An investigation on a diesel jet's ignition characteristics under cold-start conditions

The diesel engine is widely regarded as a critical power source for transportation vehicles, construction machinery vehicles, and military-equipment due to its advantages of significant horse power, higher fuel efficiency, greater reliability. However, diesel engines do suffer from ignition issues in extremely cold environments. Ignition characteristics during the start-up process directly impact both the start-ability and emission levels of the diesel engine. In this study, various optical diagnostics, such as Mie scattering, shadowgraphy method, and high-speed imaging, were applied to investigate the spray's vapor and liquid phases as well as the initial flame development, while an optical constant volume chamber was used to observe the in-cylinder condition. During the experiments, the ambient temperature, injection pressure, and fuel temperature was varied in a large range to simulate different stages of the cold-start or warm-up processes. The results show that the penetration of the spray's liquid and vapor phases, flame lift-off length, and ignition delay all observably increase with a decrease in the ambient density, ambient temperature, and fuel temperature. Furthermore, the chemical process is the dominant factor for ignition at a low ambient temperature. A low ambient temperature may lead to a misfire due to the separation between the spray's vapor phase and the expected initial flame location. Additionally, a low ambient density could lead to misfires due to the low heating capacity of the ambient gas. The study results also show that the injection pressure does not significantly influence the ignition delay or the flame lift-off length, but a higher pressure could result in a lower ignition success rate due to over-mixing of the air-fuel. Additionally, the fuel temperature significantly influences the ignition, which advances the initial ignition time 0.1 ms with every increase of 10 °C under an ambient temperature of 535 °C.