Combustion and Emission Behavior of Ethanol Fuelled Homogeneous Charge Compression Ignition (HCCI) Engine

Transcription

1 8-8-6 Combustion and Emission Behavior of Ethanol Fuelled Homogeneous Charge Compression Ignition (HCCI) Engine Copyright 8 SAE International Rakesh Kumar Maurya, Avinash Kumar Agarwal Engine Research laboratory, Department of Mechanical Engineering Indian Institute of Technology Kanpur Kanpur 86, India Corresponding author s akag@iitk.ac.in ABSTRACT The Homogeneous charge compression ignition (HCCI) is the third alternative for the combustion in the reciprocating engine. HCCI a hybrid of well-known spark ignition (SI) and compression ignition (CI) engine concepts and has potential of combining the best features of both. A two cylinder, four stroke, direct injection diesel engine was modified to operate one cylinder on the compression ignition by detonation of homogeneous mixture of ethanol and air. The homogeneous mixture of the charge is prepared by port injection of ethanol in the preheated Intake air. This study presents results of experimental investigations of HCCI combustion of ethanol at intake air temperature of C and at different air-fuel ratios. In this paper, the combustion parameters, pressure time history, rate of pressure rise, rate of heat release, mean temperature history in the combustion chamber is analyzed and discussed. The HCCI operating region criteria is defined based on the cycle-to-cycle variation of indicated mean effective pressure (IMEP) and rate of pressure rise. The results presented in this study for air fuel ratio, which satisfies the HCCI operating region criteria. The results show that controlled HCCI combustion is possible with extremely low emission and high efficiency. INTRODUCTION Petroleum fuel supply concerns and increasingly stringent exhaust emission regulations require development of new propulsion systems that provide high performance, excellent fuel economy, as well as reduced exhaust gas emissions. Homogeneous charge compression ignition (HCCI) is new combustion concept that may be developed as an alternative to compression ignition with higher efficiency, lower NOx and particulate emissions. HCCI means that the fuel and air should be mixed homogeneously before combustion starts and the mixture is auto-ignited due to increase in temperature at the end of the compression stroke. Thus HCCI is similar to SI in the sense that both engines use a premixed charge and HCCI is similar to CI as both rely on autoignition for combustion initiation. However, the combustion process is totally different for the three types. The principle of HCCI combustion consists of (a) Preparing a highly diluted air/fuel mixture by burnt gases recirculation to give reasonable burn rate. (b) The temperature after compression stroke should equal the auto-ignition temperature of the fuel/air mixture to promote simultaneous ignition in the whole combustion chamber. (c) Controlling precisely the combustion heat release to achieve the best compromise in terms of efficiency and pollutant emissions. The earliest experience with HCCI is a report by Onishi et al. [] on a unique combustion behavior intermediate between SI and CI. It was called Active Thermo Atmosphere Combustion (ATAC). The HCCI combustion process has been studied with certain success in two stroke [-] and four stroke engines [-9], and with liquid [-7] and gaseous [8-9] fuels. The HCCI family can be distinguished according to the fuel introduction strategy employed []. This distinction include port injection [-], early in- cylinder injection [6, -], and late in-cylinder injection [], dual fuel introduction (both in-cylinder and port injection) []. The main problem with the HCCI is that the ignition is completely controlled by chemical kinetics, and is therefore affected by the fuel composition, equivalence ratio, and thermodynamic state of the mixture []. There is no external control such as the fuel injection or spark timing that are used on diesel or SI engines. Achieving the required level of control during transient engine operation is even more challenging since charge temperatures has to be correctly matched to the operating condition during rapid transients with a high repeatability as the speed and load is changing. Another problem with HCCI is the low BMEP. The relatively high intake air temperature and very lean air to fuel (A/F) ratios reduce the output compared with the conventional engines. However considering the advantage of HCCI and emission legislation limits, HCCI combustion attracts the attention of researchers globally.

