Energy & Fuels 2004, 18, 1213-1219
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Effect of Premixed Gasoline Fuel on the Combustion Characteristics of Compression Ignition Engine Dae Sik Kim, Myung Yoon Kim, and Chang Sik Lee* Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Sungdong-ku, Seoul 133-791, Korea Received January 26, 2004. Revised Manuscript Received April 6, 2004
The effect of the premixed ratio and supply condition of premixed fuel on the combustion and emission characteristics of a partial homogeneous charge compression ignition (HCCI) engine has been investigated experimentally and numerically. The fuel, which is supplied by an additional port fuel injector in the intake manifold, is premixed with air. The homogeneous charge is compressed and ignited by directly injected diesel fuel in the combustion chamber. In this work, the experimental investigation is made by varying the supply conditions of the premixed fuel, such as the injection pressure and the intake temperature. Results show that a partial HCCI engine with a premixed gasoline fuel shows single-stage ignition and the increase in premixed ratio of gasoline fuel results in the advance of ignition timing and the remarkable reduction of nitrogen oxide emission of the engine. Also, heating the intake air is useful for reducing the HC and CO emissions, which are regarded as one of the problems in the partial HCCI engine.
Homogeneous charge compression ignition (HCCI) combustion has the potential to provide the best features of both spark ignition and compression ignition. As in a spark ignition (SI) engine, the charge is wellmixed, which minimizes particulate emissions, and, as in a compression ignition engine, the charge is compressed and ignited spontaneously and also has no throttling losses, which leads to high efficiency. However, unlike either of these conventional engines, the combustion of HCCI engine occurs simultaneously throughout the cylinder volume rather than in a flame front. This important attribute of HCCI in the engine allows combustion to occur at much lower temperatures, dramatically reducing the emissions of the oxides of nitrogen (NOx).1-3 Much research on HCCI combustion has concentrated the attention on the simultaneous reduction of nitrogen oxides (NOx) and particulate matter (PM) and the control factors of the engine operation. However, despite these various efforts, several technical barriers must be overcome, so that HCCI combustion can be widely applied. One major difficulty is that the stable ignition control of HCCI has not been demonstrated yet, because HCCI combustion is achieved by controlling the temperature, pressure, and composition of the fuel and air mixture in the combustion chamber. Also, HCCI operation has encountered difficulties at high equivalence
ratios (high loads), because combustion can become very rapid and intense, causing unacceptable noise, potential engine damage, and eventually unacceptable levels of NOx emissions. Currently, MK combustion4 and AR combustion5 have been applied to a direct injection (DI) diesel engine and a two-stroke engine, respectively; however, they could only realize HCCI combustion at a partial load. To resolve these difficulties in HCCI engines, the recent several researches investigated the partial HCCI combustion as a combination of HCCI and diesel combustion under various names. The concept of partial HCCI combustion can be divided into two categories. One category is to provide the premixed charge prior to ignition through a multi-injection in the cylinder using a gasoline fuel injector6 or electronic high-pressure direct diesel injection system.7,8 However, this category of HCCI system revealed some problems, including too early ignition and inadequate homogeneity of the premixture. The other category is to form a premixed airfuel charge in the intake port, using an additional port fuel injector. In this category, a homogeneous air-fuel mixture is compressed in the cylinder and then ignited by a small amount of fuel with a high cetane number injected directly into the combustion chamber at the end of the compression stroke. Suzuki et al.9 investigated the combustion of partial HCCI as the concept of homogeneous charge diesel combustion (HCDC), using the port injection of pre-
* Author to whom correspondence should be addressed. Telephone: +82-2-2290-0427. Fax: +82-2-2281-5286. E-mail address: cslee@ hanyang.ac.kr. (1) Christensen, M.; Johansson, B. SAE Tech. Pap. Ser. 1998, 982454. (2) Chen, J. Y.; Dibble, R. W.; Kolbu, J.; Homma, R. Combust. Sci. Technol. 2003, 175, 373-392. (3) Tanaka, S.; Ayala, F.; Keck, J. C.; Heywood, J. B. Combust. Flame 2003, 132, 219-239.
(4) Kimura, S.; Aoki, O.; Kitahara, Y.; Aiyoshizawa, E. SAE Tech. Pap. Ser. 2001, 2001-01-0200. (5) Ishibashi, Y.; Asai, M. SAE Tech. Pap. Ser. 1996, 960742. (6) Yamamoto, S.; Satou, T.; Ikuta, M. SAE Tech. Pap. Ser. 2002, 2002-01-0113. (7) Hasegawa, R.; Yanagihara, H. SAE Tech. Pap. Ser. 2003, 200301-0745. (8) Yokota, H.; Kudo, Y.; Nakajima, H.; Kakegawa, T.; Suzuki, T. SAE Tech. Pap. Ser. 1997, 970891.
