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With a high thermal efficiency and low CO2 (carbon dioxide) emissions, Diesel engines become leader of transport market. However, the exhaust-gas legislation evolution leads to a drastic reduction of NOx (nitrogen oxide) standards with very low particulate, HC (unburned hydrocarbons) and CO (carbon monoxide) emissions, while combustion noise and fuel consumption must be kept under control. The reduction of the volumetric compression ratio (CR) is a key factor to reach this challenge, but it is today limited by the capabilities to provide acceptable performances during very cold operation: start and idle below −10°C. This paper focuses on the understanding of the main parameter’s impacts on cold operation. Effects of parameters like hardware configuration and calibration optimization are investigated on a real 4 cylinder Diesel 14:1 CR engine, with a combination of specific advanced tools.

A methodology which uses simultaneously 3D calculations and advanced experimental tools has been developed in order to characterize air-fuel mixing and combustion of gasoline direct injection engines at every stage of development. The analysis of Mitsubishi GDI engine has been carried out in order to validate this approach. The experimental tools used in this analysis underline the great cycle-to-cycle variability and show that the air-fuel ratio variations at spark plug correlate closely with the fluctuations of combustion starting and development. Despite this variability, average measurements are reproducible and in good agreement with 3D computational results obtained with KIVA-MB code. The common use of both kinds of tools allows to get a very fine understanding of Mitsubishi wall-guided concept.