The Importance of Late-Cycle Combustion to Thermal Efficiency of Compression-Ignition Engine: A Study using Single-Cylinder Engine Experiment

Compression-ignition engines usually have higher thermal efficiency than spark-ignited engines due to the design characteristics and operating mechanism. Under compression-ignition operation, fuel spray development and spray-piston interaction are essential topics to study to improve combustion performance fundamentally. This paper considers different combustion systems of piston designs and injector configurations to look at the combustion process, including spray development, premixed, diffusion, and late-cycle combustion. The experiment was performed using a single-cylinder research engine (SCE). The compression ratio (CR) was 17:1. Combustion profiles (heat release rate) processed from high-speed cylinder pressure measurement are compared from experiments with different combustion systems having varying levels of spray-wall interaction: wave (strong), open-bowl (low). Open-bowl system experiments were carried out under 12-hole and 16-hole injector configurations. The level of spray-wall interaction is stronger with the 12-hole system. Late-cycle combustion, a period where the combustion recession process begins, plays an important part in both combustion and emissions performance. It was found that late-cycle combustion occurred earlier under the open-bowl system than wave system. This was explained by the dependence of late-cycle combustion on spray-driven turbulence (more on the open-bowl system) or flame-piston interaction (more on wave system). There was ~6 degrees longer in combustion duration and ~6% point lower in gross thermal efficiency in open-bowl as compared to wave system. It was found that increasing injection pressure could improve the thermal efficiency of the open-bowl system more effectively than in the wave system. Potential improvements in combustion system design and operating strategies are considered to achieve higher thermal efficiency and minimize harmful emissions. Specifically, this study suggests that flame-wall interaction should occur near the peak of mixing-controlled combustion, especially before the start of late-cycle combustion. This provides an opportunity to combine the spray-driven turbulence that remains from the end of injection (EOI). The flame motion is guided by piston geometry and/or downstream flame-flame interaction. It was realized that an optimal combustion design should have a short late-cycle combustion duration (a period from MFB at the start of late-cycle combustion to MFB90) as defined by combustion aggressiveness analysis.