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HCCI (Homogeneous Charge Compression Ignition) combustion has attracted attention as a new combustion technology, owing to its capability of achieving both low NOx and high efficiency. However, issues of ignition control and high-load operations still remain. As a possible solution to these problems, the present authors have attempted to devise a combustion control method using EGR on natural gas-fueled HCCI engines. Effects of EGR on HCCI combustion have been studied from experimental and numerical simulation fronts. Results of knock suppressing effects using EGR to increase operation load of HCCI combustion were reported elsewhere. In the present study, effects of inhomogeneity of EGR inside the cylinder on HCCI combustion are reported by performing experiments, CFD and detailed chemical kinetics calculations.

A computer model has been developed to investigate the effect of temperature and air/fuel ratio inhomogeneity on HCCI combustion. Engine tests were performed to aid the model validation. After obtaining appropriate agreement between experimental pressure profiles and the computer simulation, it was found that two very important factors impact the pressure profiles; (1) the onset timing of the overall reaction, which is mainly governed by the highest temperature in the cylinder, and (2) the time lag between initial ignition and combustion in other regions, resulting from temperature and species inhomogeneity.

This paper proposes a concept of “lower temperature and higher pressure combustion” for natural gas engines in order to simultaneously achieve high performance, high reliability and low emissions. This concept should not only improve engine performance but also reduce engine thermal load (improve reliability) by adopting low engine speed specifications with the Miller cycle or EGR system while maintaining power output. This paper experimentally examines the effects of engine speed on performance, such as engine efficiency, friction loss, pump loss, heat loss, exhaust loss, blow-by loss, time loss, combustion efficiency, knock limit, combustion duration, combustion temperature and specific heat ratio.

The present levels of the BMEP for natural gas fueled spark ignition engines, the BMEP of 1.0MPa for stoichiometric burn and 1.2MPa for lean burn, are lower than those of diesel engines. This paper discusses the reasons. The factors that limit the BMEP are mainly engine knocking and thermal loading such as exhaust temperature and boost pressure. The Miller cycle and cooled EGR were applied to a turbo-charged, 324kW natural gas engine for co-generation. A lower compression ratio prevents engine knocking and a higher expansion ratio reduces the exhaust temperature in the Miller cycle. The EGR also improves the knock limit by reducing the exhaust temperature. In the Otto cycle, the BMEP is limited by the EGR ratio (COV_IMEP) which is used to control the engine knocking and decrease the exhaust temperature, but in the Miller cycle with its high expansion ratio and low compression ratio, is limited by the boost pressure.