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An experimental examination of the effects of n-hexane and n-heptane fuel blends on combustion, performance and emissions characteristics in a HCCI engine
Journal Pre-proof An Experimental Examination of the Effects of n-Hexane and n-Heptane Fuel Blends on Combustion, Performance and Emissions Characteristics in a HCCI Engine
Bilal Aydoğan PII:
S0360-5442(19)32295-9
DOI:
https://doi.org/10.1016/j.energy.2019.116600
Reference:
EGY 116600
To appear in:
Energy
Received Date:
03 June 2019
Accepted Date:
22 November 2019
Please cite this article as: Bilal Aydoğan, An Experimental Examination of the Effects of n-Hexane and n-Heptane Fuel Blends on Combustion, Performance and Emissions Characteristics in a HCCI Engine, Energy (2019), https://doi.org/10.1016/j.energy.2019.116600
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Journal Pre-proof An Experimental Examination of the Effects of n-Hexane and n-Heptane Fuel Blends on Combustion, Performance and Emissions Characteristics in a HCCI Engine Bilal Aydoğan Burdur Mehmet Akif Ersoy University, High Vocational School of Technical Sciences, 15100 Burdur, Turkey [email protected] Abstract Homogenous charge compression ignition (HCCI) engines are low-temperature combustion engines with high thermal efficiency. The operation range of HCCI engines, which are not directly controlled on combustion, is limited by the problems of knocking and misfiring. At this point, it is aimed to eliminate the problem of knocking at the high engine loads by the charge mixture composition and various operating parameters. In this study, the experiments were performed in a single cylinder, four stroke, port injection HCCI gasoline engine to investigate the effects of n-hexane, n-heptane and n-hexane/n-heptane blends. Combustion characteristics, engine performance and exhaust emissions were determined at different engine speeds (800-1800 rpm), lambda (λ=1.5-3.0) and inlet air temperatures (60 and 80 oC). Operating range of the HCCI engine was determined by using both pure n-hexane and pure nheptane fuels. The experiments showed that n-hexane, which has higher octane number than that of n-heptane, has more resistance to knocking. In-cylinder pressure decreased with the increase of lambda for all test fuels. Thermal efficiency increased about 28% by using nhexane compared to n-heptane at constant lambda (λ=2.35). The results showed that H75N25 provided larger operating range compared with other test fuels. Keywords: HCCI, n-hexane, n-heptane, combustion, performance, emissions
Nomenclature -
HCCI HRR EGR Imep SOC Wnet Vd
Homegenous charge compression ignition Heat release rate (J/oCA) Exhaust gas recurcilation Indicated mean effective pressure (bar) Start of combustion Net work (joule) Cylinder swept volume (m3)
Journal Pre-proof -
P dV 𝑑𝑄 𝑑𝜃 𝑑𝑄ℎ𝑒𝑎𝑡 𝑑𝜃
A2 hc n Tg Tw Ti Vc nc k mfuel QLVH ∆R xn
Cylinder pressure (bar) Variation of cylinder volume (m3) Heat release (J) Crank angle (oCA) Heat transfer to cylinder walls (J/oCA) Heat transfer surface area (m2) Heat transfer coefficient (W/m2K) Engine speed (rpm) In-cylinder average gas temperature (K) Combustion chamber wall temperature (K) In-cylinder temperature versus crank angle In-cylinder volume polytrophic index Ratio of specific heat values Consumed fuel per cycle (g) Lower heating value (kJ/kg) Uncertainty of the measurement Independent variables of the function and uncertainty
1.Introduction Petroleum based fuels are widely used in many fields in all over the world. Crude oil reserves have been decreased and environmental pollution resulted from motored vehicles rapidly increases and damages atmosphere [1,2]. The use of petroleum-based fuels is also increasing rapidly to meet the increasing energy need with developing technology. Nevertheless, the fact that known fossil fuel reserves can meet the needs of less than fifty years with the current utilization rate and environmental degradation and global warming accelerated the work on alternative energy sources and alternative combustion modes [3,4]. So, alternative combustion modes should be researched in view of cleaner exhaust emissions with higher thermal efficiency in the internal combustion engines [5,6]. In recent years, there has been an increasing interest in HCCI combustion which is defined as low temperature combustion has an important potential to reduce exhaust emissions and higher thermal efficiency [7-9]. HCCI operating is limited by knocking that engine is unexpectedly run with higher pressure rise rate and misfiring limit that present higher cyclic variations [10,11]. Thus, volatile fuel should be selected for better homogenity of charge mixture in order to characterise HCCI combustion [12-14]. Chemical kinetics, composition of the charge mixture, and in-cylinder temperature are the main parameters affecting the HCCI combustion [15-17]. But knocking and misfiring are the most important problems which limits the usage of HCCI combustion in internal combustion engines [18,19]. Because, simultaneous and spontenous auto-ignition can result in knocking. Moreover, HCCI combustion results in higher CO and HC emissions because of the
Journal Pre-proof leaner charge mixture and lower combustion temperature [20-22]. Up to now, a number of studies have investigated how to control the combustion phasing and eliminate knocking by different methods such as changing inlet air temperature, using fuel with higher octane number, EGR, variable compression ratio, variable valve timing, using glow plug were used [23-28]. Among them, it is seen that HCCI combustion is highly sensitive to chemical properties of the fuel. The start of combustion and pressure-temperature history that are directly changed during combustion due to difficulty of vaporization compared to n-heptane. There are relatively many historical studies in the area of achieving stable HCCI combustion and extend operating range with iso-octane and n-heptane at different ratios. Furthermore, volatile and more flammable fuels like solvent based fuel should be used in order to prevent misfiring which is one of the most important handicap to tackle on HCCI combustion. In a study investigating the effects of compression ratio on combustion and emissions in a HCCI engine, Calam et al. [29] reported that when the mixture getting leaner rate of heat release and in-cylinder pressure decreased. They used RON20 composed of 20% isooctane and 80% nheptane, and RON40 composed of 40% isooctane and 60% n-heptane in a four-cycle, single cylinder Ricardo Hydra experimental engine in their experiments. The experiments were carried out by changing compression ratio (9:1, 10:1, 11:1 and 12:1) at different lambda. Combustion duration was extended with the high octane number of the fuel while it was decreased by increasing the compression ratio. CO and HC emissions decreased with the increase of compression ratio while NOx emisions were increased. In a follow-up study, Cınar et al. [30] evaluated the effects of valve lift on the combustion and emissions of a HCCI gasoline engine with the fuel RON80 (20% isooctane-80% n-heptane). The tests were performed at full HCCI combustion mode, at different lambda (λ=0.5-2) and inlet air temperatures (20-120 oC) between 800 and 1900 rpm engine speeds in a four stroke, port injection, single cylinder, Ricardo Hydra gasoline research engine. It has been reported that low lift cams extended the operating range of HCCI engine on misfiring and knocking operating zones. When the inlet air temperature increased, test engine could be run at high lambda values. However leaner air/fuel mixture decreases thermal efficiency at lower engine speed. Beside this, more stable HCCI combustion occured when the valve lifts were reduced. In a study conducted by Mack et al. [31], butanol isomer combustion in HCCI engine with wide range of intake pressure and equivalence ratios was investigated. The experiments showed that n-butanol showed more stable combustion and later misfiring than isobutanol. However, n-butanol showed more knocking combustion. In a study which set out to determine the engine parameters on combustion and performance of HCCI engine fueled with n-heptane,
Journal Pre-proof Hasan et al. [32] found that the combustion phase was advanced and the combustion duration was shortened when the engine speed decreased and intake air temperature was increased. In an investigation into the effects of n-butanol on combustion and emission characteristics of single cylinder port fuel injection four stroke HCCI engine, He et al. [33] reported that onset of autoignition was advanced and autoignition timing was delayed when the engine speed increased. Khandal et al. [34] investigated the effects of hydrogen (H2) and diesel/biodiesel fuel blends on the performance of HCCI engine. Engine showed severe knocking working at 80% load without EGR (exhaust gas recirculation) for biodiesel/diesel fuel blend. However, biodiesel/diesel fuel blend with 7% hydrogen fuel energy ratio showed 65-67% lower smoke and 98-99% lower NOx emissions at 80% load. In another study, Turkcan et al. [35] studied the effects of different injection parameters on HCCI combustion fueled with gasoline/ethanol and methanol/gasoline blends and compared the results with gasoline reference fuel. The experimental results showed that when start of the first injection timing was earlier, maximum pressure rate increased with the usage of alcohol/gasoline blends. However, they have reported that second injection timing had more impact on the HCCI combustion, namely thermal efficiency, indicated that mean effective pressure and maximum cylinder gas pressure could be controlled with the second injection timing. Appplication of HCCI combustion in the internal combustion engines is not seen to perform easily. This new combustion mode has some difficulties such as uncontrollable combustion phasing, narrow operating range and changing engine operating parameters. Besides, higher HC and CO are still significant handicap in view of achieving recent engine technology. On the contrary, both NOx and soot emissions could be reduced due to combustion of leaner charge with higher thermal efficiency. This experimental study was performed with n-hexane and n-heptane fuel blends on HCCI combustion which presents lower NOx and soot emissions with lean charge mixtures. HCCI combustion occurs spontaneously in the combustion chamber. Low temperature combustion (LTC) is seen with leaner charge mixture. So, NOx emissions are reduced, because lower gas temperature is obtained due to combustion of leaner mixture. Furthermore, soot and NOx formation released from CI engines can be reduced simultaneously with HCCI engines. SI engines have to be operated near the stoichiometric ratio. In addition, catalitic convertor and exhaust gas reduction systems that expensive and non-practical should be used to reduce exhaust gas. So, HCCI can be defined as environmentally friendly combustion mode when compared other conventional cycles. If the comparison was made, the operating parameters of SI engine should be modified for the selected test fuel.
