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Experimental assessment of performance and emissions maps for biodiesel fueled compression ignition engine
Applied Energy 161 (2016) 320–329
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Experimental assessment of performance and emissions maps for biodiesel fueled compression ignition engine Kezrane Cheikh a,b, Awad Sary b,⇑, Loubar Khaled b, Liazid Abdelkrim c, Tazerout Mohand b a
Laboratoire d’Energétique, Mécanique et Ingénieries, UMBB, Boumerdes 35000, Algeria GEPEA, UMR 6144, Département Systèmes Energétiques et Environnement, Ecole des Mines de Nantes, La chantrerie, 04 rue Alfred Kastler, B.P. 20722, F-44307 Nantes Cedex 3, France c LTE Laboratory, BP 1523 El-Mnaouer, ENSET, 31000 Oran, Algeria b
h i g h l i g h t s Study of combustion characteristics for biodiesel, diesel and theirs blends in CI engine. Effects of engine speed and load on performance and emissions maps are investigated. The BSFC for biodiesel shows an inverse behavior to petrol diesel and its blends. UHC emissions increase with engine speed. The effect of load is noticeable for blends. CO and PM emissions are correlated with load. They are less sensitive to engine speed.
a r t i c l e
i n f o
Article history: Received 6 July 2015 Received in revised form 23 September 2015 Accepted 5 October 2015
Keywords: Performance mapping Combustion characteristics CI engine Biodiesel Emissions
a b s t r a c t Attempts to use many types of biofuel have been tried throughout the past. Waste cooking oils (WCO) represent attractive alternatives to edible fat and oils for the production of biodiesel fuel from different points of view: they are inexpensive feedstock for biodiesel production. At the same time, using this kind of feedstock eliminates food versus fuel competition. Finally, Their use supposes an environmentally friendly approach. Internal combustion engines operate correctly on a wide range of speeds and loads. However, only few studies on biodiesel fueled engines are undertaken for performance mapping. In the present study, the effects of entire operational range of speed/load on engine performance and emission levels of an engine are investigated when neat WCO biodiesel (B100) and its blends (B25) and (B50) are used. The obtained results are compared to those of conventional diesel (B0). The suitability of WCO biodiesel has been established by many researchers. However their results report a wide disparity on emission levels. Combustion characteristics, performance and emission maps will be performed to give appropriate indications explaining the divergence reported in literature. The map indicates that B50 and B25 exhibit similar trends of BSFC to diesel fuel, across the speed variation albeit with difference in response levels. Contrariwise B100 shows an inverse behavior with speed increase. Results show also that Unburned Hydro-Carbons (UHC) emissions are highly correlated with engine speed. However, CO and PM emissions are extremely correlated with load and they are less sensitive to engine speed. NOx emissions are generally higher with biodiesel except some extreme zones of the map. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The rising worldwide demand for fossil energies i and its related pollution problems require urgent development of renewable and environmentally friendly energy sources. Therefore, biomass ⇑ Corresponding author. Tel.: +33 251858561; fax: +33 251858299. E-mail address: [email protected] (A. Sary). http://dx.doi.org/10.1016/j.apenergy.2015.10.042 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
derived fuels like biodiesel are required to substitute diesel fuel. Biodiesel is technically suitable for use in compression ignition engines. It is comprised of mono-alkyl esters of long-chain fatty acids derived from vegetable oils or animal fats. It is designated as B100 and it meets the requirements of European Standard EN-14214 and/or the American Society for Testing and Materials D-675 [1].
K. Cheikh et al. / Applied Energy 161 (2016) 320–329
Bio-fuels might contribute to meet the future energy supply demands as well as helping to the reduction of green house gas emissions (GHG). However, extensive adoption of biofuel required more land to cultivate energy feedstock. Natural forests are snatched in some countries for plantation purposes of biodiesel industry. In addition, this can increase the conversion of cultivable land usages from food to biofuel production [2,3], causing food shortage [4]. Many investigations are currently led on the numerous sustainable energy options in agriculture including the use of more energy efficient technologies and the replacement of fossil fuels using renewable energy powered technologies [5]. Ester of fatty acids (biodiesel) differs from diesel fuel in its composition and properties which could lead to differences in engine performance, combustion characteristics and exhaust emissions. Literature sources present exhaustive lists of biodiesel advantages: it helps decreasing GHG emissions to the atmosphere, it does not contain sulfur or aromatic compounds, it is renewable in nature and safer to handle, in addition, its oxygen content reduces emissions related to poor combustion conditions [6,7]. Biodiesel use results in some drawbacks comparing to diesel fuel including: high specific fuel consumption, substantial emissions of oxygenatedhydrocarbons, poor flow properties at low-temperature conditions, drop in brake thermal efficiency and high costs of production [8]. The use of waste fats or vegetable oils as feedstock for transesterification process can partially solve the problem of biofuel production costs. Talebian-Kiakalaieh et al. [9], claimed that using WCO as feed stock can reduce biodiesel production costs up to 60–90%. Frying oil leads to various chemical and physical changes resulting in the formation of some polymerized tri-glycerides and free fatty acid. These undesirable compounds increase the molecular weight of the oil and reduce its volatility. Thus, fatty acid esters of fried oils influence fuel properties such as viscosity and burning characteristics resulting in greater amount of carbon residue [10,11]. Nevertheless, the suitability of WCO methyl ester as a biodiesel has been ascertained by many researchers, even the obtained results are mixed, mainly about emissions. In spite of the high consensus in trends of performance and emissions recognized in the literature when using biodiesel, there is still a lack of correspondence in the conclusions made by some authors. This may be due to various factors like biodiesel feedstock, blending ratio and engine operating conditions which affect the combustion characteristics and pollutant formation processes [12]. Lapuerta et al. [13] reported that ethyl esters obtained from WCO emit lower UHC at medium load conditions. Nevertheless no clear tendency was established at low load conditions. Such trends are in agreement with An et al. [14] observations. The UHC emissions are found to be lower at all engine operation conditions, except under partial load at very low speed. It has been proved that load conditions have a significant effect on carbon monoxide (CO) emissions. Although Lin et al. [15], Durbin et al. [16] reported an augmentation of particulate matter (PM) emissions when fueling with WCO biodiesel, an evident decrease in particulate matter emissions with the biodiesel is the noticeable trend in literature. Most of the published studies revealed a slight increase in nitrogen oxides (NOx) emissions when using WCO biodiesel. However, a number of authors claimed that NOx increased only under some conditions such as alcohol-base of tested ester and certain conditions of load. Usually, compression ignition engines are designed to operate well within useful operational speed–load condition, however, performing accurately transient load/speed tests are extremely complicated and quite expensive (these require an entirely automated test-bench with electronically control of motoring and dissipating dynamometer, fast responses exhaust gas emissions analyzers, etc.). Individual load or speed accelerations tests
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are of wide acceptance rather than examination of the engine performance during the entire transient Cycle [17]. To characterize the responses of diesel-engined vehicles, during a transient/driving cycle, many relatively straightforward approaches are presented in literature [18–22]. These approaches use as a basis the steady-state engine tests, rectification coefficients are applied to account for transient discrepancies based on individual transient data. Therefore, in spite of the steady applications like generators or the marine sector, utilization of a steady-state test cycle gives an alternative to transient test when engine performance and emissions are of special interest instead of the investigation on the overall driving cycle [7]. Only few studies of biodiesel fueled engine cover the full speed– load spectrum. Thus, the objective of this work is to investigate the effects of engine speed and load, over the entire operational range of a direct injection compression ignition engine, when fueled with biodiesel. Construction of combustion characteristics, performance and emissions maps gives a closer look contributing to explain some divergences reported in literature. 2. Materials and methods For the present investigation, a naturally aspirated, direct injection (DI) diesel engine developing 7.5 kW at 2500 rpm was used. Table 1 gives the engine specifications. An eddy current dynamometer is coupled to the engine for converting the generated power to electricity that will be directly injected to the network. Acquisition and control of measured signals were performed by two systems. The first one commands the engine dynamometer and measures engine speed, torque, exhaust emissions, temperature and pressure in the collectors at a low-frequency. A second system allows the acquisition of high-frequency measurements (pressure in the cylinder, angular position of the crankshaft and fuel injection). Cylinder pressure was measured each 0.2 °CA using a water cooled piezoelectric pressure sensor, type AVL-QH32D. The pressure of injection measurement was performed by a piezoelectric transducer, type AVL-QH33D, placed between the fuel injector and the injection pump. An encoder, type AVL-364C, located in the flywheel measured the crankshaft angular position. The intake air flow was measured by a differential pressure transmitter, type LPX-5481. The test setup was equipped with a number of thermocouples type K for temperature measurements of engine parts. An active transmitter for humidity and temperature, type HD-2012-TC/150 allows measure of ambient temperature. The measure of fuel flow was performed using a Coriolis mass-flow meter. Regarding emissions, a Crystal COSMA 500 gas analyzer placed on the line of the engine exhaust gas was used to analyze the main pollutants. Emissions of UHC were measured by FID (flame ionization detector) using a heated hydrocarbon analyzer (model GRAPHITE 52 M) while those of nitrogen oxides (NOx) were measured via a chemiluminescence nitrogen oxide analyzer
