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. In-cylinder high speed visualizations allow to observe locally injection and combustion processes and to complete the standard combustion analysis operated from heat release which is a more global approach. Moreover, the CFD (Computational Fluid Dynamics) calculation, which combines the local and global approaches, is really helpful when phenomena take place out of the visualization area.
Very local phenomena specific to cold operation as combustion of the spray close to the glow plug, or combustion in the hot burned gases coming from the Pilot injection combustion have been highlighted. Thus, concerning hardware configuration, glow plug and spray interaction appears to be the first order parameter and glow plug temperature shows a great influence on cold idle. Concerning calibration, the use of Pilot injection improves significantly cold operation (especially opacity and engine speed stability), by deeply modifying the fuel and temperature stratification in the combustion chamber.
