Optical study on the fuel spray characteristics of high-pressure isobaric combustion using four consecutive injections

High-pressure isobaric combustion using the double compression expansion concept was proposed to obtain higher brake thermal efficiency than the conventional diesel combustion. Experiments in metal engines have shown that four consecutive injections delivered by a single injector can achieve isobaric combustion. Improved understanding of the detailed fuel-air mixing with multiple consecutive injections is needed to optimize the isobaric combustion and reduce engine emissions. In this study, we explored the fuel spray characteristics of the four consecutive injections strategy using fuel-tracer planar laser-induced fluorescence (PLIF) imaging on a heavy-duty optical engine under non-reactive condition. Toluene of 2% by volume was added into the n-heptane fuel and served as the tracer. The fourth harmonic of a 10 Hz Nd:YAG laser was applied for excitation of toluene. One of the fuel sprays was bisected by the vertical laser sheet and visualized by a camera from the side view. The PLIF images of the premixed mixture formed by port fuel injection were used to normalize the direct-injection PLIF images and correct the effect of non-uniformity of the laser sheet. The focus shift of the camera lens was removed by using a Sheimpflug adapter and image distortion caused by the optical piston and windows were corrected by image post-processing. The effect of the later fuel injection on the overall fuel-air mixture distribution was evaluated by comparing the in-cylinder fuel distribution with different combinations of the four injections. The present study also provides insight into the potential interaction between the fuel spray and piston bowl wall and piston top. This interaction was found alleviated with the increasing of the in-cylinder pressure.