon emission. The present experimental work explores the potential of using CNG under dual fuel operation by utilizing two different blends of Dieseleethanolebiodiesel as the pilot fuel. The present study reveals that the increased percentages of ethanol and biodiesel in the pilot fuel triggers an increase in the brake thermal efficiency of the engine. In this study, the NOx emission was also found to decrease with a corresponding increase in the percentages of ethanol and biodiesel in the pilot fuel. The study reveals definite encouraging aspects of using the D45E15B40 and D30E20B50 blends as the pilot fuel because it can extend the range of usage of the CNG to 9000 ms of injection duration. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Dieseleethanolebiodiesel blends Oxygenated pilot fuel Peak cylinder pressure Mass fraction burn NOX reduction NOx-Bth-BSEC tradeoff
1. Introduction Over the past few decades, the rapid growth of industrialization and urbanization has drastically increased the usage of personal and industrial vehicles, which run on petroleum-based fuels. Simultaneously, this has also intensified the emission of noxious gases into the atmosphere. It is estimated that the combustion of fossil fuels releases about 7 tons of greenhouse gas annually [1]. Due to this unrestrained usage of fossil fuel, the natural reservoir of crude petroleum that can be economically excavated is dwindling by the day. However, the growing concern over the deteriorating environmental condition due to industrial and vehicular emissions has prompted the enforcement of stringent emission laws on a global scale. All these restrictions have attracted several researchers to the field of emission control and performance enhancement methods for conventional vehicles. In our modern era, a major part of the transportation sector utilizes the Diesel engine due to its greater efficiency and higher
* Corresponding author. E-mail address: [email protected] (A. Paul).
power output, which results in saving fuel costs. However, the Diesel engine is also accountable for the high emissions of NOX, particulate matters and other polycyclic aromatic hydrocarbons. In order to cope with the strict guidelines of emission norms and the gradual reduction in the fuel supplies, several alternatives in fuel technology have been investigated in recent years. A feasible alternative that has emerged from these researches is the dual fuel operation of the conventional CI engine where a gaseous fuel is used as the primary fuel and a small amount of liquid fuel as the pilot fuel. Thus far, natural gas has been the most extensively used gaseous fuel for the IC engine. Although a good proportion of these studies have been done on the application of natural gas in the spark ignition engine, the use of natural gas in the compression ignition engine has also attracted a significant number of researchers. The usage of natural gas in the Diesel engine produces cleaner combustion as it contains a minimal quantity of impurities due to its gaseous state [2]. It also offers several other advantages such as better availability, lower market price, high octane number etc. In addition it has a high self-ignition temperature of 540 C [3]. All these properties enable the use of natural gas in Diesel engines in high compression ratios without knocking. Hence, a significant
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Nomenclature CNG compressed natural gas NO nitric oxide BSEC brake specific energy consumption CO carbon monoxide PPME Pongamia pinnata methyl ester CO2 carbon dioxide mSec micro second HC hydrocarbon DAQ data acquisition 20%L diesel 20%D100 ¼ 20% load with Diesel as pilot fuel
number of research works have been conducted by different researchers in the field of gaseDiesel engines based on the properties of natural gas. Mustafi et al. [4], studied the combustion and emission characteristics of a dual fuel CI engine, operated on gaseous fuels such as natural gas and biogas. It was found that the maximum net heat release was 27% higher for the natural gas dual fuel operation and 30% higher for the biogas dual fuel operation as compared with the baseline Diesel. Significantly lower PM and NOx emissions were observed with both the gaseous fuels. Hallquist et al. [5], discovered that the buses running on CNG emitted more amounts of fine particulates but less quantity of particulates when compared with the Diesel-fueled buses. Karabektas et al. [6], and Serrano et al. [7], separately concluded that the dual fuel mode of the CI engine operation yielded higher CO and HC (Hydrocarbon) emissions at all loads, besides lower NO emissions other than high loads. Cheenkachorn et al. [8], concluded that a maximum of 77.90% of natural gas at 1300 rpm could be used in dual fuel engine operation. Cordiner et al. [9], concluded that for high degrees of CNG substitution, a significant improvement in PM emission could be achieved. All the researches cited in this study reveal that the increase in the Diesel fuel supplementary ratio resulted in reduced NOx and soot emissions compared with the normal Diesel operation. Both CO and HC emissions were observed to be higher than the normal Diesel operation in the previous studies. Based on these findings, an experimental work was conducted by the same researchers using 5 and 10% ethanol blends in the Diesel as the pilot fuel together with CNG to improve the CO and HC emissions [10]. The results revealed a promising trend with the CNG dual fuel operation and Dieseleethanol blend as the pilot fuel. For lesser CNG injection strategies, an increase in the ethanol percentage in the pilot fuel produced a higher hbth than the base diesel operation, with a commendable decrease in the BSEC. In terms of the emission parameters too, the CNG dual fuel operation with the pilot Dieseleethanol blends produced better results with an increasing ethanol content. A reasonable decreasing trend in the NOx emission for lesser CNG injection strategies was observed with an increasing percentage of ethanol in the pilot fuel. The hydrocarbon emission from the engine was seen to increase with the CNG e Diesel operation. The hydrocarbon emission was found to decrease with the addition of ethanol to the pilot fuel. The D90E10 blend used with 10% ethanol produced an almost identical hydrocarbon emission compared with the base Diesel operation. However, the use of ethanol in Diesel is greatly hindered by the limited miscibility of the ethanol in the Diesel. In this way, the phase separation restricts the usage of ethanol in Diesel beyond 10% at normal temperature. To solve this problem and to increase the ethanol participation in Diesel, a co-solvent can be utilized so that the properties of the blend may be kept as close to that of the Diesel. Biodiesel derived
BTDC before top dead center 20%L E15 20%E15 ¼ 20% load with D45E15B40 as pilot fuel ROHR rate of heat release 20%L E2020%E20 ¼ 20% load with D30E20B50 as pilot fuel DBE blends dieselebiodieseleethanol blends D45E15B40 45% (v/v) diesel, 15% (v/v) ethanol and 40% biodiesel D30E20B50 30% (v/v) diesel, 20% (v/v) ethanol and 50% biodiesel hbth brake thermal efficiency cSt centistokes LHV lower heating value NOx oxides of nitrogen
from different sources is reported to act as a co-solvent to enhance the ethanol miscibility in the Diesel [11e13]. Several additives such as alkanols, decaglycerol mono-oleate (MO750), and alkanolamides have been used over the years to increase the miscibility of the ethanol in the Diesel [14e16]. However, biodiesel has become the most commonly used co-solvent to increase miscibility of ethanol in Diesel. Besides increasing the miscibility, the biodiesel also improves the cetane content of the blend as it has a higher cetane number than Diesel [17,18]. Biodiesel is also enriched with oxygen due to the presence of about 11% of dissolved oxygen [18]. This property is instrumental in reducing the soot and hydrocarbon emissions [19,20]. Due to these favorable properties of biodiesel in the Dieseleethanol blends, a significant number of research studies have been conducted in the field of natural gas dual fuel operation. Works by Namasivayam et al. [21,22], Selim et al. [23], and Korakianitis et al. [24], utilized different types of biodiesel with natural gas dual fuel operation. All of these researches revealed significant improvement in the thermal efficiency with a distinct reduction in the hydrocarbon and carbon monoxide emissions. However, they also observed an increase in the NOx emission. The main objective of this investigation is to replace the Diesel to the greatest possible degree and use the biodiesel as the cosolvent for the addition of a higher percentage of ethanol in the pilot fuel for CNG dual fuel operation, which is an extension of the previous work [10]. Increase of ethanol percentage in pilot fuel showed remarkable results as mentioned above, hence increase in ethanol percentage may reveal further improvement in performance, combustion and emission of the engine. Since the benefits of CNG dual fuel operation with Dieseleethanolebiodiesel blends are yet to be investigated as per the knowledge of the authors, so the present work is a new approach in improving the performance and emission of the engine. 2. Experimental setup and procedure 2.1. Test fuels and their properties The base liquid fuels used in this study are mineral Diesel, ~ nata. anhydrous ethyl alcohol and the methyl ester of Pongamia pin The Diesel and ethanol are collected from a local fueling station and chemist shop, respectively. As discussed earlier, ethanol has miscibility of up to 10% by volume in Diesel. Any further increase in the percentage of ethanol in the Diesel produces an insoluble twophase heterogeneous mixture. In order to increase the miscibility of the higher percentage of ethanol in the Diesel, PPME (Pongamia ~ nata Methyl Ester) is used as a co-solvent. PPME is combination Pin of methyl esters of different chain lengths. The reason for the better solubility of the higher percentage of ethanol in the Diesel in the presence of PPME is because the PPME can act as an amphiphile
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and form micelles with a non-polar tail and a polar head. In the continuous phase, the polar head of the micelle orients itself to the ethanol, whereas the non-polar tail orients itself to the Diesel [17]. A comprehensive study to discover the minimum amount of PPME required to stabilize a higher quantity of ethanol in the Diesel has been achieved in this work. As a part of this procedure, the ethanol percentage in the blend is at first maintained constant at 15% and only the percentage of the PPME is increased from 5% with increments of 5%. It was found that 40% PPME with 15% ethanol and 45% Diesel is the optimum ratio for a stable blend. Further, the ethanol percentage is then increased to 20% and the PPME percentage is also increased from 5% with an increment of 5%. In this state, the first stable blend is found with 50% PPME, 30% Diesel and 20% ethanol. The two stable blends viz. D45E15B40 (45%Diesel, 15% ethanol and 40% PPME) and D30E15B50 (30% Diesel, 20% ethanol and 50% PPME) are chosen for the engine test. The entire range of blends tested (stable and unstable) are shown in Fig. 1. The properties of the test blends are calculated on the basis of base fuel properties and are shown in Table 1. The CNG used in this study is collected from local CNG fueling station. The properties of CNG are listed in Table 2.
