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A large number of small size gas-fired cogeneration engines operate with homogenous lean air-fuel mixture. It allows for engine operation at high efficiency and low NOx emissions. As a result of the rising amount of installed cogeneration units, however, a tightening of the governmental emission limits regarding NOx is expected. While engine operation with further diluted mixture reduces NOx emissions, it also decreases engine efficiency. This leads to lower mean effective pressure, in particular for naturally aspirated engines. In order to improve the trade-off between engine efficiency, NOx emissions and mean effective pressure, numerical investigations of an alternative combustion process for a series small cogeneration engine were carried out. In a first step, Miller and Atkinson cycles were implemented by advanced or retarded inlet valve closing timings, respectively.

Lean burn operation allows small cogeneration engines to achieve both high efficiency and low NOx emissions. While further mixture dilution enables future emission standards to be met, it leads to retarded combustion phasing and losses in indicated engine efficiency. In the case of naturally aspirated engines, IMEP drops due to lower fuel fraction, increasing brake specific fuel consumption. In this work, an alternative engine configuration was investigated that improves the trade-off between engine efficiency, NOx emissions and IMEP. It combines well-established means such as Miller/Atkinson valve timing and optimised intake system for a single-cylinder cogeneration engine, operating with homogenous lean air-natural gas mixture. First, the engine configuration was analysed using a detailed 1D CFD model, implying a significant potential in reaching the project target.