Table 1 Specifications of test engine. Maximum power Cooling system Number of cylinders Volumetric ratio Bore Stroke Connecting rod length Swept volume Injection pressure Injection timing
7.5 kW @ 2500 rpm Air cooled 01 18–1 95.30 mm 85.50 mm 165.30 mm 630 cc 250 bar 13 °CA before TDC
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(model TOPAZE 32 M). Emissions of carbon monoxide (CO), carbon dioxide (CO2) and oxygen (O2) were measured by absorption of infrared radiation using a 2 M MIR analyzer. Measurement of Particulate emissions were performed via a TEOM model 1105 which is based on a novel technique enabling real-time, continuous and high sensitivity measurements of raw exhausts PM emissions without diluting the exhaust gas. A schematic of test engine setup is shown in Fig. 1. The sensitivity of instruments used in this study and the uncertainties in the calculated parameters, performed according to Kline and McClintock method [23,24], are given in Table 2. To assess the engine response in overall operating conditions through combustion, emission and performance characteristics the biodiesel was synthesised from waste cooking oil by alkalicatalyzed reaction using methanol as an alcohol and caustic potash as a catalyst. Properties of WCO biodiesel, diesel fuel and their blends used in this work are listed in Table 3. Experiments were conducted firstly for diesel fuel B0, thereafter for the different tested fuels, neat waste cooking oil biodiesel B100 and its blends B25 (25% biodiesel in diesel fuel) and B50 (50% biodiesel in diesel fuel). The majority of studies are conducted under a single engine speed using 4–5 selected loads to characterize these operating regimes. In the present work, 4 speeds are used: 1500, 1800, 2200 and 2500 rev/min. At each engine speed, eight part loads were studied starting from 25% to 100% of full load were studied. Hence the test covers the full range of useful load and speed in order to realize the maps of combustion characteristics, engine performance and emissions. Thereafter, a factorial analysis of variance ANOVA (Post-hoc analysis at the 95% confidence level) was performed with Tukey’s HSD (honest significant difference) test, on the experimental data, to determine whether the perceived differences in emissions between fuels are statistically significant and to locate means that are significantly different from each other.
3. Results and discussions 3.1. Combustion characteristics 3.1.1. Heat release rate and cylinder pressure Variations of in-cylinder gas pressure and heat release rate, for diesel fuel and its blends with 25%, 50% of volume, and neat WCO biodiesel, denoted in the figures as ‘B0’, ‘B25’, ‘B50’ and ‘B100’, will be shown only for the case of 75% of load at two engine speeds 1500 and 2500 rev/min (Fig. 2a and b). As expected, the premixed burning starts earlier for biodiesel and its blends for almost all operating engine speeds, due to their higher cetane numbers as compared to that of diesel fuel (DCN are 56.1, 54.1, 51.9 and 51for B100, B50, B25 and B0 respectively). However, the rise rate of heat release doesn’t keep the same levels, e.g. at high speed, maximum HRR (HRRmax) is 40 J/°CA and 37 J/°CA, while at low speed it was 49 J/°CA and 53 J/°CA for diesel and biodiesel respectively. Hence, HRRmax decreases about 18% and 30% for diesel and biodiesel respectively, due to the reduction in premixed part of combustion caused by the reduction of ignition delay from 5.6 °CA ffi 0.62 ms and 3.6 °CA ffi 0.40 ms at 1500 rev/min to 7.4 °CA ffi 0.49 ms and 5.8 °CA ffi 0.38 ms at 2500 rev/min. The location of HRRmax was slightly shifted towards the TDC at high speeds compared to the same loads at low speeds. HRRmax is lower for biodiesel and its blends. At high speeds this result can be explained by the degradation of the premixed combustion part, because of the deterioration of atomization and combustion processes with the increase of biodiesel portion in the blend [25]. The diffusion burning phase indicated under the second peak is
earlier at low speeds. This is consistent with the expected effects of reduction of fuel–air mixing rates due to reduced air entrainment. At ignition time, a smaller amount of fuel–air mixture is ready to burn with the biodiesel. Hence more burning occurs in the diffusion stage rather than in the uncontrolled stage to meet the same requirements of brake power, i.e. second peak of HRR is always elevated for biodiesel and its blends than diesel fuel. This is mainly attributed to the effect of oxygen-bounded in biodiesel which improves the diffusion combustion process. For low engine speeds Fig. 2a, it was observed that maximum in-cylinder pressure (Pmax) increased when blend ratio increased. It is 78.5 bar with diesel fuel, 80.7 bar with B25, 85.2 bar with B50 and 88.3 bar with B100. At high engine speeds Fig. 2b, the dominance of properties that improve combustion such as cetane number and content of oxygen does become not quite obvious, so Pmax doesn’t follow the concentration of blends linearly. Unlike neat biodiesel, B25 and B50 have slightly lower peak pressures than diesel fuel. From the above results, no significant correlation can be set between biodiesel blend ratio and the peak of heat release rate. It is thought that these tendencies are originated from three factors: portion of fuel which takes part in the premixed combustion, quality of atomization at local scale, and the global air–fuel mixing quality, extremely affected by speeds and loads conditions [26–29]. These factors are in turn influenced by the higher viscosity and lower volatility of biodiesel and its chemically bound oxygen which have an opposite effect on the spray envelope of injected fuel and the delay period. 3.1.2. Ignition delay map Ignition delay (ID) in the compression ignition engines has great influence on performance, noise, vibrations, mechanical stress and polluting emissions. Usually, delay period, in terms of crank angle, becomes shorter with engine loads and longer with engine speed. From the map of different fuels studied, a general aspect can be noticed on the variation of ignition delay; its increase with the speed of the engine can be justified by the fact that at greater speed, fuel takes more crankshaft degree to complete the mixing and combustion process. However delay period seems not very sensitive to the load at high rotation speed. The high cetane numbers of biodiesel and its blends compared to diesel fuel and oxygen content of fuel cause a decrease in ignition delay [23,24]. The lower thermal capacity of biodiesel leading to rapid warming and fuel evaporation has an effect on decrease of ignition delay [30]. From Fig. 3, at low speeds, it can be noticed that ignition delay values of biodiesel and blends were shorter than those of the diesel fuel, e.g. for 3 bar of BMEP at 1800 rev/min, ignition delays are 5 °CA, 4.6 °CA, 4.2 °CA and 3.6 °CA respectively for B0, B25, B50 and B100. At this load the ID difference is about 0.4 °CA advance for each 25% addition of biodiesel in the total volume of fuel. Increasing the engine load at the same speed, results in shortening ignition delay and earlier start of combustion (SOC) with the increase of in-cylinder gas temperatures. At full load the delay period becomes shorter about 0.2 °CA for each 25% addition of biodiesel in the total volume of fuel. A lot of other parameters such as air–fuel ratio, intake air temperature and pressure, swirl and tumble affected by engine speed and their interactions with the quality of fuel proprieties influence the ID [31]. At high engine speeds ID becomes less sensitive to both fuels type and load increase. That we can see at low BMEP, blends of biodiesel recorded a similar or slightly longer ignition delay compared to those of diesel. 3.1.3. Heat loss map The normalized heat losses (fraction of primary energy forgone by heat loss during the closed part of the cycle), for different studied fuels are shown in Fig. 4, those are higher with neat biodiesel
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1. Diesel engine
6. Injection pump
11. Ambient air conditions sensor
a. Unburned hydrocarbons analyser
2. Dynamometer
7. Fuel injector
12. Intake air flow rate sensor
b. Nitric oxides analyser
3. Diesel Fuel tank
8. Fuel line pressure sensor
13.Exhaust gas collector
c. CO, CO2 and O2 analyser
4. Biodiesel tank
9. In-cylinder pressure sensor
14. Particulate matter analyser
C&C Control and command signals
5. Fuel flow rate sensor
10. Angular encoder
15. Exhaust gas analyser
L.F/ H.F: Low/ High frequency signals
Fig. 1. Experimental setup scheme.