2.2. Experimental setup The experimental study is conducted on a 3.5 kW 4 stroke water-cooled, naturally aspirated CI engine. An eddy current dynamometer (Make: Saj test plant Pvt. Ltd., Model-AG10) is synchronized to the engine for load measurement. The engine RPM is measured with a crank angle sensor (Make: Kubler-Germany, Model 8.3700.1321.0360). The crank angle sensor is calibrated in terms of 1-degree intervals. Two piezoelectric pressure transducers (Make: KISTLER, Type 6056A31U20) measure the in-cylinder pressure and the fuel injection pressure. The CNG is inducted into the engine through the intake manifold along with the incoming
3
Table 1 Properties of base fuels and fuel blends. Property
Diesel Ethanol PPME
D40E15B45
D30E20B50
Density (kg/m3) Kinematic viscosity (cSt) Calorific value (KJ/Kg) Flash point ( C) Fire point ( C) Cetane number
820 2.51 42650 52 64 46
845.05 5.222173244 37250.46648 120.3655 126.625 45
846.8 5.47319556 36175.30468 126.654 131.9 43.3
789 1.09 26950 12.77 13.5 7
886 8.68 35866 217 220 35
air. The intake manifold of the engine is marginally modified to accommodate the CNG injection system. Considering the risks involved in working with a gaseous fuel, a number of safety and flow control devices are built into the CNG injection system. The CNG injection system consists of a CNG cylinder, a gas flow regulator, a pressure gauge, a gas flow meter and a gas injector. The CNG cylinder houses the gas at about 196 bar pressure whereas, the working pressure of the gas injector (Make DYMCO Corp, Modeli1000) is of 1.2 bars. Hence, a pressure regulator (Make: CONCOA, USA, Model-3123322-01-B04)is connected to the CNG flow pipe to reduce the CNG pressure from 196 bars to 1.2 bars. A pressure gauge (Make OMEGA, Model- PGC-25L-600) is connected to the path of the CNG flow line to monitor the gas pressure constantly. A gas flow meter (Make CLESSE, Type- G1.6) is connected after the pressure gauge to measure the volume flow rate of the CNG. The gas flow meter is used to measure the flow rate of CNG. This is done by measuring the time taken for a fixed volume (30CC) flow of CNG. From this data, the flow rate of CNG is calculated. The injector is mounted on the intake manifold at a distance of 1.5 D (where D is the manifold outer diameter) from the engine inlet valve to ensure the formation of a homogeneous mixture of inducted CNG to the incoming air. The gas injector is synchronized to the crank angle rotation by means of a second angle encoder to enable a precise induction of the CNG at any desired crank angle and for any desired
Fig. 1. Miscibility test of the E15 and E20 blends.
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A. Paul et al. / Energy xxx (2015) 1e14 Table 2 Properties of CNG. Properties Density (kg/m3) Flammability limits (volume % in air) Flammability limits (Ø) Auto ignition temperature in air ( C) Quenching distance (mm) Stoichiometric fuel/air mass ratio Stoichiometric volume fraction % Calorific value (kJ/kg)
0.72 4.3e15 0.4e1.6 723 2.1 0.069 9.48 45765
induction duration. The complete circuit is shown in Fig. 2A and the gas injection circuit is shown separately in Fig. 2B [35]. The major specifications of the engine and the instruments used are shown in Table 3. An AVL DiGAS 444 exhaust gas analyzer is used to measure the emission from the engine. An engine speed dependent gas injection strategy is formulated to control and alter the CNG flow into the engine. As the objective is to study the effect of different amounts of CNG on the performance and emission of the engine, the CNG injection strategies are formulated to ensure that the quantity of the CNG can be increased in subsequent steps. The formulation of CNG injection strategies are solely based on the injection duration of the engine. As per the valve timing diagram shown in Fig. 3, it can be seen that the intake
Fig. 2. A: Complete engine setup diagram. B: CNG injection circuit.
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2.3. Experimental methodology
Table 3 Engine specifications. Engine type
Kirloskar, Model TV-1, 4-stroke water-cooled, VCR engine. (Assembled by APEX Innovations Pvt. Ltd. Sangli, Maharastra)
Bore and stroke Max. power Compression ratio Swept volume Fuel injection pressure Dynamometer
87.5 mm and 110 mm 3.5 kW (1500 RPM) 17.5 661 cc 205 bars Model-AG10, Eddy Current Type, Make: Saj test plant Pvt. Ltd.) Model 8.3700.1321.0360,Make: Kubler Model-6056A31U20, Piezoelectric type, Make: KISTLER.
Crank angle sensor Pressure transducer
valve of the engine remain open for (180 þ 35.5 þ 4.5 ) ¼ 220 of crank rotation. Within this intake duration, there is a valve overlap of (4.5 þ 4.5 ) ¼ 9 that reduces the effective intake duration to (220 e 9 ) ¼ 211. This intake duration is divided into 5 parts. As a result, 5 injection strategies with increasing angular duration are obtained. Hence, CNG was injected for 42 , 84 , 126 , 168 and 210 of crank rotation, which means, for the 1st injection strategy, CNG was injected for 42 of crank rotation, for 2nd injection strategy CNG was injected for 84 and so on. These injection durations are converted into time-scale by using Eq. (1). Hence, CNG is injected into the intake manifold of the engine for 4500 ms, 9000 ms, 13500 ms, 18000 ms and 22500 ms. These increasing CNG injection durations allowed five strategies in which the volume flow rate of the CNG could be systematically increased. The complete calculation of CNG injection duration has been shown in supplementary material-4 [10].
Injection Duration ðmSec:Þ ¼ where,
5
60 q 106 N 360
q ¼ Degree of crank rotation for a Specific injection strategy: N ¼ RPM for the same strategy:
(1)
The engine tests were conducted at a constant speed of 1500 RPM and at a constant liquid fuel injection angle of 23 BTDC. Initially the engine was run with Diesel with six different loading options (20%, 40%, 60%, 80%, 100% and 120% of full load). This provides the baseline data set for comparison. Following this, the engine was tested under the CNG-Diesel duel fuel operation under the same loading conditions. After testing the engine with the Diesel-CNG combination, the engine was allowed to run on Diesel alone to completely remove all the liquid fuel from the liquid fuel supply line. This was done to avoid any contamination of the DBE blends. Once the testing with the pilot Diesel was completed, the D45E15B40 and D30E20B50 blends were introduced into the engine simultaneously and the engine was tested with the different CNG strategies as discussed in Section 2.2. The engine was allowed to run for about 5 min so that the steady state condition of the engine was achieved and only after that were the performance and emission data recorded. Any cyclic variation that might have occurred during the data acquisition was avoided by averaging the data for 80 cycles. Moreover, each set of data was taken six times and averaged to increase the accuracy of the data measured. The CNG injection pressure was carefully maintained at 1.2 bars. The engine speed was also maintained constant (±10 RPM). Special care was taken to maintain a constant rate of water flow into the engine and the calorimeter. The emission analyzer was introduced on to the exhaust pipe after the steady state condition of engine was achieved. The engine tests were conducted at an ambient temperature of 30 C and at a relative humidity of 70%. 3. Measured data uncertainty analysis All measurements of the physical quantities include some degree of uncertainty owing to their different sources, namely the instrumentation used, its calibration, observation accuracy and the experimentation methodology [10,25]. Hence, it is of prime importance to establish an uncertainty analysis with respect to the repeatability and precision of the experimentation. The combined uncertainty analysis for the performance parameters has been performed on the basis of the root mean square method, where the total uncertainty U of a quantity Q has been estimated, depending on the independent variables x1, x2,…,xn (i.e., Q ¼ f [x1,x2,…,xn]) having individual errors Dx1, Dx2,…,Dxnas given by Eq. (2) [26]. The percentages of uncertainty of the performance parameters are shown in Table 4.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 vU vU vU DU ¼ DX1 þ DX2 þ/þ DXn vX1 vX2 vXn
(2)
The measuring range and accuracy of the AVL DiGAS 444 emission analyzer is given in Table 5. 4. Result and discussion 4.1. Combustion analysis The combustion of the engine is evaluated on the basis of its cylinder peak pressure, maximum heat release rate, angle of maximum heat release rate and mass fraction burn.
Fig. 3. Valve timing diagram of the engine.
4.1.1. Maximum cylinder pressure Variations in the maximum cylinder pressure with different CNG injection durations for the pilot operations using the Diesel, D45E15B40 and D30E20B50 blends are shown in Fig. 4. It is evident that at each and every load condition and at each and every
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Table 4 Total percentage of uncertainty of the computed performance parameters. Computed performance Measured variables Instrument involved in parameter measurement BP (brake power)
Load, RPM
Pilot fuel flow
SFC (liquid fuel)
BSEC
SFC (liquid fuel) BP CNG flow
Load sensor, load indicator, speed measuring unit. Fuel measuring unit, fuel flow transmitter As for SFC measurement as for BP measurement CNG flow controller.
% Uncertainty of the Calculation measuring instrument [10] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0.2, 0.1, 1.0. ð0:2Þ2 þ ð0:1Þ2 þ ð1:0Þ2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0.065, 1.5. ð0:065Þ2 þ ð1:5Þ2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1.501 1.02 0.25 ð1:501Þ2 þ ð1:02Þ2 þ ð0:25Þ2
Total % uncertainty of the computed parameters 1.02. 1.501 1.83
Table 5 Accuracy of the emission measuring. Instrument
Measuring range
Accuracy
AVL DiGAS 444 five gas analyzer Carbon monoxide (CO) Carbon dioxide (CO2) Hydrocarbon (HC) NOx Oxygen (O2)
0e10% vol 0e20% vol 0e20000 ppm vol 0e5000 ppm vol 0e22% vol
<0.6% vol: ±0.03% vol > 0.6% vol: ±5% <10% vol: ±0.5% vol > 10% vol: ±5% vol <200 ppm vol: ±10 ppm vol > 200 ppm vol: ±5% <500 ppm vol: ±50 ppm vol. 500 ppm vol: ±10% <2% vol: ±0.1% vol 2% vol: ±5% vol
CNG injection strategy, the pilot operation of the Dieseleethanolebiodiesel blends produces higher cylinder pressure than does the pilot Diesel operation. It is also found that the D30E20B50 blend pilot operation produces a higher cylinder pressure than does the D45E15B40 blend. The cylinder pressure can be regarded as an index of the quality of the combustion inside the cylinder. Better combustion produces a higher kinetic energy of the gas molecules inside the combustion chamber, which results in a higher cylinder pressure. The increase in the maximum cylinder pressure with the Dieseleethanolebiodiesel blends indicates that these blends cause better combustion of the charge, which may be possible due to the higher oxygen content of the pilot fuel. The oxygen released as a result of the decomposition of the pilot fuels partially compensates for the loss of oxygen intake due to the CNG supplementation. Again, because the D30E20B50 has a higher oxygen content than the D45E15B40, the greater heat release with the D30E20B50 is quite acceptable. It is also observed that the cylinder pressure decreases with an increasing CNG induction, regardless of the pilot fuel. This is because of the retardation in the rate of combustion due to the reduced oxygen content inside the combustion chamber.
Again, it is also observed that the maximum cylinder pressure increases with the load for all the pilot fuel operations. This is because, with any increase in the load the BMEP increases (constant RPM), which produces a favorable condition for combustion, and that in turn results in the higher in-cylinder pressure. 4.1.2. Maximum rate of heat release The blends of Dieseleethanol and biodiesel (PPME) are used as pilot fuel in this study. The blend ratio, calorific value and mass flow rate of the blends are known. Further, the mass flow rate of CNG and its calorific value are also known. The cumulative heat input is calculated from these parameters. The procedure of calculation is show in supplementary material-3. The heat release from the engine has been calculated as per Eq. (3) [27].
ROHR ¼
dQ CV ¼ ¼ dq R
p:
dV dp PV dM þ V: dq dq m dq
þP
dV dq
(3)
The variations observedinthe maximum heat release rates for the CNGeDiesel, CNG-D45E15B40 blend and CNG-D30E20B50
Fig. 4. Variation of maximum cylinder pressure with CNG injection duration for different pilot fuels.
