Experimental Investigation of Homogeneous Charge Compression

Jan 2, 2014 - ABSTRACT: In parallel to the interest in renewable fuels, there has also been increased interest in homogeneous charge compression ignit...
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Experimental Investigation of Homogeneous Charge Compression Ignition Combustion of Biodiesel Fuel with External Mixture Formation in a CI engine D. Ganesh,* G. Nagarajan, and S. Ganesan Internal Combustion Engineering Division, Department of Mechanical Engineering, College of Engineering Guindy, Anna University, Sardar Patel Road, Chennai, Tamil Nadu 600 025, India ABSTRACT: In parallel to the interest in renewable fuels, there has also been increased interest in homogeneous charge compression ignition (HCCI) combustion. HCCI engines are being actively developed because they have the potential to be highly efficient and to produce low emissions. Even though HCCI has been researched extensively, few challenges still exist. These include controlling the combustion at higher loads and the formation of a homogeneous mixture. To obtain better homogeneity, in the present investigation external mixture formation method was adopted, in which the fuel vaporiser was used to achieve excellent HCCI combustion in a single cylinder air-cooled direct injection diesel engine. In continuation of our previous works, in the current study a vaporised jatropha methyl ester (JME) was mixed with air to form a homogeneous mixture and inducted into the cylinder during the intake stroke to analyze the combustion, emission and performance characteristics. To control the early ignition of JME vapor−air mixture, cooled (30 °C) Exhaust gas recirculation (EGR) technique was adopted. The experimental result shows 81% reduction in NOx and 72% reduction in smoke emission.

1. INTRODUCTION The HCCI concept has been extensively investigated with a particular focus on gasoline combustion due to its potential to improve fuel consumption and decrease NOx emissions in leanburn engines. This concept is also employed on diesel and alternative fuelled engines to reduce NOx and PM emissions.1−3 Chiara et al.,4 have characterized the mixed-mode HCCI-DI combustion with external mixture formation on a high speed diesel engine. Experimental analysis was carried on a four cylinder 2.5 L diesel engine by integrating conventional common rail injection with diesel fueled HCCI combustion and an external diesel fuel vaporizer was used to achieve the HCCI mode of combustion. This study revealed the extension of HCCI mode of operation with the aid of a mixed mode combustion process. By integrating with DI the BMEP limit of pure HCCI operation had been overcome. Junjun Ma et al.,5 have studied the port injected n-heptane HCCI in combination with in-cylinder diesel fuel direct injection. The effects of the premixed ratio and direct injection timing on HCCI-DI combustion characteristics and emissions were investigated. It was found that NOx emissions decreased dramatically with partial premixing and exhibited a decreasing trend as a function of increment in premixed ratio. The results also revealed that the HCCI-DI combustion effectively improved the thermal efficiency at the operating ranges of low to medium loads. Canova et al.,6 have done a theoretical and experimental investigation of HCCI combustion with external mixture formation. They have created a control-oriented combustion © 2014 American Chemical Society

model for HCCI mode with external mixture formation. The HCCI combustion with external mixture formation has been experimented7−9 which characterize the HCCI combustion with a dedicated fuel atomizer and are found effective to achieve the highly homogeneous charge, which leads to ultralow NOx and PM emissions. Biodiesel is an alternative fuel for diesel engine which can reduce HC, CO, CO2, PM, SO2, PAH emissions with marginal increase in brake specific fuel consumption (BSFC) and NOx emissions. However, significant improvements are needed to reduce both PM and NOx emissions to meet the emission regulations. Low temperature combustion (LTC) is a promising technique to meet the above requirement.10−14 The LTC concept includes the HCCI combustion, premixed compression ignition combustion (PCI) and modulated kinetics combustion (MK). HCCI technology has high fuel flexibility and can be applied for a wide range of fuel. There has been a significant growth in the diversity of the fuels that is used in HCCI engines. HCCI fuels range from biofuels, to hydrocarbon fuels and reforming fuels. Also, being a tropical country, India has a potential to produce the jatropha curcas plant. Several vegetable oils have been tested in engines. Among these, jatropha oil is promising. It is nonedible and has a high calorific value and cetane number. It is Received: Revised: Accepted: Published: 3039

