Energy Fuels 2009, 23, 4966–4973 Published on Web 09/17/2009
: DOI:10.1021/ef9004626
Combustion and Emission Characteristics of Diesel Partial Homogeneous Charge Compression Ignition (p-HCCI) by Adding Fuel Injection in Negative Valve Overlap Lei Shi,* Shuan Qu, Yong Gui, and Kangyao Deng Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai City 200240, China Received May 15, 2009. Revised Manuscript Received September 1, 2009
This paper presents a dual-injection strategy to achieve diesel partial homogeneous charge compression ignition (p-HCCI), which is the combination of HCCI-like combustion and traditional diesel combustion. The dual-injection strategy involves a negative valve overlap (NVO) fuel injection and a traditional fuel injection. The NVO injection occurs during the negative valve overlap period to prepare the homogeneous mixture for HCCI combustion. The traditional injection occurs at the end of the compression stroke to achieve diffused combustion. The results indicate that the NVO injection affects the combustion and emission characteristics in a different way compared to the traditional injection. The increase of NVO injection turns the combustion into HCCI-like combustion and advances the combustion phase. The increase of traditional injection makes the maximum heat release rate (HRR) higher. The emissions results indicate that p-HCCI can greatly reduce the NOx emissions compared to the baseline engine and maintain the same thermal efficiency. However, the NOx emissions increase with the increase of total injection. More traditional injection deteriorates the smoke emissions easily because of little time for fuel to mix with the intake air. The NVO injection ratio has been optimized at medium load according to the emissions and the thermal efficiency, and the optimum result is in the range of 30-40%, which can reduce the NOx emissions by about 40% and the smoke emissions by about 30%. However, the NVO injection should not be used at low load to stabilize the combustion.
achieving a homogeneous mixture, including external mixture formation, dual-injection, and multi-injection. External mixture formation combined with in-cylinder direct injection (DI) has been used to achieve p-HCCI by Ganesh and Nagarajan, and the engine combustion with a homogenized mixture via a fuel vaporizer was demonstrated.8 Mixed-mode HCCI/DI combustion has been studied using the same method, and the benefits of HCCI combustion were extended to a broader range of engine-operating conditions.9 A narrow fuel spray angle and in-cylinder dual-injection strategy have been used to improve the exhaust emissions in a HCCI engine, and it was found that a dual-injection strategy consisting of an early timing of the first injection for HCCI combustion and a late timing of the second injection was effective to reduce the NOx emissions, while it suppressed the deterioration of the combustion efficiency.10 Lee et al. performed an experimental study of two-stage combustion of a DI-type HCCI engine, and it was found that the combustion showed the combination of cool flame, premixed combustion, and diffused combustion.11 The uniform bulky combustion system (UNIBUS) also used dual injection to control p-HCCI, and the injection timings and ratios of dual injections were optimized.12,13 Kook et al. studied the HCCI combustion using the first
1. Introduction Today, the low emissions and high efficiency are the development orientations of an internal combustion engine. Homogeneous charge compression ignition (HCCI) engines have been proven to have the potential to reduce NOx and smoke emissions simultaneously.1,2 Pioneer work has shown that it is difficult to control the HCCI combustion process and extend the operational window. Diesel HCCI has special problems compared to HCCI combustion using gasoline or other fuels. The formation of the homogeneous mixture is difficult because of the low volatility of diesel, and the control of combustion is difficult because of the tendency of early auto-ignition related to the high fuel cetane number.3-5 Therefore, HCCI combustion still cannot be used in all load and speed ranges because of the above problems. The concept of partial homogeneous charge compression ignition (p-HCCI) has been proposed, which can combine the low emissions of HCCI and the high thermal efficiency of traditional diesel combustion.6,7 Many studies have been carried out on diesel p-HCCI using different methods for *To whom correspondence should be addressed. Telephone: þ86-02134206589. Fax: þ86-021-34206589. E-mail:
[email protected]. (1) Zhao, F. Homogeneous Charge Compression Ignition (HCCI) Engines: Key Research and Development Issues; Society of Automotive Engineering: Warrendale, PA, 2003. (2) Stanglmaier, R. H.; Roberts, C. E. SAE Tech. Pap. 1999-01-3682, 1999. (3) Ishibashi, Y.; Asai, M. SAE Tech. Pap. 960742, 1996. (4) Kim, D. S.; Lee, C. S. Fuel 2006, 85, 695–704. (5) Akira, I.; Hideo, S. SAE Tech. Pap. 2007-01-1886, 2007. (6) Fang, T.; Coverdill, R. E.; Lee, C. F. Fuel 2008, 87, 3232–3239. (7) Tanaka, S.; Ayala, F.; Keck, J. C.; Heywood, J. B. Combust. Flame 2003, 132, 219–239. r 2009 American Chemical Society
(8) Ganesh, D.; Nagarajan, G. SAE Tech. Pap. 2009-01-0924, 2009. (9) Canova, M.; Chiara, F.; Cowgill, J.; Midlam-Mohler, S.; Guezennec, Y.; Rizzoni, G. SAE Tech. Pap. 2007-24-0085, 2007. (10) Kim, M. Y.; Lee, C. S. Fuel 2007, 86, 2871–2880. (11) Lee, K.; Lee, C.; Ryu, J.; Kim, H. Energy Fuels 2005, 19, 393–402. (12) Yanagihara, H. Ignition timing control at Toyota “UNIBUS” combustion system. International Congress on a New Generation of Engine Combustion Processes for the Future, IFP, 2001 (13) Hasegawa, R.; Yanagihara, H. SAE Tech. Pap. 2003-01-0745, 2003.
