Investigation of diesel pre-injection effects on combustion and emission characteristics in a small-displacement ammonia/diesel dual-fuel engine

As a zero‑carbon fuel, ammonia is characterized by a high ignition temperature and slow laminar flame speed. Ammonia/diesel dual-fuel (ADDF) mode effectively improves ammonia combustion characteristics. This study employs a three-dimensional computational fluid dynamics model to systematically investigate the effects of diesel injection strategies on the combustion and emission characteristics of ADDF engine at low ammonia energy ratios (ammonia energy ratios below 30 %). The findings reveal that the pre-injection strategy markedly enhances the combustion efficiency and reduces emissions compared to the single-injection strategy. When the start of diesel pre-injection (SODI-pre) and the diesel split ratio (DSR) are −47.2 °CA and 20 %, the indicated thermal efficiency of the engine reaches 47.04 %, with an improvement of 1.47 % over the single-injection strategy. Meanwhile, greenhouse gas emissions are reduced by 16.92 %. The combustion of the diesel pre-injection generates a high-temperature environment at the SODI-pre of −17.2 °CA. This promotes the evaporation and combustion of the main-injected diesel, thereby increasing the peak in-cylinder pressure. However, regarding the cases of the SODI-pre of −27.2 °CA and − 37.2 °CA, the combustion phase advances significantly. The extended interval between the onset of combustion and the main-injection suppresses the combustion of the main-injected diesel, leading to a reduction in peak in-cylinder pressure. The in-cylinder combustion is improved when the SODI-pre is advanced to −47.2 °CA. In the combustion process, a notable increase in combustion zones at the center of the combustion chamber is observed. Meanwhile, a significant reduction in low-temperature combustion regions contributes to a substantial decrease of N₂O emissions. Additionally, the increase of DSR enhances the in-cylinder mixture uniformity, allowing more of the combustible mixture to burn, resulting in an increase of peak in-cylinder pressure.