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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.

The Late Intake-Valve Closing (LIVC) Miller cycle was applied to a turbo-charged stationary gas engine for co-generation. The engine, with a power of 324 kW, was operated under stoichiometric conditions and equipped with a three-way catalyst. The LIVC Miller cycle was aimed to improve the thermal efficiency and lower the exhaust gas temperature by increasing an expansion ratio, while avoiding engine knocking by reducing an effective compression ratio. This part of the study employed an exhaust gas recirculation (EGR) to improve the thermal efficiency of the LIVC Miller cycle engine. The EGR was expected to improve the knocking limit and reduce thermal damage to the engine's exhaust train. The experiments clarified the basic characteristics of EGR and its effect on the performance of the gas engine. The LIVC Miller cycle with EGR operating at stoichiometric conditions demonstrated a high thermal efficiency of 38 % (LHV), rivaling that of existing lean burn gas engines.

This study conducts a series of numerical simulation to investigate systematically the improvement of power as well as thermal efficiency of the Miller cycle gas engines. Novel approach of the numerical simulation was to develop full chemical-kinetics methods to predict knocking phenomena for practical use. The method was successfully incorporated into the one-dimensional cycle simulator of the TSpark code developed by UMIST. The study predicted the performance of the Miller cycle, with particular attention to the improvement of thermal efficiency with higher expansion ratio and of engine power with lower compression ratio. Results show the Miller cycle substantially improves either efficiency or power by optimizing the compression and expansion ratios. Trade-off relation between the improved power and efficiency is evaluated.