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Fig. 5. Variation in the maximum rate of heat release with the CNG injection duration for different pilot fuels.
blend are shown in Fig. 5. It is clear that the maximum rate of heat release from the engine decreases with the pilot operation of the Dieseleethanolebiodiesel blends. The heat released from a fuel depends upon several factors such as the calorific value of the fuel, combustion quality, equivalence ratio etc. Due to the lower calorific value of the ethanol, the calorific value of the blend decreases. This results ina decrease in the total heat input to the engine in spite of a better quality of combustion. Hence, the pilot fuel flow represented in Fig. 10 is found to increase. The maximum rate of heat release is also found to decrease with the increasing CNG induction inside the combustion chamber. This diminution is found due to the presence of the high fuel rich mixture inside the chamber that retards the combustion process and restricts the heat release rate [28]. This fuel rich mixture also reduces the combustion quality and increases the unburned hydrocarbon emission as shown in Fig. 13. It can also be seen from Fig. 6 that the maximum heat release for the Dieseleethanolebiodiesel blends occurs at an earlier crank angle than it does for the pilot Diesel operation. This suggests that the ignition of the Dieseleethanolebiodiesel blends occurs at an earlier crank angle, which subsequently causes the maximum heat release at an earlier crank angle. It is also seen that the maximum heat release
from the D30E20B50 blend pilot operation occurs at an earlier crank angle than that of the D45E15B40 blend pilot operation. Therefore, it can be understood that the increase in the percentage of the biodiesel in the pilot fuel is beneficial for the dual fuel application. This occurs because biodiesel has the highest cetane number compared with the other fuels used in the study, which signifies a lower ignition temperature. Therefore, the fuel with the higher cetane number ignites earlier and burns relatively quicker, resulting in the maximum heat release at an earlier crank angle.
4.1.3. Mass fraction burn The mass fraction burned has been calculated according to Rassweiler and With row procedure [29] and is expressed in Eq. (4) [29].
Pj Dpc ðkÞ mb ðjÞ ¼ Pk¼0 MFB ¼ M mb ðtotalÞ k¼0 Dpc ðkÞ
(4)
where, M ¼ The total number of crank angle intervals, '0' denotes the start of combustion, 'N' denotes the end of combustion, Dpc ðkÞ is calculated as Eq. (5) [29],
Fig. 6. Variation in the angle of maximum rate of heat release with CNG injection duration for different pilot fuels.
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Fig. 7. Variation in the angle of 50% mass fraction burn with CNG injection duration for different pilot fuels.
Fig. 8. Variation in the angle of 90% mass fraction burn with CNG injection duration for different pilot fuels.
Fig. 9. Variation in the brake thermal efficiency with CNG injection duration for the different pilot fuels.
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Fig. 10. Variation in the pilot fuel flow with CNG injection duration for the different pilot fuels.
Fig. 11. Variation in the NOx emission with CNG injection duration for the different pilot fuels.
Fig. 12. Variation in the CO emission with CNG injection duration for the different pilot fuels.
Please cite this article in press as: Paul A, et al., An experimental study of the performance, combustion and emission characteristics of a CI engine under dual fuel mode using CNG and oxygenated pilot fuel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.050
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Fig. 13. Variation in the hydrocarbon emission with CNG injection duration for the different pilot fuels.
Dpc ðjÞ ¼ pjþ1 pj
Vj Vjþ1
!n (5)
Figs. 7 and 8 show respectively, the 50% and 90% mass fraction burn of the charge in terms of the crank angle for all the fuel combinations tested. The graphs indicate that the rate of burning for the pilot operation of the Dieseleethanolebiodiesel blends is higher as they burn at a lower crank angle. It is also found that the D30E20B50 blend pilot operation initiates the burning of the charge at lower crank angles compared with the D45E15B40 pilot operation. It also found that the D30E20B50 blend burns from 50% to 90% at a lower crank angle rotation (lower combustion duration) compared with the D45E15B40 pilot blend operation and the pilot Diesel operation. It is also found that with the increase in the CNG induction needs a higher crank angle (combustion duration) for the 50%e90% mass fraction burned. It can be seen that for the 4500 ms CNG injection at 100% load condition, 50% burning of the charge occurs at 370 of the crank angle for the pilot Diesel operation, at 369 of the crank angle for the pilot D45E15B40 blend operation and at 368 of the crank angle for the pilot D30E20B50 blend operation. But with the 22500 ms CNG injection and the 100% load condition, the 50% burn angle rises up to the 379 crank angle for the pilot Diesel operation, the 378 crank angle for the pilot D45E15B40 blend operation and to the 377 crank angle for the pilot D30E20B50 blend operation. The higher cetane number of the blend enables the charge to ignite at an earlier crank angle and subsequently the burning process completes at the earlier crank angle. On the other hand, the CNG being a fuel of high octane number ignites at later stage of combustion when it achieves selfignition temperature. 4.2. Performance analysis The performance of the engine is evaluated on the basis of the brake thermal efficiency and the pilot fuel flow for the estimated CNG injection durations under the load conditions tested. 4.2.1. Brake thermal efficiency Brake thermal efficiency of an engine is the ratio of brake power to the input fuel energy in appropriate units [30]. It has been calculated as per Eq. (6).
hbth ¼
Brake Power ðFuel flow rate=SÞ ðCalorific value of fuelÞ
(6)
Fig. 9 shows the variations in the brake thermal efficiency of the engine at the load conditions tested for the CNG-Diesel, CNGD45E15B40 blend and the CNG-D30E20B50 blend operations. It is evident that both the DieseleethanolePPME blends perform better than the Diesel with CNG induction. It can be seen from the graph that the addition of the ethanol and biodiesel in the pilot fuel enables a better utilization of the CNG and as the percentage of ethanol and biodiesel increases, the hbth of the engine increases simultaneously. It is found that the D45E15B40 blend with 22500 ms CNG injection increases the hbth by 50.18% at a 20% load, whereas the D30E20B50 blend increases it by 57.66% when compared with the baseline Diesel operation. At 60% load, the D45E15B40 blend increases the hbth by 2.5% while the D30E20B50 blend increases it by 11.07%. Similarly, at full load, the D45E15B40 blend increases the hbth by 23.06% while the D30E20B50 blend increases it by 46.54%. This significant rise in the hbth is due to the oxygenated content present in the blend, which are the ethanol and PPME. The oxygen liberated from these compounds partially compensates for the decrease in the intake of oxygen due to the CNG substitution. As a result, the engine gets more oxygen to burn the charge completely and this results in a higher hbth. 4.2.2. Pilot fuel flow The pilot fuel plays an important role in the dual fuel combustion system, as it is responsible for the initiation of combustion. Hence, the calorific value and burning quality of the pilot fuel significantly influences the total combustion process. Fig. 10 shows the pilot fuel flow under the load conditions tested for the CNGDiesel, CNG-D45E15B40 blend and CNG-D30E20B50 blend operations. It is found that a greater amount of the pilot fuel is required for both the Dieseleethanolebiodiesel blends under the CNG dual fuel operation. This may be due to the fact that the CNG requires a particular temperature of a 540 C [3] for self-ignition. When the test is conducted using Diesel, it is found that a lesser amount of fuel is required for burning to achieve that particular temperature which may be the cause for the higher calorific value of Diesel. However, the calorific value of the D45E15B40 and D30E20B50 blends is lower and consequently a higher quantity of fuel is required to achieve the desired ignition temperature. It is also
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A. Paul et al. / Energy xxx (2015) 1e14
observed that with the increase in the load, the pilot fuel flow also increases. This is because, with an increase in the load, the engine requires a higher amount of energy to overcome the load at the same RPM. As the amount of CNG being injected at a particular strategy does not increase, the energy shortage is compensated for by the pilot fuel. 4.3. Emission analysis 4.3.1. NOx emission The NOx emission from an engine depends on several factorsincludinghigh in-cylinder temperature, equivalence ratio, oxygen content of the fuel, nitrogen content of the fuel, nitrogen present in the air etc [31]. In Fig. 11, the variations in the NOx emissions at different load conditions for the dual fuel combinations tested are seen. It is evident from the graph that the NOx emission from the engine decreases with the increase in the CNG percentage, irrespective of the pilot fuel variation. This decrease may be attributed to several factors. The CNG has a very low percentage of nitrogen in it. Therefore, the fuel NOx formation is quite low for the CNG. Simultaneously, the injection of CNG produces a cooling effect due to throttling. This reduces the in-cylinder temperature and reduces the thermal NOx emission. It can also be seen that the addition of the ethanol and PPME further reduces the NOx emission in the dual fuel mode. This reduction in the NOx emission can also be attributed to the charge cooling by the ethanol, which evaporates inside the combustion chamber and absorbs the heat from the charge. As a result, it reduces the in-cylinder temperature. As the higher temperature prominently influences the NOx emission, the decrease in the temperature reduces the NOx emission. 4.3.2. CO emission The CO emission from the engine is shown in Fig. 12. It is evident from the Figure that the CO emission from the engine decreases with the increasing load. This is because, as the load increases, the BMEP of the engine increases along with it. This increase in the BMEP is conducive to better combustion inside the combustion chamber. As the CO emission is a result of the incomplete combustion of the fuel carbon, any improvement in the quality of combustion reduces the CO emission. It can also be seen that the addition of ethanol and PPME to the pilot fuel reduces the CO emission. It is a well-known fact that CO oxidizes to produce CO2 and the reactions are shown below [32]. CO þ O2 / CO2 þ O
(7)
O þ OH / OH þ OH
(8)
CO þ OH / CO2 þ H
(9)
H þ O2 / OH þ O
(10)
Equations (7) and (9) are the main oxidation reactions, whereas Equation (8) and10are the chain branching reactions. The first reaction shown in Eq. (7) is the initialization reaction, but it is slow. The second reaction shown in Eq. (9) is the main oxidation step [32]. Hence, it is evident that the increase in the 'H' molecule and the ‘OH’ radical is conducive to a reduction in the CO emission. Therefore, a reduction in the CO emission by the DieseleethanolePPME blends may be due to the fact that these blends release significant amounts of the ‘OH’ radical and ‘H’ molecules because ‘OH’ is present in the functional groups of ethanol (C2H5OH) and PPME which consist of hydrocarbons of different chain lengths, and can release the ‘H’ molecules during combustion.
