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Hydrogen may be used to feed a fuel cell or directly an internal combustion engine as an alternative to current fossil fuels. The latter option offers the advantages of already existing hydrocarbon fuel engines - autonomy, pre-existing and proven technology, lifetime, controlled cost, existing industrial tools and short time to market - with a very low carbon footprint and high tolerance to low purity hydrogen. Hydrogen is expected to be relevant for light and heavy duty applications as well as for off road applications, but currently most of research focus on small engine and especially spark ignition engine which is easily adaptable. This guided us to select modern high-efficient gasoline-based engines to start the investigation of hydrogen internal combustion engine development. This study aims to access the properties and limitations of hydrogen combustion on a high-efficiency spark ignited single cylinder engine with the support of the 3D-CFD computation.

The present paper is concerned with part of the work performed by Renault, IFPEN and Politecnico di Torino within a research project founded by the European Commission. The project has been focused on the development of a dedicated CNG engine featuring a 25% decrease in fuel consumption with respect to an equivalent Diesel engine with the same performance targets. To that end, different technologies were implemented and optimized in the engine, namely, direct injection, variable valve timing, LP EGR with advanced turbocharging, and diluted combustion. With specific reference to diluted combustion, it is rather well established for gasoline engines whereas it still poses several critical issues for CNG ones, mainly due to the lower exhaust temperatures. Moreover, dilution is accompanied by a decrease in the laminar burning speed of the unburned mixture and this generally leads to a detriment in combustion efficiency and stability.