2 EXPERIMENTAL SETUP A two cylinder, four stroke, air cooled, naturally aspirated, bowl shaped combustion chamber; direct injection diesel engine was modified for the experiment. The engine specification was given in table. One of the two cylinders of the engine is modified to operate in HCCI mode, while the other cylinder is operated like an ordinary diesel engine, thus motoring the first cylinder for achieving HCCI. A schematic diagram of the experimental setup is shown in Figure. Table : Detailed Engine specification Engine characteristics Specification Model Indec PH Diesel Engine Injection Type Direct Injection Number of Cylinders Two Bore / Stroke 87. / mm Power per Cylinder.85 5 rpm Compression Ratio 6.5 Displacement 8 cc Fuel Injection Timing before TDC Fuel Injection Pressure rpm : Intake Air, : Air Box, : Heater, : Injection Timing Circuit, 5: Electronic fuel Injector, 6: Piezo-electric Pressure Transducer, 7: Charge Amplifier, 8: Shaft Encoder, 9: Dynamometer, : Emission Analyzer, : Combustion Analyzer Figure : Schematic diagram of experimental setup. Test fuel used for this investigation is ethanol. A premixed fuel system was installed in the intake manifold. This system consists of electronic injector and an injection timing and injection duration controlling electronic circuit. Fresh air entering the engine is heated by an air pre-heater positioned upstream of the intake manifold. The intake air heater is operated by a closed loop controller, which maintains the intake air temperature at C. The in-cylinder pressure was measured using a water-cooled piezo-electric pressure transducer (Make: Kistler, Switzerland; Model: 66B) which is mounted flush in the cylinder head. The pressure transducer minimizes thermal shock error by using a double walled diaphragm and integral water cooling system. To measure the a precision shaft encoder (Make: Encoders India, Model: ENC58/6-7ABZ/5-V) is coupled with the crank shaft using a helical coupling. 9 The cylinder pressure history data acquisition and combustion analysis is done using a program based on LabVIEW, developed at Engine Research Laboratory IIT Kanpur. The raw exhaust gas analysis for NO x, CO and THC were carried out using Exhaust gas emission analyzer (Make: AVL, India; Model: DiGAS ). Experiments were conducted on the modified engine at constant speed of 5 rpm and intake air temperature of C. RESULT AND DISCUSSION In this section the experimental results at different engine load at constant engine speed are presented with ethanol as fuel at intake air temperature of the C. RELATIVE AIR/FUEL RATIO () is the ratio of the actual air/fuel ratio to the stoichiometric air/ fuel ratio. The results in this investigation are presented with respect to different relative air/fuel ratios (), which are present in the HCCI operating region. OPERATING REGION To study the HCCI combustion criteria as to what constitutes HCCI combustion must be defined. The HCCI operation region is limited by the misfire and knocking. The operation boundaries are associated with these factors (misfire and knock). The first boundary defines the lower limit for the HCCI combustion. At low loads, fuel flow rate decreases hence the net heat release also decreases. It is believed that the resulting gradual reduction of average combustion temperature results in more unburned charge that is characterized by high CO and THC emissions and by increase in cycle-tocycle variation. Cycle-to-cycle variation of the combustion process in an engine can be monitored by cylinder pressure transducer. Fluctuation of both maximum cylinder pressure and the indicated mean effective pressure (IMEP) were used as a measure of cycle-to-cycle variations and were expressed as COV pmax and COV IMEP. The Coefficient of Variation (COV) of IMEP and COV of maximum cylinder pressure (Pmax) was used in this investigation and COV is calculated for consecutive engine cycles was calculated as standard deviation ( ) divided by mean value (IMEP) as a percentage [5]. COV IMEP IMEP % IMEP IMEP Since the drivability problems in automobiles normally arise when COV IMEP exceeds percent [5], this study used this value for the misfire boundary. When the fuelling rates is increased (lower ), the HCCI combustion rates also increase and intensify, and gradually cause unacceptable noise and may

