Effects of altitude on combustion and ignition characteristics of speed-up period during cold start in a diesel engine

Altitude has a significant effect on the combustion of diesel engines during cold start, especially during speed-up. In this study, combustion characteristics of speed-up period were investigated using an experimental test on a heavy-duty diesel engine with an intake and exhaust pressure controlled by the plateau simulation test system to simulate altitude conditions including 0 m, 3000 m and 4500 m. Further effects of altitude on ignition characteristics of diesel engine were accomplished through a zero-dimensional thermodynamic model coupled with a detailed kinetic model. Results indicated that as the altitude rose from 0 m to 3000 m, the pressure in the cylinder was reduced, the number of diesel engine speed-up cycles increased, and the rising speed ratio decreased during speed-up. There was a misfire after the fifth cycle at an altitude of 4500 m, and the rising speed ratio was significantly reduced. Through analysis of fuel injection, air intake and ignition phase, we confirmed that the misfire at the high 4500 m altitude was caused by the ignition delay, which was mainly controlled by chemical reaction during cold start. In the detailed kinetic study, the hot-flame and blue-flame reactions slowed down in the chemical reaction of the mixture as the altitude rose. The higher altitude enhanced the H-atom abstraction, but weakened the second O2 addition and the peroxyalkylhydroperoxide isomerization. The decomposition reaction was enhanced. As a result, the overall reaction rate and the heat release fraction deteriorated, extending significantly the negative temperature coefficient (NTC) duration of the ignition. •Combustion characteristics of speed-up during cold start under different altitudes were compared.•Misfires at altitude of 4500 m led to a rapid decrease in starting performance.•Misfire and firing after first misfire was mainly due to ignition delay extension.•High altitude enhanced ignition negative temperature and coefficient reaction is given by chemical kinetic analysis.