July 23, 2013 November 1, 2013 January 2, 2014 January 2, 2014 dx.doi.org/10.1021/es403104f | Environ. Sci. Technol. 2014, 48, 3039−3046

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Figure 1. Experimental setup scheme.

nontoxic and water requirement for the jatropha plant is negligible and also it can grow anywhere even on sandy soil. The calorific value and cetane number of jatropha oil are comparable to diesel but the density is higher. Considering the above facts and the biodiesel properties, in the present experimental investigation jatropha methyl ester (JME) is used as a fuel for HCCI operation. In the present study, a homogeneous mixture of jatropha fuel and air was prepared externally by using the fuel vaporizer and the experiments were conducted with jatropha vapor induction. The obtained results were compared with conventional mode of DI-diesel and DIbiodiesel operation.

Table 1. Test Engine Specifications

2. DESCRIPTION OF THE EXPERIMENTAL SETUP AND ENGINE MANAGEMENT SYSTEM The experimental investigation on mixed-mode jatropha HCCI combustion was performed on a single cylinder, four stroke, air cooled direct injection diesel engine, coupled with eddy current dynamometer, ECU controlled fuel vaporiser system, and the AVL combustion analysis system. In cylinder pressure and crank angle signals (using AVL pressure sensor of measuring range 0−250 bar, sensitivity 0.35 pC/bar and AVL crank angle encoder with a resolution of 0.1 °CA) were acquired and stored on a high speed computer based digital data acquisition system called AVL Indismart. The data from 100 consecutive cycles were recorded. These were processed with AVL combustion analysis software to obtain combustion parameters such as HRR, CHRR, SOC, EOC, PV, and log PVdiagrams. The experimental schematic is shown in Figure 1. The test engine specifications and the properties of biodiesel are given in Tables 1 and 2. The surge tank fitted with the inlet manifold to ensure steady flow of air to the engine.

Table 2. Properties of Diesel, Jatropha Oil, and JME

parameters

specifications

general details

single cylinder, 4-stroke, compression Ignition, constant speed, vertical, air-cooled, DI engine 87.5 mm 110 mm 662 cm3 23° bTDC (Static) 17.5:1 4.4 kW @ 1500 rpm 1500 rpm 200 bar

bore stroke swept volume injection timing compression ratio rated power output rated speed nozzle opening pressure

S.No 1 2 3 4 5

property 3

density (kg/m ) calorific value (kJ/kg) viscosity (cSt) cetane number flash point (°C)

diesel

jatropha oil

JME

815 43 350 4.30 47 50

918.6 39 774 49.93 40−45 240

880 38 450 5.65 50 170

Intake charge and exhaust gas temperatures were measured using K-Type thermocouple. In order to provide an accurate characterization of emissions, a five species exhaust gas measurement was performed with AVL DiGas 444 gas analyzer. In addition, a Bosch smoke meter was used to measure the Smoke level in the exhaust gas. Water cooled piezoelectric pressure transducer of range 0−250 bar was used to measure in-cylinder pressure. Intake charge pressure and exhaust manifold pressure was measured by using a piezo-resistive pressure sensor of range 0−5 bar and 0−10 bar, respectively. 3040

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Figure 2. Functional scheme of ECU controlled fuel vaporizer system.

and the EGR control valve. The EGR rate is determined by measuring the concentration of CO2 in the inlet and exhaust pipes. The expression used for evaluating the EGR ratio is as follows:

