Literature Review: Ammonia as a Fuel for Compression Ignition Engines

The diesel engine, also known as the compression ignition (CI) engine, has been a workhorse of the modern energy economy for more than a hundred years. Its role in the coming sustainable energy economy will be determined by its ability to co-evolve with climate-friendly fuels. Two researchers from the National Institute of Advanced Industrial Science and Technology in Japan have now examined the fit between ammonia and the CI engine. 

Pavlos Dimitriou and Rahat Javaid arrive at a two-part conclusion in their paper, “A review of ammonia as a compression ignition engine fuel,” published in January in the International Journal of Hydrogen Energy. Part one is good news: “Ammonia as a compression ignition fuel can be currently seen as a feasible solution.” Part two is a dose of qualifying reality: to manage emissions of N2O, NOx, and unburnt NH3, “aftertreatment systems are mandatory for the adaptation of this technology,” which means that ammonia-fueled CI engines are likely to be feasible “only for marine, power generation and possibly heavy-duty applications where no significant space constraints exist.”

The paper provides a detailed and readable account of efforts over the last eight decades to develop a viable version of an ammonia-fueled CI engine. The authors state that, “to the best of [their] knowledge, this is the first review approach focusing entirely on ammonia utilisation for compression ignition.” A major challenge confronted by the development efforts derives from ammonia’s “poor combustion characteristics … such as high autoignition temperature, low flame speed, narrow flammability limits and high heat of vaporization.” They continue, “successful ammonia compression ignition operation could only be observed for engine designs that featured extremely high compression ratios from 35:1 to 100:1.”

To address this challenge, most researchers resorted to the expedient of co-combustion. With the addition of fuels like diesel, biodiesel, and dimethyl ether, “the combustion of ammonia … is a realistic conception, as the secondary fuel, with lower Autoignition temperature, can be used to trigger the combustion of the mixture.” Different researchers have used a variety of fuel ratios under a variety of conditions. Ammonia ratios as high as 95% have been achieved, but numbers in the range of 40 to 80% are more prevalent in the literature. Hydrogen has been used successfully as the complementary fuel. This includes hydrogen derived from on-board ammonia cracking, but the authors of one study determined that “the introduction of pure hydrogen [from an off-board source] seems to be the most promising in terms of emissions reduction and engine performance enhancement.”

The dual-fuel approach opens the door to CI for ammonia, but another challenge soon arises: when ammonia is burned as a CI fuel, it tends to produce problematic levels of nitrogen oxides and unburned ammonia. To compound the issue, NOx tends to be a product of high combustion temperatures and unburned ammonia of low temperatures – and there is no “sweet spot” temperature where neither species is a problem. 

Contemporary researchers are attacking this problem with two methods. The first is with advanced fuel injection techniques. By injecting fuel at several points during the engine’s compression stroke, with fine control of the fuel increments, it is possible to achieve “simultaneous reduction of N2O and NH3 emissions in ammonia dual-fuel engines.” The second method is exhaust after-treatment. Selective catalytic reduction (SCR) technologies have been found that can reduce both NOx and unburned ammonia to acceptable levels, a result furthered by “the effect of ammonia in NOx reduction as observed in the modern after-treatment systems.”

The authors’ conclusion that ammonia is unlikely to become a major fuel for passenger cars will not come as a surprise to most members of the ammonia energy community. Ships, of course, are a different story.

Engine manufacturer MAN Energy Solutions expects to bring its dual-fueled maritime engine to market in 2024. (An Ammonia Energy update on the MAN ammonia engine appeared in January.) Success in the maritime realm will certainly encourage development of engines scaled for off-grid and back-up power generation. For other transportation applications, long the near-exclusive province of internal combustion, CI might have its hands full fighting off electrification via battery and fuel cell.