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The study presented here is one of the first investigations to explore the behaviors of n-hexane fuel blends on HCCI mode in view of detailed combustion analysis and engine performance. In this study, n-heptane (N100), n-hexane (H100), 50% n-hexane/50% n-heptane (H50N50) and 75% n-hexane/25% n-heptane were used in HCCI engine and combustion, performance and emission characteristics were investigated. The experiments were performed at different lambda values (between 1.5 and 3.0), engine speeds (between 800-1800 rpm) and inlet air temperatures (60 and 80 oC). The aim of this experimental research is to extend HCCI operating range and eliminate knocking tendency in a HCCI engine fueled with high octane number n-hexane and n-heptane fuel blends. 2.Experimental Setup and Procedures The experiments were performed with a single cylinder, port injection, four stroke gasoline HCCI engine. DC dynamometer which was rated 30 kW/6500 rpm engine speed was connected to the test engine. The test engine could not be run with original cam mechanism on HCCI mode. In addition, dilution of charge mixture is essential to prevent knocking and reduce pressure rise rate in HCCI combustion mode. Low lift cam mechanism were used for HCCI mode. Some residual gases were trapped to dilute charge composition. The test engine was stably operated on HCCI mode between 1.5 and 3.0 lambda values and 60 and 80 oC inlet air temperatures. HCCI combustion was not achieved apart from mentioned parameters due to knocking and misfiring. Combustion characteristics were discussed at 1000 rpm, 60 or 80 °C
and constant lambda. The table below illustrates the technical properties of the HCCI
engine. The schematic diagram of the experimental setup is given in Fig. 1. Table 1. The technical properties of the test engine Model Number of Cylinders Cylinder bore (mm) Stroke (mm) Volume (cc) Compression ratio Power output (kW) Maximum engine speed (rpm) Valve timing Valve lift
Ricardo-Hydra 1 80.26 88.90 540 13:1 15 5400 IVO/EVC 12o BTDC/56o ATDC Intake/exhaust 5.5/3.5
Journal Pre-proof Engine speed, fuel injection pulse, engine oil, engine coolant and inlet air temperatures can be measured and controlled by the dynamometer control panel. Air heating system was adapted in the access of the intake manifold. The experiments were performed at two inlet air temperatures including 60 and 80 oC. Temperatures were measured with K-type thermocouple and fixed at constant value by closed-loop controller. Kistler model 6121 piezoelectric pressure transducer was used to measure in-cylinder pressure. Cussons P4110 combustion analysis device scaled up the pressure data and National Instruments USB 6259 data acquisition card converted the data to digital signals with a timing resolution of 0.36 crank angle degrees. The pressure data of the cylinder were recorded in the computer. In-cylinder pressure of 50 consecutive cycles were averaged in order to minimize the cyclic variations for each test condition.
Fig. 1. The schematic diagram of the experimental setup The experiments were performed between λ=1.5 and λ=3 fueling ratios and different engine speeds from 800 to 1800 rpm at constant compression ratio (13:1). Exhaust gas analyzer apparatus measured HC, CO and NO emissions and air/fuel ratio. Firstly, the engine was operated on SI mode and warmed up. Then, spark ignition was switched off to achieve HCCI combustion. N-heptane, n-hexane and n-heptane/n-hexane blends were used in the experiments. The results obtained from the experiments were compared with n-heptane which was used as the base fuel. Table 2 provides the chemical properties of the test fuels.
Journal Pre-proof Table 2. Chemical properties of the test fuels [36] Chemical Formula Density (kg/m3) Octane number Heat of Combustion (kJ/mol) Boiling point (oC) Molar mass (g/mol)
n-Hexane C6H14 659 26 4163.2 69 86.17
n-Heptane C7H16 679.5 0 4817 98 100.16
An algorithm arranged in Matlab was used to process raw cylinder pressure data. The heat transfer from gas to cylinder walls were taken into consideration in calculations. It was assumed that there is no leakage between piston and cylinder and charge mixture in the cylinder was constant. Imep was calculated with Equation 1. Imep shows the mean pressure that exerted on the piston during a cycle. So, it is recognized as a significant engine performance parameter. Equation 2 and 3 were used to calculate net work and HRR, respectively. Cyclic variations were aimed to reduce using the average of 50 consequtive cycles and then in-cylinder pressure and heat release rate were determined. The calculated heat release is the total heat release. Heat transfer was computed seperately. The heat transfer to the cylinder wall was determined with Eq. 4. The heat transfer to the cylinder wall is a small portion of total heat release (<2 J/oCA). The exhaust gas temperature was measured in the exhaust manifold just after combustion chamber. The tests were performed at constant coolant and engine oil temperatures in order to provide durability for stable HCCI combustion. In-cylinder wall temperature changes during a cycle. The variations of incylinder temperature were estimated with the Eq. 5. In-cylinder wall temperature was assumed to be a constant value as 500 K due to exhaust gas temperature in order to make the calculations easier. 𝑖𝑚𝑒𝑝 =
𝑊𝑛𝑒𝑡
(1)
𝑉𝑑
(2)
𝑊𝑛𝑒𝑡 = ∫𝑃𝑑𝑉 𝑑𝑄 𝑑𝜃
𝑘
𝑑𝑉
1
𝑑𝑃
= 𝑘 ― 1𝑃𝑑𝜃 + 𝑘 ― 1𝑉𝑑𝜃 +
𝑑𝑄ℎ𝑒𝑎𝑡 𝑑𝜃
1
= 6𝑛ℎ𝑐𝐴2(𝑇𝑔 ― 𝑇𝑤)
( )
𝑇𝑖 + 1 = 𝑇𝑖
𝑉𝑟
𝑉𝑐 + 1
𝑛𝑐
𝑑𝑄ℎ𝑒𝑎𝑡 𝑑𝜃
(3) (4) (5)
Thermal efficiency is a dimensionless parameter in the internal combustion engines and calculated with the ratio between the net work and released energy from fuel (Equation 6).
Journal Pre-proof 𝑊𝑛𝑒𝑡
(6)
𝜂𝑇 = 𝑚𝑓𝑢𝑒𝑙1.𝑄𝐿𝐻𝑉1 + 𝑚𝑓𝑢𝑒𝑙2.𝑄𝐿𝐻𝑉2
Various error sources such as the random fluctuation of the instruments, the calibration of the measuring devices, the calculated accuracies and the technics of the tests affect the uncertainties of the observed parameters. The measurement equipments were calibrated before the tests. The accuracy and the operating range of the gas analyzer and cylinder pressure transducer were given at Table 3 and 4, respectively. The gas analyzer was purged and calibrated before performing the next work on a selected test condition. The uncertainty of the performance parameters were calculated on the basis of the root mean square method (Eq. 7). The accuracies of the measurements and the uncertainties of the calculated results were given at Table 5. Table 3. The specifications of the exhaust gas analyzer Operating range 0.000-10.00 0.00-18.00 0-9999 0.00-22.00 0.500-9.999 0-5000
CO (% vol) CO2 (% vol) HC (ppm) O2 (% vol) λ NO (ppm vol)
Accuracy 0.001 0.01 1 0.001 0.001 ≤1
Table 4. Technical properties of the cylinder pressure transducer Model Operating range (bar) Measurement precision (pC/bar) Operating temperature (oC) Accuracy (±%) ∆𝑅 =
[(
2 ∂𝑅 ∆𝑥 1 ∂𝑥1
) ( +
Kistler 6121 piezo electric 0-250 14.7 -50-350 0.5
2 ∂𝑅 ∆𝑥 2 ∂𝑥2
)
+…+
(
2 0.5 ∂𝑅 ∆𝑥 𝑛 ∂𝑥𝑛
)]
(7)
Table 5. Accuracies of the measurements and the uncertainties in the calculated results Accuracy Time (s) Temperature (°C) Fuel (g) Heating value (kJ/kg)
±0.5% ±1°C ±0.1 g ±0.1 %
Uncertainty (%) -
Journal Pre-proof Engine speed (rpm) Load (N) Torque (Nm) BTE BSFC (g/kW-h)
±1% ±0.25% -
±0.09 ±1.38 ±1.25
3.Results and Discussion 3.1. Combustion Evaluation Air/fuel ratio, mixture composition and inlet air temperature are the important parameters for HCCI combustion. In this study, the experiments were performed at different air/fuel ratios, inlet air temperatures and mixture compositions at full load condition. Stable HCCI combustion could be achieved between λ=1.5 and λ=3. And the test engine was smoothly run with 60 and 80 oC inlet air temperatures on HCCI mode. At lower and higher inlet air temperatures stable HCCI combustion was deteriorated due to misfiring and knocking, respectively. So, range of the test parameters were selected as mentioned. The extend to which higher inlet air temperature caused to knocking with low octane number n-hexane. Hence, inlet air temperature could not be increased too much. This is already an important key for HCCI that provides the combustion of very lean mixture. So, air/fuel ratio could not be dropped below λ=1.5 because of unstable HCCI combustion.
Fig. 2 shows the in-cylinder
pressure and HRR versus to crank angle with the variation of lambda for the test fuels. In Fig. 2 there is a clear trend of increasing in-cylinder pressure for the low values of lambda and the knocking occured for all test fuels. This result maybe explained by the fact that when the lambda decreased, more fuel quantity is injected into the combustion chamber to generate richer air/fuel mixtures that increased the maximum in-cylinder pressure after combustion [37]. It can be clearly seen from the Fig. 2 that maximum in-cylinder pressure decreased and knocking disappeared with the increase of lambda values. A possible explanation for this might be that when charge mixture is getting lean, combustion phasing is retarded and incylinder pressure decreases. HRR is a sign of the speed of the energy conversion from chemical energy to thermal energy during combustion. Rapid heat release shows the higher pressure rise rate in case of knocking [38-39]. The force exerted into the piston increased too much with knocking. The pressure applied to the piston increased for each crank angle variation. Oil gap between engine bearing and crank journal closes rapidly. Pressure waves and undesirable combustion noise were observed due to knocking. It is apparent from Fig.2 that HRR decreased for high value of the lambda, because less amount of the fuel is injected into the combustion chamber. It was clearly seen that knocking was seen with n-heptane. The
Journal Pre-proof addition of n-hexane into n-heptane resulted in more stable HCCI combustion especially with H75N25 due to higher octane number of n-hexane. Combustion delayed with the increase of lambda for all test fuels.