AVL crank angle encoder is used to sense the TDC position. Output of the crank angle encoder and pressure transducer were acquired through a data acquisition system and the real time output data is recorded in a computer. In the mixed-mode HCCI operation, fuel vaporizer system was mounted on the intake manifold. The fuel vaporizer system is used to achieve homogeneous mixture of air and jatropha fuel vapor. 2.1. Fuel Vaporizer System. The fuel vaporizer system is used for changing the phase of the fuel (biodiesel) from liquid to vapor. The intake manifold was modified to house the fuel vaporizer and it is shown in Figure 2. The design details of the fuel vaporizer are discussed in.2,3 2.2. Experimental Procedure. All experiments were performed at variable load and constant speed of 1500 rpm. At each load, the air flow rate, fuel flow rate, exhaust gas temperature, cylinder pressure, HC, NOx, CO, CO2, O2, and smoke emission readings were measured and recorded. During experimentation, the engine was first operated in a direct injection diesel mode for warm-up and later switched to HCCI mode operation. Once the ECU gives signal to inject the fuel into the vaporiser which was maintained at a temperature above the boiling point of jatropha methyl ester converts the biodiesel into vapor form and admits the vaporised fuel into the intake manifold. In the manifold biodiesel vapor mixed with air to form a homogeneous mixture. The homogeneous mixture was inducted inside the cylinder during the intake stroke. The moment engine attains the rated speed through biodiesel vapor-air mixture induction, engine governor cut off the fuel supply to the conventional fuel injector thereon the engine operated completely in a homogeneous mixture of biodiesel vapor-air mixture induction (BDVI) mode and readings were taken up to 75% load condition. At full load condition, engine was switched to conventional mode operation. It is well-known that EGR is the best way to vary the cylinder gas temperature and the ignition timing could be delayed. In this investigation the temperature of the exhaust gas recirculated (EGR) was held at 30 °C by an EGR cooling device. EGR cooling would increase the EGR heat absorbing capacity that would further reduce NOx. In addition cooler EGR temperature means that less volume is occupied in the inlet system. A lower EGR volume displaces a smaller fraction of fresh filtered intake air, thus displacing less O2, which helps in maintaining the combustion efficiency. In this study EGR was varied (10%, 20%, and 30%) and its effects on the engine performance, emission and combustion were studied. The quantity of cooled exhaust gas recirculation was adjusted by the backpressure valve

EGR ratio =

[CO2 ]inlet × 100% [CO2 ]exhaust

3. RESULTS ANS DISCUSSION In this section, results are presented from the studies of the performance, combustion and emission characteristics of HCCI combustion of biodiesel with external mixture formation using fuel vaporizer system. 3.1. Combustion Analysis. 3.1.1. Cylinder Pressure History. For combustion analysis, the cylinder pressure histories are measured for the DI diesel, DI biodiesel and biodiesel vapor induction mode and the peak cylinder pressure, instantaneous heat release rate, cumulative heat release rate, start of combustion and end of combustion are deduced from the measured cylinder pressures. Figure 3 shows the variation of cylinder pressure with crank angle.

Figure 3. Variation of cylinder pressure with crank angle.

It is observed that the in-cylinder pressure is higher in the case of biodiesel vapor induction mode (BDVI) when compared to DI diesel and DI biodiesel operation. The HCCI operation (biodiesel vapor induction mode) resulted in maximum in-cylinder pressure of 74 bar at 1 °CA bTDC for BDVI without EGR. In this study the cylinder pressure is retarded and suppressed with the use of cooled EGR, which is clearly shown in Figure 4. It can be seen that the maximum 3041

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accelerate the combustion better compared to conventional DI biodiesel and DI diesel operation. And also it is noticed that the ignition delay period is minimized to most significant level in the case of HCCI operation compared DI operation which is very clearly seen in the cylinder pressure history. The reason for significant reduction in delay period in the case of HCCI operation is due to fuel vaporiser which helps to prepare the homogeneous biodiesel vapor-air mixture. The variations of peak pressure at different brake mean effective pressure are shown in Figure 4. At low loads, that is, up to BMEP 1.32 bar, the peak pressure for the DI biodiesel and HCCI operation is less than the DI diesel operation. After that region the peak pressure increases for DI biodiesel and HCCI operation than the DI diesel operation. The reason for the difference in variation of peak pressure between these two modes of operation is at low loads the conditions prevailing for combustion (like cylinder temperature, mixture composition etc.,) for DI biodiesel, HCCI operation is inferior to the DI diesel operation. This condition is changed when the load increases due to increased cylinder temperature, improved mixing conditions as the load increases and the oxygen present in the biodiesel also supports and enhances the oxidation reaction that leads to increased peak pressure at the part load conditions. In the current study the effect of cooled EGR on HCCI is also investigated. It is noticed that the application of cooled EGR delays the initiation of cool flame combustion which intern helped in pushing the high temperature reaction further and suppressed the pressure by absorbing energy. This is clearly seen in the figure that the peak pressure for BDVI with 10% EGR, 20% EGR, 30% EGR decreased compared to BDVI without EGR. 3.1.2. Heat Release Rate. The heat release rate estimations are obtained from the measured cylinder pressure histories by using the first law of thermodynamics applicable to closed part of engine cycle.15−19 Figure 5 shows the typical comparison of heat release rates of two fuels operated in different modes. The test engine was operated in conventional mode as well as HCCI mode and its heat release rate were estimated and compared here. It is observed that the energy is released well before TDC

