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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.

Lean burn gas engines are expected to reduce NOx emission while improving engine performances such as output and thermal efficiency. Recently, an ignition method using a small quantity of diesel fuel (pilot fuel) as an ignition source for lean-burn gas engines has introduced further improvement of their performance. Generally, this method has been used for pre-chamber engines because it could not successfully lead to reduce NOx and Particulate emissions when adopted for open-chamber engines. However, the possibility of improvement of performances of open-chamber engines with this ignition method has also been expected(1). An experimental study was conducted to investigate the performance of an open-chamber gas engine with pilot fuel for ignition source. Experiments were conducted by using a single cylinder gas engine equipped with a common-rail injection system.

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.

An experimental study was conducted to investigate the effect of EGR on combustion and exhaust emissions characteristics of a spark-ignited, super-charged, stoichiometric gas engine in order to achieve high BMEP equivalent to that of diesel engines. A four-stroke-cycle single-cylinder test engine was used. EGR was completely mixed with intake air before being introduced into the compressor. The results indicate that dry EGR utilizing drained exhaust gas improved the maximum mean effective pressure, as well as specific fuel consumption over the whole load due to improved knock characteristics of the unburnt mixture, increased specific heat ratio (κ), and reduced heat loss. Further experiments were conducted to identify the effect of humidity in the mixture on engine performance. The lean burn method was compared with the EGR method.

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.

Existing air-fuel ratio control systems for lean burn gas engines are rather complicated. In order to develop small lean burn gas engine, it was necessary to develop a gas mixer which provides mixture of the required air-fuel ratio over the various load without any feed back control. Venturi type gas mixers were employed, because they have simple structure and relatively constant air-fuel ratio characteristic over wide range of flow conditions. Various venturi mixers, with different throat diameter, different position of the throat, different position of the fuel gas inlet, different shape of the fuel gas inlet, were tested by steady flow test installation. Based on these test results, a prototype gas mixer for a 40 kw premixed turbocharged open chamber lean burn gas engine was designed and tested by steady flow test installation and real gas engine.