compared to diesel at partial loads. As will be explained in the next section, engine consumes relatively more fuel when running on biodiesel. For elevated loads, in-cylinder pressure, temperature and heat release rate are higher leading to an increase in heat losses. It can be noticed that the normalized heat losses almost become similar in the shape for different fuels. They reach a maximum at 1800 rev/min and 6 bar BMEP and a minimum in the central area around 3.5 bars BMEP. For B0, B25 and B50, differences are not very significant, but between B0 and B100 differences become quite noticeable. This is mainly due to the longer combustion duration with biodiesel. At 1500 rpm and a BMEP of 5 bar, heat losses are about 25% for B100 and 20% for B0 of the primary energy, no difference can be discerned with the same load at high speed. This can be explained by the enhanced quality of mixing and hence the combustion with the increase of amount of fuel to burn especially at full speeds. Extremely related on heat release rate and combustion characteristics, the normalized heat losses are higher or lower about 1– 3%, for biodiesel blends compared to diesel depending on engine speeds and loads. We already know that the supplied fuel energy increase with the increase of engine load. Similarly, the heat losses increase with engine load, but the increment is not the same for all fuels. This increment is greater in the vicinity of 1800 rev/min especially at medium and high loads. Despite the well known increase in losses due to friction, starting from a set value of the load, if one increases the engine speed, the normalized heat loss decreases because of simultaneous reduction in in-cylinder pressure and temperature as seen in Fig. 2. In summary, the normalized heat losses seem to be inversely correlated to ignition delays. This is well justified by the fact that
shorter ignition delay leads to less amount of fuel burned during premixed combustion known by its high efficiency. So on an increase in diffusion combustion occurs later in expansion stroke to meet the same requirements of brake power and hence more heat losses. Furthermore, fuels with shorter ignition delays record higher cycle pressures and temperatures, which can be reflected on a higher temperature difference between burned gas and engine walls, leading to an increase in heat losses.
3.2. Engine performance 3.2.1. BSFC and BTE maps Figs. 5 and 6 show respectively the brake specific fuel consumption (BSFC) and brake thermal efficiency (BTE) maps of the different tested fuels. It can be observed that at fixed engine speed, increasing engine load decreases BSFC, e.g. at medium speeds the BSFC of biodiesel decreased from approximately 550 g/kW h to less than 325 g/kW h as the BMEP increased from 1.5 bar to 5 bar. Friction losses are mainly related to the speed of mechanical parts of the engine, so at constant speed, when increasing engine load, efficiency is gained, as an increase in the mechanical power without actually changing the friction losses. According to the contours slope in the maps, if one increases the engine speed, BSFC increases slightly for the diesel and its blends. However, for neat biodiesel, BSFC remains unchanged or even decreases faintly when the engine speed increases. This could be the result of an increase in cylinder temperatures offsetting the viscosity effect coupled with splitting of biodiesel molecules, for instance linoleic and oleic fatty acid methyl esters, into lower
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Table 2 Accuracy of the measurements and uncertainties in calculated parameters. Measurements
Sensor type
Accuracy
Torque (Tm) Engine speed Injection timing Intake air flow rate
Effort sensor (FN 3148) AVL 364C AVL 364C Differential pressure transmitter (LPX5841) Coriolis type mass flow meter (RHM015) Piezo-electric (AVL QH32D) Piezo-electric (AVL QH33D) Differential pressure transmitter (LPX5841) K type thermocouple
±0.1 N m ±3 rpm ±0.05 °CA ±1% of measured value ±0.5% of measured value ±2 bars ±2.5 bars ±1.6 K
K type thermocouple
±1.6 K
HD 2012 TC/150
±0.2 K
HD 2012 TC/150 FID (GRAPHITE 52 M)
±2% ±10 ppm
TOPAZE 32 M
±20 ppm
Infra-red detector (MIR 2 M)
±0.2%
Infra-red detector (MIR 2 M) Infra-red detector (MIR 2 M)
±0.25% 50 ppm
TEOM 1105
±1 lg/m3
Isoperibol calorimeter (PARR 6200CLEF)
±0.25% of measured value
Fuel flow rate In cylinder pressure Injection pressure Intake air temperature Fuel injection temperature Exhaust gas temperature Ambient air temperature Relative humidity Hydrocarbon emissions Nitric oxides emissions Carbon dioxide emissions Non-reacted oxygen Carbon monoxide emissions Particulate matter emissions Fuel lower heating value
±1.6 K
Calculated parameters
Uncertainty range (%)
Brake power BSFC BTE
0.4–1.9 0.6–2.0 0.7–2.0
Table 3 Properties of petrol diesel, WCO biodiesel and their blends. Properties
B0
B100
B25
B50
Kinematic viscosity (mm2/s at 40 °C) Density (kg/m3 at 20 °C) Lower heating value (MJ/kg) Chemical composition Flash point (°C) Derived cetane number
2.3 840 43.45 C21H44 59 51
4.0 860 38.15 C17H31O2 155 56.1
2.8 846 41.20 – 71 51.9
3.2 849 40.80 – 83 54.1
molecules, having better volatility, during injection and the presence of oxygen in biodiesel. At a specific load, the injection pump delivers the same volume of fuel, but the higher density of biodiesel and its blends compared to diesel (860, 849, and 846 versus 840 kg/m3) led to an increase of fuel mass flow rate. The heating value of B100, B50 and B25 are respectively lower by 12.19%, 6.1% and 3% than that of diesel fuel led also to an increase of BSFC. As shown in Fig. 5, at low speeds, going from BSFC map for diesel to those of its blends, BSFC increases in an apparent correlation with biodiesel content as confirmed by many research works [32– 34]. The maps showed tangible increase in BSFC when biodiesel blending ratio increased from 50% to 100%. It is to be noted that BTE is simply the inverse of the product of the BSFC and the lower heating value of the fuel. This increases the brake specific fuel consumption for the biodiesel in order to compensate the corresponding lower energy content. Regarding B100 map, high engine speeds and loads is area of low specific consumption corresponding exactly to the area of lower heat losses, and hence high brake thermal efficiency.
When the engine runs at partial load conditions, it operates in lean conditions, thus premixed combustion is more significant than diffusion. At these conditions, oxygen content of biodiesel does not have significant effects on combustion. However, its lower volatility leads to longer combustion duration and, by consequence, to a relative decrease in BTE. 3.3. Emissions 3.3.1. Unburned hydrocarbon emissions (UHC) map Fig. 7 shows variations of UHC versus load and speed for biodiesel, diesel and their blends. A sharp reduction in emitted UHC when substituting diesel fuel with WCO biodiesel fuel can be drawn. Maps show about 40% mean diminution with neat biodiesel compared to diesel fuel. In addition to the oxygen present in the intake air, a supplementary source of oxygen chemically bonded with carbon in biodiesel (about 12%) enhances oxidation process. Thereby, engine emits lower UHC with neat biodiesel due to better oxygenation. According to the biodiesel portion, the UHC reduced nearly in all case of load or engine speed. At full load and low speed, UHC emissions for diesel, B25, B50 and biodiesel are respectively 325 ppm, 325 ppm, 300 ppm and 225 ppm. However at full load and high speed there is not much difference in UHC emissions between diesel and its blends, they are about 425 ppm for B0, B25, B50 and 325 ppm for B100. When looking at regions of high efficiency of biodiesel corresponding to low specific consumption of biodiesel (Fig. 5), neat biodiesel keeps constantly very low UHC emissions compared to the other tested fuels, even at these extreme operating conditions. Regardless the effect of fuel type, starting from low to high speed, shortening ignition delay reduces UHC emissions. In summary some interesting aspects can be drawn from UHC maps: (1) The emissions of UHC are almost independent of the load with the diesel and the dependence starts to appear using biodiesel. (2) At low loads, there is a minimum area of UHC emissions with biodiesel and blends which moves from right to left by increasing the amount of biodiesel, which revealed that UHC emissions are slightly sensitive to engine speed. 3.3.2. Carbon monoxide emission (CO) map Variations in emitted CO versus engine speed and load for different tested fuels are given in Fig. 8. Mostly neat biodiesel emits relatively lower CO emissions comparing to diesel and their blends. This is likely attributed to additional oxygen in biodiesel and higher combustion temperature that enhances the oxidation of CO to CO2. In the CO maps, it can be identified that region of minimal CO emission (less than 150 ppm) is limited for diesel to partial load and low speed, but it goes to full speed and more elevated loads for neat biodiesel. In the zone of maximum loads, it can be noticed a large variation in CO emissions. At medium speeds no significant difference between fuels, CO emission reached levels higher than 800 ppm. At low speed, a further increase in CO emission for neat biodiesel and its blends, they emitted higher levels of carbon monoxide (800 ppm) compared to diesel fuel (700 ppm). This region is characterized by a significant difference in BSFC between diesel and the other fuels. 3.3.3. Nitrogen oxides (NOx) map As widely recognized, the formation of thermal NOx is mainly favoured by two parameters: high oxygen concentration and high
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Fig. 2. Heat release rate and cylinder pressure.