Fig. 2. The variation of in-cylinder pressure and HRR a) N100 b) H100 c) H50N50 d) H75N25 At leaner charge mixtures, fuel molecules decrease in the combustion chamber. In addition, fuel molecules could not easily gather with oxygen molecules due to lower fuel concentration during combustion. So, oxidation rate reduces with the lack of fuel. HCCI combustion was obtained with H75N25 until λ=1.81 lambda value. Test engine could not be operated at richer charge mixture with H75N25. Similar maximum in-cylinder pressure was measured with test fuels owing to close calorific value of n-hexane and n-heptane. Fig. 3 represents the incylinder and HRR with different inlet air temperatures (Tin=60 oC and Tin=80 oC) at constant lambda (λ=2.2) for the test fuels. When the inlet air temperature is increased at constant lambda, maximum in-cylinder pressure increased because of the higher initial temperature of the charge. Auto-ignition occurs earlier due to the high inlet air temperature and combustion phase is advanced. In high inlet air temperatures, the chemical reactions between fuel and
Journal Pre-proof oxygen molecules occur easier and faster and more molecules take part in reactions [40]. As it can be seen from the Fig. 3, low and high temperature combustion zones are seen on HCCI mode. Moreover, it was depicted from Fig. 3 that maximum in-cylinder pressure of n-heptane is higher than that of n-hexane for both inlet air temperatures. N-heptane is ignited earlier than n-hexane due to the lower octane number. It is possible to say that when the amount of nhexane was increased in the fuel mixtures, maximum in-cylinder pressure decreased compared to n-heptane. Similarly, HRR decreased with the addition of n-hexane in the fuel blends. Released heat energy decreases because of lower heating value of n-hexane.
Fig.3 The variation of in-cylinder pressure and HRR a) Inlet air temperature= 60 oC b) Inlet air temperature= 80 oC CA10 defines the 10% of burned charge mixture according to crank angle. CA10 can be supposed as the start of combustion (SOC) in many studies in the literature on HCCI mode. SOC is strongly dependent on the thermodynamical situation at the end of compression stroke especially with inlet air temperature on HCCI mode. Higher inlet air temperature increases the in-cylinder temperature at the end of compression stroke. Hence, oxidation reactions are accelerated with higher heat energy in the combustion chamber. CA10 can not be directly controlled and auto-ignition is totally controlled by chemical kinetics of the fuel and incylinder temperature in HCCI combustion [41]. The variation of CA10 versus lambda for different inlet air temperatures were given in Fig. 4. What stands out in this figure is the retarding combustion by increasing the lambda for all test fuels, because lower heat is released due to lower fraction of fuel during the burning of leaner charge mixture. As it can be seen from the Fig.4 H50N50 showed the highest C10 value. When the inlet air temperature was increased from 60 oC to 80 oC, CA10 was obtained earlier for all test fuels compared to 60 oC. Moreover, n-heptane showed significant effect on CA10 due to lower resistance of
Journal Pre-proof combustion compared that fuel blends. So, combustion was advanced with the increase of inlet air temperature in case n-heptane was used as test fuel.
Fig.4 The variation of start of combustion a) Inlet air temperature= 60 oC b) Inlet air temperature= 80 oC Fig. 5 illustrates the variation of the in-cylinder gas temperatures versus the crank angle for constant lambda and engine speed for different inlet air temperatures (60-80 oC). In-cylinder gas temperature is an important parameter for HCCI combustion, because it initiates the autoignition reactions. N-heptane showed the highest maximum in-cylinder gas temperature for 60 oC inlet air temperature (≈1730 K). The corresponding temperature decreased by increasing the amount of the n-hexane in the fuel mixtures. It can be mentioned that lower heating value and density of n-hexane caused to obtain lower in-cylinder gas temperature. On the contrary, when the inlet air temperature was increased n-heptane showed the lowest incylinder gas temperature and it increased with the increasing amount of the n-hexane in the mixture. A possible explanation for this might be that partial of n-heptane is vaporized at higher inlet air temperature during combustion. So, maximum in-cylinder gas temperature increases according to the amount of n-hexane fraction in the fuel blends.
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Fig.5. The variation of in-cylinder gas temperatures of HCCI combustion at constant lambda λ=2.2 3.2. Engine Performance Combustion duration and thermal efficiency of the test fuels at constant engine speed and inlet air temperature for different lambda were given at Fig. 6. CA50 is an indicator for the crank angle where the half of the fuel mixture was combusted. What is striking about the figure is that CA50 closes to the top dead center (TDC) as the lambda increased. In-cylinder gas temperature decreases at leaner charge mixtures in the combustion chamber and chemical oxidation reactions are deteriorated. This event causes to retard combustion and CA50. Cylinder wall temperature also decreases and combustion chamber temperature reduces that prevents to complete combustion. On the contrary, CA50 moved away from TDC with increasing the amount of n-hexane in the mixture. As it is expected, higher octane number of n-hexane resisted to auto-ignition. Fig. 6b. shows the variation of thermal efficiency which is defined as how much the chemical energy of the fuel was converted to the mechanical energy. Thermal efficiency increased with the increasing the lambda for a given value and then started to decrease. Whole fuel molecules can be reacted and oxidized in the combustion chamber due to higher oxygen concentration at lambda values of λ=2.4, λ=2.6. Fuel molecules can gather with oxygen molecules until mentioned lambda values. Nevertheless, thermal efficiency decreased with very lean mixture (λ=2.6 and λ=2.7) due to lower released heat energy. Hence, thermal efficiency decreased. Thermal efficiency is highly affected by calorific value and density of the fuel. As it is seen from Fig. 6 that, lower thermal efficiency was obtained with H50N50 especially towards to stoichiometric ratio (λ=1.96, λ=1.80, λ=1.66). However, it was seen that H75N25 showed the highest thermal efficiency at leaner charge mixtures compared to H50N50 due to higher fraction of n-heptane. Charge mixture
Journal Pre-proof could be ignited due to lower octane number of n-heptane at leanaer mixtures. It can be pointed out that thermal efficiency increased with the increase of n-hexane addition in the fuel blends. Thermal efficiency increased by about 28% with H100 compared to n-heptane at λ=2.35. Because, stable auto-ignition reactions were observed with the addition of n-hexane. Knocking tendency increased with lower octane number fuels. Thus, thermal efficiency decreases.
Fig.6. The variation of a) combustion duration (CA50) and b) thermal efficiency of HCCI combustion Residual gases could not be discharged from the combustion chamber due to higher engine speed and valve overlap at the end of exhaust stroke. Some exhaust gases are trapped in the combustion chamber and mixes with fresh charge for the next cycle. So, charge composition and thermodynamic situation varies at the end of compression stroke. Consequently, combustion characteristics change cycle by cycle. This phonomena resulted in unstable combustion and prevented the durability of engine operation. It is recomended that cyclic variation does not exceeds 10% for stable combustion [10]. In the present study, valve mechanism of the test engine was modified and low lift cams were used in order to prevent knocking with trapped residual gasses especially with richer mixtures and to achieve stable HCCI combustion. Heat capacity of charge composition should be increased on HCCI mode avoiding higher pressure rise rate. Fig. 7 shows the variation of COVimep versus lambda at 1000 rpm constant engine speed and 80 oC inlet air temperature. The values of COVimep are 6.54, 9.82, 8.71, 7.60 at the leanest mixture (λ=1.7) for N100, H100, H50N50 and H75N25, respectively. The lowest COVimep was calculated with n-heptane among the other test fuels. COVimep was calculated for N100, H100, H50N50 and H75N25 as 2.4, 3.07, 3.33, 2.85 at λ=2.2 that was mentioned as the ideal fuel/air mixture in this study. Furthermore, COVimep decreased with the addition of n-hexane in the fuel blends. It was seen that knocking tendency
Journal Pre-proof decreases with n-hexane addition. This event also provided more stable combustion. Sudden and rapid combustion and heat release are prevented due to higher resistance of knocking with n-hexane.
Fig 7. The variation of COVimep with the test fuels Fig. 8. illustrates the variation of the combustion duration versus lambda in HCCI combustion at constant engine speed and inlet air temperature. It is observed that combustion duration increases as the amount of the n-hexane increases in the mixture because of higher resistance to auto-ignition. Combustion duration decreased with H75N25 with the increase of lambda. No significant differences were found between H50N50 and n-hexane for combustion duration according to lambda values. It can be pointed out that combustion duration decreased with n-heptane as the lambda increased. The maximum combustion duration observed for H75N25 test fuel as about 41.56 oCA at λ= 1.93. The combustion duration of N100, H100, H50N50 and H75N25 are 41.56, 39.24, 38.9, and 34.6 at λ= 1.93. The corresponding value decreased for all test fuels when the lambda was increased to 2.56. The maximum decrease was obtained about 5.8 % for H75N25.
Fig. 8. The variation of the combustion duration versus lambda
Journal Pre-proof RI is one of the most important parameters which controls the operating limits of combustion noise [42]. The variation of ringing intensity (RI) with maximum pressure rise rate at constant engine speed, inlet air temperature and for λ= 2.2 was given in Fig. 9. RI changes according to engine speed, combustion rate and maximum in-cylinder pressure [43,44]. MPRR defines the maximum in-cylinder pressure that applied on the piston for each crank angle degree variation. RI increased with increased MPRR for all test fuels. The lowest RI values were obtained with n-heptane. RI decreased with the increase of n-hexane in the mixture.
Fig. 9. The variation of a) Ringing intensity and b) MPRR versus lambda Imep is an important parameter to indicate performance for internal combustion engines [45,46]. Fig. 10 shows the variation of imep versus 50 consecutive cycles. Imep varies according to heating value of fuel, because higher heat energy releases and higher pressure is exerted on the piston. So, imep increases. More stable combustion without knocking was observed for H50N50 test fuel. The highest imep values were obtained with n-heptane owing to higher calorific value. The addition of n-hexane caused to decrease imep due to lower calorific value. However, more reasonable and stable HCCI combustion was obtained with fuel blends even if n-hexane has lower calorific value compared that n-heptane. Knocking tendency reduced with the addition of n-hexane. This improves the chemical oxidation reactions and engine performance. However, lower density of n-hexane may not be applicable to obtain higher imep. Particle size of n-hexane fuel molecules remain small and fuel molecules could not well proceed in the inlet air compared to n-heptane. Thus, homogenous charge mixture could not be obtained and auto-ignition reactions do not occur towards to combustion chamber. So, obtained heat and in-cylinder pressure forced to the piston decreased and lower imep was obtained with n-hexane.
