Life Science Journal 2013;10(7s) http://www.lifesciencesite.com 1141 Boost Pressure Effects on HCCI Combustion Performance Dr. P. M. Diaz 1 , N. Austin 2 , Dr. K. Maniysundar 3 and M. Julie Emerald Jiju 4 1 Professor, Ponjesly College of Engineering, Tamilnadu, India 2 Research Scholar, Sathyabama University, Tamilnadu, India 3 Principal, Ponjesly College of Engineering, Tamilnadu, India 4 Professor, CSI Institute of Technology, Tamilnadu, India pauldiaz71@gmail.com Abstract: This paper describes use of reformer gas (RG) to alter and control combustion in a CNG-fueled HCCI engine. Experimental work used a mixture of simulated RG (75% H 2 and 25% CO) to supplement base CNG fueling in a COMET engine upgraded to achieve high compression ratios. RG was used to improve the engine’s operating performance and to control combustion onset in experiments. The building compressed air supply was used to supercharge a CNG fueled COMET engine operating in HCCI mode. This sufficiently raised the engine's indicated power that it could overcome internal friction for even leaner mixtures and thus produced a significantly wider operating range. Operation near the lean boundary could be extended to the point where misfiring and partial burning cycles were identified. As well as illustrating the potential for widened operating range through pressure boosting, this study also examined the direct effects of pressure on the HCCI combustion processes. [P. M. Diaz, N. Austin, K. Maniysundar and M. Julie Emerald Jiju. Boost Pressure Effects on HCCI Combustion Performance. Life Sci J 2013;10(7s):1141-1144]. (ISSN: 1097-8135). http://www.lifesciencesite.com . 181 Keywords: Homogeneous Charge Compression Ignition, Compressed Natural Gas, Reformer Gas, Air/Fuel ratio, Exhaust Gas Recirculation, Carbon Monoxide, Effective Pressure, Valve Timing, Crank Angle. 1. Introduction Homogeneous Charge Compression Ignition (HCCI) combustion engines have recently gained attention from automotive researchers because of the potential for high efficiency with low NO x emissions [1, 2]. Unfortunately, they suffer from a narrow operating range because they lack a means to control combustion timing. Ignition is governed by the mixture temperature history during compression and the auto-ignition chemistry of the fuel/air/residual mixture. This makes the study of ignition mechanisms for such mixtures important because of the potential to lead to improved engine control techniques. There have been many attempts to control ignition timing using techniques such as changing the exhaust gas recirculation (EGR) ratio [3-5], adjusting initial operating conditions [6], varying engine compression ratio [7], and using different fuel blends [8-10]. EGR quantity control using variable valve timing (VVT) has been proven to control HCCI ignition [11]. However, the cost and complication of VVT which would enable such a technique makes it less attractive. A variable fuel blend is more attractive because it can be adjusted on a cycle-by-cycle basis by changing the fraction of each fuel injected [12-14]. Moreover, using a reformer gas produced from the base fuel as the blending agent [15], offers a substantial fuel change with less inconvenience than supplying and storing two fuels. Fuel reformers convert base fuels to low- molecular weight gases dominated by hydrogen (H 2 ) and carbon monoxide (CO). The reformer gas (RG) may also contain variable amounts of unreformed fuel as well as inerts like carbon dioxide and nitrogen. In a comprehensive experimental and modeling study of HCCI combustion of natural gas, ethanol, and iso-octane by Christensen et al (1998) [1], a maximum IMEP of 14 bar was reported on natural gas with intake pressure boosted up to 2 bar gauge and intake temperature adjusted accordingly. In another experimental study by the same group (Christensen and Johansen (2000) [2]), a boost pressure of 1.5 bar was used to expand the operating region of an HCCI engine. The engine achieved 16 bar IMEP at high EGR rate and λ close to stoichiometric for natural gas HCCI combustion. Hyvonen et al (2003) [16] also presented results of the intake pressure variation (both boosting and throttling) of an HCCI engine. They found that maximum load and brake efficiency were higher with turbo charging than with mechanical supercharging. Using a turbocharger on HCCI engines leads to practical considerations as the exhaust temperature is lower than that of conventional engines and also exhaust back pressure affects internal EGR and consequently combustion timing. Those practical considerations of using a turbocharger or mechanical supercharger have been discussed in a paper by Olsson et al (2004) [17]. Recently in an experimental study by Yap and Megaritis (2005) [18], a high IMEP of 7.5 bar was achieved in an HCCI engine fueled