1. Introduction
10.1021/ef049971g CCC: $27.50 © 2004 American Chemical Society Published on Web 07/07/2004
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Kim et al.
Figure 1. Schematic diagram of the experimental apparatus.
mixed fuel, and satisfactory results were shown in reduction of NOx and soot. Simescu et al.10 performed an experimental investigation on the partial premixed charge compression ignition (PCCI) combustion of the heavy duty diesel engine, using port fuel injection to control the amount of diesel fuel. Their experimental results showed that the operation range at which the effect of a simultaneous reduction in NOx and PM emissions could be achieved was limited, in comparison to those from the partial HCCI engine of the premixed gasoline fuel,9 because the characteristics of vaporization and mixing with air of premixed diesel fuel are inferior to those of the premixed gasoline fuel. Also, Zaidi et al.11 introduced a partial fumigation system and showed the influence of partial premixing fumigation of the diesel fuel in the intake manifold on the exhaust emissions and the engine performance parameters in a DI diesel engine. Despite these several researches, an understanding of the correlation between combustion characteristics and emissions of the partial HCCI engine is still required. In addition, the effect of the supply condition of the premixed fuel on the combustion in a partial HCCI engine remains a question yet to be resolved. The aim of this work is to provide new data on the effect of the equivalence ratio and premixed ratio of fuel on the characteristics of the combustion and exhaust emission using a partial HCCI engine and to determine the key factors that affect the operating region of the partial HCCI engine. For this purpose, the experimental investigation is made under various supply conditions of premixed fuel, such as injection pressure and intake air temperature. Also, the numerical approach is used in an attempt to estimate the combustion process of premixed fuel in the combustion chamber with a single-zone model modified by introducing some factors under the influence of a premixed charge in a nondimensional compression ignition model. (9) Suzuki, H.; Koike, N.; Odaka, M. SAE Tech. Pap. Ser. 1997, 970313. (10) Simescu, S.; Ryan, T. W.; Neely, G. D.; Matheaus, A. C.; Surampudi, B. SAE Tech. Pap. Ser. 2002, 2002-01-0964. (11) Zaidi, K.; Andrews, G. E.; Greenhaugh, J. H. SAE Tech. Pap. Ser. 2002, 2002-01-1157.
Table 1. Engine Specifications combustion chamber number of cylinder compression ratio displacement bore × stroke rated output rated speed
direct injection 1 19 673 cm3 95 mm × 95 mm 9.55 kW 2400 rpm
Table 2. Test Conditions engine speed engine load (equivalence ratio) intake air temperature fuel premixed directly injected injection pressure premixed directly injected injection pressure premixed directly injected premixed ratio a
1000-1800 rpm 0-30 N m (φ ) 0.1-0.5) 0-120 °C gasoline diesel 4 and 55 bar 220 bar TDCa at compression stroke BTDC,b 22 ° 0-maximum operating regime
Top dead center. b Before top dead center.
2. Experimental Apparatus and Procedure A schematic diagram of the experimental system is shown in Figure 1. The test engine is a single-cylinder, direct-injection (DI), four-stroke-cycle diesel engine with a piston displacement of 673 cm3; the details are specified in Table 1. The experimental apparatus is composed of the combustion analyzer, the exhaust gas analyzer system, the electronic air-temperature control system, and the premixed fuel injection system. A premixture chamber with a volume of 9000 cm3 and a diameter of 0.2 m is installed upstream of the intake port, to minimize the inhomogeneity effect of premixed charge and ensure a sufficient mixing of air and premixed fuel. To analyze the influence of premixed charge on the exhaust emissions and combustion characteristics, the injection amount of premixed fuel is controlled by a port fuel injection system. Also, to examine the effect of exhaust gas recirculation (EGR) gas on the combustion and emission characteristics of the PCCI engine, an EGR system that was operated mechanically was installed in the intake manifold. The tests covered a wide range of equivalence ratio and various engine speeds, and the intake air temperature was heated to 120 °C; the test conditions in this research are summarized in Table 2.
