Ammonia as Fuel for Low-Carbon Spark-Ignition Engines of Tomorrow's Passenger Cars

Abstract

Faced with the problem of reducing greenhouse gas emissions and transitioning toward a greater use of renewable energies, ammonia, as an energy carrier, is increasingly seen as a potential "green" fuel for transportation, in particular marine applications. However, its combustion characteristics (high minimum ignition energy and auto-ignition temperature, low combustion speed in comparison to usual hydrocarbon fuels) are drawbacks that have so far limited its use. Due to the evolution of different pollutant standards for road transportation, spark-ignition engines and thus the combustion process itself have been subjected to many changes over the last 20 years (e.g., gasoline direct injection, downsizing). The objective of this article is to discuss the potential of ammonia as a fuel for spark-ignition engines, thanks to the studies carried out so far and to point out directions for future work.

Review on Ammonia as a Potential Fuel: From Synthesis to Economics

“…As a result of the specific features of ammonia as a fuel, i.e., its narrow flammability limits, high autoignition temperature, and low combustion speed, even in spark-ignition (SI) or compression-ignition (CI) engines, the feasibility to consider pure NH 3 requires some adaptations to achieve favorable ignition conditions, by either increasing the compression ratio or using a turbocharger or dedicated ignition system or using a more reactive quantity of fuel, as summarized recently in an International Energy Agency (IEA) report. 216,217 As a result of its high autoignition temperature and its narrow flammability limits, as summarized by Dimitriou et al 218 in their recent review focused on CI engines, dual-fuel modes are the most attractive way to use ammonia as a fuel in a CI engine, thus feeding the engine with a pilot injection of more reactive fuels, such as diesel, acetylene, butane, LNG, liquefied petroleum gas (LPG), DME, etc. In most cases, the port-fuel injection of ammonia is better than direct liquid injection of ammonia, which is very limited in conventional diesel mode, as highlighted recently by Lesmana et al 219 Analogously, in SI engines, the ignition is easier thanks to the ignition device, making possible the combustion at a load range with pure ammonia and its blends with another fuel (such as hydrogen or standard or alternative gasoline fuels), thus enhancing the combustion process 217,220 with reasonable compression ratios.…”

Analysing the Performance of Ammonia Powertrains in the Marine Environment

“…Beyond industry research, examples of ammonia-fuelled ICEs appear at select academic institutions. Up to this point, most academic institutions working on ammonia combustion have investigated the characteristics of the fuel in laboratory conditions [41]. Many institutions have generated robust chemical mechanism pathways and associated computer simulations from these experiments [21,[42][43][44][45][46][47].…”

Operating Limits for Ammonia Fuel Spark-Ignition Engine

“…To counterbalance, ignition could be promoted by a more reactive fuel, as H 2 or gasoline type fuel and the in-cylinder temperature can be increased by a higher compression ratio (CR) or supercharged conditions without any risk of knocking. A recent review [16] reported only 16 studies investigating the use of ammonia fuel in spark-ignition engine. About half of them were conducted in CFR engine with CR ranging from 6.1 to 10, which is representative of current vehicles [6,10,[17][18][19][20][21].…”

“…To improve the combustion process, the best carbon-free promoter is hydrogen but also due to its combustion velocity and wide flammability range. In [16], Mounaïm-Rousselle and Brequigny concluded from previous studies [6,[23][24][25]28] that with conventional CR (about 10:1), only small quantities of hydrogen (around 5 to 10%) are necessary to ensure stable operation and best efficiencies. Those small quantities can even be obtained by the means of an thermal converter to dissociate on-board ammonia into H 2 and N 2 [22,29].…”