11
Therefore, much of the CO is oxidized to CO2 and subsequently the CO emission gets reduced. 4.3.3. HC emission Hydrocarbons are one of the primary and necessary constituents of conventional fuels. Under ideal conditions the entire hydrocarbon content present in the fuel should burn completely and produce CO2 and water vapor. Factors such as the presence of a fuel rich mixture, lubricant contamination, charge cooling etc., causes deterioration in the combustion quality and results in the emission of the unburned hydrocarbon [33e35]. Fig. 13 shows the HC emission from the engine. It is evident from the Figure that the hydrocarbon emission increases with the increase in the CNG injection for all the pilot fuel combinations. This occurs because the induction of the CNG into the combustion chamber reduces the mass of the intake air of the engine and produces a fuel-rich charge. This charge fails to burn completely in the absence of sufficient air and the unburned and partially burned hydrocarbons are released via the exhaust. This increase in the hydrocarbon emission due to the CNG induction is found to be high during the longer durations of the CNG induction which implies that there is an excess of the CNG injection than the requirement. It is also clear that the hydrocarbon emission reduces with the increasing load conditions, irrespective of the pilot fuel. This occurs because with the increase in the load, the BMEP of the engine increases, creating a favorable environment for the combustion inside the combustion chamber. As a result, the combustion quality improves and the hydrocarbon emission reduces. The hydrocarbon emission is found to increase for both the DieseleethanolePPME blend pilot operations. This increase may be due to the cooling of charge caused by the evaporation of the ethanol inside the combustion chamber. 5. Performance-emission tradeoff A tradeoff study is basically a comparative assessment among three or more parameters to find the effect of a certain parameter on the behavior of the other parameters. The following is a tradeoff analysis among the brake thermal efficiency, NOx emission and BSEC for the different durations of the CNG injection for different pilot fuel combinations. Figs. 14e18 show the comparison between the brake thermal efficiency, NOX emission, and BSEC for the durations of the CNG injection of 4500 ms, 9000 ms, 13500 ms, 18000 ms, and 22500 ms,respectively. Fig. 14 shows the tradeoff between NOX emission, brake thermal efficiency and BSEC of the engine with CNG injection duration for 4500 ms. It is observed that at low load conditions, the differentiation between pilot Diesel operation, pilot D45E15B40 (represented as E15) blend operation and pilot D30E20B50 (represented as E20) blend operation is marginal as the tradeoff points for the said pilot blends are clustered around each other for a particular load. A more minute observation reveals that the pilot blends are able to yield higher thermal efficiency and lower NOX emission than pilot Diesel operation. The BSEC of the engine however remained almost same for all the mentioned pilot fuels. At medium load conditions, the improvement in brake thermal efficiency by pilot D45E15B40 and pilot D30E20B50 blend operations are more prominently observed as the tradeoff points are found in the higher hbth zone. However, at this load conditions, the NOX emission for pilot D45E15B40 and pilot D30E20B50 blend is found to be higher that pilot Diesel operation which signifies better combustion of the charge. This also points that CNG with pilot D45E15B40 and pilot D30E20B50 blend offers better performance characteristics than pilot Diesel operation. The BSEC for both the pilot blend operation is observed to be marginally higher than pilot Diesel operation. At high load conditions, a distinct advantage of the proposed blends as pilot fuel can
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A. Paul et al. / Energy xxx (2015) 1e14
Fig. 14. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 4500 ms.
Fig. 16. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 13500 ms.
be observed as the both the blends are observed to produce significantly higher brake thermal efficiency with reduced or similar NOX emission. The variation of BSEC for the different pilot fuels is also found to be almost same. The tradeoff analysis between brake thermal efficiency, NOX emission and BSEC for CNG injection duration of 9000 ms is shown in Fig. 15. Here also the pilot operation with the tested blends is found to improve the brake thermal efficiency of the engine as compared to pilot Diesel operation. Along with that, the NOX emission from the engine is found to decrease appreciably, thus drawing the tradeoff zone downward to a low NOX region. This decrease in the NOx emission may be attributed to the cooling effect produced by the evaporation of ethanol [10]. It is again observed that the NOx emission from the D30E20B50 blend is slightly higher than that of the D45E15B40 blend. This may be due to the presence of the higher percentage of biodiesel, which
promotes better combustion and increases the NOx formation by producing a higher in-cylinder temperature [36,37]. BSEC is again found to be almost same for all the tested pilot fuel blends. Fig. 16 shows the comparison between the brake thermal efficiency, NOx emission and BSEC for the CNG injection duration of 13500 ms. The decrease in the brake thermal efficiency with increasing the CNG induction is more prominent here, as an increase in the CNG induction has significantly reduced the oxygen induction in the cylinder. However, the oxygen rich blends are again found to be promising, being able to compensate for a part of the oxygen deficiency due to which the brake thermal efficiency is found to be higher than the pilot Diesel operation at the same load level. The considerable decrease in the NOx emission for both the pilot Diesel as well as the pilot blend operation is evident from this graph. For the CNG Diesel operation, this drop off is observed
Fig. 15. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 9000 ms.
Fig. 17. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 18000 ms.
Please cite this article in press as: Paul A, et al., An experimental study of the performance, combustion and emission characteristics of a CI engine under dual fuel mode using CNG and oxygenated pilot fuel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.050
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Fig. 18. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 22500 ms.
because of the reduced combustion temperature due to the incomplete combustion caused by the oxygen deficiency. For the CNG-blend operation, this decrease is also aggravated by the cooling effect of the ethanol. The BSEC of the engine is again found to be similar for all the pilot fuel combinations and it is affected only by increasing the loading conditions. Figs. 17 and 18 show the tradeoff zones at 18000 ms and 22500 ms of the CNG injection durations, respectively. These CNG injection strategies represent a prominent decrease in the brake thermal efficiency and NOx emission. These decreases are conspicuous due to the retardation of the combustion due to the paucity of oxygen in the inducted air. It can also be seen that the D30E20B50 blend shows the best brake thermal efficiency and a significantly lower NOx emission when compared with the pilot Diesel operation under high load conditions. The increase in the brake thermal efficiency with this blend occurs because of the presence of the higher percentage of ethanol (20%) and biodiesel (50%) which can release a higher percentage of the oxygen to support combustion. The BSEC here also shows no variance with a change in the pilot fuel and inversely changes with an increasing load. The study confirms the attractive potential of the Dieseleethanolebiodiesel blends tested as the pilot fuel for the CNG dualfuel operation. It is clearly evident from the above discussion that these blends are quite capable of improving engine performance and emission. It is also observed that the blends are most effective under high load conditions. It is also observed that the D45E15B40 blend is more effective in NOx reduction, whereas the D30E20B50 blend is more efficient in increasing the brake thermal efficiency. The best tradeoff zone is observed at the 120% load condition with the CNG-D30E20B50 blend dual fuel operation with the CNG injection duration of 9000 ms.
6. Conclusion The experimental work presented here is conducted on a partially modified CI engine. The conventional pilot fuel Diesel in a dual fuel operation is replaced with two blends of Dieseleethanolebiodiesel in the ratios of 45:15:40 (D45E15B40) and 30:20:50 (D30E20B50),respectively. The CNG is used as the primary fuel and is indirectly injected into the engine through the intake
13
manifold. The CNG injection duration is varied from 4500 ms to 23000 ms with a step increment of 4500 ms. The engine performance and emission are evaluated using the pilot fuels mentioned earlier and the CNG injection durations. Further, a performanceemission tradeoff study is also done to study the variation in the NOx emission and BSEC with increasing brake thermal efficiency for the CNG-Diesel, CNG-D45E15B40 blend and CNG-D30E20B50 blend dual fuel combinations. The pilot operation of the D45E15B40 and D30E20B50 blends is found to be beneficial in improving engine performance as it improves the brake thermal efficiency. However, due to the reduced calorific value of the blends, the pilot fuel flow is found to increase marginally for the mentioned Dieseleethanolebiodiesel blends. Again, the blends are found to decrease the NOx emission of the engine, and the major decrease is observed with the D30E20B50 blend pilot operation. The CNG dual fuel operation with the blends is also found to produce lower CO emissions than the CNG-Diesel dual fuel operation. The hydrocarbon emission from the engine with the CNG-Diesel dual fuel operation is found to increase with the increasing CNG injection durations. The tradeoff study indicates that the CNG injection upto 9000 ms is best suited for achieving an optimal performance at a reduced NOx emission. It also highlights that the D30E20B50 blend is more suited for the higher durations of the CNG injection. It can be concluded from the study that the inclusion of a higher percentage of ethanol and biodiesel in the pilot fuel can extend the maximum limit of the CNG injection duration to 9000 ms with a significant corresponding increase in the brake thermal efficiency and a commendable decrease in all the emission parameters. Appendix ASupplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.energy.2015.04.050. References [1] Yu Byeonghun, Kum SM, Lee CE, Lee S. Effects of exhaust gas recirculation on the thermal efficiency and combustion characteristics for premixed combustion system. Energy 1 January 2013;49. ISSN: 0360-5442:375e83. http://dx. doi.org/10.1016/j.energy.2012.10.057. [2] Karabektas M, Ergen G, Hosoz M. The effects of using diethylether as additive on the performance and emissions of a diesel engine fuelled with CNG. Fuel January 2014;115. ISSN: 0016-2361:855e60. http://dx.doi.org/10.1016/j.fuel. 2012.12.062. [3] Das LM, Gulati R, Gupta PK. A comparative evaluation of the performance characteristics of a spark ignition engine using hydrogen and compressed natural gas as alternative fuels. Int J Hydrogen Energy 1 August 2000;25(8). ISSN: 0360-3199:783e93. http://dx.doi.org/10.1016/S0360-3199(99)00103-2. [4] Mustafi NN, Raine RR, Verhelst S. Combustion and emissions characteristics of a dual fuel engine operated on alternative gaseous fuels. Fuel July 2013;109. ISSN: 0016-2361:669e78. http://dx.doi.org/10.1016/j.fuel.2013.03.007. € M, Fallgren H, Westerlund J, Sjo €din Å. Particle and [5] Hallquist ÅM, Jerksjo gaseous emissions from individual diesel and CNG buses. Atmos Chem Phys 2013;13:5337e50. http://dx.doi.org/10.5194/acp-13-5337-2013. [6] Karabektas M, Ergen G, Hosoz M. The effects of using diethylether as additive on the performance and emissions of a diesel engine fuelled with CNG. Fuel January 2013;13. ISSN: 0016-2361. Available online, http://dx.doi.org/10. 1016/j.fuel.2012.12.062. [7] Serrano D, Bertrand L. Exploring the potential of dual fuel diesel-CNG combustion for passenger car engine. In: Proceedings of the FISITA 2012 World Automotive Congress, Lecture Notes in Electrical Engineering, vol. 191; 2013. p. 139e53. [8] Cheenkachorn K, Poompipatpong C, Ho CG. Performance and emissions of a heavy-duty Diesel engine fuelled with diesel and LNG (liquid natural gas). Energy 1 May 2013;53. ISSN: 0360-5442:52e7. http://dx.doi.org/10.1016/j. energy.2013.02.027. [9] Cordiner S, Rocco V, Scarcelli R, Gambino M, Iannaccone S. Experiments and multi-dimensional simulation of dual-fuel diesel/natural gas engines. 2007. http://dx.doi.org/10.4271/2007-24-0124. SAE Technical Paper 2007-24-0124. [10] Paul A, Bose PK, Panua RS, Banerjee R. An experimental investigation of performance-emission trade off of a CI engine fueled by dieselecompressed natural gas (CNG) combination and dieseleethanol blends with CNG enrichment. Energy 7 May 2013 ISSN: 0360-5442. Available online, http://dx.doi. org/10.1016/j.energy.2013.04.002.