Figure 4. Variation of cylinder peak pressure with brake mean effective pressure.

peak pressure is obtained with HCCI mode fuelled with biodiesel vapor induction without EGR. The reason is the availability of highly premixed charge at the start of combustion. This premixed charge decreases as quantity of EGR is increased, which results in the dilution of the charge leading to lower peak pressure. The peak pressure value of biodiesel under conventional operation exceeds the diesel operation due to the higher local oxygen concentration in the biodiesel fuel during combustion. The maximum cylinder pressure value for BDVI with 10% EGR, 20% EGR, and 30% EGR is 72.5, 71, and 70 bar and the crank angle at which the maximum cylinder pressure occurs is at TDC, 1 °CA aTDC and 2 °CA aTDC, respectively. Maximum cylinder pressures recorded for conventional DI diesel operation and DI biodiesel operations are 61.4 bar at 4 °CA aTDC and 63.2 bar at 3 °CA aTDC, respectively. The reason for increased cylinder pressure in the case of HCCI operation is due to better mixture preparation and the fuel used is a biodiesel which has high oxygen content that helps to

Figure 5. Variation of heat release rate with crank angle. 3042

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for HCCI operation (BDVI with 0%, 10%, 20%, and 30% EGR conditions). The maximum energy released for BDVI with 0%, 10%, 20%, and 30% EGR conditions is 64 kJ/m3deg CA at 21 °CA bTDC, 60 kJ/m3deg CA at 20 °CA bTDC, 51 kJ/m3deg CA at 17 °CA bTDC and 46 kJ/m3deg CA at 16 °CA bTDC, respectively, whereas for DI diesel and biodiesel operation the maximum energy release rates are 67 kJ/m3deg CA at 6 °CA bTDC and 48 kJ/m3deg CA at 7 °CA bTDC, respectively. The maximum heat release rate of DI-Biodiesel is lowered compared to DI-diesel owing to its lower calorific value. In-cylinder fuel− air mixing of diesel fuel and biodiesel may not be identical because of an expected decrease in vaporization due to the high viscosity of JME (biodiesel). Even if the mixing levels are presumed to be identical, the lower calorific value of biodiesel compared with diesel fuel should lower the maximum heat release rate. Hence, a lower degree of mixing and the lower calorific value with biodiesel may be responsible for the lower heat release rate of DI-Biodiesel when compared to DI-diesel case. In this investigation EGR was used to reduce the combustion temperature and pressure. As the mixture is formed externally for HCCI mode, combustion timing can only be influenced by diluting the cylinder charge with exhaust gas this is seen very clearly in the heat release rate pattern. The energy released in the case of HCCI mode decreases as well as retarded with increase in EGR percentage. This is due to the higher specific heat of exhaust gas, which absorbs more of energy, reduces the in-cylinder temperature. At the same time exhaust gas dilutes oxygen concentration in the inlet gas and thus decreases the speed of reaction. Many experimental and numerical results show that HCCI exhibits a two stage combustion consisting of cool flame region and the high temperature HCCI combustion region.23,24 This kind of feature can be observed clearly in Figure 5. It is worth noting the low temperature reaction and high temperature reaction heat release rates as the EGR percentage increases. Figure 5 clearly shows that both low temperature reaction (LTR) and high temperature reaction (HTR) peak is suppressed as the EGR percentage increases. This is due to the effect of EGR on peak pressure rise and cylinder mean temperature. The increased EGR rate may delay both start of LTR and HTR. Figure 5 shows the effect of EGR on the interval between HTR and LTR. As the EGR rate increases the interval increases. It can be concluded that EGR can retard HTR longer than LTR and prolong the duration of the negative temperature coefficient area in HCCI combustion because more exhaust gas absorbs more energy released by the LTR. With the combustion before top dead center, the temperature will be increased both by the chemical reactions and the compression due to piston motion. Thus for a given autoignition temperature, combustion onset before TDC will result in faster reactions. With the conditions changed to give combustion onset close to TDC, the temperature will not be increased by piston motion; the only temperature driver would be the chemical reactions. This gives a more sensitive system and the later the combustion phasing the more sensitive the system is. This is the underlying problem with HCCI combustion control. It is desired to have a late combustion phasing to reduce the burn rate and hence pressure rise rate and peak pressure. 3.1.3. Start of Combustion. Figure 6 details the start of combustion (SOC) variation at different brake mean effective pressure. It is observed that the start combustion is advanced

