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The performance of boosted gasoline engines is limited at high loads by knock, stochastic Low Speed Pre-Ignition, and Megaknock. An investigation has been carried out on the occurrence of abnormal combustion and megaknock in a 1.6 L GTDI engine with the aim to determine the causes of such phenomena. A classification of abnormal combustion events and causes is presented in order to facilitate a consistent terminology. The experiments specifically focus on the effects of exhaust residual gas on occurrence of megaknock in multi-cylinder engines. The results showed that while a misfire will not lead to megaknock, a very late combustion in one cycle, in one cylinder may lead to megaknock in the following cycle in the same or adjacent cylinder. Additionally, a recently developed multi-zone model was used to analyze the role of residual gas on auto-ignition.

A fundamental requirement for natural gas (NG) and renewable methane (e.g. bio-methane or power-to-gas methane) as automotive fuel is reliable knock resistance; to enable optimization of dedicated NG engines with high compression ratio and high turbocharger boost (which enables considerable engine downsizing factors). In order to describe the knock resistance of NG, the Methane Number (MN) has been introduced. The lowest MN which generally can be found in any NG is 65, and the vast majority of NG (~ 99.8%) is delivered with a MN above 70. The MN of bio-methane and power-to-gas methane is usually far above 80. Thus, from an automotive point of view any methane fuel should at least provide a minimum Methane Number of 70 at any point of sale. But the European draft standard describing the automotive CNG fuel quality so far proposes a minimum MN limit of 65.