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Fig. 10. The variation of imep versus consecutive 50 cycles The effect of engine speed on torque, power output and SFC for n-heptane and n-hexane was given in Fig. 11. Power output increases with the increase of engine speed until a specific value and then started to decrease. Power output decreases due to friction losses and unsufficient oxygen at higher engine speed. Moreover, the test engine could not take sufficient oxygen due to limited duration at higher speeds and volumetric efficiency decreases. So, power output decreases. Likely, brake torque first increased and then started to decrease. After maximum torque speed, brake torque decreases owing to gas leakages and heat losses at higher engine speeds. It is apparent from Fig. 11 that, torque first increased with the increase of engine speed for both fuels. N-heptane showed higher torque values compared with n-hexane. As it can be seen from the Fig. 11.b that HCCI combustion could not be achieved at low engine speed (800 rpm) for n-hexane. Torque of n-heptane is higher about 23.4% than n-hexane for 1000 rpm engine speed. At higher engine speeds (1400 and 1600 rpm) the value of torque between n-hexane and n-heptane remained nearly same. Power output of n-hexane and n-heptane were 0.49 kW and 0.37 kW for 1000 rpm engine speed, respectively. It was nearly same for n-hexane and n-heptane for 1400 rpm engine speed but nhexane showed higher power output value about 19% than n-heptane for 1600 rpm engine speed. As the engine speed increased, SFC decreased until 1400 rpm engine speed. After that engine speed, SFC got started to increase. SFC was obtained as 0.58 kg/kWh and 0.46 kg/kWh for n-heptane and n-hexane at 1000 rpm engine speed, respectively. Higher fuel molecules are injected into the cylinder with n-heptane due to higher density, because injected mass fraction of n-heptane by volume is higher than n-hexane. It means that higher fuel molecules should be combusted in order to obtain same power output with n-heptane.
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Fig. 11. The variation of torque, power output and SFC versus engine speed a) N100 b) H100 3.3. Emissions Fig. 12 displays the variation of the HC emissions at different lambda values at constant engine speed (1000 rpm) and inlet air temperature (60 oC). HCCI combustion has very low NOx emissions because it has low combustion temperature. Nevertheless, low combustion temperature causes to insufficient oxidation of the fuel and this leads to higher HC emissions. What stands out in the Fig. 12 is increasing HC emissions by increasing value of the lambda due to the low temperature of burnout in-cylinder gasses as the amount of the fuel taken into the cylinder decreased. This result may be explained by the fact that an incomplete combustion increases with the low temperature of the in-cylinder gasses and HC emissions increase. When the air/fuel mixture closed to the stochiometric ratio, in-cylinder temperature and combustion efficiency increased since more fuel was burned in these conditions. So, HC emissions decreased. HC emissions decreased about 7.16 % for H50N50 (324 ppm) compared to n-heptane (349 ppm) at λ=2.2.
Fig. 12. The variation of HC emissions versus lambda
Journal Pre-proof The generation of CO emissions strongly depends on the incomplete combustion. Lambda is the main parameter which controls the CO emissions. Insufficient oxygen and lower incylinder gas temperature cause to form CO. The influence of lambda on CO emissions for the test fuels can be seen in Fig. 13. It is possible to say that CO emissions increased for the high values of lambda. This is because, in leaner mixture conditions less amount of the fuel was taken into the combustion chamber and lead to more incomplete combustion and low temperatures. Low temperature slows down the oxidation reactions and oxidation could not be performed well. So, CO2 formation can not be performed. Hence, higher CO is formed. It is seen that CO increases with the usage of n-hexane. In fact, there is slight difference between CO values with test fuels due to similar chemical composition such as molar mass and chemical formula. Chemical structure of the fuels are close to eachother. CO is affected more likely from the air/fuel ratio than fuel properties. CO emissions of N100, H100, H50N50 and H75N25 were determined as 0.061, 0.078, 0.074, and 0.073 % at λ=2.2, respectively.
Fig. 13. The variation of CO emissions versus lambda Unfortunately, HCCI combustion is restricted due to misfiring and knocking that are the most remarkable handicap. Knocking problem can be eliminated with higher octane number fuels. Fig. 14 illustrates the HCCI operating range for the test fuels versus lambda. As it can be seen from Fig. 14, the narrowest HCCI operating range was obtained for n-hexane. It was seen that HCCI combustion could not be achieved with H100 in a large misfiring and knocking zones. However, it was clearly observed that operating range was expanded with the addition of nheptane. So, n-heptane caused to achieve HCCI combustion with lower octane number. This can lead to occur auto-ignition easier. It was first realized that H75N25 presented larger operating range according to other test fuels. HCCI combustion occured with H75N25
Journal Pre-proof towards to λ=3 that means the combustion of very lean charge mixture. HCCI mode was generally provided between λ=1.6 and λ=2.8 with test fuels. N-heptane presented reasonable operating range due to zero octane number, because there is no knocking resistance. It was found that HCCI combustion can stably run with lean charge mixture due to addition of nhexane. It was also found that operating range extended in misfiring and knocking boundaries with H75N25 compared to n-heptane. N-hexane addition resisted to knocking and the test engine could run with richer charge mixture and higher imep. Furthermore, H75N25 provided larger HCCI operating range in misfiring zone with lower imep. At this point, H75N25 is seen to be the best test fuel in view of larger HCCI operating range. The richest (λ=1.4) HCCI combustion was achieved with H50N50. The increase of inlet air temperature also enlarged the HCCI combustion. Warmer combustion chamber improved the thermodynamic situation at the end of compression stroke. So, auto-ignition chemical reactions can be easily achieved due to better condition for HCCI combustion. Initial conditions are highly significant for HCCI combustion. Higher heat energy in the cylinder accelerates the combustion reactions and misfiring problem disappeared. Hence, HCCI operating range could be extended. Enlarging of HCCI operating range in missfiring boundary is an important aspect of n-hexane addition as seen in Figure 14. H50N50 showed less remarkable effect on operating range compared to n-heptane. What is suprising is that H75N25 presented larger area on HCCI mode. Unfortunately, the usage of poor n-hexane on HCCI mode is not a practical way to avoid missfiring. Moreover, desired findings could not be obtained with n-hexane addition due to lower anti-knock effect compared to other fuels.
Fig. 14. The effects of tests fuels on HCCI operating range versus lambda
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The effect of the test fuels on HCCI operating range versus engine speed was given in Fig. 15. Stable HCCI combustion was achieved with test fuels between 800 and 1600 rpm. Apart from the mentioned engine speed, HCCI combustion could not be performed owing to misfiring, partial combustion and knocking. It can be clearly seen that the largest operating range was obtained with H75N25. The narrowest region was obtained with H100 and H50N50. At 800 rpm, the test engine was run with a wide range of lambda values on stable HCCI combustion mode. Generally HCCI combustion was achieved with richer charge mixture between 800 and 1200 rpm resulting in higher imep as the test engine was run with H75N25. So, operating range was extended in knocking zones with H75N25. In a similar way, test engine was operated with very lean mixtures during wide engine speed range. This phonomena resulted in lower imep. Thus, HCCI combustion was extended in large misfiring region. That means economical and environmentally friendly alternative combustion mode. Optimum test fuel is thought to be H75N25 in view of larger HCCI operating range. The addition of n-hexane into n-heptane presented stronger resistance to knocking. It caused to run the test engine with a wide operating range on knocking zones. In addition, test engine needed higher fuel concentration in order to achieve auto-ignition when n-hexane fuel blends were used due to knocking resistance. Thus, imep increased. H100 showed stronger resistance for auto-ignition resulting in narrower operating range. Auto-ignition reactions can not be performed at richer and leaner mixtures with H100.
Fig. 15. The effect of test fuels on HCCI operating range versus engine speed
Journal Pre-proof 4.Conclusions In the present study, the combustion, performance and emission characteristics of HCCI engine fueled with pure n-heptane, pure n-hexane and n-hexane/n-heptane blends were investigated with varying lambda, engine speed and inlet air temperature. The most important results are; -
Both in-cylinder pressure and HRR decreased by increasing the value of lambda. The maximum in-cylinder pressure was obtained for n-heptane due to the lower octane number caused earlier ignition.
-
The highest CA10 value was obtained for H50N50. CA10 was obtianed earlier when the inlet air temperature was increased from 60 oC to 80 oC.
-
N-heptane showed the highest maximum in-cylinder gas temperature for 60 oC inlet air temperature. In-cylinder gas temperature decreased by increasing the amount of the n-hexane in the fuel mixtures.
-
CA50 was obtained close to TDC when the lambda was increased due to leaner charge mixture which cause lower in-cylinder gas temperature. Combustion was advanced with the increase of inlet air temperature. Moreover, the addition of n-hexane caused to retard combustion at a given lambda value due to higher octane number of nhexane.
-
Along with the increase of lambda thermal efficiency was decreased owing to lower released heat energy. Thermal Efficiency increased by 18.5% and 14.8% with H100 and H75N25 compared to n-heptane at λ=2.5, respectively.
-
The lowest COVimep value was obtained with n-heptane. COVimep was calculated for N100 and H100 as 2.4 and 3.07, respectively, at λ=2.2.
-
N-hexane showed higher combustion duration compared to n-heptane due to higher resistance of auto-ignition. N-heptane showed higher imep values owing to higher calorific value.
-
N-heptane showed higher torque values compared with n-hexane. Torque of n-heptane is higher about 23.4% than n-hexane for 1000 rpm engine speed. SFC values of nhexane and n-heptane were obtained as 0.58 kg/kWh and 0.46 kg/kWh, respectively, at 1000 rpm engine speed. N-heptane showed higher SFC because of higher density which cause injection of higher fuel molecules into the cylinder.
-
The lowest HC was measured with H50N50 compared to other test fuels. HC was measured as 309 ppm with H50N50 whereas 331 ppm with n-heptane at λ=2 at 1000
Journal Pre-proof rpm. Lower CO was measured with H50N50 especially at λ=1.54, λ=1.66, λ=1.80 compared to n-heptane. -
Narrower operating range was obtained with H50N50 and H100 compared to nheptane due to lower calorific value of n-hexane and density. However, H75N25 showed the largest operating range compared to all test fuels. It is possible to say that, HCCI combustion can be achieved with very lean charge mixture (λ≈3) for H75N25. Similarly, H75N25 expanded the operating range for both knocking and misfiring zones due to higher resistance to knocking owing to higher octane number of nhexane.
