Homogenous Charge Compression Ignition (HCCI) combustion offers efficiency improvements compared to conventional gasoline engines. However, due to the nature of HCCI, traditional HCCI combustion can be realized only in a limited operating range. The HCCI operation at high load is limited by excessive combustion noise.In order to maximize the fuel economy benefits of HCCI its operating range needs to be extended to higher loads. In particular, one immediate benefit of an increased load range on the NEDC driving cycle is the avoidance of transitions between SI operation and HCCI operation.In this research a detailed investigation of the fundamental reasons for high combustion noise was performed. Sparkassisted HCCI combustion was found to be a key factor to reduce combustion noise at high load condition. Injection timing for charge cooling, spark timing for the onset of combustion through flame propagation, smaller NVO and higher EGR are the enablers to achieve the appropriate sparkassisted HCCI combustion at high load operation. The high load limit was successfully extended while maintaining good efficiency and complying with emissions regulations.The effect of double injection strategy was also investigated. It is found that even though slight improvement in the tradeoff between combustion noise and combustion stability can be achieved through double injection strategy, fuel consumption is deteriorated due to incomplete combustion.

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In order to meet future emissions regulations, new combustion concepts are being developed. Among them, the development of low-temperature diesel combustion systems has received considerable attention. Low NO x emissions are achieved through minimization of peak temperatures during the combustion process. Concurrently, soot formation is inhibited due to a combination of low combustion temperatures and extensive fuel-air premixing. In this study, the effect of late-cycle mixing enhancement by post-injection strategies on combustion and engine-out emissions in the low-temperature (low soot and NO x emissions) combustion regime was experimentally investigated. The baseline operating condition considered for low-temperature combustion was 1500 rpm, 3 bar IMEP with 50% EGR rate, and extension to high loads was considered by means of post injection. Post-injection strategies gave very favorable emission results in the lowtemperature combustion regime at all loads tested in this study. Since post injection leads to late-cycle mixing improvement, further reductions in soot emissions were achieved without deteriorating the NO x emissions. With smaller fuel injected amounts for the second pulse, better soot emissions were found. However, the determination of the dwell between the injections was found to be very important for the emissions.

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In order to meet future emissions regulations, new combustion concepts are being developed. Among them, the development of low-temperature diesel combustion systems has received considerable attention. Low NOx emissions are achieved through minimization of peak temperatures occurring during the combustion process. Concurrently, soot formation is inhibited due to a combination of low combustion temperatures and extensive fuel-air pre-mixing. In this study, the effect of late-cycle mixing enhancement by post injection strategies on combustion and engine-out emissions in the low-temperature combustion regime was investigated experimentally and numerically. The baseline operating condition considered for low-temperature combustion was 1500 rev/min, 3bar IMEP with 50% EGR rate, and extension to high loads was considered by means of post injection. Post injection strategies gave very favorable emission results in the low temperature combustion regime at all loads. With small second fuel injected amounts, better soot emissions were found. However, the determination of the dwell between the injections was found to be very important for the emissions. Since post injection leads to late-cycle mixing improvement, further reductions in soot emissions were achieved without deteriorating the NOx emissions. To explain these results, numerical analysis was also done using the KIVA-CHEMKIN code. The simulations show that optimal combustion requires that the post injection fuel avoid fuel rich regions formed from the main injection.

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