Fig. 3. Ignition delay periods for different fuels (°CA).
Fig. 4. Heat loss map for different fuels.
charge temperature. A most important difference between biodiesel and diesel fuel is the 12% oxygen content in the biodiesel. One reason for the higher NOx emissions is the higher cetane numbers for B100, B50 and B25 (DCN = 56.1, 54.1 and 51.9 respectively) compared to B0 (DCN = 51). Higher cetane number leads to a shorter ignition delay after the fuel is injected into the cylinder, this results in earlier SOC, which tends to increase NOx formation, the earlier SOC also results in reduced amount of fuel participating in the uncontrolled (premixed) combustion, which lower NOx formation. These two effects somewhat offset each other. As shown in Fig. 9, region of high loads and low speed, characterized by higher cycle temperature and maximums in-cylinder pressure, as mentioned in Section 3.1.2, presents the maximum NOx emissions
which are slightly higher for B25 and B50 compared to diesel and neat biodiesel. Changes in physical properties such as speed of sound, bulk modulus and viscosity may be involved in higher NOx emissions [35]. In summary, the results show a rough increase of NOx with BMEP increase and slight decrease with speed. This is extremely related to maximum pressure of cycle, which has the same tendencies with load–speed variation. If following the equi-value of 800 ppm, it shifts down on going from diesel to biodiesel maps. Similar trends can be observed for almost all operation ranges; nevertheless an exception occurs in a small zone. e.g. at low to medium speed and full load where very closer values are recorded for diesel and biodiesel (1150 ppm), however blends recorded slightly higher
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Fig. 5. BSFC map for different fuels (g/kW h).
Fig. 6. Brake thermal efficiency map.
Fig. 7. UHC emissions map (ppm).
NOx emissions (1200 ppm). Therefore, NOx emissions have a nonlinear behavior with biodiesel blends. This tendency could be explained by opposite effects of oxygen content and higher viscosity of biodiesel. Oxygen content enhances combustion and free oxygenated radicals creation, leading to higher NOx formation. And viscosity leads to poorer volatility and reduces premixed combustion. Thus, it can be concluded that B25 and B50 have the optimal situation to produce NOx i.e.: good volatility and oxygen content. 3.3.4. Particulate matter (PM) map Reduction in PM emission with biodiesel content can be clearly stated. The presence of oxygen in the fuel improves the air fuel mixing rates which contribute to prepare more quantity of fuel
for premixed combustion and, by consequence, to decrease the PM emission. Excess oxygen is required to improve conversion of carbon particles to CO2, which is a major advantage of oxygenated fuels. Likewise, the absence of sulfur from the bio-fuels should lead to lower (PM) emissions [36]. Fig. 10 provides the variation of PM emissions for different fuels versus speed and load. According to the map, at low and moderate loads, PM concentration of biodiesel is lower in comparison to diesel fuel and blends. As an example, the region where PM emissions are under 5 mg/m3 is extended to medium loads, with all operating speeds for biodiesel. PM concentrations are inversely correlated with the portion of biodiesel. At full load, they are 40 mg/m3 for B0, 30 mg/m3 both for B25 and B50 and 25 mg/m3 for B100. These
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Fig. 8. CO emissions map (ppm).
Fig. 9. NOx emissions map (ppm).
Fig. 10. PM emissions map (mg/m3).
levels tend to decrease with engine speed increase to a certain value and then it goes up, in similar scenario with carbon monoxide emission. 3.3.5. Comparison between fuels under different load/speed conditions Covering the total of data presented in Figs. 7–10 the variation in numbers were: – Compared to diesel fuel, the UHC emission of the neat biodiesel was decreased by 40%, while the UHC emissions of B50 and B25 were reduced by 16.4% and 12.9%, respectively.
– CO emission of the neat biodiesel was decreased by 28.9% compared to diesel fuel, while the CO emissions of B50 and B25 were reduced by 8.2% and 7.5%, respectively. – Regarding PM emission of the neat biodiesel was reduced by 43.2% compared to diesel fuel, while the PM emissions of B50 and B25 were decreased by 25% and 13.9%, respectively. – NOx emissions of the neat biodiesel are greater than those of diesel fuel by 6%, while they are greater by 5% and 2.1% for B50 and B25 respectively.
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The single-step multiple comparison procedure ‘‘Tukey grouping test” at the 95% confidence level was performed on the exhaust emissions data. The results are summarized as follow: – Under all load/speed ranges, PM and NOx emissions changes from the diesel fuel were found to be statistically significant for the biodiesel and its blends. This includes the decrease in NOx relative to diesel fuel observed for the two blends at low to medium speed and full load. – At high speeds and maximum loads, the reductions in UHC and CO, compared to diesel fuel, were found to be statistically highly significant (the probabilities: p-values are 0.0016 and 0.00167 for UHC and CO respectively) for neat biodiesel, while the reductions observed for the blends were also statistically significant for UHC and only borderline significant for CO emissions (e.g. p-value = 0.04938 for B50). In contrariwise of CO emissions, no statistically significant difference in UHC emissions was also found between B100 and its blends at these operating conditions. – At low speed and maximum loads, the reduction in UHC of the neat biodiesel compared to diesel fuel was statistically significant, however the CO emission for the biodiesel and its two blends are statistically similar to those of diesel fuel. – At medium loads, the reductions in UHC were found to be statistically significant for neat biodiesel and its blends and there were no statistically significant differences between the fuels in the observed reductions of CO emissions. – At low loads, the Tukey’HSD analysis shows that the reduction in UHC was found to be statistically significant for neat biodiesel, while the reductions observed for the two blends were only borderline significant. Regarding CO emissions, there were no statistically significant differences between the two blends and the baseline diesel fuel.
4. Conclusion In this study, experiments were conducted using diesel, biodiesel fuels and their blends to investigate the impact of biodiesel on the combustion characteristics, performance and emissions of a compression ignition engine under the overall engine operating conditions. The experimental engine maps permit a comprehensive examination of the convoluted interactions between operating conditions and responses of the engine. Results revealed that the biodiesel performance is not quantitatively governed by a variation in a particular fuel property, but somewhat is the outcome of a competition of different mechanisms. Depending on fuel properties and combustion characteristics, the influence of these mechanisms may tend to boost or neutralise each other under various engine operating conditions. Even so, all these parameters appear to be key factors in helping to explain the biodiesel emissions variation under all engine operating conditions. With regard to the experimental results discussed above, the main conclusion to be drawn is that: In terms of combustion characteristics: – In-cylinder pressure increases slightly when using biodiesel at low engine speeds, no deterministic trend is found at high speed with biodiesel addition. – The use of biodiesel leads to shortening ignition delay, which, in turn, seems to be inversely correlated with heat loss. Compared to diesel, the BSFC of B100 and its blends was almost higher, a noticeable increase in BSFC was observed when the biodiesel blend ratio raised from 50% to 100%. However an improved performance was also observed under high engine loads.
For neat biodiesel, opposite trend of BSFC and BTE was identified with respect to engine speed variation, which once more showed the significant effect of engine speed on the combustion process.On the other hand, the results revealed that the variation of engine speed and load conditions affected considerably the engine emissions. – Increasing engine speed leads to an increase of UHC emissions. The rate of this increase is more affected by the load when more biodiesel is present in the blend. – Increasing engine speed with a specified load leads to an increase of CO and PM until maximums, often takes place at medium speed and then a sharp decrease on CO and PM occurs. – Almost, NOx emissions increase slightly with the biodiesel and blends, nevertheless an exception occurs for some engine operating conditions.