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Conflict of Interest and Authorship Conformation Form Please check the following as appropriate: √ All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version. √ This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. √ The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript o The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript: Author’s name
Affiliation
Journal Pre-proof Highlights; -
-
Optimum test fuel is thought to be H75N25 in view of larger HCCI operating range Thermal efficiency was decreased owing to lower released heat energy with the increase of lambda Knocking tendency was observed with n-heptane, n-hexane and H50N50 at richer charge mixtures
Bilal Aydoğan PII:
S0360-5442(19)32295-9
DOI:
https://doi.org/10.1016/j.energy.2019.116600
Reference:
EGY 116600
To appear in:
Energy
Received Date:
03 June 2019
Accepted Date:
22 November 2019
Please cite this article as: Bilal Aydoğan, An Experimental Examination of the Effects of n-Hexane and n-Heptane Fuel Blends on Combustion, Performance and Emissions Characteristics in a HCCI Engine, Energy (2019), https://doi.org/10.1016/j.energy.2019.116600
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Journal Pre-proof An Experimental Examination of the Effects of n-Hexane and n-Heptane Fuel Blends on Combustion, Performance and Emissions Characteristics in a HCCI Engine Bilal Aydoğan Burdur Mehmet Akif Ersoy University, High Vocational School of Technical Sciences, 15100 Burdur, Turkey [email protected] Abstract Homogenous charge compression ignition (HCCI) engines are low-temperature combustion engines with high thermal efficiency. The operation range of HCCI engines, which are not directly controlled on combustion, is limited by the problems of knocking and misfiring. At this point, it is aimed to eliminate the problem of knocking at the high engine loads by the charge mixture composition and various operating parameters. In this study, the experiments were performed in a single cylinder, four stroke, port injection HCCI gasoline engine to investigate the effects of n-hexane, n-heptane and n-hexane/n-heptane blends. Combustion characteristics, engine performance and exhaust emissions were determined at different engine speeds (800-1800 rpm), lambda (λ=1.5-3.0) and inlet air temperatures (60 and 80 oC). Operating range of the HCCI engine was determined by using both pure n-hexane and pure nheptane fuels. The experiments showed that n-hexane, which has higher octane number than that of n-heptane, has more resistance to knocking. In-cylinder pressure decreased with the increase of lambda for all test fuels. Thermal efficiency increased about 28% by using nhexane compared to n-heptane at constant lambda (λ=2.35). The results showed that H75N25 provided larger operating range compared with other test fuels. Keywords: HCCI, n-hexane, n-heptane, combustion, performance, emissions
Nomenclature -
HCCI HRR EGR Imep SOC Wnet Vd
Homegenous charge compression ignition Heat release rate (J/oCA) Exhaust gas recurcilation Indicated mean effective pressure (bar) Start of combustion Net work (joule) Cylinder swept volume (m3)
Journal Pre-proof -
P dV 𝑑𝑄 𝑑𝜃 𝑑𝑄ℎ𝑒𝑎𝑡 𝑑𝜃
A2 hc n Tg Tw Ti Vc nc k mfuel QLVH ∆R xn
Cylinder pressure (bar) Variation of cylinder volume (m3) Heat release (J) Crank angle (oCA) Heat transfer to cylinder walls (J/oCA) Heat transfer surface area (m2) Heat transfer coefficient (W/m2K) Engine speed (rpm) In-cylinder average gas temperature (K) Combustion chamber wall temperature (K) In-cylinder temperature versus crank angle In-cylinder volume polytrophic index Ratio of specific heat values Consumed fuel per cycle (g) Lower heating value (kJ/kg) Uncertainty of the measurement Independent variables of the function and uncertainty
1.Introduction Petroleum based fuels are widely used in many fields in all over the world. Crude oil reserves have been decreased and environmental pollution resulted from motored vehicles rapidly increases and damages atmosphere [1,2]. The use of petroleum-based fuels is also increasing rapidly to meet the increasing energy need with developing technology. Nevertheless, the fact that known fossil fuel reserves can meet the needs of less than fifty years with the current utilization rate and environmental degradation and global warming accelerated the work on alternative energy sources and alternative combustion modes [3,4]. So, alternative combustion modes should be researched in view of cleaner exhaust emissions with higher thermal efficiency in the internal combustion engines [5,6]. In recent years, there has been an increasing interest in HCCI combustion which is defined as low temperature combustion has an important potential to reduce exhaust emissions and higher thermal efficiency [7-9]. HCCI operating is limited by knocking that engine is unexpectedly run with higher pressure rise rate and misfiring limit that present higher cyclic variations [10,11]. Thus, volatile fuel should be selected for better homogenity of charge mixture in order to characterise HCCI combustion [12-14]. Chemical kinetics, composition of the charge mixture, and in-cylinder temperature are the main parameters affecting the HCCI combustion [15-17]. But knocking and misfiring are the most important problems which limits the usage of HCCI combustion in internal combustion engines [18,19]. Because, simultaneous and spontenous auto-ignition can result in knocking. Moreover, HCCI combustion results in higher CO and HC emissions because of the
Journal Pre-proof leaner charge mixture and lower combustion temperature [20-22]. Up to now, a number of studies have investigated how to control the combustion phasing and eliminate knocking by different methods such as changing inlet air temperature, using fuel with higher octane number, EGR, variable compression ratio, variable valve timing, using glow plug were used [23-28]. Among them, it is seen that HCCI combustion is highly sensitive to chemical properties of the fuel. The start of combustion and pressure-temperature history that are directly changed during combustion due to difficulty of vaporization compared to n-heptane. There are relatively many historical studies in the area of achieving stable HCCI combustion and extend operating range with iso-octane and n-heptane at different ratios. Furthermore, volatile and more flammable fuels like solvent based fuel should be used in order to prevent misfiring which is one of the most important handicap to tackle on HCCI combustion. In a study investigating the effects of compression ratio on combustion and emissions in a HCCI engine, Calam et al. [29] reported that when the mixture getting leaner rate of heat release and in-cylinder pressure decreased. They used RON20 composed of 20% isooctane and 80% nheptane, and RON40 composed of 40% isooctane and 60% n-heptane in a four-cycle, single cylinder Ricardo Hydra experimental engine in their experiments. The experiments were carried out by changing compression ratio (9:1, 10:1, 11:1 and 12:1) at different lambda. Combustion duration was extended with the high octane number of the fuel while it was decreased by increasing the compression ratio. CO and HC emissions decreased with the increase of compression ratio while NOx emisions were increased. In a follow-up study, Cınar et al. [30] evaluated the effects of valve lift on the combustion and emissions of a HCCI gasoline engine with the fuel RON80 (20% isooctane-80% n-heptane). The tests were performed at full HCCI combustion mode, at different lambda (λ=0.5-2) and inlet air temperatures (20-120 oC) between 800 and 1900 rpm engine speeds in a four stroke, port injection, single cylinder, Ricardo Hydra gasoline research engine. It has been reported that low lift cams extended the operating range of HCCI engine on misfiring and knocking operating zones. When the inlet air temperature increased, test engine could be run at high lambda values. However leaner air/fuel mixture decreases thermal efficiency at lower engine speed. Beside this, more stable HCCI combustion occured when the valve lifts were reduced. In a study conducted by Mack et al. [31], butanol isomer combustion in HCCI engine with wide range of intake pressure and equivalence ratios was investigated. The experiments showed that n-butanol showed more stable combustion and later misfiring than isobutanol. However, n-butanol showed more knocking combustion. In a study which set out to determine the engine parameters on combustion and performance of HCCI engine fueled with n-heptane,
Journal Pre-proof Hasan et al. [32] found that the combustion phase was advanced and the combustion duration was shortened when the engine speed decreased and intake air temperature was increased. In an investigation into the effects of n-butanol on combustion and emission characteristics of single cylinder port fuel injection four stroke HCCI engine, He et al. [33] reported that onset of autoignition was advanced and autoignition timing was delayed when the engine speed increased. Khandal et al. [34] investigated the effects of hydrogen (H2) and diesel/biodiesel fuel blends on the performance of HCCI engine. Engine showed severe knocking working at 80% load without EGR (exhaust gas recirculation) for biodiesel/diesel fuel blend. However, biodiesel/diesel fuel blend with 7% hydrogen fuel energy ratio showed 65-67% lower smoke and 98-99% lower NOx emissions at 80% load. In another study, Turkcan et al. [35] studied the effects of different injection parameters on HCCI combustion fueled with gasoline/ethanol and methanol/gasoline blends and compared the results with gasoline reference fuel. The experimental results showed that when start of the first injection timing was earlier, maximum pressure rate increased with the usage of alcohol/gasoline blends. However, they have reported that second injection timing had more impact on the HCCI combustion, namely thermal efficiency, indicated that mean effective pressure and maximum cylinder gas pressure could be controlled with the second injection timing. Appplication of HCCI combustion in the internal combustion engines is not seen to perform easily. This new combustion mode has some difficulties such as uncontrollable combustion phasing, narrow operating range and changing engine operating parameters. Besides, higher HC and CO are still significant handicap in view of achieving recent engine technology. On the contrary, both NOx and soot emissions could be reduced due to combustion of leaner charge with higher thermal efficiency. This experimental study was performed with n-hexane and n-heptane fuel blends on HCCI combustion which presents lower NOx and soot emissions with lean charge mixtures. HCCI combustion occurs spontaneously in the combustion chamber. Low temperature combustion (LTC) is seen with leaner charge mixture. So, NOx emissions are reduced, because lower gas temperature is obtained due to combustion of leaner mixture. Furthermore, soot and NOx formation released from CI engines can be reduced simultaneously with HCCI engines. SI engines have to be operated near the stoichiometric ratio. In addition, catalitic convertor and exhaust gas reduction systems that expensive and non-practical should be used to reduce exhaust gas. So, HCCI can be defined as environmentally friendly combustion mode when compared other conventional cycles. If the comparison was made, the operating parameters of SI engine should be modified for the selected test fuel.
