Ammonia can be utilized directly in a combustion engine without conversion to hydrogen, and the infrastructure already exists as it is one of the most produced and transported chemicals. Thus, ammonia—one of the most cost-efficient ways to carry and store renewable hydrogen—can facilitate the introduction of hydrogen into the existing vast infrastructure propelled by cost effective internal combustion engines, extending to ship propulsion, road transport and peak power production.
Ammonia as a fuel for internal combustion engines—both SI and CI—has been studied since the 1960s. Most studies reported poor SI engine performance for 100% ammonia operation and concluded that neat ammonia was suboptimal as an engine fuel without the addition of an ignition promoter such as hydrogen. This has been a major obstacle in the practical application of ammonia for SI engines due to safety and system complexity issues.
Now, a team of researchers from the Technical University of Denmark reports in an open-access paper in the journal Fuel on their investigation of the lean-burn characteristics of ammonia in a pre-mixed SI (Spark Ignition) engine and the influence of spark energy and discharge characteristics on engine performance and emissions in order to mitigate the need for an ignition improver.
All experiments were performed with 100% neat ammonia in an upgraded single-cylinder Cooperative Fuel Research (CFR) engine. The engine is equipped with a smart ignition coil from MSD (8289 Smart Coil) with an integrated igniter circuit. This enables external control of spark timing and dwell time for the coil, which influences the spark discharge profile and ignition energy.
Engine setup schematic. Jespersen et al.
With present engine modifications, emissions of unburned ammonia was measured to be between 5000 and 10000 ppm and with a combustion efficiency above 95%. The team believed the unburned ammonia to originate from crevices, particularly at the ring-pack.
The NOx emission was between 4000 and 800 ppm even at high excess air ratios. It is critical to minimize emissions of N2O, as it is a strong greenhouse gas. It was measured to be between 20 and 80 ppm and appeared to be related to post-oxidation reactions of ammonia released from crevices during expansion.
Advancing the ignition timing proved to be an efficient handle for balancing the emissions of NH3 and NOx. These emissions will be reduced to H2O and N2 in an SCR catalyst if they are correctly balanced. Advancing ignition timing also minimizes the formation of N2O.
The team identified four key parameters to be most critical in premixed ammonia combustion:
Indicated efficiency should be high to minimize fuel consumption.
CoV (Coefficient of variation) of IMEP (Indicated Mean Effective Pressure) should be low to ensure smooth engine operation and stable exhaust composition.
NH3/NOx molar ratio in the exhaust composition should be close to 1 to enable complete conversion of both unburned ammonia and NOx in a SCR catalyst.
N2O should be as low as possible, preferably zero, as it is a strong greenhouse gas. The global warming from use of green ammonia may actually become higher than from fossil diesel if concentrations of N2O is too high.
Successful ignition of neat ammonia at lean condition can be achieved using excessive energy charging of an inductive spark ignition system. Excess charge allow the arc to stretch longer and move along with the swirling flow in the CFR engine before it collapses and restrikes several times during a single spark event.
The optimal air–fuel equivalence ratio was found to be 1.25. At this air excess an optimum indicated efficiency, low CoV and low NH3/NOx ratio close to unity was found. Increasing the intake pressure reduces the NH3/NOx ratio and lower the N2O as well. With the present engine, final adjustment to obtain a NH3/NOx ratio of unity and minimum N2O emission was made by advancing the spark timing and compromising the engine efficiency slightly. A NH3/NOx ratio of unity and minimum N2O emission are critical criteria’s for NH3 operation of engines in order to achieve complete NH3 and NOx removal with an SCR catalyst and minimize global warming from N2O.
The investigation provides evidence that ammonia slip from the present engine primarily originates from unburned fuel trapped in crevices during combustion, and released during the expansion. There is also evidence that N2O is formed during a part of the expansion stroke when crevice releases are mixed with burned gas just hot enough for partly oxidation of NH3. The present test engine has a rather large crevice volume relative to combustion chamber. A smaller crevice would lower the slip of ammonia and this would also lower the N2O emission as well as the NH3/NOx ratio because the NOx would not be affected by the smaller crevices volumes.—Jespersen et al.
Mads Carsten Jespersen, Thomas Østerby Holst Rasmussen, Anders Ivarsson (2023) “Widening the operation limits of a SI engine running on neat ammonia,” Fuel, Volume 358, Part B doi: 10.1016/j.fuel.2023.130159.