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3. Numerical Model 3.1. Definition of Premixed Ratio. The premixed ratio (rp) is defined as the ratio of energy of premixed fuel to the total energy, which can be obtained from the following equation:
rp )
mphup mphup + mdhud
(1)
where mp is the premixed fuel mass, md is the diesel fuel mass, hu is the lower heating value of diesel fuel, and the subscripts p and d denote premixed and diesel fuel, respectively. 3.2. Thermodynamic Analysis of the Engine System. The combustion analysis of the engine is theoretically investigated through a filling and emptying model that assumes a control volume in the cylinder. The foundation of the model is based on the first law of thermodynamics focused on the combustion processes. It is assumed that the entire control volume contains a homogeneous equilibrium mixture of air and combustion products at each instant. The thermodynamic balance equation for energy is given by
dQ dt
)
dmu dt
+P
( ) dV
-
dt
∑m˘ ihi
(2)
where dQ/dt is the rate of heat transfer into the system through the system boundary, dmu/dt is the rate of change of total internal energy in the system with mass m, P(dV/dt) is the rate of mechanical work done by the system on its boundary, and m ˘ ihi is the energy converted into or out of the system at a given flow rate m ˘ i. The equivalence ratio of mixture can be calculated by considering that the total mass in a cylinder is the summation of the air mass flow through the intake and exhaust valve and the mass of fuel injected into the combustion chamber. The total mass of gas in a cylinder and the equivalence ratio of the mixture in cylinder can be written as follows:
Figure 2. Effect of the premixed ratio on the ignition delay.
of the fuel type and air:fuel ratio, and P and T are the pressure and temperature of the compressed air, respectively. The ignition delay is strongly dependent on the premixed ratio in the partial HCCI engine, as indicated by Simescu et al.10 In addition, if we can determine the effect of the premixed ratio on ignition delay, eq 5 can be correlated to the additional function of premixed ratio by
( )
θid ) AP-n exp
Ea × Cid RT
(6)
where Ea is the apparent activation energy for fuel auto ignition process, A and n are constant but are functions
where Cid ) Cid(rp). Ignition delay is defined as the interval between the injection timing of diesel fuel and the point of the cycle at which the calculated cumulative heat release changes from a negative or zero value and remains positive for the remainder of the cycle. An extensive analysis was performed to determine the change of ignition delay under different engine loads and initial temperatures, as shown in Figure 2. The premixed ratio has a noticeable effect on the ignition delay. An increase in the premixed ratio advanced the ignition timing almost linearly under every condition of load and intake air temperature, despite the reduction of DI diesel fuel, which was used as an ignition source. Pekalski et al.13 reported that the oxidation reactions of the premixed fuel start at ∼140 °C. Hereby, it can be inferred that this oxidation of premixed fuel proceeds through many intermediate compounds prior to the ignition of the fuel and that these intermediate species and the improved gas temperature result in a shorter ignition delay for the DI fuel. Also, Chen et al.2 and Tanaka et al.3 showed that the ignition delay decreases as the equivalence ratio and intake temperature each increase, and this trend is also applied to the partial HCCI combustion in this study. The relationship between ignition delay and premixed ratio is linear, with an approximately similar slope, regardless of the equivalence ratio and intake temperature. In the range of intake temperatures of >100 °C, the chemical reactions become more activated and ignition delay showed a sharp reduction with increases in the premixed ratio. In the regimes except the case of
(12) Hiroyasu, H. In Proceedings of the First International Symposium (COMODIA), Tokyo; 1985; 53-75.
(13) Pekalski, A. A.; Zevenbergen, J. F.; Pasman, H. J.; Lemkowitz, S. M.; Dahoe, A. E.; Scarlett, B. J. Hazard. Mater. 2002, 93, 93-105.
m ) ma + mf φ ) mf
(
)
1 + φfst mfst
(3) (4)
where f is the fuel-to-air ratio and the subscript st denotes the stoichiometric condition. 3.3. Estimation of Ignition Delay. An evaluation of ignition timing was assessed to estimate the combustion process because the ignition timing was a major controlling factor of combustion in this system. For the calculation of ignition delay, many researchers introduced the following type of formula, which is characterized by the Arrhenius equation:12
( )
θid ) AP-n exp
Ea RT
(5)
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Figure 3. Effect of the premixed ratio on the combustion pressure and the rate of heat release (Tin ) 20 °C, Pinj•pre ) 4 bar).
Kim et al.