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Experimental assessment of performance and emissions maps for biodiesel fueled compression ignition engine Kezrane Cheikh a,b, Awad Sary b,⇑, Loubar Khaled b, Liazid Abdelkrim c, Tazerout Mohand b a
Laboratoire d’Energétique, Mécanique et Ingénieries, UMBB, Boumerdes 35000, Algeria GEPEA, UMR 6144, Département Systèmes Energétiques et Environnement, Ecole des Mines de Nantes, La chantrerie, 04 rue Alfred Kastler, B.P. 20722, F-44307 Nantes Cedex 3, France c LTE Laboratory, BP 1523 El-Mnaouer, ENSET, 31000 Oran, Algeria b
h i g h l i g h t s Study of combustion characteristics for biodiesel, diesel and theirs blends in CI engine. Effects of engine speed and load on performance and emissions maps are investigated. The BSFC for biodiesel shows an inverse behavior to petrol diesel and its blends. UHC emissions increase with engine speed. The effect of load is noticeable for blends. CO and PM emissions are correlated with load. They are less sensitive to engine speed.
a r t i c l e
i n f o
Article history: Received 6 July 2015 Received in revised form 23 September 2015 Accepted 5 October 2015
Keywords: Performance mapping Combustion characteristics CI engine Biodiesel Emissions
a b s t r a c t Attempts to use many types of biofuel have been tried throughout the past. Waste cooking oils (WCO) represent attractive alternatives to edible fat and oils for the production of biodiesel fuel from different points of view: they are inexpensive feedstock for biodiesel production. At the same time, using this kind of feedstock eliminates food versus fuel competition. Finally, Their use supposes an environmentally friendly approach. Internal combustion engines operate correctly on a wide range of speeds and loads. However, only few studies on biodiesel fueled engines are undertaken for performance mapping. In the present study, the effects of entire operational range of speed/load on engine performance and emission levels of an engine are investigated when neat WCO biodiesel (B100) and its blends (B25) and (B50) are used. The obtained results are compared to those of conventional diesel (B0). The suitability of WCO biodiesel has been established by many researchers. However their results report a wide disparity on emission levels. Combustion characteristics, performance and emission maps will be performed to give appropriate indications explaining the divergence reported in literature. The map indicates that B50 and B25 exhibit similar trends of BSFC to diesel fuel, across the speed variation albeit with difference in response levels. Contrariwise B100 shows an inverse behavior with speed increase. Results show also that Unburned Hydro-Carbons (UHC) emissions are highly correlated with engine speed. However, CO and PM emissions are extremely correlated with load and they are less sensitive to engine speed. NOx emissions are generally higher with biodiesel except some extreme zones of the map. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The rising worldwide demand for fossil energies i and its related pollution problems require urgent development of renewable and environmentally friendly energy sources. Therefore, biomass ⇑ Corresponding author. Tel.: +33 251858561; fax: +33 251858299. E-mail address: [email protected] (A. Sary). http://dx.doi.org/10.1016/j.apenergy.2015.10.042 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
derived fuels like biodiesel are required to substitute diesel fuel. Biodiesel is technically suitable for use in compression ignition engines. It is comprised of mono-alkyl esters of long-chain fatty acids derived from vegetable oils or animal fats. It is designated as B100 and it meets the requirements of European Standard EN-14214 and/or the American Society for Testing and Materials D-675 [1].
K. Cheikh et al. / Applied Energy 161 (2016) 320–329
Bio-fuels might contribute to meet the future energy supply demands as well as helping to the reduction of green house gas emissions (GHG). However, extensive adoption of biofuel required more land to cultivate energy feedstock. Natural forests are snatched in some countries for plantation purposes of biodiesel industry. In addition, this can increase the conversion of cultivable land usages from food to biofuel production [2,3], causing food shortage [4]. Many investigations are currently led on the numerous sustainable energy options in agriculture including the use of more energy efficient technologies and the replacement of fossil fuels using renewable energy powered technologies [5]. Ester of fatty acids (biodiesel) differs from diesel fuel in its composition and properties which could lead to differences in engine performance, combustion characteristics and exhaust emissions. Literature sources present exhaustive lists of biodiesel advantages: it helps decreasing GHG emissions to the atmosphere, it does not contain sulfur or aromatic compounds, it is renewable in nature and safer to handle, in addition, its oxygen content reduces emissions related to poor combustion conditions [6,7]. Biodiesel use results in some drawbacks comparing to diesel fuel including: high specific fuel consumption, substantial emissions of oxygenatedhydrocarbons, poor flow properties at low-temperature conditions, drop in brake thermal efficiency and high costs of production [8]. The use of waste fats or vegetable oils as feedstock for transesterification process can partially solve the problem of biofuel production costs. Talebian-Kiakalaieh et al. [9], claimed that using WCO as feed stock can reduce biodiesel production costs up to 60–90%. Frying oil leads to various chemical and physical changes resulting in the formation of some polymerized tri-glycerides and free fatty acid. These undesirable compounds increase the molecular weight of the oil and reduce its volatility. Thus, fatty acid esters of fried oils influence fuel properties such as viscosity and burning characteristics resulting in greater amount of carbon residue [10,11]. Nevertheless, the suitability of WCO methyl ester as a biodiesel has been ascertained by many researchers, even the obtained results are mixed, mainly about emissions. In spite of the high consensus in trends of performance and emissions recognized in the literature when using biodiesel, there is still a lack of correspondence in the conclusions made by some authors. This may be due to various factors like biodiesel feedstock, blending ratio and engine operating conditions which affect the combustion characteristics and pollutant formation processes [12]. Lapuerta et al. [13] reported that ethyl esters obtained from WCO emit lower UHC at medium load conditions. Nevertheless no clear tendency was established at low load conditions. Such trends are in agreement with An et al. [14] observations. The UHC emissions are found to be lower at all engine operation conditions, except under partial load at very low speed. It has been proved that load conditions have a significant effect on carbon monoxide (CO) emissions. Although Lin et al. [15], Durbin et al. [16] reported an augmentation of particulate matter (PM) emissions when fueling with WCO biodiesel, an evident decrease in particulate matter emissions with the biodiesel is the noticeable trend in literature. Most of the published studies revealed a slight increase in nitrogen oxides (NOx) emissions when using WCO biodiesel. However, a number of authors claimed that NOx increased only under some conditions such as alcohol-base of tested ester and certain conditions of load. Usually, compression ignition engines are designed to operate well within useful operational speed–load condition, however, performing accurately transient load/speed tests are extremely complicated and quite expensive (these require an entirely automated test-bench with electronically control of motoring and dissipating dynamometer, fast responses exhaust gas emissions analyzers, etc.). Individual load or speed accelerations tests
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are of wide acceptance rather than examination of the engine performance during the entire transient Cycle [17]. To characterize the responses of diesel-engined vehicles, during a transient/driving cycle, many relatively straightforward approaches are presented in literature [18–22]. These approaches use as a basis the steady-state engine tests, rectification coefficients are applied to account for transient discrepancies based on individual transient data. Therefore, in spite of the steady applications like generators or the marine sector, utilization of a steady-state test cycle gives an alternative to transient test when engine performance and emissions are of special interest instead of the investigation on the overall driving cycle [7]. Only few studies of biodiesel fueled engine cover the full speed– load spectrum. Thus, the objective of this work is to investigate the effects of engine speed and load, over the entire operational range of a direct injection compression ignition engine, when fueled with biodiesel. Construction of combustion characteristics, performance and emissions maps gives a closer look contributing to explain some divergences reported in literature. 2. Materials and methods For the present investigation, a naturally aspirated, direct injection (DI) diesel engine developing 7.5 kW at 2500 rpm was used. Table 1 gives the engine specifications. An eddy current dynamometer is coupled to the engine for converting the generated power to electricity that will be directly injected to the network. Acquisition and control of measured signals were performed by two systems. The first one commands the engine dynamometer and measures engine speed, torque, exhaust emissions, temperature and pressure in the collectors at a low-frequency. A second system allows the acquisition of high-frequency measurements (pressure in the cylinder, angular position of the crankshaft and fuel injection). Cylinder pressure was measured each 0.2 °CA using a water cooled piezoelectric pressure sensor, type AVL-QH32D. The pressure of injection measurement was performed by a piezoelectric transducer, type AVL-QH33D, placed between the fuel injector and the injection pump. An encoder, type AVL-364C, located in the flywheel measured the crankshaft angular position. The intake air flow was measured by a differential pressure transmitter, type LPX-5481. The test setup was equipped with a number of thermocouples type K for temperature measurements of engine parts. An active transmitter for humidity and temperature, type HD-2012-TC/150 allows measure of ambient temperature. The measure of fuel flow was performed using a Coriolis mass-flow meter. Regarding emissions, a Crystal COSMA 500 gas analyzer placed on the line of the engine exhaust gas was used to analyze the main pollutants. Emissions of UHC were measured by FID (flame ionization detector) using a heated hydrocarbon analyzer (model GRAPHITE 52 M) while those of nitrogen oxides (NOx) were measured via a chemiluminescence nitrogen oxide analyzer
