Optical Study on the Fuel Spray Characteristics of the Four-Consecutive-Injections Strategy Used in High-Pressure Isobaric Combustion

High-pressure isobaric combustion used in the double compression expansion engine (DCEE) concept was proposed to obtain higher engine brake thermal efficiency than the conventional diesel engine. Experiments on the metal engines showed 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 high-speed imaging with background illumination and fuel-tracer planar laser-induced fluorescence (PLIF) imaging in a heavy-duty optical engine under non-reactive conditions. Toluene of 2% by volume was added to the n-heptane and served as the tracer. The fourth harmonic of a 10 Hz Nd:YAG laser was applied for the excitation of toluene. The PLIF image distortion caused by the side window curvature and the optical piston was mitigated using a correction lens and corrected with a grid mapping technique. The effects of hydraulic delay and injection dwell on the in-cylinder liquid-phase fuel penetration and vapor-phase fuel distribution were evaluated under different combinations of the four direct injections. The high-speed imaging of the liquid-phase spray shows that a short injection dwell reduces the hydraulic delay of the injector, resulting in an increase in both the peak liquid-phase penetration length and the injection duration. The fuel-tracer PLIF imaging clarifies the spatial fuel distribution of the four consecutive injections involved with the interaction between the vapor-phase spray and the piston bowl wall and the squish region. The intensity distribution in the PLIF images confirms that a longer injector hydraulic delay leads to a shorter peak vapor-phase spray penetration length and a reduced flow rate.