Numerical Simulation of Monodisperse Lube Oil Multiple Droplet Evaporation and Autoignition under Nonconstant Cylinder Conditions of Low-Speed Two-Stroke Gas Engines.

Abstract

The possibility of lube oil droplets’ existence in cylinders for two-stroke low-speed gas engines is higher because of the much higher lube oil consumption rate. Some droplets are directly injected into cylinders by lube oil injectors, and some are blown into cylinders through the scavenging ports. Autoignition of cylinder oil droplets is the main cause of preignition. This research study indicates that under in-cylinder conditions, overlarge single droplets cannot autoignite due to the long evaporation time, and overly small single droplets cannot autoignite because of the low vapor concentration. To find out what kinds of oil droplet groups could autoignite and cause preignition, 3-D computational fluid dynamics simulation in OpenFOAM was carried out. The model predictions were validated against the experimental results, including the evaporation rate of n-heptane droplets and the ignition delay of lube oil droplets. Also, the simulation was used to investigate the characteristics of multiple droplets under different ambient temperatures and pressures. The evaporation lifetime and the ignition delay of a large single droplet are dozens of times longer than that of multiple droplets, which confirms that the droplet group is more dangerous than a large single droplet. The evaporation rate and ignition delay are affected by the distance and number of droplets. A larger number causes a lower average evaporation rate. A smaller distance causes a shorter ignition delay. The local vapor concentration and temperature could be greatly reduced due to the existence of multiple evaporation and ignition cores. Additionally, these findings of multiple droplets were confirmed under nonconstant cylinder conditions of a natural gas engine. This research provides a guide to design the lube oil injectors and scavenging ports so as to reduce the preignition caused by lube oil autoignition.