3 potentially cause engine damage, and eventually leads to unacceptably high level of NO x emissions. Therefore knocking combustion can be defined as being at the upper limit of HCCI combustion. In this investigation, the upper limit of HCCI combustion was defined as being when the rate of pressure rise in a cylinder exceed. Mpa pre crank angle degree ( ) (dp/dθ max =. Mpa / ) for each individual cycle. The recorded pressure in the cylinder determined the values of COV IMEP, COV Pmax and dp/dθ max. Therefore, the HCCI operation region is the area in which the values of COV IMEP and COV Pmax are less than percent and values of dp/dθ max are less than. Mpa/. This definition is applied for present investigation and HCCI operating region was found for of,.5, and.5. The variation of the COV IMEP, COV Pmax and rate of pressure rise at shown in the Figure and Figure at different relative air fuel ratio () in HCCI operating range. COV IMEP COV Pmax Maximum Pressure (bar) Rate of Pressure rise (bar/) Pressure (bar) IMEP (bar) down the speed of the chemical reactions sufficiently so that engine is not damaged and this leads to slow combustion. With lean operation, this will significantly reduces the output for a given air flow through the engine. The rich side limit for IMEP is limited by the rate of combustion and hence that of rate of pressure rises. The maximum IMEP encountered in this investigation is. bars. The Variation of IMEP at different relative air fuel ratios () in HCCI mode is shown in Figure Figure : COV IMEP and COV Pmax for successful HCCI operating region = =.5 = = Figure: Rate of rate of pressure rise variation for successful HCCI operating region ENGINE LOAD, IMEP One major limitation of HCCI combustion is the requirement of a highly diluted mixture in order to slow Figure : IMEP for HCCI operating region. CYLINDER PRESSURE The cylinder pressure was measured for all operating conditions. The cylinder pressure was recorded for cycles, with a resolution of.5 crank angle degrees. Figure 5 shows the pressure trace for different relative air fuel ratio (). For all plots, the trace with the highest maximum pressure corresponds to the operating condition with the richest mixture, as given by figure 6, and the lowest maximum pressure corresponds to the leanest mixture = =.5 = = Figure 5: P-θ diagram at different relative air fuel ratios () Relative Air/Fuel ratio ( ) Figure 6: Maximum cylinder pressure for HCCI mode at different relative air fuel ratios (). RATE OF COMBUSTION / HEAT RELEASE

4 Indicated Thermal efficiency Temperature (K) Max Rate of Heat Release (J/) Rate of Heat Release (J/) The cylinder pressure was analyzed using a single zone heat release model, which gives the rate of heat release. Details concerning the model can be found in research by brunt et. Al. [6]. Figure 7 shows the rate of heat release. It can be noticed that the start of combustion is sensitive to the temperature history during the compression stroke. The mean temperature history in the combustion chamber is shown in Figure 8. It can be noticed from the Figure that the maximum temperature in the combustion chamber is for the richest mixture, which also demonstrated the highest rate of heat release. For all plots, the trace with the highest maximum rate of heat release corresponds to the operating condition with the richest mixture, as given by Figure 9, and the lowest rate of heat release corresponds to the leanest mixture (=.5) = =.5 = = Figure 7: Rate of heat release for different relative airfuel ratio mixtures combusting in HCCI mode = =.5 = = Figure 9: Mean temperature history in the combustion chamber for different relative air-fuel ratio mixtures combusting in HCCI mode INDICATED THERMAL EFFICIENCY As the experiments were carried out with a twocylinder engine, which was converted to operate in HCCI mode on one cylinder only, brake thermal efficiencies can be measured with relatively lower confidence. The engine friction of the first cylinder working in HCCI mode becomes high compared to power produced from the same cylinder. Therefore only indicated efficiency calculations are reported in the present research. Gross Indicated thermal efficiency is defined as the ratio between the work on the piston during the compression and expansion stroke (W i,g ) to the input fuel energy. W ig, mq f ig, LHV Where m f is fuel mass per cycle and q LHV is the lower heating value of the fuel. Net Indicated Thermal Efficiency is the gross indicated efficiency adjusted for pumping work. It is the ratio between the work on the piston for all the four strokes, W i,n, to the input fuel energy. W in, mq f in, LHV Figure shows the measured Gross and net Indicated thermal efficiency. It shows that the indicated thermal efficiency decreases with increasingly rich mixture. In present study the maximum indicated thermal efficiency is achieved for =.5, for HCCI combustion mode Figure 9: Maximum rate of heat release for different relative air-fuel ratio mixtures combusting in HCCI mode Gross Net.5.5 Figure: Measured indicated thermal efficiency for different relative air-fuel ratio mixtures Indicated specific fuel consumption (ISFC) is the ratio of fuel consumed and the indicated power. The indicated power is calculated from the pressure volume curve. The measured ISFC is shown in Figure. The figure shows that the minimum fuel consumption is for =.5. This observation is justified as the maximum