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Energy journal homepage: www.elsevier.com/locate/energy
An experimental study of the performance, combustion and emission characteristics of a CI engine under dual fuel mode using CNG and oxygenated pilot fuel blends Abhishek Paul a, *, Raj Sekhar Panua a, Durbadal Debroy a, Probir Kumar Bose b a b
Department of Mechanical Engineering, National Institute of Technology, Agartala 799055, India Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2014 Received in revised form 1 April 2015 Accepted 14 April 2015 Available online xxx
During the past few decades, many researchers have proposed a partial replacement of Diesel in the CI engine by substituting it with natural gas in order to reduce the exhaust emission without altering the performance characteristics of the engine too much. Most of these researches have focused on the combustion of natural gas in a dual fuel mode by using Diesel as the pilot fuel. However, the dual fuel operation of natural gas with pilot Diesel reduces the brake thermal efficiency and increases the hydrocarbon emission. The present experimental work explores the potential of using CNG under dual fuel operation by utilizing two different blends of Dieseleethanolebiodiesel as the pilot fuel. The present study reveals that the increased percentages of ethanol and biodiesel in the pilot fuel triggers an increase in the brake thermal efficiency of the engine. In this study, the NOx emission was also found to decrease with a corresponding increase in the percentages of ethanol and biodiesel in the pilot fuel. The study reveals definite encouraging aspects of using the D45E15B40 and D30E20B50 blends as the pilot fuel because it can extend the range of usage of the CNG to 9000 ms of injection duration. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Dieseleethanolebiodiesel blends Oxygenated pilot fuel Peak cylinder pressure Mass fraction burn NOX reduction NOx-Bth-BSEC tradeoff
1. Introduction Over the past few decades, the rapid growth of industrialization and urbanization has drastically increased the usage of personal and industrial vehicles, which run on petroleum-based fuels. Simultaneously, this has also intensified the emission of noxious gases into the atmosphere. It is estimated that the combustion of fossil fuels releases about 7 tons of greenhouse gas annually [1]. Due to this unrestrained usage of fossil fuel, the natural reservoir of crude petroleum that can be economically excavated is dwindling by the day. However, the growing concern over the deteriorating environmental condition due to industrial and vehicular emissions has prompted the enforcement of stringent emission laws on a global scale. All these restrictions have attracted several researchers to the field of emission control and performance enhancement methods for conventional vehicles. In our modern era, a major part of the transportation sector utilizes the Diesel engine due to its greater efficiency and higher
* Corresponding author. E-mail address: [email protected] (A. Paul).
power output, which results in saving fuel costs. However, the Diesel engine is also accountable for the high emissions of NOX, particulate matters and other polycyclic aromatic hydrocarbons. In order to cope with the strict guidelines of emission norms and the gradual reduction in the fuel supplies, several alternatives in fuel technology have been investigated in recent years. A feasible alternative that has emerged from these researches is the dual fuel operation of the conventional CI engine where a gaseous fuel is used as the primary fuel and a small amount of liquid fuel as the pilot fuel. Thus far, natural gas has been the most extensively used gaseous fuel for the IC engine. Although a good proportion of these studies have been done on the application of natural gas in the spark ignition engine, the use of natural gas in the compression ignition engine has also attracted a significant number of researchers. The usage of natural gas in the Diesel engine produces cleaner combustion as it contains a minimal quantity of impurities due to its gaseous state [2]. It also offers several other advantages such as better availability, lower market price, high octane number etc. In addition it has a high self-ignition temperature of 540 C [3]. All these properties enable the use of natural gas in Diesel engines in high compression ratios without knocking. Hence, a significant
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A. Paul et al. / Energy xxx (2015) 1e14
Nomenclature CNG compressed natural gas NO nitric oxide BSEC brake specific energy consumption CO carbon monoxide PPME Pongamia pinnata methyl ester CO2 carbon dioxide mSec micro second HC hydrocarbon DAQ data acquisition 20%L diesel 20%D100 ¼ 20% load with Diesel as pilot fuel
number of research works have been conducted by different researchers in the field of gaseDiesel engines based on the properties of natural gas. Mustafi et al. [4], studied the combustion and emission characteristics of a dual fuel CI engine, operated on gaseous fuels such as natural gas and biogas. It was found that the maximum net heat release was 27% higher for the natural gas dual fuel operation and 30% higher for the biogas dual fuel operation as compared with the baseline Diesel. Significantly lower PM and NOx emissions were observed with both the gaseous fuels. Hallquist et al. [5], discovered that the buses running on CNG emitted more amounts of fine particulates but less quantity of particulates when compared with the Diesel-fueled buses. Karabektas et al. [6], and Serrano et al. [7], separately concluded that the dual fuel mode of the CI engine operation yielded higher CO and HC (Hydrocarbon) emissions at all loads, besides lower NO emissions other than high loads. Cheenkachorn et al. [8], concluded that a maximum of 77.90% of natural gas at 1300 rpm could be used in dual fuel engine operation. Cordiner et al. [9], concluded that for high degrees of CNG substitution, a significant improvement in PM emission could be achieved. All the researches cited in this study reveal that the increase in the Diesel fuel supplementary ratio resulted in reduced NOx and soot emissions compared with the normal Diesel operation. Both CO and HC emissions were observed to be higher than the normal Diesel operation in the previous studies. Based on these findings, an experimental work was conducted by the same researchers using 5 and 10% ethanol blends in the Diesel as the pilot fuel together with CNG to improve the CO and HC emissions [10]. The results revealed a promising trend with the CNG dual fuel operation and Dieseleethanol blend as the pilot fuel. For lesser CNG injection strategies, an increase in the ethanol percentage in the pilot fuel produced a higher hbth than the base diesel operation, with a commendable decrease in the BSEC. In terms of the emission parameters too, the CNG dual fuel operation with the pilot Dieseleethanol blends produced better results with an increasing ethanol content. A reasonable decreasing trend in the NOx emission for lesser CNG injection strategies was observed with an increasing percentage of ethanol in the pilot fuel. The hydrocarbon emission from the engine was seen to increase with the CNG e Diesel operation. The hydrocarbon emission was found to decrease with the addition of ethanol to the pilot fuel. The D90E10 blend used with 10% ethanol produced an almost identical hydrocarbon emission compared with the base Diesel operation. However, the use of ethanol in Diesel is greatly hindered by the limited miscibility of the ethanol in the Diesel. In this way, the phase separation restricts the usage of ethanol in Diesel beyond 10% at normal temperature. To solve this problem and to increase the ethanol participation in Diesel, a co-solvent can be utilized so that the properties of the blend may be kept as close to that of the Diesel. Biodiesel derived
BTDC before top dead center 20%L E15 20%E15 ¼ 20% load with D45E15B40 as pilot fuel ROHR rate of heat release 20%L E2020%E20 ¼ 20% load with D30E20B50 as pilot fuel DBE blends dieselebiodieseleethanol blends D45E15B40 45% (v/v) diesel, 15% (v/v) ethanol and 40% biodiesel D30E20B50 30% (v/v) diesel, 20% (v/v) ethanol and 50% biodiesel hbth brake thermal efficiency cSt centistokes LHV lower heating value NOx oxides of nitrogen
from different sources is reported to act as a co-solvent to enhance the ethanol miscibility in the Diesel [11e13]. Several additives such as alkanols, decaglycerol mono-oleate (MO750), and alkanolamides have been used over the years to increase the miscibility of the ethanol in the Diesel [14e16]. However, biodiesel has become the most commonly used co-solvent to increase miscibility of ethanol in Diesel. Besides increasing the miscibility, the biodiesel also improves the cetane content of the blend as it has a higher cetane number than Diesel [17,18]. Biodiesel is also enriched with oxygen due to the presence of about 11% of dissolved oxygen [18]. This property is instrumental in reducing the soot and hydrocarbon emissions [19,20]. Due to these favorable properties of biodiesel in the Dieseleethanol blends, a significant number of research studies have been conducted in the field of natural gas dual fuel operation. Works by Namasivayam et al. [21,22], Selim et al. [23], and Korakianitis et al. [24], utilized different types of biodiesel with natural gas dual fuel operation. All of these researches revealed significant improvement in the thermal efficiency with a distinct reduction in the hydrocarbon and carbon monoxide emissions. However, they also observed an increase in the NOx emission. The main objective of this investigation is to replace the Diesel to the greatest possible degree and use the biodiesel as the cosolvent for the addition of a higher percentage of ethanol in the pilot fuel for CNG dual fuel operation, which is an extension of the previous work [10]. Increase of ethanol percentage in pilot fuel showed remarkable results as mentioned above, hence increase in ethanol percentage may reveal further improvement in performance, combustion and emission of the engine. Since the benefits of CNG dual fuel operation with Dieseleethanolebiodiesel blends are yet to be investigated as per the knowledge of the authors, so the present work is a new approach in improving the performance and emission of the engine. 2. Experimental setup and procedure 2.1. Test fuels and their properties The base liquid fuels used in this study are mineral Diesel, ~ nata. anhydrous ethyl alcohol and the methyl ester of Pongamia pin The Diesel and ethanol are collected from a local fueling station and chemist shop, respectively. As discussed earlier, ethanol has miscibility of up to 10% by volume in Diesel. Any further increase in the percentage of ethanol in the Diesel produces an insoluble twophase heterogeneous mixture. In order to increase the miscibility of the higher percentage of ethanol in the Diesel, PPME (Pongamia ~ nata Methyl Ester) is used as a co-solvent. PPME is combination Pin of methyl esters of different chain lengths. The reason for the better solubility of the higher percentage of ethanol in the Diesel in the presence of PPME is because the PPME can act as an amphiphile
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and form micelles with a non-polar tail and a polar head. In the continuous phase, the polar head of the micelle orients itself to the ethanol, whereas the non-polar tail orients itself to the Diesel [17]. A comprehensive study to discover the minimum amount of PPME required to stabilize a higher quantity of ethanol in the Diesel has been achieved in this work. As a part of this procedure, the ethanol percentage in the blend is at first maintained constant at 15% and only the percentage of the PPME is increased from 5% with increments of 5%. It was found that 40% PPME with 15% ethanol and 45% Diesel is the optimum ratio for a stable blend. Further, the ethanol percentage is then increased to 20% and the PPME percentage is also increased from 5% with an increment of 5%. In this state, the first stable blend is found with 50% PPME, 30% Diesel and 20% ethanol. The two stable blends viz. D45E15B40 (45%Diesel, 15% ethanol and 40% PPME) and D30E15B50 (30% Diesel, 20% ethanol and 50% PPME) are chosen for the engine test. The entire range of blends tested (stable and unstable) are shown in Fig. 1. The properties of the test blends are calculated on the basis of base fuel properties and are shown in Table 1. The CNG used in this study is collected from local CNG fueling station. The properties of CNG are listed in Table 2.
