Combustion Characteristics of Ammonia in a Modern Spark-Ignition Engine

Abstract

Ammonia is now recognized as a very serious asset in the context of the hydrogen energy economy, thanks to its non-carbon nature, competitive energy density and very mature production, storage and transport processes. If produced from renewable sources, its use as a direct combustion fuel could participate to the flexibility in the power sector as well as help mitigating fossil fuel use in certain sectors, such as long-haul shipping. However, ammonia presents unfavorable combustion properties, requiring further investigation of its combustion characteristics in practical systems. In the present study, a modern single-cylinder spark-ignition engine is fueled with gaseous ammonia/air mixtures at various equivalence ratios and intake pressures. The results are compared with methane/air and previous ammonia/hydrogen/air measurements, where hydrogen is used as combustion promoter. In-cylinder pressure and exhaust concentrations of selected species are measured and analyzed. Results show that ammonia is a very suitable fuel for SI engine operation, since high power outputs were achieved with satisfying efficiency by taking advantage of the promoting effects of either hydrogen enrichment or increased intake pressure, or a combination of both. The performances under NH3 fueling compare well with those obtained under methane operation. High NOx and unburned NH3 exhaust concentrations were also observed under fuel-lean and fuel-rich conditions, respectively, calling for additional mitigation measures. A detailed combustion analysis show that hydrogen mainly acts as an ignition promoter. In the engine, pure ammonia combustion is assumedly mainly driven by the ignition kinetics of ammonia and the flame response to turbulence rather than by the laminar burning velocity.

An experimental investigation of the performance, emission and combustion stability of compression ignition engine powered by diesel and ammonia solution (NH4OH)

“…Ammonia is difficult to use directly in internal combustion engines due to the insufficiently fast flame spread, but it is possible. 11,12 In a compression ignition (CI) engine, ammonia may be co-combusted with conventional engine fuel (DF). Two supply systems are used in this solution, one for DF supplied to the engine cylinder and the other for alternative fuel, supplied most often to the intake manifold.…”

Operating Limits for Ammonia Fuel Spark-Ignition Engine

“…From this review, it is striking to realize that only a few have explored the feasibility of running an SI engine with ammonia only. At this state it can be concluded that pure ammonia is possible mainly at full load operation, as underlined in [22] and recently confirmed by Lhuillier et al [24,25]. To improve the combustion process, the best carbon-free promoter is hydrogen but also due to its combustion velocity and wide flammability range.…”

“…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]. The other half were conducted with modern single cylinder engine [22][23][24][25] or multi-cylinder engine [26,27]. Among those studies, only two investigated higher CR, i.e., about 14:1 [6,22].…”

Alternative Fuels for Internal Combustion Engines

“…Others report RON of over 130 when lean mixtures are used. When the intake is at atmospheric temperature hydrogen shows a very high RON, enabling compression ratios (CR) higher than 14.5:1 without knock, probably helped by its very high combustion velocities [53].…”

“…Supercharging seems to be a good option [102] and some researchers used plasma ignition with good results [103]. As one of the major problems is its slow combustion, engine conditions of low charge and high speed can only be achieved using a combustion "promotor" such as hydrogen [53], petrol or even diesel fuel, enabling a better ignibility and increased combustion velocity. It can be burned in CI engines when mixed with diesel, but it would be advantageous to be mixed with biodiesel or DME as these fuels have higher cetane numbers (CN) [104].…”