Table 1 Specifications of test engine. Maximum power Cooling system Number of cylinders Volumetric ratio Bore Stroke Connecting rod length Swept volume Injection pressure Injection timing
7.5 kW @ 2500 rpm Air cooled 01 18–1 95.30 mm 85.50 mm 165.30 mm 630 cc 250 bar 13 °CA before TDC
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(model TOPAZE 32 M). Emissions of carbon monoxide (CO), carbon dioxide (CO2) and oxygen (O2) were measured by absorption of infrared radiation using a 2 M MIR analyzer. Measurement of Particulate emissions were performed via a TEOM model 1105 which is based on a novel technique enabling real-time, continuous and high sensitivity measurements of raw exhausts PM emissions without diluting the exhaust gas. A schematic of test engine setup is shown in Fig. 1. The sensitivity of instruments used in this study and the uncertainties in the calculated parameters, performed according to Kline and McClintock method [23,24], are given in Table 2. To assess the engine response in overall operating conditions through combustion, emission and performance characteristics the biodiesel was synthesised from waste cooking oil by alkalicatalyzed reaction using methanol as an alcohol and caustic potash as a catalyst. Properties of WCO biodiesel, diesel fuel and their blends used in this work are listed in Table 3. Experiments were conducted firstly for diesel fuel B0, thereafter for the different tested fuels, neat waste cooking oil biodiesel B100 and its blends B25 (25% biodiesel in diesel fuel) and B50 (50% biodiesel in diesel fuel). The majority of studies are conducted under a single engine speed using 4–5 selected loads to characterize these operating regimes. In the present work, 4 speeds are used: 1500, 1800, 2200 and 2500 rev/min. At each engine speed, eight part loads were studied starting from 25% to 100% of full load were studied. Hence the test covers the full range of useful load and speed in order to realize the maps of combustion characteristics, engine performance and emissions. Thereafter, a factorial analysis of variance ANOVA (Post-hoc analysis at the 95% confidence level) was performed with Tukey’s HSD (honest significant difference) test, on the experimental data, to determine whether the perceived differences in emissions between fuels are statistically significant and to locate means that are significantly different from each other.
3. Results and discussions 3.1. Combustion characteristics 3.1.1. Heat release rate and cylinder pressure Variations of in-cylinder gas pressure and heat release rate, for diesel fuel and its blends with 25%, 50% of volume, and neat WCO biodiesel, denoted in the figures as ‘B0’, ‘B25’, ‘B50’ and ‘B100’, will be shown only for the case of 75% of load at two engine speeds 1500 and 2500 rev/min (Fig. 2a and b). As expected, the premixed burning starts earlier for biodiesel and its blends for almost all operating engine speeds, due to their higher cetane numbers as compared to that of diesel fuel (DCN are 56.1, 54.1, 51.9 and 51for B100, B50, B25 and B0 respectively). However, the rise rate of heat release doesn’t keep the same levels, e.g. at high speed, maximum HRR (HRRmax) is 40 J/°CA and 37 J/°CA, while at low speed it was 49 J/°CA and 53 J/°CA for diesel and biodiesel respectively. Hence, HRRmax decreases about 18% and 30% for diesel and biodiesel respectively, due to the reduction in premixed part of combustion caused by the reduction of ignition delay from 5.6 °CA ffi 0.62 ms and 3.6 °CA ffi 0.40 ms at 1500 rev/min to 7.4 °CA ffi 0.49 ms and 5.8 °CA ffi 0.38 ms at 2500 rev/min. The location of HRRmax was slightly shifted towards the TDC at high speeds compared to the same loads at low speeds. HRRmax is lower for biodiesel and its blends. At high speeds this result can be explained by the degradation of the premixed combustion part, because of the deterioration of atomization and combustion processes with the increase of biodiesel portion in the blend [25]. The diffusion burning phase indicated under the second peak is
earlier at low speeds. This is consistent with the expected effects of reduction of fuel–air mixing rates due to reduced air entrainment. At ignition time, a smaller amount of fuel–air mixture is ready to burn with the biodiesel. Hence more burning occurs in the diffusion stage rather than in the uncontrolled stage to meet the same requirements of brake power, i.e. second peak of HRR is always elevated for biodiesel and its blends than diesel fuel. This is mainly attributed to the effect of oxygen-bounded in biodiesel which improves the diffusion combustion process. For low engine speeds Fig. 2a, it was observed that maximum in-cylinder pressure (Pmax) increased when blend ratio increased. It is 78.5 bar with diesel fuel, 80.7 bar with B25, 85.2 bar with B50 and 88.3 bar with B100. At high engine speeds Fig. 2b, the dominance of properties that improve combustion such as cetane number and content of oxygen does become not quite obvious, so Pmax doesn’t follow the concentration of blends linearly. Unlike neat biodiesel, B25 and B50 have slightly lower peak pressures than diesel fuel. From the above results, no significant correlation can be set between biodiesel blend ratio and the peak of heat release rate. It is thought that these tendencies are originated from three factors: portion of fuel which takes part in the premixed combustion, quality of atomization at local scale, and the global air–fuel mixing quality, extremely affected by speeds and loads conditions [26–29]. These factors are in turn influenced by the higher viscosity and lower volatility of biodiesel and its chemically bound oxygen which have an opposite effect on the spray envelope of injected fuel and the delay period. 3.1.2. Ignition delay map Ignition delay (ID) in the compression ignition engines has great influence on performance, noise, vibrations, mechanical stress and polluting emissions. Usually, delay period, in terms of crank angle, becomes shorter with engine loads and longer with engine speed. From the map of different fuels studied, a general aspect can be noticed on the variation of ignition delay; its increase with the speed of the engine can be justified by the fact that at greater speed, fuel takes more crankshaft degree to complete the mixing and combustion process. However delay period seems not very sensitive to the load at high rotation speed. The high cetane numbers of biodiesel and its blends compared to diesel fuel and oxygen content of fuel cause a decrease in ignition delay [23,24]. The lower thermal capacity of biodiesel leading to rapid warming and fuel evaporation has an effect on decrease of ignition delay [30]. From Fig. 3, at low speeds, it can be noticed that ignition delay values of biodiesel and blends were shorter than those of the diesel fuel, e.g. for 3 bar of BMEP at 1800 rev/min, ignition delays are 5 °CA, 4.6 °CA, 4.2 °CA and 3.6 °CA respectively for B0, B25, B50 and B100. At this load the ID difference is about 0.4 °CA advance for each 25% addition of biodiesel in the total volume of fuel. Increasing the engine load at the same speed, results in shortening ignition delay and earlier start of combustion (SOC) with the increase of in-cylinder gas temperatures. At full load the delay period becomes shorter about 0.2 °CA for each 25% addition of biodiesel in the total volume of fuel. A lot of other parameters such as air–fuel ratio, intake air temperature and pressure, swirl and tumble affected by engine speed and their interactions with the quality of fuel proprieties influence the ID [31]. At high engine speeds ID becomes less sensitive to both fuels type and load increase. That we can see at low BMEP, blends of biodiesel recorded a similar or slightly longer ignition delay compared to those of diesel. 3.1.3. Heat loss map The normalized heat losses (fraction of primary energy forgone by heat loss during the closed part of the cycle), for different studied fuels are shown in Fig. 4, those are higher with neat biodiesel
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1. Diesel engine
6. Injection pump
11. Ambient air conditions sensor
a. Unburned hydrocarbons analyser
2. Dynamometer
7. Fuel injector
12. Intake air flow rate sensor
b. Nitric oxides analyser
3. Diesel Fuel tank
8. Fuel line pressure sensor
13.Exhaust gas collector
c. CO, CO2 and O2 analyser
4. Biodiesel tank
9. In-cylinder pressure sensor
14. Particulate matter analyser
C&C Control and command signals
5. Fuel flow rate sensor
10. Angular encoder
15. Exhaust gas analyser
L.F/ H.F: Low/ High frequency signals
Fig. 1. Experimental setup scheme.