Figure 6. Variation of start of combustion with brake mean effective pressure.

with increase in engine BMEP for both DI diesel and DI biodiesel operation. Even at HCCI mode, the start of combustion is advanced with increase in engine BMEP. DI of diesel and biodiesel alone taken for comparison, it is clearly seen that biodiesel combustion start before diesel combustion start. The reason may be due to the presence of oxygen in biodiesel it helps in accelerating the combustion of biofuel than conventional diesel fuel. A similar trend was observed in the case of BDVI mode. In BDVI mode since the mixture is prepared/mixed outside the cylinder with the help of dedicated fuel vaporiser, the charge burns well before the DI of biodiesel. It is seen that up to 50% load (BMEP of 2.64 bar) the difference in start of combustion between DI of biodiesel and BDVI mode is very less. This is also predicted and discussed in Figure 6. For DI diesel and DI biodiesel the SOC varies between 1 to 5 °CA bTDC and 2 to 7 °CA bTDC from no load to full load condition. For HCCI operation the SOC is still advanced and it is between 2 to 22 °CA bTDC for BDVI without and with EGR case from no load to 75% load. Full load condition is limited in the case of HCCI operation due to difficulty in controlling the combustion. 3.2. Performance Analysis. 3.2.1. Brake Thermal Efficiency. The effect of brake mean effective pressure on brake thermal efficiency for DI diesel fuel, DI biodiesel fuel and biodiesel vapor induction is shown in Figure 7. There is a steady rise in brake thermal efficiency as the load increases in all

Figure 7. Variation of brake thermal efficiency with brake mean effective pressure. 3043

