Design of engines and associated nominal settings are normally computed and optimized so as to provide maximum performance, i.e. compromise between efficiency and emissions. For Combined Heat and Power (CHP) applications fuelled by gas network, this conception is adapted for a large range of gas chemical composition, up to a limit. The indicator used to describe gas quality is the methane number (MN). Hence, engine manufacturers normally supply specific information such as nominal engine settings and its associated limit value for the methane number (MNL). When the engine is operating under its nominal settings, a low grade gas (MN < MNL) can lead to engine knock. Knock is caused by auto-ignition of the end gas ahead of the flame in spark-ignition engine. Heavy knock can severely damage engine piston, constituting a main constraint for optimization of engine operating conditions. For an engine setting ES, (the vector ES includes the spark advance SA, the air fuel ratio AFR and the load), methane number requirement (MNR) is defined such as the minimum value of MN above which no-knock is ensured. The objective of this paper is to predict MNR as a function of engine setting for three engines. Simulation results show that the critical value of the considered knock criterion varies from an engine to another. A new normalized knock indicator based on the energy ratio is proposed to enable this comparison: laminar flame speed is assumed to be more sensible to MN variation than internal fluid dynamics (swirl, tumble, and squish) due to engine design.

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