Toward ammonia-to-NOx ratio control in an ammonia/diesel dual direct-injection engine under low-load conditions

An excessive ammonia-to-NO x ratio (ANR) under low-load operation poses a challenge to the efficient operation of aftertreatment systems in ammonia-fueled engines. In this study, experiments were conducted on a single-cylinder ammonia/diesel dual direct-injection (DDI) engine to investigate the effects of ammonia substitution ratio (ASR), intake pressure, dual-fuel injection pressure, and injection timing (−28 to −12°CA ATDC) on NH 3 emissions, NO x emissions, and the ANR under a low-load condition of 4 bar indicated mean effective pressure (IMEP). The results show that increasing ASR is the dominant factor driving the increase in the ANR. When the ASR was maintained below 60%, coordinated optimization of intake pressure, dual-fuel injection pressure, and injection timing under the late diesel injection strategy reduced the ANR to 1.1–1.25 while maintaining a high indicated thermal efficiency (ITE), thereby reducing the ANR to a level compatible with the efficient operating window of the aftertreatment system, and engine-out N 2 O emissions showed no evident deterioration. Furthermore, by substantially advancing diesel injection timing (−340 to −220°CA ATDC) to establish a thermally favorable in-cylinder environment before ammonia combustion, the ANR could be further reduced to below 1.0. However, a trade-off between ASR and ITE together with a marked increase in engine-out N 2 O emissions was observed under this strategy. The study systematically elucidates the regulation characteristics of key combustion control parameters on the ANR under low-load conditions and proposes practical ANR control strategies for ammonia/diesel DDI engines. The findings provide new insights into the optimization of ammonia utilization and energy conversion efficiency, and offer useful guidance for in-cylinder combustion control with aftertreatment requirements in ammonia-fueled engines.