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This may seem to be opposed to the well-established holding that diesel NOx is a thermal phenomenon. In general, earlier the maximum temperature is achieved; the longer the conditions are conducive for NOx formation. It is well noticed in the study, that the higher bulk modulus of compressibility for biodiesel cause an advanced SOI timing leading to higher NOx emission. It is observed very clearly between direct injection of diesel and biodiesel in the test engine. Figure clearly shows that NOx emission is higher for direct injection of biodiesel compared to direct injection of diesel (NOx emission values at BMEP 3.96 bar for diesel and biodiesel are 10 g/kWh and 18 g/kWh, respectively). The above-discussed issues are solved in this study by mixing biodiesel vapor with air by use of dedicated fuel vaporiser which does not depend on mechanically controlled fuel injection system and bulk modulus issues. In HCCI combustion low NOx emission is one of the primary advantages. Since the homogeneous mixture autoignites, HCCI combustion starts in the entire cylinder, so that higher combustion temperature and rich fuel region are eliminated. From Figure 8, it is observed that DI biodiesel operation having more NOx emissions compared to biodiesel vapor induction mode. In biodiesel vapor induction operation, the NOx emissions are substantially less due to the lean and homogeneous fuel vapor-air mixture which results in low combustion temperatures. EGR also used in this investigation to study its effect on HCCI combustion. It is observed that EGR substitution beyond 10% does not bring down NOx to that extent the reason may be due to the presence of oxygen in the biodiesel would have released more energy that could have effected on rate of heat absorbing heat capacity of species that present in the recycled exhaust gas. At 75% loading (BMEP 3.96 bar) condition, the percentage reduction in oxides of nitrogen emission compared to DI biodiesel for HCCI operation (BDVI mode) without EGR, 10%, 20%, and 30% EGR are 76%, 77%, 79%, and 81%, respectively. The cooled EGR on premixed biodiesel vapor induction acts as a heat sink, results in further reduction of NOx emission. 3.3.2. Smoke Emission. Biodiesel, like other oxygenated diesel fuels can reduce the amount of soot formed in the spray flame and can lead to reduced total particulate emissions. However, as has been reported,23,24 ester molecular structures, such as the methyl that comprise bio diesel, are less effective soot suppressants than ether structures. Methyl esters undergo decarboxylation, which yields a CO2 molecule directly from ester and means that the oxygen in the fuel is used less effectively to remove carbon from the pool of soot precursors. Despite this less effective performance as a soot suppressant, biodiesel is observed to impart substantial benefits to the characteristics of soot that is formed during combustion. It is noticed from Figure 9 that smoke emission is lower for direct injection of biodiesel compared to direct injection of diesel and it is further reduced in the case of biodiesel vapor induction mode, because of the absence of diffusion combustion and localized fuel rich mixture discourage the smoke emission behavior of the engine operated with biodiesel vapor air mixture compared to direct injection of diesel and biodiesel operation. Smoke emissions are attributed to flame propagation or fuel−air mixture with nonhomogeneity. In the current study the fuel−air mixture is prepared externally and hence the obtained mixture is homogeneous this results in the disappearance of rich region of mixture around the combustion chamber and better combustion resulting in reduced smoke

the cases. The brake thermal efficiency decreases in the case of biodiesel vapor induction mode at all engine BMEP when compared to conventional mode of operation due to increased fuel consumption. The reason for increased fuel consumption is due to the fuel conversion loss in the fuel vaporiser and the lower calorific value of the biodiesel when compared to diesel. The increase in brake specific energy consumption between biodiesel vapor induction mode and the direct injection of biodiesel under conventional mode is mainly due to the fuel conversion loss in the fuel vaporiser that in turn increased the fuel required to maintain the same BMEP. The fuel (biodiesel) characteristics like low volatility, higher viscosity and density could be the reason for decreased brake thermal efficiency. Also the brake thermal efficiency presented here is based on the total fuel injected/supplied and not the fuel that has participated in the combustion, since it is very difficult to estimate accurately the later. From the test results it is noticed that among the biodiesels tested, the maximum brake thermal efficiency recorded at BMEP 3.96 bar (75% load) for DI biodiesel and biodiesel vapor induction without and with EGR is 26% and 24%, 22%, 20%, 19%, respectively compared to 28% brake thermal efficiency of diesel. Biodiesel vapor induction mode of operation resulted in the maximum decrease in brake thermal efficiency of about 9% and 7%, respectively, at 75% load compared with conventional mode of operation of DI diesel and DI biodiesel. 3.3. Emission Analysis. 3.3.1. Oxides of Nitrogen Emission. The variation of NOx emission with brake mean effective pressure is shown in Figure 8. In the case of biodiesel

Figure 8. Variation of oxides of nitrogen with brake mean effective pressure.

the start of injection (SOI) timing was found to advance as the fuel bulk modulus increases, and other researchers have previously reported that biodiesel fuels have higher bulk moduli than petroleum derived diesel fuels.16 Thus, SOI and fuel bulk modulus are interrelated in this type of mechanical fuel injection system, and it is well-known that NOx emission increases as the SOI timing is advanced for a given fuel. Once the inadvertent advance in SOI timing is corrected for, the NOx emissions are no longer fuel composition dependent. Therefore it can be concluded that the primary mechanism by which biodiesel increases NOx emissions is by an inadvertent advance in SOI timing,20−22 caused by a higher bulk modulus, in diesel engines with mechanically controlled fuel injection systems. In addition, neither the maximum heat release rate nor the maximum cylinder temperature correlated to NOx emissions. 3044