compared to diesel at partial loads. As will be explained in the next section, engine consumes relatively more fuel when running on biodiesel. For elevated loads, in-cylinder pressure, temperature and heat release rate are higher leading to an increase in heat losses. It can be noticed that the normalized heat losses almost become similar in the shape for different fuels. They reach a maximum at 1800 rev/min and 6 bar BMEP and a minimum in the central area around 3.5 bars BMEP. For B0, B25 and B50, differences are not very significant, but between B0 and B100 differences become quite noticeable. This is mainly due to the longer combustion duration with biodiesel. At 1500 rpm and a BMEP of 5 bar, heat losses are about 25% for B100 and 20% for B0 of the primary energy, no difference can be discerned with the same load at high speed. This can be explained by the enhanced quality of mixing and hence the combustion with the increase of amount of fuel to burn especially at full speeds. Extremely related on heat release rate and combustion characteristics, the normalized heat losses are higher or lower about 1– 3%, for biodiesel blends compared to diesel depending on engine speeds and loads. We already know that the supplied fuel energy increase with the increase of engine load. Similarly, the heat losses increase with engine load, but the increment is not the same for all fuels. This increment is greater in the vicinity of 1800 rev/min especially at medium and high loads. Despite the well known increase in losses due to friction, starting from a set value of the load, if one increases the engine speed, the normalized heat loss decreases because of simultaneous reduction in in-cylinder pressure and temperature as seen in Fig. 2. In summary, the normalized heat losses seem to be inversely correlated to ignition delays. This is well justified by the fact that
shorter ignition delay leads to less amount of fuel burned during premixed combustion known by its high efficiency. So on an increase in diffusion combustion occurs later in expansion stroke to meet the same requirements of brake power and hence more heat losses. Furthermore, fuels with shorter ignition delays record higher cycle pressures and temperatures, which can be reflected on a higher temperature difference between burned gas and engine walls, leading to an increase in heat losses.
3.2. Engine performance 3.2.1. BSFC and BTE maps Figs. 5 and 6 show respectively the brake specific fuel consumption (BSFC) and brake thermal efficiency (BTE) maps of the different tested fuels. It can be observed that at fixed engine speed, increasing engine load decreases BSFC, e.g. at medium speeds the BSFC of biodiesel decreased from approximately 550 g/kW h to less than 325 g/kW h as the BMEP increased from 1.5 bar to 5 bar. Friction losses are mainly related to the speed of mechanical parts of the engine, so at constant speed, when increasing engine load, efficiency is gained, as an increase in the mechanical power without actually changing the friction losses. According to the contours slope in the maps, if one increases the engine speed, BSFC increases slightly for the diesel and its blends. However, for neat biodiesel, BSFC remains unchanged or even decreases faintly when the engine speed increases. This could be the result of an increase in cylinder temperatures offsetting the viscosity effect coupled with splitting of biodiesel molecules, for instance linoleic and oleic fatty acid methyl esters, into lower
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Table 2 Accuracy of the measurements and uncertainties in calculated parameters. Measurements
Sensor type
Accuracy
Torque (Tm) Engine speed Injection timing Intake air flow rate
Effort sensor (FN 3148) AVL 364C AVL 364C Differential pressure transmitter (LPX5841) Coriolis type mass flow meter (RHM015) Piezo-electric (AVL QH32D) Piezo-electric (AVL QH33D) Differential pressure transmitter (LPX5841) K type thermocouple
±0.1 N m ±3 rpm ±0.05 °CA ±1% of measured value ±0.5% of measured value ±2 bars ±2.5 bars ±1.6 K
K type thermocouple
±1.6 K
HD 2012 TC/150
±0.2 K
HD 2012 TC/150 FID (GRAPHITE 52 M)
±2% ±10 ppm
TOPAZE 32 M
±20 ppm
Infra-red detector (MIR 2 M)
±0.2%
Infra-red detector (MIR 2 M) Infra-red detector (MIR 2 M)
±0.25% 50 ppm
TEOM 1105
±1 lg/m3
Isoperibol calorimeter (PARR 6200CLEF)
±0.25% of measured value
Fuel flow rate In cylinder pressure Injection pressure Intake air temperature Fuel injection temperature Exhaust gas temperature Ambient air temperature Relative humidity Hydrocarbon emissions Nitric oxides emissions Carbon dioxide emissions Non-reacted oxygen Carbon monoxide emissions Particulate matter emissions Fuel lower heating value
±1.6 K
Calculated parameters
Uncertainty range (%)
Brake power BSFC BTE
0.4–1.9 0.6–2.0 0.7–2.0
Table 3 Properties of petrol diesel, WCO biodiesel and their blends. Properties
B0
B100
B25
B50
Kinematic viscosity (mm2/s at 40 °C) Density (kg/m3 at 20 °C) Lower heating value (MJ/kg) Chemical composition Flash point (°C) Derived cetane number
2.3 840 43.45 C21H44 59 51
4.0 860 38.15 C17H31O2 155 56.1
2.8 846 41.20 – 71 51.9
3.2 849 40.80 – 83 54.1
molecules, having better volatility, during injection and the presence of oxygen in biodiesel. At a specific load, the injection pump delivers the same volume of fuel, but the higher density of biodiesel and its blends compared to diesel (860, 849, and 846 versus 840 kg/m3) led to an increase of fuel mass flow rate. The heating value of B100, B50 and B25 are respectively lower by 12.19%, 6.1% and 3% than that of diesel fuel led also to an increase of BSFC. As shown in Fig. 5, at low speeds, going from BSFC map for diesel to those of its blends, BSFC increases in an apparent correlation with biodiesel content as confirmed by many research works [32– 34]. The maps showed tangible increase in BSFC when biodiesel blending ratio increased from 50% to 100%. It is to be noted that BTE is simply the inverse of the product of the BSFC and the lower heating value of the fuel. This increases the brake specific fuel consumption for the biodiesel in order to compensate the corresponding lower energy content. Regarding B100 map, high engine speeds and loads is area of low specific consumption corresponding exactly to the area of lower heat losses, and hence high brake thermal efficiency.
When the engine runs at partial load conditions, it operates in lean conditions, thus premixed combustion is more significant than diffusion. At these conditions, oxygen content of biodiesel does not have significant effects on combustion. However, its lower volatility leads to longer combustion duration and, by consequence, to a relative decrease in BTE. 3.3. Emissions 3.3.1. Unburned hydrocarbon emissions (UHC) map Fig. 7 shows variations of UHC versus load and speed for biodiesel, diesel and their blends. A sharp reduction in emitted UHC when substituting diesel fuel with WCO biodiesel fuel can be drawn. Maps show about 40% mean diminution with neat biodiesel compared to diesel fuel. In addition to the oxygen present in the intake air, a supplementary source of oxygen chemically bonded with carbon in biodiesel (about 12%) enhances oxidation process. Thereby, engine emits lower UHC with neat biodiesel due to better oxygenation. According to the biodiesel portion, the UHC reduced nearly in all case of load or engine speed. At full load and low speed, UHC emissions for diesel, B25, B50 and biodiesel are respectively 325 ppm, 325 ppm, 300 ppm and 225 ppm. However at full load and high speed there is not much difference in UHC emissions between diesel and its blends, they are about 425 ppm for B0, B25, B50 and 325 ppm for B100. When looking at regions of high efficiency of biodiesel corresponding to low specific consumption of biodiesel (Fig. 5), neat biodiesel keeps constantly very low UHC emissions compared to the other tested fuels, even at these extreme operating conditions. Regardless the effect of fuel type, starting from low to high speed, shortening ignition delay reduces UHC emissions. In summary some interesting aspects can be drawn from UHC maps: (1) The emissions of UHC are almost independent of the load with the diesel and the dependence starts to appear using biodiesel. (2) At low loads, there is a minimum area of UHC emissions with biodiesel and blends which moves from right to left by increasing the amount of biodiesel, which revealed that UHC emissions are slightly sensitive to engine speed. 3.3.2. Carbon monoxide emission (CO) map Variations in emitted CO versus engine speed and load for different tested fuels are given in Fig. 8. Mostly neat biodiesel emits relatively lower CO emissions comparing to diesel and their blends. This is likely attributed to additional oxygen in biodiesel and higher combustion temperature that enhances the oxidation of CO to CO2. In the CO maps, it can be identified that region of minimal CO emission (less than 150 ppm) is limited for diesel to partial load and low speed, but it goes to full speed and more elevated loads for neat biodiesel. In the zone of maximum loads, it can be noticed a large variation in CO emissions. At medium speeds no significant difference between fuels, CO emission reached levels higher than 800 ppm. At low speed, a further increase in CO emission for neat biodiesel and its blends, they emitted higher levels of carbon monoxide (800 ppm) compared to diesel fuel (700 ppm). This region is characterized by a significant difference in BSFC between diesel and the other fuels. 3.3.3. Nitrogen oxides (NOx) map As widely recognized, the formation of thermal NOx is mainly favoured by two parameters: high oxygen concentration and high
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Fig. 2. Heat release rate and cylinder pressure.