5 CO (g/kwhr) NOx (g/kwhr) HC (g/kwhr) ISFC (g/kwhr) indicated thermal efficiency is at the same air fuel equivalence ratio in the HCCI operating region Figure : Measured Indicated specific fuel consumption for ethanol fuel in HCCI mode EXHAUST GAS EMISSIONS Nitric oxide (NO) and nitrogen dioxide (NO ) are usually grouped together as NOx emissions. Nitric oxide is the predominant oxide of nitrogen produced inside the engine cylinder. The principal source of NO is the oxidation of atmospheric (molecular) nitrogen. The critical equivalence ratio for NO formation in high temperature, high pressure burned gases (typical of engines) is close to stoichiometric. Oxides of nitrogen are formed during combustion when localized temperatures in the combustion chamber exceed the critical temperature and molecules of oxygen and nitrogen combine. With the homogeneous combustion of a premixed charge, the temperature is expected to be same throughout the combustion chamber, except near the walls. This in combination with very lean mixtures gives a low maximum temperature during the cycle. NOx formation is very sensitive to the temperature history during the cycle. At temperatures over 8 K, the NOx formation rate increases rapidly with increased temperature. The NO formation rate is governed by Zeldovich mechanism [5]. Figure shows the NO x emission in the HCCI operating region. The trend shows that the NO x emissions decrease as the mixture become leaner. This is due to the lowering of the combustion chamber temperature which is also explained by mean gas temperature (Fig.8). It is also observed that the NO x emission over entire HCCI combustion region is extremely low (. g/kwhr)) as compared to the CI or SI engines. Unburnt hydrocarbons form as a consequence of incomplete combustion of the hydrocarbon fuel. The level of unburned hydrocarbons in the exhaust gas is specified in terms of the total hydrocarbon concentration expressed in PPM. Total hydrocarbon emission is a useful measure of the combustion inefficiency; it is not necessarily a significant index of pollutant emissions. Engine exhaust gases contain a wide variety of hydrocarbon compounds. Lower combustion chamber temperature prevents NOx formation, but at times, this becomes too low to fully oxidize the fuel, resulting in high-unburned hydrocarbons emission. Even if the mixture is well prepared and close to homogeneous, the combustion rate near the walls will probably be slower, due to higher heat loss from the walls. If we assume that the combustion rate is slower near the walls, much fuel in this region will not be able to burn completely, as the overall temperature decreases due to the volume increase during the expansion stroke. Large part of the HC emissions will probably originate from the wall regions [7]. Figure shows the HC emissions from the engine operating in HCCI mode. The trend shows that HC emissions increase when the mixture becomes leaner. Leaner mixture lowers the temperature of the combustion chamber, thus emitting higher hydrocarbons NOx HC.5.5 Figure : Indicated specific NO x and HC emissions for different relative air-fuel ratio mixtures in HCCI mode Carbon monoxide emissions from internal combustion engines are controlled primarily by the fuelair equivalence ratio Figure : Indicated specific CO Emission for different relative air-fuel ratio mixtures combusting in HCCI mode In HCCI mode, CO emission depends on relative air fuel ratio () and preheating of the intake charge. Close to the rich limit for HCCI and with early combustion phasing, very little CO is generated. However closer to the lean limit, higher CO is generated. With increased relative air-fuel ratio, CO increases drastically, as can be seen in figure. This depends mainly on the lower combustion temperature and the later combustion phasing. At the end of combustion, the temperature becomes too low for complete oxidation and high amount of CO is generated. CONCLUSIONS