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in HC and CO emissions. At 75% load for HCCI operation (BDVI mode) without EGR, with 10%, 20%, and 30% EGR the observed HC values are 0.32 g/kWh, 0.35 g/kWh, 0.37 g/kWh and 0.39 g/kWh, respectively, when compared to HC values of 0.47 g/kWh, 0.42 g/kWh of DI diesel and biodiesel at the same load condition. 3.3.4. Carbon Monoxide Emission. CO is an intermediate product in the combustion of hydrocarbons. It is formed mainly due to incomplete combustion, which is exacerbated by a lack of oxidants, temperature and residence time. As combustion proceeds to completion, oxidation of CO to CO2 occurs. Combustion of fuel rich mixtures usually produces high CO emissions. The variation of carbon monoxide emission with brake mean effective pressure for DI diesel fuel, DI biodiesel fuel and biodiesel vapor induction is shown in Figure 11. Figure 9. Variation of smoke with brake mean effective pressure.

emissions. At 75% loading condition, the percentage reduction of smoke emission for HCCI operation without EGR, 10%, 20%, and 30% EGR compared to DI diesel and DI biodiesel are 50%, 54%, 63%, 72%, and 25%, 35%, 46%, 58%, respectively. 3.3.3. Hydrocarbon Emission. The variation of hydrocarbon emissions with brake mean effective pressure for DI diesel, DI biodiesel and biodiesel vapor induction is shown in Figure 10.

Figure 11. Variation of carbon monoxide with brake mean effective pressure.

According to the most literatures, the trend is common that carbon monoxide emissions reduce when diesel fuel is replaced by pure biodiesel. The reason being the oxygen content present in the biodiesel enabling the recombination reaction better way in which CO is converted to CO2, resulting in low CO emissions.27−30 From Figure 11, it can be observed that the CO in the case of biodiesel vapor induction is reduced due to lean mixture and more oxygen molecules present in biodiesel when compared with DI biodiesel and DI diesel operation. Also the percentage reduction in CO is decreased with increase in EGR due to the reason discussed in the previous section. At 75% load condition, the observed values of CO emission for HCCI operation without EGR, with 10%, 20%, and 30% EGR are 2.1 g/kWh, 2.54 g/kWh, 3.35 g/kWh, 3.86 g/kWh, respectively, compared to 5.1 g/kWh and 4.1 g/kWh of DI diesel and biodiesel.

Figure 10. Variation of hydrocarbon with brake mean effective pressure.

It is observed that the formation of HC is lower for DI biodiesel and biodiesel vapor induction operation when compared to DI diesel operation. This is due to the presence of oxygen in the fuel which promotes the HC oxidation process. The incomplete combustion of HC due to several reasons will lead to unburned hydrocarbon emissions. As biodiesel contains oxygen in the structure itself, it is believed to enhance the oxidation reactions. Also in biodiesel vapor induction operation, the mixture of fuel−air is very lean and the presence of oxygen in the biodiesel which help to better combustion results in reduced HC emissions. It is clearly noticed in Figure 10 that as the percentage of EGR increases the hydrocarbon emission is increased,25,26 it is indicated in literatures that increase in EGR from 0 to 50%, reduced O2 from 21% to 14%, and increased CO2 by 5%. With O2 being displaced (reduced), the soot oxidation rate drops and leads to higher concentrations of carbonaceous particulate matter and also it was found that the increase in CO2 and soot was also accompanied by an increase

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CONCLUSION The results presented in this study show that HCCI combustion of biodiesel fuel with external mixture preparation is a feasible option to achieve a considerable decrease in NOx smoke, HC and CO emissions. ● NOx (81%) and smoke (72%) have found to be reduced compared to conventional mode of operation. ● Even though there is a decrease in brake thermal efficiency observed from the experimental studies, it is possible to improve the efficiency of the system by proper fuel metering through ECU. 3045

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● HC and CO emissions are reduced due to the presence of oxygen in the biodiesel. ● Increase in EGR rate results in increased HC and CO emission, but reduced NOx and PM emissions. ● Thus the advantage of HCCI combustion with fuel vaporizer was utilized to exploit the benefits of lower NOx and smoke emissions were achieved.

