Variable valve timing strategies for ammonia-hydrogen fuelled spark ignition engine combustion

Ammonia-hydrogen combustion has emerged as a promising carbon-free option for internal combustion engines, yet its practical application is constrained by low fuel reactivity and the associated challenges in combustion control at high ammonia substitution levels. In this study, intake and exhaust valve timings were varied using a variable valve timing (VVT) system to examine their influences on combustion behavior, nitrogen-based emissions, and energy distribution in a modified four-cylinder spark-ignition engine operating at a high ammonia energy ratio of 92%. The results show that delaying the intake valve open or advancing the exhaust valve open timing increases the cycle-averaged air charge by enhancing fresh-air trapping or reducing the residual-gas fraction, respectively. As a result, higher peak in-cylinder pressure and heat release rate are achieved, accompanied by an advanced combustion phase. Appropriate valve timing adjustments enable controllable load variation and improve brake thermal efficiency (BTE), with a maximum BTE of 35.3%, corresponding to an absolute increase of 2.3%. Appropriate valve timing strategies also markedly reduce unburned NH3 and N2O emissions, with unburned NH3 decreasing by up to 13.2% to a minimum of 4438 ppm, and N2O reduced by as much as 54.3%, mainly due to enhanced thermal decomposition and suppressed low-temperature formation pathways. Energy analysis indicates that valve timing strategies influence BTE primarily through the redistribution of energy losses. While friction and unburned energy losses are reduced under optimized valve timing, increased wall heat transfer associated with intensified combustion emerges as the main constraint on further efficiency improvement.