Performance characteristics of a high-speed four-stroke engine with ammonia direct injection and diesel pilot ignition

As a zero-carbon fuel, ammonia holds significant promise for decarbonizing internal combustion engines due to its established storage and transport infrastructure. However, challenges such as high unburned ammonia and nitrogen oxide emissions limit its widespread adoption. This study experimentally investigated the combustion and emission characteristics of a high-speed four-stroke engine using diesel ignition of high-pressure direct injection (HPDI) of liquid ammonia. The effects of ammonia injection timing and ammonia energy ratio (AER) were systematically evaluated under constant engine load conditions. The results indicate that, due to lower combustion temperatures and in-cylinder selective non-catalytic reduction (SNCR) reactions, ammonia HPDI can effectively reduce NOX emissions, but CO emissions increase significantly. However, by optimizing ammonia injection timing and increasing AER, CO emissions can be reduced to levels comparable to those in pure diesel ignition conditions. Engine performance was found to be highly sensitive to ammonia injection timing: delaying ammonia injection shifted the combustion mode from premixed to diffusion, which improved both thermal efficiency and operational stability, while reducing ammonia slip. The optimal injection window for ammonia was found to be between 4° and 12° CA after diesel injection, balancing thermal efficiency and emissions effectively. Under the same load, ITE peaked at 64 % AER and then declined due to incomplete ammonia combustion. At an AER of 78 %, although N2O formation and ammonia slip were more pronounced, greenhouse gas (GHG) emissions decreased by 55 %. These findings underscore the trade-off between carbon reduction and combustion efficiency in ammonia–diesel dual-fuel engines.