Fig. 3. Ignition delay periods for different fuels (°CA).
Fig. 4. Heat loss map for different fuels.
charge temperature. A most important difference between biodiesel and diesel fuel is the 12% oxygen content in the biodiesel. One reason for the higher NOx emissions is the higher cetane numbers for B100, B50 and B25 (DCN = 56.1, 54.1 and 51.9 respectively) compared to B0 (DCN = 51). Higher cetane number leads to a shorter ignition delay after the fuel is injected into the cylinder, this results in earlier SOC, which tends to increase NOx formation, the earlier SOC also results in reduced amount of fuel participating in the uncontrolled (premixed) combustion, which lower NOx formation. These two effects somewhat offset each other. As shown in Fig. 9, region of high loads and low speed, characterized by higher cycle temperature and maximums in-cylinder pressure, as mentioned in Section 3.1.2, presents the maximum NOx emissions
which are slightly higher for B25 and B50 compared to diesel and neat biodiesel. Changes in physical properties such as speed of sound, bulk modulus and viscosity may be involved in higher NOx emissions [35]. In summary, the results show a rough increase of NOx with BMEP increase and slight decrease with speed. This is extremely related to maximum pressure of cycle, which has the same tendencies with load–speed variation. If following the equi-value of 800 ppm, it shifts down on going from diesel to biodiesel maps. Similar trends can be observed for almost all operation ranges; nevertheless an exception occurs in a small zone. e.g. at low to medium speed and full load where very closer values are recorded for diesel and biodiesel (1150 ppm), however blends recorded slightly higher
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Fig. 5. BSFC map for different fuels (g/kW h).
Fig. 6. Brake thermal efficiency map.
Fig. 7. UHC emissions map (ppm).
NOx emissions (1200 ppm). Therefore, NOx emissions have a nonlinear behavior with biodiesel blends. This tendency could be explained by opposite effects of oxygen content and higher viscosity of biodiesel. Oxygen content enhances combustion and free oxygenated radicals creation, leading to higher NOx formation. And viscosity leads to poorer volatility and reduces premixed combustion. Thus, it can be concluded that B25 and B50 have the optimal situation to produce NOx i.e.: good volatility and oxygen content. 3.3.4. Particulate matter (PM) map Reduction in PM emission with biodiesel content can be clearly stated. The presence of oxygen in the fuel improves the air fuel mixing rates which contribute to prepare more quantity of fuel
for premixed combustion and, by consequence, to decrease the PM emission. Excess oxygen is required to improve conversion of carbon particles to CO2, which is a major advantage of oxygenated fuels. Likewise, the absence of sulfur from the bio-fuels should lead to lower (PM) emissions [36]. Fig. 10 provides the variation of PM emissions for different fuels versus speed and load. According to the map, at low and moderate loads, PM concentration of biodiesel is lower in comparison to diesel fuel and blends. As an example, the region where PM emissions are under 5 mg/m3 is extended to medium loads, with all operating speeds for biodiesel. PM concentrations are inversely correlated with the portion of biodiesel. At full load, they are 40 mg/m3 for B0, 30 mg/m3 both for B25 and B50 and 25 mg/m3 for B100. These
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Fig. 8. CO emissions map (ppm).
Fig. 9. NOx emissions map (ppm).
Fig. 10. PM emissions map (mg/m3).
levels tend to decrease with engine speed increase to a certain value and then it goes up, in similar scenario with carbon monoxide emission. 3.3.5. Comparison between fuels under different load/speed conditions Covering the total of data presented in Figs. 7–10 the variation in numbers were: – Compared to diesel fuel, the UHC emission of the neat biodiesel was decreased by 40%, while the UHC emissions of B50 and B25 were reduced by 16.4% and 12.9%, respectively.
– CO emission of the neat biodiesel was decreased by 28.9% compared to diesel fuel, while the CO emissions of B50 and B25 were reduced by 8.2% and 7.5%, respectively. – Regarding PM emission of the neat biodiesel was reduced by 43.2% compared to diesel fuel, while the PM emissions of B50 and B25 were decreased by 25% and 13.9%, respectively. – NOx emissions of the neat biodiesel are greater than those of diesel fuel by 6%, while they are greater by 5% and 2.1% for B50 and B25 respectively.
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The single-step multiple comparison procedure ‘‘Tukey grouping test” at the 95% confidence level was performed on the exhaust emissions data. The results are summarized as follow: – Under all load/speed ranges, PM and NOx emissions changes from the diesel fuel were found to be statistically significant for the biodiesel and its blends. This includes the decrease in NOx relative to diesel fuel observed for the two blends at low to medium speed and full load. – At high speeds and maximum loads, the reductions in UHC and CO, compared to diesel fuel, were found to be statistically highly significant (the probabilities: p-values are 0.0016 and 0.00167 for UHC and CO respectively) for neat biodiesel, while the reductions observed for the blends were also statistically significant for UHC and only borderline significant for CO emissions (e.g. p-value = 0.04938 for B50). In contrariwise of CO emissions, no statistically significant difference in UHC emissions was also found between B100 and its blends at these operating conditions. – At low speed and maximum loads, the reduction in UHC of the neat biodiesel compared to diesel fuel was statistically significant, however the CO emission for the biodiesel and its two blends are statistically similar to those of diesel fuel. – At medium loads, the reductions in UHC were found to be statistically significant for neat biodiesel and its blends and there were no statistically significant differences between the fuels in the observed reductions of CO emissions. – At low loads, the Tukey’HSD analysis shows that the reduction in UHC was found to be statistically significant for neat biodiesel, while the reductions observed for the two blends were only borderline significant. Regarding CO emissions, there were no statistically significant differences between the two blends and the baseline diesel fuel.
4. Conclusion In this study, experiments were conducted using diesel, biodiesel fuels and their blends to investigate the impact of biodiesel on the combustion characteristics, performance and emissions of a compression ignition engine under the overall engine operating conditions. The experimental engine maps permit a comprehensive examination of the convoluted interactions between operating conditions and responses of the engine. Results revealed that the biodiesel performance is not quantitatively governed by a variation in a particular fuel property, but somewhat is the outcome of a competition of different mechanisms. Depending on fuel properties and combustion characteristics, the influence of these mechanisms may tend to boost or neutralise each other under various engine operating conditions. Even so, all these parameters appear to be key factors in helping to explain the biodiesel emissions variation under all engine operating conditions. With regard to the experimental results discussed above, the main conclusion to be drawn is that: In terms of combustion characteristics: – In-cylinder pressure increases slightly when using biodiesel at low engine speeds, no deterministic trend is found at high speed with biodiesel addition. – The use of biodiesel leads to shortening ignition delay, which, in turn, seems to be inversely correlated with heat loss. Compared to diesel, the BSFC of B100 and its blends was almost higher, a noticeable increase in BSFC was observed when the biodiesel blend ratio raised from 50% to 100%. However an improved performance was also observed under high engine loads.
For neat biodiesel, opposite trend of BSFC and BTE was identified with respect to engine speed variation, which once more showed the significant effect of engine speed on the combustion process.On the other hand, the results revealed that the variation of engine speed and load conditions affected considerably the engine emissions. – Increasing engine speed leads to an increase of UHC emissions. The rate of this increase is more affected by the load when more biodiesel is present in the blend. – Increasing engine speed with a specified load leads to an increase of CO and PM until maximums, often takes place at medium speed and then a sharp decrease on CO and PM occurs. – Almost, NOx emissions increase slightly with the biodiesel and blends, nevertheless an exception occurs for some engine operating conditions.
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