Figure 4. Effect of the premixed ratio on the combustion pressure and rate of heat release (Tin ) 20 °C, Pinj•pre ) 55 bar).
an intake temperature of >100 °C, the best fitting of eq 6 is suggested as follows:
Cid ) 1 - 0.35rp
(0 e rp e operational range) (7)
Accordingly, this numerical fitting is limited in intake temperatures of 0.6, soot emission became almost zero, because the rich region of mixture around the fuel spray in diesel combustion disappeared and the airfuel mixture became homogeneous in the entire combustion chamber. 4.3. Effect of Intake Air Temperature. As shown in Figure 10, for the higher initial temperature (Tin ) 80 °C), the combustion of the HCCI engine shows clearly the stronger dependence of the premixed ratio than that for the lower initial temperature (Tin ) 20 °C) in Figure 4. When the inlet temperature increases, the air mass per stroke in the cylinder was reduced from 0.689 g/stroke at the intake temperature of 20 °C to 0.584 g/stroke at 80 °C for the case of the same injection amount of fuel and a much-richer air-fuel mixture was formed at an intake air temperature of 80 °C. Also, heating of the air-fuel mixture promotes the lowtemperature reactions of premixed fuel, as stated in the preceding paragraph. Hence, the early stage of combustion was activated and then the homogeneous and
Figure 10. Effect of the premixed ratio on the combustion characteristics (Tin ) 80 °C).
premixed combustion was accompanied over the entire cylinder at a high premixed ratio. Figure 11 shows the influence of the intake air temperature on the NOx and soot emissions at an equivalence ratio of 0.35. As the intake air temperature increases, both NOx and soot emissions are increased, because the heating effect of the intake temperature causes an increment in the equivalence ratio and combustion gas temperature. Also, in this study, the induction of premixed charge in the diesel engine without heating intake charge has an adverse influence on the HC and CO emissions as shown in Figures 12 and 13, respectively. HC emissions of HC increase as the premixed ratio increases. How-
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Energy & Fuels, Vol. 18, No. 4, 2004 1219
emissions. As shown in the HC results, the heating of intake air is determined to be a useful method for the reduction of CO in a partial HCCI engine. Also, in contrast, further heating over 80 °C decreases the CO emissions when the premixed ratio increases >0.5. This different trend between CO and HC emissions at a premixed ratio of >0.5 can be attributed to the combustion characteristics, as shown in Figure 10. For a premixed ratio of >0.5 in these graphs, the rapid progress of combustion was observed prior to expansion, because of the shortened ignition delay. Accordingly, this advanced combustion prior to expansion stroke is effective for the completion of the reaction of CO to CO2,19 and it would have an influence on the reduction of CO emission. Figure 11. Effect of intake air temperature on NOx and soot emissions.
Figure 12. Effect of the intake air temperature on HC emission.
Figure 13. Effect of the intake air temperature on carbon monoxide (CO) emission.
ever, heating the intake air reduces the rate of HC increase remarkably. The reasons seem to be because the premixed fuel is vaporized and mixed with air more perfectly at a high intake temperature.18 Figure 13 shows the influence of premixing on the emissions of carbon monoxide (CO). An increase of the premixed ratio resulted in an increase in the CO (18) Yamasaki, Y.; Iida, N. SAE Tech. Pap. Ser. 2003, 2003-01-1091.
5. Conclusions Combustion and emission characteristics of the partial homogeneous charge compression ignition (HCCI) engine have been analyzed under various conditions of engine operation range and various supplies of premixed fuel, experimentally and numerically. The results of this work have demonstrated the reduction of NOx and soot emission, using a method to apply the premixed gasoline fuel that is based on a direct injection (DI) diesel engine. The following conclusions are summarized from these results: (1) At a low injection pressure of premixed fuel, the peak value of combustion pressure is decreased and the timing of peak pressure is retarded as the premixed ratio increases. However, at a high injection pressure of premixed fuel, the ignition delay is slightly decreased and the combustion pressure is almost constant. (2) The rates of increase of HC and CO decrease remarkably, by almost half, at an injection pressure of 55 bar, in comparison to the case of an injection pressure of 4 bar. (3) With premixed gasoline fuel, the increase of the premixed ratio advanced the ignition timing almost linearly in every operational condition. (4) NOx concentrations are reduced linearly as the premixed ratio increases. When exhaust gas recirculation (EGR) is applied in a partial HCCI system, NOx emission of the partial HCCI engine with an EGR system is reduced, to a value one-fourth lower than that of diesel injection only. The soot concentration is decreased as the premixed ratio increases at overall equivalence, and at a premixed ratio of >0.6, soot emission became almost zero. (5) Heating of the air-fuel mixture promotes the lowtemperature reactions of premixed fuel, and then the early stage of combustion was activated and the homogeneous and premixed combustion happened over the entire cylinder at a high premixed ratio. (6) The induction of a premixed charge in the diesel engine without heating the intake charge has an adverse influence on the HC and CO emissions. However, heating of the intake charge can prevent HC and CO emissions from increasing rapidly. Acknowledgment. This work is supported by the Korea Research Foundation (under Grant KRF-2002042-D00025). EF049971G (19) Dec, J. E.; Sjoberg, M. SAE Tech. Pap. Ser. 2003, 2002-01-0752.