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The study presented here is one of the first investigations to explore the behaviors of n-hexane fuel blends on HCCI mode in view of detailed combustion analysis and engine performance. In this study, n-heptane (N100), n-hexane (H100), 50% n-hexane/50% n-heptane (H50N50) and 75% n-hexane/25% n-heptane were used in HCCI engine and combustion, performance and emission characteristics were investigated. The experiments were performed at different lambda values (between 1.5 and 3.0), engine speeds (between 800-1800 rpm) and inlet air temperatures (60 and 80 oC). The aim of this experimental research is to extend HCCI operating range and eliminate knocking tendency in a HCCI engine fueled with high octane number n-hexane and n-heptane fuel blends. 2.Experimental Setup and Procedures The experiments were performed with a single cylinder, port injection, four stroke gasoline HCCI engine. DC dynamometer which was rated 30 kW/6500 rpm engine speed was connected to the test engine. The test engine could not be run with original cam mechanism on HCCI mode. In addition, dilution of charge mixture is essential to prevent knocking and reduce pressure rise rate in HCCI combustion mode. Low lift cam mechanism were used for HCCI mode. Some residual gases were trapped to dilute charge composition. The test engine was stably operated on HCCI mode between 1.5 and 3.0 lambda values and 60 and 80 oC inlet air temperatures. HCCI combustion was not achieved apart from mentioned parameters due to knocking and misfiring. Combustion characteristics were discussed at 1000 rpm, 60 or 80 °C
and constant lambda. The table below illustrates the technical properties of the HCCI
engine. The schematic diagram of the experimental setup is given in Fig. 1. Table 1. The technical properties of the test engine Model Number of Cylinders Cylinder bore (mm) Stroke (mm) Volume (cc) Compression ratio Power output (kW) Maximum engine speed (rpm) Valve timing Valve lift
Ricardo-Hydra 1 80.26 88.90 540 13:1 15 5400 IVO/EVC 12o BTDC/56o ATDC Intake/exhaust 5.5/3.5
Journal Pre-proof Engine speed, fuel injection pulse, engine oil, engine coolant and inlet air temperatures can be measured and controlled by the dynamometer control panel. Air heating system was adapted in the access of the intake manifold. The experiments were performed at two inlet air temperatures including 60 and 80 oC. Temperatures were measured with K-type thermocouple and fixed at constant value by closed-loop controller. Kistler model 6121 piezoelectric pressure transducer was used to measure in-cylinder pressure. Cussons P4110 combustion analysis device scaled up the pressure data and National Instruments USB 6259 data acquisition card converted the data to digital signals with a timing resolution of 0.36 crank angle degrees. The pressure data of the cylinder were recorded in the computer. In-cylinder pressure of 50 consecutive cycles were averaged in order to minimize the cyclic variations for each test condition.
Fig. 1. The schematic diagram of the experimental setup The experiments were performed between λ=1.5 and λ=3 fueling ratios and different engine speeds from 800 to 1800 rpm at constant compression ratio (13:1). Exhaust gas analyzer apparatus measured HC, CO and NO emissions and air/fuel ratio. Firstly, the engine was operated on SI mode and warmed up. Then, spark ignition was switched off to achieve HCCI combustion. N-heptane, n-hexane and n-heptane/n-hexane blends were used in the experiments. The results obtained from the experiments were compared with n-heptane which was used as the base fuel. Table 2 provides the chemical properties of the test fuels.
Journal Pre-proof Table 2. Chemical properties of the test fuels [36] Chemical Formula Density (kg/m3) Octane number Heat of Combustion (kJ/mol) Boiling point (oC) Molar mass (g/mol)
n-Hexane C6H14 659 26 4163.2 69 86.17
n-Heptane C7H16 679.5 0 4817 98 100.16
An algorithm arranged in Matlab was used to process raw cylinder pressure data. The heat transfer from gas to cylinder walls were taken into consideration in calculations. It was assumed that there is no leakage between piston and cylinder and charge mixture in the cylinder was constant. Imep was calculated with Equation 1. Imep shows the mean pressure that exerted on the piston during a cycle. So, it is recognized as a significant engine performance parameter. Equation 2 and 3 were used to calculate net work and HRR, respectively. Cyclic variations were aimed to reduce using the average of 50 consequtive cycles and then in-cylinder pressure and heat release rate were determined. The calculated heat release is the total heat release. Heat transfer was computed seperately. The heat transfer to the cylinder wall was determined with Eq. 4. The heat transfer to the cylinder wall is a small portion of total heat release (<2 J/oCA). The exhaust gas temperature was measured in the exhaust manifold just after combustion chamber. The tests were performed at constant coolant and engine oil temperatures in order to provide durability for stable HCCI combustion. In-cylinder wall temperature changes during a cycle. The variations of incylinder temperature were estimated with the Eq. 5. In-cylinder wall temperature was assumed to be a constant value as 500 K due to exhaust gas temperature in order to make the calculations easier. 𝑖𝑚𝑒𝑝 =
𝑊𝑛𝑒𝑡
(1)
𝑉𝑑
(2)
𝑊𝑛𝑒𝑡 = ∫𝑃𝑑𝑉 𝑑𝑄 𝑑𝜃
𝑘
𝑑𝑉
1
𝑑𝑃
= 𝑘 ― 1𝑃𝑑𝜃 + 𝑘 ― 1𝑉𝑑𝜃 +
𝑑𝑄ℎ𝑒𝑎𝑡 𝑑𝜃
1
= 6𝑛ℎ𝑐𝐴2(𝑇𝑔 ― 𝑇𝑤)
( )
𝑇𝑖 + 1 = 𝑇𝑖
𝑉𝑟
𝑉𝑐 + 1
𝑛𝑐
𝑑𝑄ℎ𝑒𝑎𝑡 𝑑𝜃
(3) (4) (5)
Thermal efficiency is a dimensionless parameter in the internal combustion engines and calculated with the ratio between the net work and released energy from fuel (Equation 6).
Journal Pre-proof 𝑊𝑛𝑒𝑡
(6)
𝜂𝑇 = 𝑚𝑓𝑢𝑒𝑙1.𝑄𝐿𝐻𝑉1 + 𝑚𝑓𝑢𝑒𝑙2.𝑄𝐿𝐻𝑉2
Various error sources such as the random fluctuation of the instruments, the calibration of the measuring devices, the calculated accuracies and the technics of the tests affect the uncertainties of the observed parameters. The measurement equipments were calibrated before the tests. The accuracy and the operating range of the gas analyzer and cylinder pressure transducer were given at Table 3 and 4, respectively. The gas analyzer was purged and calibrated before performing the next work on a selected test condition. The uncertainty of the performance parameters were calculated on the basis of the root mean square method (Eq. 7). The accuracies of the measurements and the uncertainties of the calculated results were given at Table 5. Table 3. The specifications of the exhaust gas analyzer Operating range 0.000-10.00 0.00-18.00 0-9999 0.00-22.00 0.500-9.999 0-5000
CO (% vol) CO2 (% vol) HC (ppm) O2 (% vol) λ NO (ppm vol)
Accuracy 0.001 0.01 1 0.001 0.001 ≤1
Table 4. Technical properties of the cylinder pressure transducer Model Operating range (bar) Measurement precision (pC/bar) Operating temperature (oC) Accuracy (±%) ∆𝑅 =
[(
2 ∂𝑅 ∆𝑥 1 ∂𝑥1
) ( +
Kistler 6121 piezo electric 0-250 14.7 -50-350 0.5
2 ∂𝑅 ∆𝑥 2 ∂𝑥2
)
+…+
(
2 0.5 ∂𝑅 ∆𝑥 𝑛 ∂𝑥𝑛
)]
(7)
Table 5. Accuracies of the measurements and the uncertainties in the calculated results Accuracy Time (s) Temperature (°C) Fuel (g) Heating value (kJ/kg)
±0.5% ±1°C ±0.1 g ±0.1 %
Uncertainty (%) -
Journal Pre-proof Engine speed (rpm) Load (N) Torque (Nm) BTE BSFC (g/kW-h)
±1% ±0.25% -
±0.09 ±1.38 ±1.25
3.Results and Discussion 3.1. Combustion Evaluation Air/fuel ratio, mixture composition and inlet air temperature are the important parameters for HCCI combustion. In this study, the experiments were performed at different air/fuel ratios, inlet air temperatures and mixture compositions at full load condition. Stable HCCI combustion could be achieved between λ=1.5 and λ=3. And the test engine was smoothly run with 60 and 80 oC inlet air temperatures on HCCI mode. At lower and higher inlet air temperatures stable HCCI combustion was deteriorated due to misfiring and knocking, respectively. So, range of the test parameters were selected as mentioned. The extend to which higher inlet air temperature caused to knocking with low octane number n-hexane. Hence, inlet air temperature could not be increased too much. This is already an important key for HCCI that provides the combustion of very lean mixture. So, air/fuel ratio could not be dropped below λ=1.5 because of unstable HCCI combustion.
Fig. 2 shows the in-cylinder
pressure and HRR versus to crank angle with the variation of lambda for the test fuels. In Fig. 2 there is a clear trend of increasing in-cylinder pressure for the low values of lambda and the knocking occured for all test fuels. This result maybe explained by the fact that when the lambda decreased, more fuel quantity is injected into the combustion chamber to generate richer air/fuel mixtures that increased the maximum in-cylinder pressure after combustion [37]. It can be clearly seen from the Fig. 2 that maximum in-cylinder pressure decreased and knocking disappeared with the increase of lambda values. A possible explanation for this might be that when charge mixture is getting lean, combustion phasing is retarded and incylinder pressure decreases. HRR is a sign of the speed of the energy conversion from chemical energy to thermal energy during combustion. Rapid heat release shows the higher pressure rise rate in case of knocking [38-39]. The force exerted into the piston increased too much with knocking. The pressure applied to the piston increased for each crank angle variation. Oil gap between engine bearing and crank journal closes rapidly. Pressure waves and undesirable combustion noise were observed due to knocking. It is apparent from Fig.2 that HRR decreased for high value of the lambda, because less amount of the fuel is injected into the combustion chamber. It was clearly seen that knocking was seen with n-heptane. The
Journal Pre-proof addition of n-hexane into n-heptane resulted in more stable HCCI combustion especially with H75N25 due to higher octane number of n-hexane. Combustion delayed with the increase of lambda for all test fuels.
