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A UHC emission source analysis was done for a lean burn natural gas-fired engine. The possible sources were chosen as wall quenching, crevices, too slow turbulent burning velocities, and flame quenching by excessive turbulence. By making each source analysis for each possible source, it was discussed qualitatively what the major source is, while engine geometry (crevice volume), equivalence ratio, and ignition timing were varied.

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.

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.

The Miller cycle was applied to a turbo-charged 324 kW gas engine with three-way catalyst. Three different methods of the Miller cycle were applied, the Early Rotary-Valve Closing (ERVC), Late Intake-Valve Closing (LIVC) and a combination of the ERVC and LIVC methods. The effect of the methods on engine performance was extensively investigated. The experimental results demonstrate a promising performance of the LIVC method, which substantially improves the thermal efficiency up to 38%, compared to 34% by the conventional Otto Cycle. The LIVC method is expected to improve engine performance with inexpensive modification. The combination of the ERVC and LIVC methods appears to further improve thermal efficiency.

Research and development of an advanced air/fuel ratio control system for stationary lean-burn gas engines for co-generation have been carried out. The unique feature of this control system is that the air/fuel ratio is controlled by the feedback from the fluctuation of engine speed. Air/fuel ratio is controlled as high as possible to achieve low NOx emission while maintaining the fluctuation of engine speed within its maximum tolerance. Experiments have been conducted by using a test engine equipped with the new control system. Superior performance of the system has been demonstrated.

Research and development of a low NOx lean burn combustion for open chamber gas engines have been carried out. The target NOx level has been set below 200 ppm with superior thermal efficiency to the engines with three-way-catalyst. A number of open combustion chambers were tested. Swirl combustion chambers NEBULA and TG (the latter was newly developed by the authors) and a squish chamber REENTRANT have satisfied the target while maintaining sufficient stability of combustion. It has been demonstrated that open chamber gas engines are potentially capable of achieving low NOx emission while maintaining high thermal efficiency.