2.2. Experimental setup The experimental study is conducted on a 3.5 kW 4 stroke water-cooled, naturally aspirated CI engine. An eddy current dynamometer (Make: Saj test plant Pvt. Ltd., Model-AG10) is synchronized to the engine for load measurement. The engine RPM is measured with a crank angle sensor (Make: Kubler-Germany, Model 8.3700.1321.0360). The crank angle sensor is calibrated in terms of 1-degree intervals. Two piezoelectric pressure transducers (Make: KISTLER, Type 6056A31U20) measure the in-cylinder pressure and the fuel injection pressure. The CNG is inducted into the engine through the intake manifold along with the incoming
3
Table 1 Properties of base fuels and fuel blends. Property
Diesel Ethanol PPME
D40E15B45
D30E20B50
Density (kg/m3) Kinematic viscosity (cSt) Calorific value (KJ/Kg) Flash point ( C) Fire point ( C) Cetane number
820 2.51 42650 52 64 46
845.05 5.222173244 37250.46648 120.3655 126.625 45
846.8 5.47319556 36175.30468 126.654 131.9 43.3
789 1.09 26950 12.77 13.5 7
886 8.68 35866 217 220 35
air. The intake manifold of the engine is marginally modified to accommodate the CNG injection system. Considering the risks involved in working with a gaseous fuel, a number of safety and flow control devices are built into the CNG injection system. The CNG injection system consists of a CNG cylinder, a gas flow regulator, a pressure gauge, a gas flow meter and a gas injector. The CNG cylinder houses the gas at about 196 bar pressure whereas, the working pressure of the gas injector (Make DYMCO Corp, Modeli1000) is of 1.2 bars. Hence, a pressure regulator (Make: CONCOA, USA, Model-3123322-01-B04)is connected to the CNG flow pipe to reduce the CNG pressure from 196 bars to 1.2 bars. A pressure gauge (Make OMEGA, Model- PGC-25L-600) is connected to the path of the CNG flow line to monitor the gas pressure constantly. A gas flow meter (Make CLESSE, Type- G1.6) is connected after the pressure gauge to measure the volume flow rate of the CNG. The gas flow meter is used to measure the flow rate of CNG. This is done by measuring the time taken for a fixed volume (30CC) flow of CNG. From this data, the flow rate of CNG is calculated. The injector is mounted on the intake manifold at a distance of 1.5 D (where D is the manifold outer diameter) from the engine inlet valve to ensure the formation of a homogeneous mixture of inducted CNG to the incoming air. The gas injector is synchronized to the crank angle rotation by means of a second angle encoder to enable a precise induction of the CNG at any desired crank angle and for any desired
Fig. 1. Miscibility test of the E15 and E20 blends.
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A. Paul et al. / Energy xxx (2015) 1e14 Table 2 Properties of CNG. Properties Density (kg/m3) Flammability limits (volume % in air) Flammability limits (Ø) Auto ignition temperature in air ( C) Quenching distance (mm) Stoichiometric fuel/air mass ratio Stoichiometric volume fraction % Calorific value (kJ/kg)
0.72 4.3e15 0.4e1.6 723 2.1 0.069 9.48 45765
induction duration. The complete circuit is shown in Fig. 2A and the gas injection circuit is shown separately in Fig. 2B [35]. The major specifications of the engine and the instruments used are shown in Table 3. An AVL DiGAS 444 exhaust gas analyzer is used to measure the emission from the engine. An engine speed dependent gas injection strategy is formulated to control and alter the CNG flow into the engine. As the objective is to study the effect of different amounts of CNG on the performance and emission of the engine, the CNG injection strategies are formulated to ensure that the quantity of the CNG can be increased in subsequent steps. The formulation of CNG injection strategies are solely based on the injection duration of the engine. As per the valve timing diagram shown in Fig. 3, it can be seen that the intake
Fig. 2. A: Complete engine setup diagram. B: CNG injection circuit.
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2.3. Experimental methodology
Table 3 Engine specifications. Engine type
Kirloskar, Model TV-1, 4-stroke water-cooled, VCR engine. (Assembled by APEX Innovations Pvt. Ltd. Sangli, Maharastra)
Bore and stroke Max. power Compression ratio Swept volume Fuel injection pressure Dynamometer
87.5 mm and 110 mm 3.5 kW (1500 RPM) 17.5 661 cc 205 bars Model-AG10, Eddy Current Type, Make: Saj test plant Pvt. Ltd.) Model 8.3700.1321.0360,Make: Kubler Model-6056A31U20, Piezoelectric type, Make: KISTLER.
Crank angle sensor Pressure transducer
valve of the engine remain open for (180 þ 35.5 þ 4.5 ) ¼ 220 of crank rotation. Within this intake duration, there is a valve overlap of (4.5 þ 4.5 ) ¼ 9 that reduces the effective intake duration to (220 e 9 ) ¼ 211. This intake duration is divided into 5 parts. As a result, 5 injection strategies with increasing angular duration are obtained. Hence, CNG was injected for 42 , 84 , 126 , 168 and 210 of crank rotation, which means, for the 1st injection strategy, CNG was injected for 42 of crank rotation, for 2nd injection strategy CNG was injected for 84 and so on. These injection durations are converted into time-scale by using Eq. (1). Hence, CNG is injected into the intake manifold of the engine for 4500 ms, 9000 ms, 13500 ms, 18000 ms and 22500 ms. These increasing CNG injection durations allowed five strategies in which the volume flow rate of the CNG could be systematically increased. The complete calculation of CNG injection duration has been shown in supplementary material-4 [10].
Injection Duration ðmSec:Þ ¼ where,
5
60 q 106 N 360
q ¼ Degree of crank rotation for a Specific injection strategy: N ¼ RPM for the same strategy:
(1)
The engine tests were conducted at a constant speed of 1500 RPM and at a constant liquid fuel injection angle of 23 BTDC. Initially the engine was run with Diesel with six different loading options (20%, 40%, 60%, 80%, 100% and 120% of full load). This provides the baseline data set for comparison. Following this, the engine was tested under the CNG-Diesel duel fuel operation under the same loading conditions. After testing the engine with the Diesel-CNG combination, the engine was allowed to run on Diesel alone to completely remove all the liquid fuel from the liquid fuel supply line. This was done to avoid any contamination of the DBE blends. Once the testing with the pilot Diesel was completed, the D45E15B40 and D30E20B50 blends were introduced into the engine simultaneously and the engine was tested with the different CNG strategies as discussed in Section 2.2. The engine was allowed to run for about 5 min so that the steady state condition of the engine was achieved and only after that were the performance and emission data recorded. Any cyclic variation that might have occurred during the data acquisition was avoided by averaging the data for 80 cycles. Moreover, each set of data was taken six times and averaged to increase the accuracy of the data measured. The CNG injection pressure was carefully maintained at 1.2 bars. The engine speed was also maintained constant (±10 RPM). Special care was taken to maintain a constant rate of water flow into the engine and the calorimeter. The emission analyzer was introduced on to the exhaust pipe after the steady state condition of engine was achieved. The engine tests were conducted at an ambient temperature of 30 C and at a relative humidity of 70%. 3. Measured data uncertainty analysis All measurements of the physical quantities include some degree of uncertainty owing to their different sources, namely the instrumentation used, its calibration, observation accuracy and the experimentation methodology [10,25]. Hence, it is of prime importance to establish an uncertainty analysis with respect to the repeatability and precision of the experimentation. The combined uncertainty analysis for the performance parameters has been performed on the basis of the root mean square method, where the total uncertainty U of a quantity Q has been estimated, depending on the independent variables x1, x2,…,xn (i.e., Q ¼ f [x1,x2,…,xn]) having individual errors Dx1, Dx2,…,Dxnas given by Eq. (2) [26]. The percentages of uncertainty of the performance parameters are shown in Table 4.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 vU vU vU DU ¼ DX1 þ DX2 þ/þ DXn vX1 vX2 vXn
(2)
The measuring range and accuracy of the AVL DiGAS 444 emission analyzer is given in Table 5. 4. Result and discussion 4.1. Combustion analysis The combustion of the engine is evaluated on the basis of its cylinder peak pressure, maximum heat release rate, angle of maximum heat release rate and mass fraction burn.
Fig. 3. Valve timing diagram of the engine.
4.1.1. Maximum cylinder pressure Variations in the maximum cylinder pressure with different CNG injection durations for the pilot operations using the Diesel, D45E15B40 and D30E20B50 blends are shown in Fig. 4. It is evident that at each and every load condition and at each and every
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Table 4 Total percentage of uncertainty of the computed performance parameters. Computed performance Measured variables Instrument involved in parameter measurement BP (brake power)
Load, RPM
Pilot fuel flow
SFC (liquid fuel)
BSEC
SFC (liquid fuel) BP CNG flow
Load sensor, load indicator, speed measuring unit. Fuel measuring unit, fuel flow transmitter As for SFC measurement as for BP measurement CNG flow controller.
% Uncertainty of the Calculation measuring instrument [10] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0.2, 0.1, 1.0. ð0:2Þ2 þ ð0:1Þ2 þ ð1:0Þ2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0.065, 1.5. ð0:065Þ2 þ ð1:5Þ2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1.501 1.02 0.25 ð1:501Þ2 þ ð1:02Þ2 þ ð0:25Þ2
Total % uncertainty of the computed parameters 1.02. 1.501 1.83
Table 5 Accuracy of the emission measuring. Instrument
Measuring range
Accuracy
AVL DiGAS 444 five gas analyzer Carbon monoxide (CO) Carbon dioxide (CO2) Hydrocarbon (HC) NOx Oxygen (O2)
0e10% vol 0e20% vol 0e20000 ppm vol 0e5000 ppm vol 0e22% vol
<0.6% vol: ±0.03% vol > 0.6% vol: ±5% <10% vol: ±0.5% vol > 10% vol: ±5% vol <200 ppm vol: ±10 ppm vol > 200 ppm vol: ±5% <500 ppm vol: ±50 ppm vol. 500 ppm vol: ±10% <2% vol: ±0.1% vol 2% vol: ±5% vol
CNG injection strategy, the pilot operation of the Dieseleethanolebiodiesel blends produces higher cylinder pressure than does the pilot Diesel operation. It is also found that the D30E20B50 blend pilot operation produces a higher cylinder pressure than does the D45E15B40 blend. The cylinder pressure can be regarded as an index of the quality of the combustion inside the cylinder. Better combustion produces a higher kinetic energy of the gas molecules inside the combustion chamber, which results in a higher cylinder pressure. The increase in the maximum cylinder pressure with the Dieseleethanolebiodiesel blends indicates that these blends cause better combustion of the charge, which may be possible due to the higher oxygen content of the pilot fuel. The oxygen released as a result of the decomposition of the pilot fuels partially compensates for the loss of oxygen intake due to the CNG supplementation. Again, because the D30E20B50 has a higher oxygen content than the D45E15B40, the greater heat release with the D30E20B50 is quite acceptable. It is also observed that the cylinder pressure decreases with an increasing CNG induction, regardless of the pilot fuel. This is because of the retardation in the rate of combustion due to the reduced oxygen content inside the combustion chamber.
Again, it is also observed that the maximum cylinder pressure increases with the load for all the pilot fuel operations. This is because, with any increase in the load the BMEP increases (constant RPM), which produces a favorable condition for combustion, and that in turn results in the higher in-cylinder pressure. 4.1.2. Maximum rate of heat release The blends of Dieseleethanol and biodiesel (PPME) are used as pilot fuel in this study. The blend ratio, calorific value and mass flow rate of the blends are known. Further, the mass flow rate of CNG and its calorific value are also known. The cumulative heat input is calculated from these parameters. The procedure of calculation is show in supplementary material-3. The heat release from the engine has been calculated as per Eq. (3) [27].