Fig. 2. The variation of in-cylinder pressure and HRR a) N100 b) H100 c) H50N50 d) H75N25 At leaner charge mixtures, fuel molecules decrease in the combustion chamber. In addition, fuel molecules could not easily gather with oxygen molecules due to lower fuel concentration during combustion. So, oxidation rate reduces with the lack of fuel. HCCI combustion was obtained with H75N25 until λ=1.81 lambda value. Test engine could not be operated at richer charge mixture with H75N25. Similar maximum in-cylinder pressure was measured with test fuels owing to close calorific value of n-hexane and n-heptane. Fig. 3 represents the incylinder and HRR with different inlet air temperatures (Tin=60 oC and Tin=80 oC) at constant lambda (λ=2.2) for the test fuels. When the inlet air temperature is increased at constant lambda, maximum in-cylinder pressure increased because of the higher initial temperature of the charge. Auto-ignition occurs earlier due to the high inlet air temperature and combustion phase is advanced. In high inlet air temperatures, the chemical reactions between fuel and
Journal Pre-proof oxygen molecules occur easier and faster and more molecules take part in reactions [40]. As it can be seen from the Fig. 3, low and high temperature combustion zones are seen on HCCI mode. Moreover, it was depicted from Fig. 3 that maximum in-cylinder pressure of n-heptane is higher than that of n-hexane for both inlet air temperatures. N-heptane is ignited earlier than n-hexane due to the lower octane number. It is possible to say that when the amount of nhexane was increased in the fuel mixtures, maximum in-cylinder pressure decreased compared to n-heptane. Similarly, HRR decreased with the addition of n-hexane in the fuel blends. Released heat energy decreases because of lower heating value of n-hexane.
Fig.3 The variation of in-cylinder pressure and HRR a) Inlet air temperature= 60 oC b) Inlet air temperature= 80 oC CA10 defines the 10% of burned charge mixture according to crank angle. CA10 can be supposed as the start of combustion (SOC) in many studies in the literature on HCCI mode. SOC is strongly dependent on the thermodynamical situation at the end of compression stroke especially with inlet air temperature on HCCI mode. Higher inlet air temperature increases the in-cylinder temperature at the end of compression stroke. Hence, oxidation reactions are accelerated with higher heat energy in the combustion chamber. CA10 can not be directly controlled and auto-ignition is totally controlled by chemical kinetics of the fuel and incylinder temperature in HCCI combustion [41]. The variation of CA10 versus lambda for different inlet air temperatures were given in Fig. 4. What stands out in this figure is the retarding combustion by increasing the lambda for all test fuels, because lower heat is released due to lower fraction of fuel during the burning of leaner charge mixture. As it can be seen from the Fig.4 H50N50 showed the highest C10 value. When the inlet air temperature was increased from 60 oC to 80 oC, CA10 was obtained earlier for all test fuels compared to 60 oC. Moreover, n-heptane showed significant effect on CA10 due to lower resistance of
Journal Pre-proof combustion compared that fuel blends. So, combustion was advanced with the increase of inlet air temperature in case n-heptane was used as test fuel.
Fig.4 The variation of start of combustion a) Inlet air temperature= 60 oC b) Inlet air temperature= 80 oC Fig. 5 illustrates the variation of the in-cylinder gas temperatures versus the crank angle for constant lambda and engine speed for different inlet air temperatures (60-80 oC). In-cylinder gas temperature is an important parameter for HCCI combustion, because it initiates the autoignition reactions. N-heptane showed the highest maximum in-cylinder gas temperature for 60 oC inlet air temperature (≈1730 K). The corresponding temperature decreased by increasing the amount of the n-hexane in the fuel mixtures. It can be mentioned that lower heating value and density of n-hexane caused to obtain lower in-cylinder gas temperature. On the contrary, when the inlet air temperature was increased n-heptane showed the lowest incylinder gas temperature and it increased with the increasing amount of the n-hexane in the mixture. A possible explanation for this might be that partial of n-heptane is vaporized at higher inlet air temperature during combustion. So, maximum in-cylinder gas temperature increases according to the amount of n-hexane fraction in the fuel blends.
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Fig.5. The variation of in-cylinder gas temperatures of HCCI combustion at constant lambda λ=2.2 3.2. Engine Performance Combustion duration and thermal efficiency of the test fuels at constant engine speed and inlet air temperature for different lambda were given at Fig. 6. CA50 is an indicator for the crank angle where the half of the fuel mixture was combusted. What is striking about the figure is that CA50 closes to the top dead center (TDC) as the lambda increased. In-cylinder gas temperature decreases at leaner charge mixtures in the combustion chamber and chemical oxidation reactions are deteriorated. This event causes to retard combustion and CA50. Cylinder wall temperature also decreases and combustion chamber temperature reduces that prevents to complete combustion. On the contrary, CA50 moved away from TDC with increasing the amount of n-hexane in the mixture. As it is expected, higher octane number of n-hexane resisted to auto-ignition. Fig. 6b. shows the variation of thermal efficiency which is defined as how much the chemical energy of the fuel was converted to the mechanical energy. Thermal efficiency increased with the increasing the lambda for a given value and then started to decrease. Whole fuel molecules can be reacted and oxidized in the combustion chamber due to higher oxygen concentration at lambda values of λ=2.4, λ=2.6. Fuel molecules can gather with oxygen molecules until mentioned lambda values. Nevertheless, thermal efficiency decreased with very lean mixture (λ=2.6 and λ=2.7) due to lower released heat energy. Hence, thermal efficiency decreased. Thermal efficiency is highly affected by calorific value and density of the fuel. As it is seen from Fig. 6 that, lower thermal efficiency was obtained with H50N50 especially towards to stoichiometric ratio (λ=1.96, λ=1.80, λ=1.66). However, it was seen that H75N25 showed the highest thermal efficiency at leaner charge mixtures compared to H50N50 due to higher fraction of n-heptane. Charge mixture
Journal Pre-proof could be ignited due to lower octane number of n-heptane at leanaer mixtures. It can be pointed out that thermal efficiency increased with the increase of n-hexane addition in the fuel blends. Thermal efficiency increased by about 28% with H100 compared to n-heptane at λ=2.35. Because, stable auto-ignition reactions were observed with the addition of n-hexane. Knocking tendency increased with lower octane number fuels. Thus, thermal efficiency decreases.
Fig.6. The variation of a) combustion duration (CA50) and b) thermal efficiency of HCCI combustion Residual gases could not be discharged from the combustion chamber due to higher engine speed and valve overlap at the end of exhaust stroke. Some exhaust gases are trapped in the combustion chamber and mixes with fresh charge for the next cycle. So, charge composition and thermodynamic situation varies at the end of compression stroke. Consequently, combustion characteristics change cycle by cycle. This phonomena resulted in unstable combustion and prevented the durability of engine operation. It is recomended that cyclic variation does not exceeds 10% for stable combustion [10]. In the present study, valve mechanism of the test engine was modified and low lift cams were used in order to prevent knocking with trapped residual gasses especially with richer mixtures and to achieve stable HCCI combustion. Heat capacity of charge composition should be increased on HCCI mode avoiding higher pressure rise rate. Fig. 7 shows the variation of COVimep versus lambda at 1000 rpm constant engine speed and 80 oC inlet air temperature. The values of COVimep are 6.54, 9.82, 8.71, 7.60 at the leanest mixture (λ=1.7) for N100, H100, H50N50 and H75N25, respectively. The lowest COVimep was calculated with n-heptane among the other test fuels. COVimep was calculated for N100, H100, H50N50 and H75N25 as 2.4, 3.07, 3.33, 2.85 at λ=2.2 that was mentioned as the ideal fuel/air mixture in this study. Furthermore, COVimep decreased with the addition of n-hexane in the fuel blends. It was seen that knocking tendency
Journal Pre-proof decreases with n-hexane addition. This event also provided more stable combustion. Sudden and rapid combustion and heat release are prevented due to higher resistance of knocking with n-hexane.
Fig 7. The variation of COVimep with the test fuels Fig. 8. illustrates the variation of the combustion duration versus lambda in HCCI combustion at constant engine speed and inlet air temperature. It is observed that combustion duration increases as the amount of the n-hexane increases in the mixture because of higher resistance to auto-ignition. Combustion duration decreased with H75N25 with the increase of lambda. No significant differences were found between H50N50 and n-hexane for combustion duration according to lambda values. It can be pointed out that combustion duration decreased with n-heptane as the lambda increased. The maximum combustion duration observed for H75N25 test fuel as about 41.56 oCA at λ= 1.93. The combustion duration of N100, H100, H50N50 and H75N25 are 41.56, 39.24, 38.9, and 34.6 at λ= 1.93. The corresponding value decreased for all test fuels when the lambda was increased to 2.56. The maximum decrease was obtained about 5.8 % for H75N25.
Fig. 8. The variation of the combustion duration versus lambda
Journal Pre-proof RI is one of the most important parameters which controls the operating limits of combustion noise [42]. The variation of ringing intensity (RI) with maximum pressure rise rate at constant engine speed, inlet air temperature and for λ= 2.2 was given in Fig. 9. RI changes according to engine speed, combustion rate and maximum in-cylinder pressure [43,44]. MPRR defines the maximum in-cylinder pressure that applied on the piston for each crank angle degree variation. RI increased with increased MPRR for all test fuels. The lowest RI values were obtained with n-heptane. RI decreased with the increase of n-hexane in the mixture.
Fig. 9. The variation of a) Ringing intensity and b) MPRR versus lambda Imep is an important parameter to indicate performance for internal combustion engines [45,46]. Fig. 10 shows the variation of imep versus 50 consecutive cycles. Imep varies according to heating value of fuel, because higher heat energy releases and higher pressure is exerted on the piston. So, imep increases. More stable combustion without knocking was observed for H50N50 test fuel. The highest imep values were obtained with n-heptane owing to higher calorific value. The addition of n-hexane caused to decrease imep due to lower calorific value. However, more reasonable and stable HCCI combustion was obtained with fuel blends even if n-hexane has lower calorific value compared that n-heptane. Knocking tendency reduced with the addition of n-hexane. This improves the chemical oxidation reactions and engine performance. However, lower density of n-hexane may not be applicable to obtain higher imep. Particle size of n-hexane fuel molecules remain small and fuel molecules could not well proceed in the inlet air compared to n-heptane. Thus, homogenous charge mixture could not be obtained and auto-ignition reactions do not occur towards to combustion chamber. So, obtained heat and in-cylinder pressure forced to the piston decreased and lower imep was obtained with n-hexane.
