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. In order to prove the positive effect also experimentally, comparable Miller/Atkinson valve timings (i.e. same effective compression ratio) were realised using a mass-spring model of the valve train. The results obtained from the test bed engine show that only Miller cycle holds advantages over the baseline engine (Otto cycle) with respect to the trade-off between engine efficiency, NOx emissions and IMEP. For a deeper understanding of the combustion behaviour of the investigated valve timing strategies, the experiments were accompanied by 3D CFD simulations.