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(14) FangT.LeeC. F.Jatropha effects on the combustion processes in an HSDI diesel engine employing advanced multiple injection strategies. In Proceedings of the 32nd Combustion Symposium, 2009; Vol. 32, pp 2785−2792 (15) Hanny Johanes, B.; Shizuko, H. Jatropha production from crude Jatropha curcas L. Seed oil with a high content of free fatty acid. Bioresour. Technol. 2008, 99, 1716−1721. (16) Tat, M. E.; Van Gerpen, J.H .; Soylu, S.; Canakci, M.; Monyem, A.; Wormley, S. The speed of sound and isentropic bulk modulus of biodiesel at 21 degrees C from atmospheric pressure to 35 MPa. J. Am. Oil Chem. Soc. 2000, 77 (3), 285−289. (17) Stan, C. Direct injection systems for spark ignition and compression ignition engines. Soc. Automot. Eng. 1999, 288. (18) Szybist, J. P.; Juhun, S.; Mahabubul, A.; Boehman, A. L. Biodiesel combustion, emissions and emission control. Fuel Process. Technol. 2007, 88, 679−691. (19) Anand, K.; Sharma, R. P.; Mehta, P. S. Experimental investigations on combustion, performance and emissions characteristics of neat karanji biodiesel and its methanol blend in a diesel engine. Biomass Bioenergy 2011, 35, 533−541. (20) Narayana Reddy, J.; Ramesh, A. Parametric studies for improving the performance of a jatropha oil-fuelled compression ignition engine. Renewable Energy 2006, 31, 1994−2016. (21) Tsolakis, A.; Megaritis, A.; Yap, D. Application of exhaust gas fuel reforming in diesel and homogeneous charge compression ignition (HCCI) engines fuelled with biofuels. Energy 2008, 33, 462−470. (22) Lee, C. S.; Lee, K. H.; Kim, D. S. Experimental and numerical study on the combustion characteristics of partially premixed charge compression ignition engine with dual fuel. Fuel 2003, 82, 553−560. (23) Kim, D. S.; Lee, C. S. Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR. Fuel 2006, 85, 695−704. (24) Machrafi, H.; Cavadias, S.; Guibert, P. An experimental and numerical investigation on the influence of external gas recirculation on the HCCI auto ignition process in an engine: Thermal, diluting, and chemical effects. Combust. Flame 2008, 155, 476−489. (25) Megaritis, A.; Yap, D.; Wyszynski, M. L. Effect of water blending on bioethanol HCCI combustion with forced induction and residual gas trapping. Energy 2007, 32, 2396−2400. (26) Lu, X.-C.; Chen, W.; Hou, Y.-C.; Huang, Z. Study on ignition, combustion and emissions of HCCI combustion engines fueled with primary reference fuels. Soc. Automot. Eng. 2005, 2005−01−0155. (27) Odaka, M.; Suzuki, H.; Koike, N.; Ishii, H. Search for optimizing control method of homogeneous charge diesel combustion. Soc. Automot. Eng. 1999, 1999−01−0184. (28) Jacobs Timothy, J.; Assanis Dennis, N. The attainment of premixed compression ignition low-temperature combustion in a compression ignition direct injection engine. Proc.Combust. Inst. 2007, 31, 2913−2920. (29) Miller Jothi, N. K.; Nagarajan, G.; Renganarayanan, S. LPG fueled diesel engine using diethyl ether with exhaust gas recirculation. Int. J. Therm. Sci. 2008, 47, 450−457. (30) Juttu, S.; Thipse, S.; Marathe, N. V.; Gajendra Babu, M. K. Homogeneous charge compression ignition (HCCI): A new concept for near zero NOx and particulate matter (PM) from diesel engine combustion. Soc. Automo. Eng. 2007, 2007−26−020.

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Corresponding Author

*Phone: +91-9884088394; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/es403104f | Environ. Sci. Technol. 2014, 48, 3039−3046