Mario Martins and Hua Zhao 436 / Vol. XXXIV, No. 4, October-December 2012 ABCM Mario Martins mario@mecanica.ufsm.br Federal University of Santa Maria Mechanical Engineering Department Avenida Roraima, 1000 97105-900 Santa Maria, RS, Brazil Hua Zhao Hua.Zhao@brunel.ac.uk School of Engineering and Design Brunel University Uxbridge UB8 3PH United Kingdom Performance and Emissions of a 4- Cylinder Gasoline Engine with Controlled Auto-Ignition Advanced combustion modes such as Controlled Auto-Ignition or Homogeneous Charge Compression Ignition have been under much attention due to their ability to reduce both emissions and fuel consumption. Thus, this paper aims at demonstrating the achievement of Controlled Auto-Ignition combustion on a standard 4-cylinder gasoline engine with negative valve overlap (NVO) and analyzing its performance and emissions. The engine remained with substantially original components. The only major modification was the replacement of the camshafts for a new set of bespoke ones. The results showed a fair range of load and speed under CAI combustion with reduced brake specific consumption and ultra-low levels of NOx emissions. CO was also reduced, while HC emissions showed increased values. The results also point out some of the drawbacks of CAI combustion and the technological challenges of this advanced combustion process. Keywords: CAI, HCCI, emissions, NVO Introduction 1 The past few years have shown a rising interest in alternative combustion modes such as CAI/HCCI, which are seen as promising technologies to reduce both fuel consumption and emissions from internal combustion engines. The last decade has brought intense investigation and remarkable improvements in knowledge of such technologies. CAI/HCCI shows a great potential in lowering both fuel consumption and emissions levels, in a substantially standard engine concept. Moreover, it may avoid the need for expensive and complicated exhaust after-treatment systems (Stanglmaier and Roberts, 1999). CAI combustion is a process that combines SI and CI engine characteristics. It relies on the compression and charge heating to promote auto-ignition of a premixed and often homogeneous combustible charge. This combustion process enables the auto- ignition of very lean or diluted mixtures, by controlling temperature and composition of the charge, thus lowering combustion temperature and substantially reducing NOx emissions. It allows WOT operation and virtually eliminates throttling losses, resulting in significant improvement in the part-load fuel economy of an otherwise normal gasoline spark-ignited engine. CAI combustion was first studied in the late 1970s by Onishi et al. (1979) (the ATAC paper) and Nogushi et al. (1979) working with conventional 2-stroke gasoline engines. The first results with a 4- stroke gasoline engine were achieved with intake charge heating, as reported by Najt and Foster (1983). The effects of varying A/F ratio, EGR levels, fuel type, and compression ratio on emissions and the attainable HCCI range were studied by Thring (1989). Christensen, Hultqvist, and Johansson (1999) experimented with various fuels, different compression ratios and intake charge temperatures. Lavy et al. (2000) presented results about the first 4- stroke engine that was able to achieve CAI over a limited load and speed range by means of exhaust gas trapping using bespoke camshafts for negative valve overlapping. Law et al. (2000) and Milovanovic et al. (2005) demonstrated the use of a fully variable valve train (FVVT) in a single cylinder research engine for CAI combustion. The influence of valve timing for controlling CAI combustion was studied. The most common strategies for achieving CAI combustion can be summarized as: 1. Intake charge heating; 2. Higher compression ratio; Paper received 11 January 2012. Paper accepted 17 July 2012 Technical Editor: Luís Fernando Silva 3. More auto-ignitable fuel; 4. Recycling of burnt gases. Exhaust gas recycling appears to be the most practical for obtaining CAI combustion in a gasoline engine, especially if it is done by means of trapping exhaust residuals using the negative valve overlap (NVO) approach as reported by Law et al. (2000b), Milovanovic et al. (2005b), Zhao (2007), and Li, Zhao and Ladommatos (2001). Possibly the major disadvantage of CAI combustion is its limited range of operation. Methods of increasing this envelope are very necessary. At the boundaries of the CAI range, cycle-by-cycle variations can increase considerably, eventually leading the engine to misfire and stall. In such a situation, it has been shown that spark assistance could help trigger CAI combustion (Wang et al., 2006). Also, in order to improve CAI range, forced induction has been employed. Research has been done to investigate the effects of forced induction on a gasoline engine with residual gas trapping. Boost was supplied from an external air compressor. A substantial increase in the upper limit of load range could be achieved without auxiliary intake heating, while NOx emissions were typically low (Yap, Megaritis and Wyszynski, 2005a,b). Several other methods of forced induction have been studied and demonstrated similar results (Christensen and Johansson, 2000; Olsson and Johansson, 2004; and Olsson et al., 2001, 2003). This experimental research, therefore, demonstrates the operation of a standard 4-cylinder gasoline engine with port fuel injection (PFI) under CAI combustion. The results are presented and analyzed in comparison to those of the original spark-ignited operation. Engine performance and emissions were assessed and results were discussed. Nomenclature BMEP = brake mean effective pressure, bar BSCO = brake specific carbon monoxide emissions, g/kWh BSFC = brake specific fuel consumption, g/kWh BSHC = brake specific unburned hydrocarbon emissions, g/kWh BSNO x = brake specific nitrous oxide emissions, g/kWh °CA = degrees crank angle, ° CAI = controlled auto-ignition CI = compression ignition CO = carbon monoxide EVC = exhaust valve closing EVO = exhaust valve openning FMEP = friction mean effective pressure, bar HC = unburned hydrocarbons