Analysis of multi-mode combustion and performance in a marine diesel/natural gas dual-fuel engine based on an irreversible equivalent combustion cycle theory

Diesel/natural gas dual-fuel engines offer the advantages of higher thermal efficiency and lower carbon dioxide emission while breaking the NOx-PM trade-off caused by diffusion-dominated combustion in diesel engines. However, the large reactivity gradient between diesel and natural gas leads to more complex ignition and multi-stage heat release process. Traditional pressure and heat release rate-based analyses, as well as the ideal thermodynamic cycle theory, are insufficient to identify multi-stage heat release under different combustion modes, and especially cannot estimate the correlation of specific thermodynamic processes with engine performance and emissions. In this paper, an irreversible equivalent combustion cycle theory is proposed to reveal the effects of natural gas thermal substitution ratio (NG-TSR) and high reactivity diesel injection strategies on the multi-mode combustion and performance under a typical ship propulsion condition of the engine speed of 1134 rpm and load of 25 %. The results show that as NG-TSR decreases, the heat release ratio of constant volume combustion (Q-CVC) and constant pressure combustion processes (Q-CPC) increases, while the reduction in late combustion (LC) leads to a higher indicated thermal efficiency (ITE is up to 41.4 %). At the same time, total hydrocarbon (THC) emission can be reduced by more than 70 %, while NOx emission increases. Under the single-injection strategy, CO₂ emission is dominated by NG-TSR, while the ratio of Q-CVC in the whole combustion cycle has the most important effect on NOx and THC emissions, with effect significances of +63 % and -72 %, respectively. The split-injection strategy effectively converts the LC stage to the CVC and CPC stages, and pre-injection ratio (PR) shows a strong negative correlation with THC and CO emissions, as well as brake specific energy consumption. Moreover, CO₂ and NOx emissions can be further controlled by optimizing the contributions of these three combustion stages.