4966
pubs.acs.org/EF
Energy Fuels 2009, 23, 4966–4973
: DOI:10.1021/ef9004626
Shi et al.
Figure 1. Schematic diagram of the experimental system. Table 1. Specifications of the Test Engine
injection as the main injection and the second injection as the trigger injection, and they found that the low emissions of premixed combustion could be achieved only when the first injection timing was before -100 crank-angle degrees (CAD) after top dead center (ATDC).14 A combination of adaptive injection strategies (AISs) and a two-stage combustion (TSC) strategy were proposed to solve the HCCI problems.15,16 The first stage was ideally HCCI combustion, and the second stage was diffused combustion under high temperature and low oxygen concentration conditions. AIS incorporated the use of multiple injections with different spray geometries and injection pressures. Part of the fuel was injected into the cylinder at a low injection pressure (10 MPa) and a narrow spray angle to avoid spray-wall impingement and to prepare a homogeneous mixture for the HCCI combustion. The remaining fuel was injected into the cylinder at a high injection pressure (150 MPa) and a wide spray angle to experience diffused combustion and to provide extended load capability. The results showed that TSC was able to achieve low engine-out emissions with spray-wall impingement avoided at a medium engine load. According to the multipulse fuel injection strategy, Su studied the combustion and emission characteristics of p-HCCI using different injection timings (four-, five-, and sixpulse fuel injections) and different injection modes.17,18 To date, the above methods for achieving and controlling the p-HCCI still need to be improved. An external mixture formation system should add the new fuel injection system in the inlet pipe and the heating system, and the system is more complicated. The parameters of dual- and multi-injection systems should be optimized, such as the fuel injection timing, the fuel injection angle, and the combustion chamber, to avoid wall wetting. The optimization work is more complicated. Thus, it is interesting to learn how to achieve and control the p-HCCI in a simple and effective way. The aim of this study is to propose a method for achieving the p-HCCI and study the combustion and emission characteristics of it. A dual-injection strategy was developed to achieve the combination of HCCI and traditional combustion. The first injection, named as negative valve overlap (NVO) injection, is (14) (15) (16) (17) (18)
bore stroke (mm) combustion chamber type compression ratio number of injection holes diameter of injector hole (mm) injection pressure (MPa) exhaust valve opening exhaust valve close inlet valve opening inlet valve close inlet air temperature (°C) water temperature (°C) oil temperature (°C) fuel
135 150 bowl shape 14.8:1 6 0.19 80 50 CAD BBDC 10 CAD BTDC 10 CAD ATDC 50 CAD ABDC 25 80 80 diesel
mainly used to form the homogeneous mixture and achieve the HCCI-like combustion, and it benefits the reduction of emissions. The second injection, named as traditional injection, is used to achieve diffused combustion as in a traditional diesel engine, and it benefits the thermal efficiency. The ratio of HCCI and traditional diesel combustion can be varied by adjusting the injection fuel ratio. According to this dual-injection strategy, the performance and emissions of p-HCCI will be studied and the NVO fuel injection ratio will be optimized according to the emissions and the thermal efficiency. 2. Experimental Apparatus and Procedure The experimental apparatus is shown in Figure 1. A singlecylinder, four-stroke, four-valve, and naturally aspirated diesel engine was used in these experiments. The engine specifications are listed in Table 1. A variable valve timing (VVT) control system was developed to change the opening and closing timings of inlet and exhaust valves. Early closing of the exhaust valve and late opening of the inlet valve can increase the quantity of the in-cylinder residual gas, which can increase the in-cylinder temperature. This benefits the vaporization of injected fuel and the formation of the homogeneous mixture.19,20 A common-rail electric fuel injection system was developed for achieving dual fuel injections in one cycle, and the injection timing and quantity of each injection could be adjusted independently. Combustion and emissions results were recorded in this study. NOx emissions were measured in the exhaust pipe by AVL DIGAS 4000, which uses the electrochemistry method. An excess
Kook, S.; Bae, C. SAE Tech. Pap. 2004-01-0938, 2004. Sun, Y.; Reitz, R. D. SAE Tech. Pap. 2008-01-0058, 2008. Sun, Y.; Reitz, R. D. SAE Tech. Pap. 2006-01-0027; 2006. Su, W.; Lin, T.; Pei, Y. SAE Tech. Pap. 2003-01-0741, 2003. Su, W.; Wang, H.; Liu, B. SAE Tech. Pap. 2005-01-0117, 2005.
(19) Shi, L.; Deng, K.; Cui, Y. Proc. Inst. Mech. Eng., Part D 2005, 219, 1193–1201. (20) Shi, L.; Deng, K.; Cui, Y. Energy 2006, 31, 2329–2340.
4967
Energy Fuels 2009, 23, 4966–4973
: DOI:10.1021/ef9004626
Shi et al.
Table 2. Measurement Accuracy NOx (AVL Digas4000 light) (ppm) smoke (AVL 439 opacimeter) (%) CO (AVL Digas4000 light) (%) inlet and exhaust CO2 (AVL Digas4000 light) (%) excess air ratio (AVL Digas4000 light) (%) in-cylinder pressure (AVL12QP) (%)
1 0.1 0.01 0.01 0.1