ROHR ¼
dQ CV ¼ ¼ dq R
p:
dV dp PV dM þ V: dq dq m dq
þP
dV dq
(3)
The variations observedinthe maximum heat release rates for the CNGeDiesel, CNG-D45E15B40 blend and CNG-D30E20B50
Fig. 4. Variation of maximum cylinder pressure with CNG injection duration for different pilot fuels.
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Fig. 5. Variation in the maximum rate of heat release with the CNG injection duration for different pilot fuels.
blend are shown in Fig. 5. It is clear that the maximum rate of heat release from the engine decreases with the pilot operation of the Dieseleethanolebiodiesel blends. The heat released from a fuel depends upon several factors such as the calorific value of the fuel, combustion quality, equivalence ratio etc. Due to the lower calorific value of the ethanol, the calorific value of the blend decreases. This results ina decrease in the total heat input to the engine in spite of a better quality of combustion. Hence, the pilot fuel flow represented in Fig. 10 is found to increase. The maximum rate of heat release is also found to decrease with the increasing CNG induction inside the combustion chamber. This diminution is found due to the presence of the high fuel rich mixture inside the chamber that retards the combustion process and restricts the heat release rate [28]. This fuel rich mixture also reduces the combustion quality and increases the unburned hydrocarbon emission as shown in Fig. 13. It can also be seen from Fig. 6 that the maximum heat release for the Dieseleethanolebiodiesel blends occurs at an earlier crank angle than it does for the pilot Diesel operation. This suggests that the ignition of the Dieseleethanolebiodiesel blends occurs at an earlier crank angle, which subsequently causes the maximum heat release at an earlier crank angle. It is also seen that the maximum heat release
from the D30E20B50 blend pilot operation occurs at an earlier crank angle than that of the D45E15B40 blend pilot operation. Therefore, it can be understood that the increase in the percentage of the biodiesel in the pilot fuel is beneficial for the dual fuel application. This occurs because biodiesel has the highest cetane number compared with the other fuels used in the study, which signifies a lower ignition temperature. Therefore, the fuel with the higher cetane number ignites earlier and burns relatively quicker, resulting in the maximum heat release at an earlier crank angle.
4.1.3. Mass fraction burn The mass fraction burned has been calculated according to Rassweiler and With row procedure [29] and is expressed in Eq. (4) [29].
Pj Dpc ðkÞ mb ðjÞ ¼ Pk¼0 MFB ¼ M mb ðtotalÞ k¼0 Dpc ðkÞ
(4)
where, M ¼ The total number of crank angle intervals, '0' denotes the start of combustion, 'N' denotes the end of combustion, Dpc ðkÞ is calculated as Eq. (5) [29],
Fig. 6. Variation in the angle of maximum rate of heat release with CNG injection duration for different pilot fuels.
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Fig. 7. Variation in the angle of 50% mass fraction burn with CNG injection duration for different pilot fuels.
Fig. 8. Variation in the angle of 90% mass fraction burn with CNG injection duration for different pilot fuels.
Fig. 9. Variation in the brake thermal efficiency with CNG injection duration for the different pilot fuels.
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Fig. 10. Variation in the pilot fuel flow with CNG injection duration for the different pilot fuels.
Fig. 11. Variation in the NOx emission with CNG injection duration for the different pilot fuels.
Fig. 12. Variation in the CO emission with CNG injection duration for the different pilot fuels.
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Fig. 13. Variation in the hydrocarbon emission with CNG injection duration for the different pilot fuels.
Dpc ðjÞ ¼ pjþ1 pj
Vj Vjþ1
!n (5)
Figs. 7 and 8 show respectively, the 50% and 90% mass fraction burn of the charge in terms of the crank angle for all the fuel combinations tested. The graphs indicate that the rate of burning for the pilot operation of the Dieseleethanolebiodiesel blends is higher as they burn at a lower crank angle. It is also found that the D30E20B50 blend pilot operation initiates the burning of the charge at lower crank angles compared with the D45E15B40 pilot operation. It also found that the D30E20B50 blend burns from 50% to 90% at a lower crank angle rotation (lower combustion duration) compared with the D45E15B40 pilot blend operation and the pilot Diesel operation. It is also found that with the increase in the CNG induction needs a higher crank angle (combustion duration) for the 50%e90% mass fraction burned. It can be seen that for the 4500 ms CNG injection at 100% load condition, 50% burning of the charge occurs at 370 of the crank angle for the pilot Diesel operation, at 369 of the crank angle for the pilot D45E15B40 blend operation and at 368 of the crank angle for the pilot D30E20B50 blend operation. But with the 22500 ms CNG injection and the 100% load condition, the 50% burn angle rises up to the 379 crank angle for the pilot Diesel operation, the 378 crank angle for the pilot D45E15B40 blend operation and to the 377 crank angle for the pilot D30E20B50 blend operation. The higher cetane number of the blend enables the charge to ignite at an earlier crank angle and subsequently the burning process completes at the earlier crank angle. On the other hand, the CNG being a fuel of high octane number ignites at later stage of combustion when it achieves selfignition temperature. 4.2. Performance analysis The performance of the engine is evaluated on the basis of the brake thermal efficiency and the pilot fuel flow for the estimated CNG injection durations under the load conditions tested. 4.2.1. Brake thermal efficiency Brake thermal efficiency of an engine is the ratio of brake power to the input fuel energy in appropriate units [30]. It has been calculated as per Eq. (6).
hbth ¼
Brake Power ðFuel flow rate=SÞ ðCalorific value of fuelÞ
(6)
Fig. 9 shows the variations in the brake thermal efficiency of the engine at the load conditions tested for the CNG-Diesel, CNGD45E15B40 blend and the CNG-D30E20B50 blend operations. It is evident that both the DieseleethanolePPME blends perform better than the Diesel with CNG induction. It can be seen from the graph that the addition of the ethanol and biodiesel in the pilot fuel enables a better utilization of the CNG and as the percentage of ethanol and biodiesel increases, the hbth of the engine increases simultaneously. It is found that the D45E15B40 blend with 22500 ms CNG injection increases the hbth by 50.18% at a 20% load, whereas the D30E20B50 blend increases it by 57.66% when compared with the baseline Diesel operation. At 60% load, the D45E15B40 blend increases the hbth by 2.5% while the D30E20B50 blend increases it by 11.07%. Similarly, at full load, the D45E15B40 blend increases the hbth by 23.06% while the D30E20B50 blend increases it by 46.54%. This significant rise in the hbth is due to the oxygenated content present in the blend, which are the ethanol and PPME. The oxygen liberated from these compounds partially compensates for the decrease in the intake of oxygen due to the CNG substitution. As a result, the engine gets more oxygen to burn the charge completely and this results in a higher hbth. 4.2.2. Pilot fuel flow The pilot fuel plays an important role in the dual fuel combustion system, as it is responsible for the initiation of combustion. Hence, the calorific value and burning quality of the pilot fuel significantly influences the total combustion process. Fig. 10 shows the pilot fuel flow under the load conditions tested for the CNGDiesel, CNG-D45E15B40 blend and CNG-D30E20B50 blend operations. It is found that a greater amount of the pilot fuel is required for both the Dieseleethanolebiodiesel blends under the CNG dual fuel operation. This may be due to the fact that the CNG requires a particular temperature of a 540 C [3] for self-ignition. When the test is conducted using Diesel, it is found that a lesser amount of fuel is required for burning to achieve that particular temperature which may be the cause for the higher calorific value of Diesel. However, the calorific value of the D45E15B40 and D30E20B50 blends is lower and consequently a higher quantity of fuel is required to achieve the desired ignition temperature. It is also
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A. Paul et al. / Energy xxx (2015) 1e14
observed that with the increase in the load, the pilot fuel flow also increases. This is because, with an increase in the load, the engine requires a higher amount of energy to overcome the load at the same RPM. As the amount of CNG being injected at a particular strategy does not increase, the energy shortage is compensated for by the pilot fuel. 4.3. Emission analysis 4.3.1. NOx emission The NOx emission from an engine depends on several factorsincludinghigh in-cylinder temperature, equivalence ratio, oxygen content of the fuel, nitrogen content of the fuel, nitrogen present in the air etc [31]. In Fig. 11, the variations in the NOx emissions at different load conditions for the dual fuel combinations tested are seen. It is evident from the graph that the NOx emission from the engine decreases with the increase in the CNG percentage, irrespective of the pilot fuel variation. This decrease may be attributed to several factors. The CNG has a very low percentage of nitrogen in it. Therefore, the fuel NOx formation is quite low for the CNG. Simultaneously, the injection of CNG produces a cooling effect due to throttling. This reduces the in-cylinder temperature and reduces the thermal NOx emission. It can also be seen that the addition of the ethanol and PPME further reduces the NOx emission in the dual fuel mode. This reduction in the NOx emission can also be attributed to the charge cooling by the ethanol, which evaporates inside the combustion chamber and absorbs the heat from the charge. As a result, it reduces the in-cylinder temperature. As the higher temperature prominently influences the NOx emission, the decrease in the temperature reduces the NOx emission. 4.3.2. CO emission The CO emission from the engine is shown in Fig. 12. It is evident from the Figure that the CO emission from the engine decreases with the increasing load. This is because, as the load increases, the BMEP of the engine increases along with it. This increase in the BMEP is conducive to better combustion inside the combustion chamber. As the CO emission is a result of the incomplete combustion of the fuel carbon, any improvement in the quality of combustion reduces the CO emission. It can also be seen that the addition of ethanol and PPME to the pilot fuel reduces the CO emission. It is a well-known fact that CO oxidizes to produce CO2 and the reactions are shown below [32]. CO þ O2 / CO2 þ O
(7)
O þ OH / OH þ OH
(8)
CO þ OH / CO2 þ H
(9)
H þ O2 / OH þ O
(10)
Equations (7) and (9) are the main oxidation reactions, whereas Equation (8) and10are the chain branching reactions. The first reaction shown in Eq. (7) is the initialization reaction, but it is slow. The second reaction shown in Eq. (9) is the main oxidation step [32]. Hence, it is evident that the increase in the 'H' molecule and the ‘OH’ radical is conducive to a reduction in the CO emission. Therefore, a reduction in the CO emission by the DieseleethanolePPME blends may be due to the fact that these blends release significant amounts of the ‘OH’ radical and ‘H’ molecules because ‘OH’ is present in the functional groups of ethanol (C2H5OH) and PPME which consist of hydrocarbons of different chain lengths, and can release the ‘H’ molecules during combustion.