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Fig. 10. The variation of imep versus consecutive 50 cycles The effect of engine speed on torque, power output and SFC for n-heptane and n-hexane was given in Fig. 11. Power output increases with the increase of engine speed until a specific value and then started to decrease. Power output decreases due to friction losses and unsufficient oxygen at higher engine speed. Moreover, the test engine could not take sufficient oxygen due to limited duration at higher speeds and volumetric efficiency decreases. So, power output decreases. Likely, brake torque first increased and then started to decrease. After maximum torque speed, brake torque decreases owing to gas leakages and heat losses at higher engine speeds. It is apparent from Fig. 11 that, torque first increased with the increase of engine speed for both fuels. N-heptane showed higher torque values compared with n-hexane. As it can be seen from the Fig. 11.b that HCCI combustion could not be achieved at low engine speed (800 rpm) for n-hexane. Torque of n-heptane is higher about 23.4% than n-hexane for 1000 rpm engine speed. At higher engine speeds (1400 and 1600 rpm) the value of torque between n-hexane and n-heptane remained nearly same. Power output of n-hexane and n-heptane were 0.49 kW and 0.37 kW for 1000 rpm engine speed, respectively. It was nearly same for n-hexane and n-heptane for 1400 rpm engine speed but nhexane showed higher power output value about 19% than n-heptane for 1600 rpm engine speed. As the engine speed increased, SFC decreased until 1400 rpm engine speed. After that engine speed, SFC got started to increase. SFC was obtained as 0.58 kg/kWh and 0.46 kg/kWh for n-heptane and n-hexane at 1000 rpm engine speed, respectively. Higher fuel molecules are injected into the cylinder with n-heptane due to higher density, because injected mass fraction of n-heptane by volume is higher than n-hexane. It means that higher fuel molecules should be combusted in order to obtain same power output with n-heptane.
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Fig. 11. The variation of torque, power output and SFC versus engine speed a) N100 b) H100 3.3. Emissions Fig. 12 displays the variation of the HC emissions at different lambda values at constant engine speed (1000 rpm) and inlet air temperature (60 oC). HCCI combustion has very low NOx emissions because it has low combustion temperature. Nevertheless, low combustion temperature causes to insufficient oxidation of the fuel and this leads to higher HC emissions. What stands out in the Fig. 12 is increasing HC emissions by increasing value of the lambda due to the low temperature of burnout in-cylinder gasses as the amount of the fuel taken into the cylinder decreased. This result may be explained by the fact that an incomplete combustion increases with the low temperature of the in-cylinder gasses and HC emissions increase. When the air/fuel mixture closed to the stochiometric ratio, in-cylinder temperature and combustion efficiency increased since more fuel was burned in these conditions. So, HC emissions decreased. HC emissions decreased about 7.16 % for H50N50 (324 ppm) compared to n-heptane (349 ppm) at λ=2.2.
Fig. 12. The variation of HC emissions versus lambda
Journal Pre-proof The generation of CO emissions strongly depends on the incomplete combustion. Lambda is the main parameter which controls the CO emissions. Insufficient oxygen and lower incylinder gas temperature cause to form CO. The influence of lambda on CO emissions for the test fuels can be seen in Fig. 13. It is possible to say that CO emissions increased for the high values of lambda. This is because, in leaner mixture conditions less amount of the fuel was taken into the combustion chamber and lead to more incomplete combustion and low temperatures. Low temperature slows down the oxidation reactions and oxidation could not be performed well. So, CO2 formation can not be performed. Hence, higher CO is formed. It is seen that CO increases with the usage of n-hexane. In fact, there is slight difference between CO values with test fuels due to similar chemical composition such as molar mass and chemical formula. Chemical structure of the fuels are close to eachother. CO is affected more likely from the air/fuel ratio than fuel properties. CO emissions of N100, H100, H50N50 and H75N25 were determined as 0.061, 0.078, 0.074, and 0.073 % at λ=2.2, respectively.
Fig. 13. The variation of CO emissions versus lambda Unfortunately, HCCI combustion is restricted due to misfiring and knocking that are the most remarkable handicap. Knocking problem can be eliminated with higher octane number fuels. Fig. 14 illustrates the HCCI operating range for the test fuels versus lambda. As it can be seen from Fig. 14, the narrowest HCCI operating range was obtained for n-hexane. It was seen that HCCI combustion could not be achieved with H100 in a large misfiring and knocking zones. However, it was clearly observed that operating range was expanded with the addition of nheptane. So, n-heptane caused to achieve HCCI combustion with lower octane number. This can lead to occur auto-ignition easier. It was first realized that H75N25 presented larger operating range according to other test fuels. HCCI combustion occured with H75N25
Journal Pre-proof towards to λ=3 that means the combustion of very lean charge mixture. HCCI mode was generally provided between λ=1.6 and λ=2.8 with test fuels. N-heptane presented reasonable operating range due to zero octane number, because there is no knocking resistance. It was found that HCCI combustion can stably run with lean charge mixture due to addition of nhexane. It was also found that operating range extended in misfiring and knocking boundaries with H75N25 compared to n-heptane. N-hexane addition resisted to knocking and the test engine could run with richer charge mixture and higher imep. Furthermore, H75N25 provided larger HCCI operating range in misfiring zone with lower imep. At this point, H75N25 is seen to be the best test fuel in view of larger HCCI operating range. The richest (λ=1.4) HCCI combustion was achieved with H50N50. The increase of inlet air temperature also enlarged the HCCI combustion. Warmer combustion chamber improved the thermodynamic situation at the end of compression stroke. So, auto-ignition chemical reactions can be easily achieved due to better condition for HCCI combustion. Initial conditions are highly significant for HCCI combustion. Higher heat energy in the cylinder accelerates the combustion reactions and misfiring problem disappeared. Hence, HCCI operating range could be extended. Enlarging of HCCI operating range in missfiring boundary is an important aspect of n-hexane addition as seen in Figure 14. H50N50 showed less remarkable effect on operating range compared to n-heptane. What is suprising is that H75N25 presented larger area on HCCI mode. Unfortunately, the usage of poor n-hexane on HCCI mode is not a practical way to avoid missfiring. Moreover, desired findings could not be obtained with n-hexane addition due to lower anti-knock effect compared to other fuels.
Fig. 14. The effects of tests fuels on HCCI operating range versus lambda
Journal Pre-proof
The effect of the test fuels on HCCI operating range versus engine speed was given in Fig. 15. Stable HCCI combustion was achieved with test fuels between 800 and 1600 rpm. Apart from the mentioned engine speed, HCCI combustion could not be performed owing to misfiring, partial combustion and knocking. It can be clearly seen that the largest operating range was obtained with H75N25. The narrowest region was obtained with H100 and H50N50. At 800 rpm, the test engine was run with a wide range of lambda values on stable HCCI combustion mode. Generally HCCI combustion was achieved with richer charge mixture between 800 and 1200 rpm resulting in higher imep as the test engine was run with H75N25. So, operating range was extended in knocking zones with H75N25. In a similar way, test engine was operated with very lean mixtures during wide engine speed range. This phonomena resulted in lower imep. Thus, HCCI combustion was extended in large misfiring region. That means economical and environmentally friendly alternative combustion mode. Optimum test fuel is thought to be H75N25 in view of larger HCCI operating range. The addition of n-hexane into n-heptane presented stronger resistance to knocking. It caused to run the test engine with a wide operating range on knocking zones. In addition, test engine needed higher fuel concentration in order to achieve auto-ignition when n-hexane fuel blends were used due to knocking resistance. Thus, imep increased. H100 showed stronger resistance for auto-ignition resulting in narrower operating range. Auto-ignition reactions can not be performed at richer and leaner mixtures with H100.
Fig. 15. The effect of test fuels on HCCI operating range versus engine speed
Journal Pre-proof 4.Conclusions In the present study, the combustion, performance and emission characteristics of HCCI engine fueled with pure n-heptane, pure n-hexane and n-hexane/n-heptane blends were investigated with varying lambda, engine speed and inlet air temperature. The most important results are; -
Both in-cylinder pressure and HRR decreased by increasing the value of lambda. The maximum in-cylinder pressure was obtained for n-heptane due to the lower octane number caused earlier ignition.
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The highest CA10 value was obtained for H50N50. CA10 was obtianed earlier when the inlet air temperature was increased from 60 oC to 80 oC.
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N-heptane showed the highest maximum in-cylinder gas temperature for 60 oC inlet air temperature. In-cylinder gas temperature decreased by increasing the amount of the n-hexane in the fuel mixtures.
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CA50 was obtained close to TDC when the lambda was increased due to leaner charge mixture which cause lower in-cylinder gas temperature. Combustion was advanced with the increase of inlet air temperature. Moreover, the addition of n-hexane caused to retard combustion at a given lambda value due to higher octane number of nhexane.
-
Along with the increase of lambda thermal efficiency was decreased owing to lower released heat energy. Thermal Efficiency increased by 18.5% and 14.8% with H100 and H75N25 compared to n-heptane at λ=2.5, respectively.
-
The lowest COVimep value was obtained with n-heptane. COVimep was calculated for N100 and H100 as 2.4 and 3.07, respectively, at λ=2.2.
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N-hexane showed higher combustion duration compared to n-heptane due to higher resistance of auto-ignition. N-heptane showed higher imep values owing to higher calorific value.
-
N-heptane showed higher torque values compared with n-hexane. Torque of n-heptane is higher about 23.4% than n-hexane for 1000 rpm engine speed. SFC values of nhexane and n-heptane were obtained as 0.58 kg/kWh and 0.46 kg/kWh, respectively, at 1000 rpm engine speed. N-heptane showed higher SFC because of higher density which cause injection of higher fuel molecules into the cylinder.
-
The lowest HC was measured with H50N50 compared to other test fuels. HC was measured as 309 ppm with H50N50 whereas 331 ppm with n-heptane at λ=2 at 1000
Journal Pre-proof rpm. Lower CO was measured with H50N50 especially at λ=1.54, λ=1.66, λ=1.80 compared to n-heptane. -
Narrower operating range was obtained with H50N50 and H100 compared to nheptane due to lower calorific value of n-hexane and density. However, H75N25 showed the largest operating range compared to all test fuels. It is possible to say that, HCCI combustion can be achieved with very lean charge mixture (λ≈3) for H75N25. Similarly, H75N25 expanded the operating range for both knocking and misfiring zones due to higher resistance to knocking owing to higher octane number of nhexane.
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Conflict of Interest and Authorship Conformation Form Please check the following as appropriate: √ All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version. √ This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. √ The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript o The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript: Author’s name
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Journal Pre-proof Highlights; -
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Optimum test fuel is thought to be H75N25 in view of larger HCCI operating range Thermal efficiency was decreased owing to lower released heat energy with the increase of lambda Knocking tendency was observed with n-heptane, n-hexane and H50N50 at richer charge mixtures