6 The combustion and emission characteristic of HCCI engine were investigated on a modified two cylinder engine. The inlet air was supplied at º C temperature and the engine was operated at a constant engine speed of 5 rpm, fuelled with ethanol. Successful HCCI combustion is obtained for (-.5) range. HCCI mode operation of the engine is within a narrow load range of the engine. The maximum IMEP obtained during the experiment was. bar. Knocking starts in the engine for the charge richer than relative air fuel ratio and this is the upper combustion limit for HCCI mode. The coefficient of variation of IMEP is lower for richer mixture of air and fuel. The rate of heat release is high and hence combustion duration is shorter (less than ) for all the conditions of engine running in HCCI mode. The maximum Indicated thermal efficiency was found at relative air fuel ratio of.5. Extremely low NO x (almost negligible) was emitted from the engine during HCCI mode operation. The HC and CO emission is however higher. HC emissions decrease and CO emissions increase with increase in relative air-fuel ratio. REFERENCES. Onishi S., Jo S.H., Shoda K., Jo D.P., Kato S., Active Thermo-Atmosphere Combustion (ATAC) A New Combustion Process for Internal Combustion Engines, SAE Paper No. 795, Ishibashi Y., Asai M., Improving the Exhaust Emissions of Two-Stroke Engines by Applying the Activated Radical Combustion, SAE Paper No. 967, Najt P.M., Foster D.E., Compression-Ignited Homogeneous Charge Combustion, SAE, Paper No.86, 98.. Thring R.H., Homogeneous-Charge Compression- Ignition (HCCI) Engine, SAE Paper No.8968, Aoyama T., Hattori Y., Mizuta J., Sato Y., An Experimental Study on Premixed-Charge Compression Ignition Gasoline Engine, SAE Paper No. 968, Yanagihara H.A., A Simultaneous Reduction of NOx and Soot in Diesel Engines under a New Combustion System (uniform bulky combustion system UNIBUS), 7th. Int. Motor Symposium, Vienna, Yokota H., Kudo Y., Nakajima H., Kakegawa T. Susuki T., A New Concept for Low Emission Diesel Combustion, SAE Paper No. 9789, Christensen M., Johansson B., Einewall P., Homogeneous Charge Compression Ignition (HCCI) Using Isooctane, Ethanol and Natural Gas A Comparison with Spark-Ignition Operation, SAE Paper No. 9787, Christensen M., Johansson B., Amnjus P., Mauss F., Supercharged Homogeneous Charge Compression Ignition, SAE Paper No , 998. Walter B., Gatellier B., Near Zero NOx Emissions and High Fuel Efficiency Diesel Engine: the NADITM Concept Using Dual Mode Combustion, Oil & Gas Science and Technology, Rev. IFP, Vol. 58, No., pp. -,.. Takeda Y., Keiichi Na., Keiichi Ni., Emission Characteristics of Premixed Lean Diesel Combustion with Extremely Early Staged Fuel Injection, SAE Paper No. 966, Iwabuchi Y., Kawai K., Shoji T. Takeda Y., Trial of New Concept Diesel Combustion System-Premixed Compression-Ignited Combustion, SAE Paper No , Kimura S., Aoki O., Ogawa H., Muranaka S. Enomoto Y., New Combustion Concept for Ultra- Clean and High-Efficiency Small DI Diesel Engines, SAE Paper No , Kelly-Zion, Peter L., Dec John E., A computational Study of the effect of the Fuel-Type on the Ignition Time in HCCI Engines, Proceedings of Combustion Institute. 8,. 5. Heywood J.B., Internal Combustion engine fundamentals, McGraw-Hill Book Company, NewYork, Brunt M.F.J., Rai H., Emtage, A.L., The Calculation of Heat Release Energy from Engine Cylinder Pressure Data, SAE Paper No. 985, Christensen M., Johansson B., Homogenous Charge Compression Ignition with Water Injection." SAE Paper No , 999. CONTACT Dr. Avinash Kumar Agarwal is currently working as Assistant Professor of Mechanical Engineering at Indian Institute of Technology, Kanpur since March. He is a 99 Mechanical Engineering graduate from MREC Jaipur. His area of doctoral research was developing biodiesel and related engine tribology investigations. Dr Agarwal joined Engine Research Center, University of Wisconsin, Madison, USA for pursuing his Post-Doctoral research from August 999 February. His main areas of current interest are combustion phenomenon study in IC engines, automobile emissions, biodiesel development and characterization, laser diagnostic techniques, PIV, lubricating oil consumption phenomenon, lubricating oil tribology, development of micro sensors, alternative fuels for diesel engines etc. Rakesh Kumar Maurya is a Graduate student working with Dr. Agarwal on his doctoral dissertation in mechanical engineering from IIT Kanpur. He completed his Dual Degree (B.Tech. and M.Tech.) from IIT Kanpur in Mechanical Engineering in year 6. His areas of interest include Internal Combustion Engines, Engine Instrumentation, HCCI Combustion, and emission control.