11
Therefore, much of the CO is oxidized to CO2 and subsequently the CO emission gets reduced. 4.3.3. HC emission Hydrocarbons are one of the primary and necessary constituents of conventional fuels. Under ideal conditions the entire hydrocarbon content present in the fuel should burn completely and produce CO2 and water vapor. Factors such as the presence of a fuel rich mixture, lubricant contamination, charge cooling etc., causes deterioration in the combustion quality and results in the emission of the unburned hydrocarbon [33e35]. Fig. 13 shows the HC emission from the engine. It is evident from the Figure that the hydrocarbon emission increases with the increase in the CNG injection for all the pilot fuel combinations. This occurs because the induction of the CNG into the combustion chamber reduces the mass of the intake air of the engine and produces a fuel-rich charge. This charge fails to burn completely in the absence of sufficient air and the unburned and partially burned hydrocarbons are released via the exhaust. This increase in the hydrocarbon emission due to the CNG induction is found to be high during the longer durations of the CNG induction which implies that there is an excess of the CNG injection than the requirement. It is also clear that the hydrocarbon emission reduces with the increasing load conditions, irrespective of the pilot fuel. This occurs because with the increase in the load, the BMEP of the engine increases, creating a favorable environment for the combustion inside the combustion chamber. As a result, the combustion quality improves and the hydrocarbon emission reduces. The hydrocarbon emission is found to increase for both the DieseleethanolePPME blend pilot operations. This increase may be due to the cooling of charge caused by the evaporation of the ethanol inside the combustion chamber. 5. Performance-emission tradeoff A tradeoff study is basically a comparative assessment among three or more parameters to find the effect of a certain parameter on the behavior of the other parameters. The following is a tradeoff analysis among the brake thermal efficiency, NOx emission and BSEC for the different durations of the CNG injection for different pilot fuel combinations. Figs. 14e18 show the comparison between the brake thermal efficiency, NOX emission, and BSEC for the durations of the CNG injection of 4500 ms, 9000 ms, 13500 ms, 18000 ms, and 22500 ms,respectively. Fig. 14 shows the tradeoff between NOX emission, brake thermal efficiency and BSEC of the engine with CNG injection duration for 4500 ms. It is observed that at low load conditions, the differentiation between pilot Diesel operation, pilot D45E15B40 (represented as E15) blend operation and pilot D30E20B50 (represented as E20) blend operation is marginal as the tradeoff points for the said pilot blends are clustered around each other for a particular load. A more minute observation reveals that the pilot blends are able to yield higher thermal efficiency and lower NOX emission than pilot Diesel operation. The BSEC of the engine however remained almost same for all the mentioned pilot fuels. At medium load conditions, the improvement in brake thermal efficiency by pilot D45E15B40 and pilot D30E20B50 blend operations are more prominently observed as the tradeoff points are found in the higher hbth zone. However, at this load conditions, the NOX emission for pilot D45E15B40 and pilot D30E20B50 blend is found to be higher that pilot Diesel operation which signifies better combustion of the charge. This also points that CNG with pilot D45E15B40 and pilot D30E20B50 blend offers better performance characteristics than pilot Diesel operation. The BSEC for both the pilot blend operation is observed to be marginally higher than pilot Diesel operation. At high load conditions, a distinct advantage of the proposed blends as pilot fuel can
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Fig. 14. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 4500 ms.
Fig. 16. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 13500 ms.
be observed as the both the blends are observed to produce significantly higher brake thermal efficiency with reduced or similar NOX emission. The variation of BSEC for the different pilot fuels is also found to be almost same. The tradeoff analysis between brake thermal efficiency, NOX emission and BSEC for CNG injection duration of 9000 ms is shown in Fig. 15. Here also the pilot operation with the tested blends is found to improve the brake thermal efficiency of the engine as compared to pilot Diesel operation. Along with that, the NOX emission from the engine is found to decrease appreciably, thus drawing the tradeoff zone downward to a low NOX region. This decrease in the NOx emission may be attributed to the cooling effect produced by the evaporation of ethanol [10]. It is again observed that the NOx emission from the D30E20B50 blend is slightly higher than that of the D45E15B40 blend. This may be due to the presence of the higher percentage of biodiesel, which
promotes better combustion and increases the NOx formation by producing a higher in-cylinder temperature [36,37]. BSEC is again found to be almost same for all the tested pilot fuel blends. Fig. 16 shows the comparison between the brake thermal efficiency, NOx emission and BSEC for the CNG injection duration of 13500 ms. The decrease in the brake thermal efficiency with increasing the CNG induction is more prominent here, as an increase in the CNG induction has significantly reduced the oxygen induction in the cylinder. However, the oxygen rich blends are again found to be promising, being able to compensate for a part of the oxygen deficiency due to which the brake thermal efficiency is found to be higher than the pilot Diesel operation at the same load level. The considerable decrease in the NOx emission for both the pilot Diesel as well as the pilot blend operation is evident from this graph. For the CNG Diesel operation, this drop off is observed
Fig. 15. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 9000 ms.
Fig. 17. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 18000 ms.
Please cite this article in press as: Paul A, et al., An experimental study of the performance, combustion and emission characteristics of a CI engine under dual fuel mode using CNG and oxygenated pilot fuel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.050
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Fig. 18. Tradeoff between the brake thermal efficiency, NOx emission and BSEC with CNG injection for 22500 ms.
because of the reduced combustion temperature due to the incomplete combustion caused by the oxygen deficiency. For the CNG-blend operation, this decrease is also aggravated by the cooling effect of the ethanol. The BSEC of the engine is again found to be similar for all the pilot fuel combinations and it is affected only by increasing the loading conditions. Figs. 17 and 18 show the tradeoff zones at 18000 ms and 22500 ms of the CNG injection durations, respectively. These CNG injection strategies represent a prominent decrease in the brake thermal efficiency and NOx emission. These decreases are conspicuous due to the retardation of the combustion due to the paucity of oxygen in the inducted air. It can also be seen that the D30E20B50 blend shows the best brake thermal efficiency and a significantly lower NOx emission when compared with the pilot Diesel operation under high load conditions. The increase in the brake thermal efficiency with this blend occurs because of the presence of the higher percentage of ethanol (20%) and biodiesel (50%) which can release a higher percentage of the oxygen to support combustion. The BSEC here also shows no variance with a change in the pilot fuel and inversely changes with an increasing load. The study confirms the attractive potential of the Dieseleethanolebiodiesel blends tested as the pilot fuel for the CNG dualfuel operation. It is clearly evident from the above discussion that these blends are quite capable of improving engine performance and emission. It is also observed that the blends are most effective under high load conditions. It is also observed that the D45E15B40 blend is more effective in NOx reduction, whereas the D30E20B50 blend is more efficient in increasing the brake thermal efficiency. The best tradeoff zone is observed at the 120% load condition with the CNG-D30E20B50 blend dual fuel operation with the CNG injection duration of 9000 ms.
6. Conclusion The experimental work presented here is conducted on a partially modified CI engine. The conventional pilot fuel Diesel in a dual fuel operation is replaced with two blends of Dieseleethanolebiodiesel in the ratios of 45:15:40 (D45E15B40) and 30:20:50 (D30E20B50),respectively. The CNG is used as the primary fuel and is indirectly injected into the engine through the intake
13
manifold. The CNG injection duration is varied from 4500 ms to 23000 ms with a step increment of 4500 ms. The engine performance and emission are evaluated using the pilot fuels mentioned earlier and the CNG injection durations. Further, a performanceemission tradeoff study is also done to study the variation in the NOx emission and BSEC with increasing brake thermal efficiency for the CNG-Diesel, CNG-D45E15B40 blend and CNG-D30E20B50 blend dual fuel combinations. The pilot operation of the D45E15B40 and D30E20B50 blends is found to be beneficial in improving engine performance as it improves the brake thermal efficiency. However, due to the reduced calorific value of the blends, the pilot fuel flow is found to increase marginally for the mentioned Dieseleethanolebiodiesel blends. Again, the blends are found to decrease the NOx emission of the engine, and the major decrease is observed with the D30E20B50 blend pilot operation. The CNG dual fuel operation with the blends is also found to produce lower CO emissions than the CNG-Diesel dual fuel operation. The hydrocarbon emission from the engine with the CNG-Diesel dual fuel operation is found to increase with the increasing CNG injection durations. The tradeoff study indicates that the CNG injection upto 9000 ms is best suited for achieving an optimal performance at a reduced NOx emission. It also highlights that the D30E20B50 blend is more suited for the higher durations of the CNG injection. It can be concluded from the study that the inclusion of a higher percentage of ethanol and biodiesel in the pilot fuel can extend the maximum limit of the CNG injection duration to 9000 ms with a significant corresponding increase in the brake thermal efficiency and a commendable decrease in all the emission parameters. Appendix ASupplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.energy.2015.04.050. References [1] Yu Byeonghun, Kum SM, Lee CE, Lee S. Effects of exhaust gas recirculation on the thermal efficiency and combustion characteristics for premixed combustion system. Energy 1 January 2013;49. ISSN: 0360-5442:375e83. http://dx. doi.org/10.1016/j.energy.2012.10.057. [2] Karabektas M, Ergen G, Hosoz M. The effects of using diethylether as additive on the performance and emissions of a diesel engine fuelled with CNG. Fuel January 2014;115. ISSN: 0016-2361:855e60. http://dx.doi.org/10.1016/j.fuel. 2012.12.062. [3] Das LM, Gulati R, Gupta PK. A comparative evaluation of the performance characteristics of a spark ignition engine using hydrogen and compressed natural gas as alternative fuels. Int J Hydrogen Energy 1 August 2000;25(8). ISSN: 0360-3199:783e93. http://dx.doi.org/10.1016/S0360-3199(99)00103-2. [4] Mustafi NN, Raine RR, Verhelst S. Combustion and emissions characteristics of a dual fuel engine operated on alternative gaseous fuels. Fuel July 2013;109. ISSN: 0016-2361:669e78. http://dx.doi.org/10.1016/j.fuel.2013.03.007. € M, Fallgren H, Westerlund J, Sjo €din Å. Particle and [5] Hallquist ÅM, Jerksjo gaseous emissions from individual diesel and CNG buses. Atmos Chem Phys 2013;13:5337e50. http://dx.doi.org/10.5194/acp-13-5337-2013. [6] Karabektas M, Ergen G, Hosoz M. The effects of using diethylether as additive on the performance and emissions of a diesel engine fuelled with CNG. Fuel January 2013;13. ISSN: 0016-2361. Available online, http://dx.doi.org/10. 1016/j.fuel.2012.12.062. [7] Serrano D, Bertrand L. Exploring the potential of dual fuel diesel-CNG combustion for passenger car engine. In: Proceedings of the FISITA 2012 World Automotive Congress, Lecture Notes in Electrical Engineering, vol. 191; 2013. p. 139e53. [8] Cheenkachorn K, Poompipatpong C, Ho CG. Performance and emissions of a heavy-duty Diesel engine fuelled with diesel and LNG (liquid natural gas). Energy 1 May 2013;53. ISSN: 0360-5442:52e7. http://dx.doi.org/10.1016/j. energy.2013.02.027. [9] Cordiner S, Rocco V, Scarcelli R, Gambino M, Iannaccone S. Experiments and multi-dimensional simulation of dual-fuel diesel/natural gas engines. 2007. http://dx.doi.org/10.4271/2007-24-0124. SAE Technical Paper 2007-24-0124. [10] Paul A, Bose PK, Panua RS, Banerjee R. An experimental investigation of performance-emission trade off of a CI engine fueled by dieselecompressed natural gas (CNG) combination and dieseleethanol blends with CNG enrichment. Energy 7 May 2013 ISSN: 0360-5442. Available online, http://dx.doi. org/10.1016/j.energy.2013.04.002.
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Please cite this article in press as: Paul A, et al., An experimental study of the performance, combustion and emission characteristics of a CI engine under dual fuel mode using CNG and oxygenated pilot fuel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.050