This paper presents a study of lubricating oil transport and exchange in a four-stroke spark ignition engine while undergoing transient load changes. The study consisted of experiments with a single cylinder test engine utilizing 2D LIF (Two Dimensional Laser Induced Fluorescence) techniques to view real time oil transport and exchange, along with computer modeling to describe certain phenomenon observed during the experiments. The computer modeling results included ring dynamics and corresponding gas flows through different regions of the power cylinder.
Under steady-state conditions and constant speed during the experiments, more oil was observed on the piston at low load than at high load. Therefore, a transition from low load to high load resulted in oil leaving the piston, and a transition from high load to low load resulted in oil being added to the piston.
The experiments were concentrated on the critical transients of closed throttle to wide open throttle and wide open throttle to closed throttle. It was consistently observed that a transition from closed throttle to wide open throttle would result in a sudden increase in blow-by to a value that was an order of magnitude higher than normal. The high blow-by would last for 6-15 seconds. Afterward, the high blow-by would end suddenly and a large amount of oil would transport from the top ring groove onto the piston second land and crown. Additionally, a large amount of oil was observed transporting from the piston to the liner near TDC and into free space when the high blow-by ended. This may be a contributing factor to increased oil consumption during transient engine operation.
Based on the experimental results and with the aid of computer models, it is hypothesized that after sufficient time at closed throttle, oil would block gas flow through the top ring groove. Consequently, during the transition to wide open throttle, the higher cylinder pressure caused radial collapse of the top ring, resulting in the sudden and severe increase in blow-by. Once the radial collapse ended, the blow-by quickly returned to a normal value.