Knock Mitigation and Power Enhancement of Hydrogen Spark-Ignition Engine through Ammonia Blending

Abstract

Hydrogen and ammonia are primary carbon-free fuels that have massive production potential. In regard to their flame properties, these two fuels largely represent the two extremes among all fuels. The extremely fast flame speed of hydrogen can lead to an easy deflagration-to-detonation transition and cause detonation-type engine knock that limits the global equivalence ratio, and consequently the engine power. The very low flame speed and reactivity of ammonia can lead to a low heat release rate and cause difficulty in ignition and ammonia slip. Adding ammonia into hydrogen can effectively modulate flame speed and hence the heat release rate, which in turn mitigates engine knock and retains the zero-carbon nature of the system. However, a key issue that remains unclear is the blending ratio of NH3 that provides the desired heat release rate, emission level, and engine power. In the present work, a 3D computational combustion study is conducted to search for the optimal hydrogen/ammonia mixture that is knock-free and meanwhile allows sufficient power in a typical spark-ignition engine configuration. Parametric studies with varying global equivalence ratios and hydrogen/ammonia blends are conducted. The results show that with added ammonia, engine knock can be avoided, even under stoichiometric operating conditions. Due to the increased global equivalence ratio and added ammonia, the energy content of trapped charge as well as work output per cycle is increased. About 90% of the work output of a pure gasoline engine under the same conditions can be reached by hydrogen/ammonia blends. The work shows great potential of blended fuel or hydrogen/ammonia dual fuel in high-speed SI engines.

Ammonia Can be Currently Considered as one of the Best Green Energy Ally

“…Low levels of hydrogen addition allow stability improvement, acting as an ignition promoter and increasing laminar flame speed, but this single factor does not fully explain the improvement in operation achieved [161]. The high octane number of ammonia (130) makes the engine operate under higher compression ratios than the one used for gasoline [162], and the different burning characteristics of hydrogen and ammonia allow for attaining a mixture proportion capable of reaching a similar performance regarding work output as that obtained with gasoline [163]. This approach's main disadvantage is the need for separate fuel storing tanks, making the system too complex for on-board applications.…”

Ammonia Can Be Currently Considered One of the Best Green Energy Allies

“…Low levels of hydrogen addition allow stability improvement, acting as an ignition promoter and increasing laminar flame speed, but this single factor does not fully explain the improvement in operation achieved [189]. The high octane number of ammonia (130) makes the engine operate under higher compression ratios than the one used for gasoline [190], and the different burning characteristics of hydrogen and ammonia allow for attaining a mixture proportion capable of reaching a similar performance regarding work output to that obtained with gasoline [191]. This approach's main disadvantage is the need for separate fuel storing tanks, making the system too complex for onboard applications.…”

Ammonia Combustion in a Spark-Ignition Engine Supported with Dimethyl Ether

“…One of the main disadvantages of ammonia as a fuel is its lower reactivity than conventional coal-based fuel, which complicates the combustion process and may negatively affect the emission of toxic and harmful exhaust gas components. Its relatively low calorific value (LHV = 18.8 MJ/kg), high heat of vaporization (HoE = 1370 kJ/kg), low laminar combustion speed (LFS = 0.015 m/s), high auto-ignition temperature (651 • C), and narrow flammability range (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28) hinder the use of ammonia in piston engines [21]. These properties create problems in effective combustion in both compression ignition and spark-ignition engines.…”

“…Attention was also drawn to the lack of clear information on the impact of ammonia co-burned with other fuels on PM emissions. Ge et al, in their work, used a mixture of ammonia and hydrogen to power a spark-ignition piston engine [16]. Hydrogen and ammonia, considering their properties, are extremely different fuels.…”