Optical investigation on effects of diesel injection strategy on ammonia/diesel dual fuel combustion characteristics and flame development

•The effect of the diesel injection strategy on ADDC mode was investigated with an optical engine.•The split diesel injection enhances mixture activity and promotes ammonia flame propagation.•Premixed diesel improves combustion stability across a wider range of injection phases.•Excess premixed diesel adversely affects the stability of ammonia combustion. As a zero-carbon fuel, ammonia is challenging to achieve optimal combustion when used alone in internal combustion engines (ICE) due to its high auto-ignition temperature and low laminar flame speed. The dual-fuel combustion mode, which utilizes a highly reactive fuel to ignite premixed ammonia, is expected to address this drawback. This study investigates the effects of different diesel injection strategies, specifically single and split injections, on ammonia/diesel dual-fuel combustion (ADDC). The high-speed natural flame luminosity imaging technique was employed to record the ignition and flame development of the combustion at 1000 rpm. The results show that in the ammonia/diesel dual-fuel engine, both excessively advanced and delayed injection timing in the single injection strategy is detrimental to combustion. In contrast, the split injection strategy significantly enhances mixture distribution and reactivity, and leads to a more distinct combustion flame propagation. Additionally, advancing pre-injection (SOI1) timing leads to a decrease in the proportion of diffusion combustion and a delay in the combustion phasing, reducing the intensity of flame luminescence. The impact becomes minimal when the SOI1 timing is earlier than −40° ATDC. Additionally, premixed diesel allows broader stable combustion for main injection (SOI2), and appropriately advanced SOI2 timing improves combustion performance. However, excessive pre-injected fuel reduces flame brightness and increases ammonia flame quenching, and the SOI1 ratio should not exceed 50 %. The present work contributes to basic knowledge of combustion progress, offering valuable insights for developing and optimizing ADDC engine combustion strategy.