Abstract: An environmental-adaptive design method for the minimum compression ratio of diesel engines is proposed based on the thermodynamics theory and experiments to ensure that diesel engines can start smoothly in low temperature and plateau environments. In this paper, the critical ignition temperature of diesel spray is determined through optical testing, and a fitting relationship between the critical ignition temperature and ambient density is presented. Diesel engines are tested for the actual in-cylinder pressure during cold start using the motoring test method. A mathematical relationship between the minimum compression ratio and the environmental temperature and altitude is established based on optical diagnosis, motoring tests and the thermodynamic theory. Optical tests show that with the increase of ambient density, the critical ignition temperature of diesel spray drops rapidly, then slowly, before stabilizing at about 700 K. The actual in-cylinder pressure and temperature are lower than the theoretical values for adiabatic compression due to the high-level heat transfer and blow-by during cold starting. As the speed of the motoring test increases, the blow-by in the cylinder of the compression stroke decreases, resulting in an increase in pressure and temperature at the top dead center. The results of thermodynamic analysis show that, when the environmental temperature decreases from 273 K to 238 K at an altitude of 0 m, the minimum compression ratio of a diesel engine with intake preheating needs to be increased from 13.8 to 18.0. When the intake air temperature is 263 K and the altitude rises from 500 m to 4 400 m, the minimum compression ratio of a diesel engine should be increased from 12 to 17. The optimized minimum compression ratio will help improve the cold start performance under extreme conditions.

Key words: dieselengine, coldstart, opticaltest, thermodynamicanalysis, designofminimumcompressionratio