How much can ammonia actually cut emissions?
The headline numbers are impressive. In a 2023 study of a marine engine running on ammonia and diesel, replacing natural gas with ammonia cut total greenhouse gas (GHG) emissions by 60% and CO2 by a striking 89% [1]. Another study found that using ammonia as the primary fuel with a small biodiesel pilot could replace nearly 70% of the biodiesel energy, slashing CO2, carbon monoxide (CO), and unburned hydrocarbons [4]. A 2025 study on a coal-to-liquid (CTL) and ammonia dual-fuel engine showed that with the right injection timing, CO emissions dropped notably at low ammonia fractions [6]. These results come from controlled engine tests, but they point to real potential: ammonia can dramatically reduce carbon-based emissions because it contains no carbon itself.
What's the catch? Ammonia can create new pollution problems
The main trade-off is that burning ammonia can produce nitrous oxide (N2O) and nitrogen oxides (NOx). N2O is a potent greenhouse gas—about 300 times stronger than CO2 over a century—and NOx contributes to smog and acid rain. In the same marine engine study that achieved 89% CO2 reduction, N2O emissions were a problem, arising from slow flame propagation and flame quenching near cylinder walls [1]. A 2023 shipping scenario analysis found that if ammonia becomes the dominant marine fuel by 2050, CO2 would drop 40% but CO2-equivalent emissions (which include N2O) would only fall 22%—the difference is almost entirely due to increased N2O [5]. That same scenario estimated ammonia 'slip' (unburned ammonia escaping) at 930 gigagrams in the North and Baltic Seas alone [5]. So while ammonia eliminates CO2, it introduces new emissions that must be managed.
What makes ammonia work (or fail) in a real engine?
Ammonia is difficult to burn on its own—it has a high ignition temperature, slow flame speed, and a narrow range of air-fuel mixtures that will ignite [2][7]. That's why nearly all practical engines use a 'pilot fuel' like diesel, biodiesel, or hydrogen to ignite the ammonia [1][2][4][6]. The exact blend matters a lot. In the marine engine study, using only 8.5% diesel (by energy) with ammonia gave poor combustion; increasing diesel to 24% made the engine run well and cut GHGs by 70% [1]. Engine temperature also plays a role: a 2024 optical engine study found that raising the wall temperature significantly improved combustion stability and reduced N2O and unburned ammonia, though it increased NOx [3]. Advanced injection strategies—like post-injection of fuel after the main burn—can further reduce NOx and soot while improving ammonia oxidation [6]. So the real-world success of ammonia depends on careful engineering of fuel blends, injection timing, and thermal management.
About These Sources
This answer is built on 7 peer-reviewed studies — published from 2022 to 2025, 3 from 2024 or later, 5 in Q1 journals, collectively cited 423 times — selected as the most relevant from 10 studies that passed quality screening, drawn from 57 papers retrieved from a database of over 500 million.
Sources used in this answer
Performance and emission characteristics of an ammonia/diesel dual-fuel marine engine
In a marine engine, switching from natural gas/diesel to ammonia/diesel (24% diesel energy) cut total GHG by 60% and CO2 by 89%, but N2O from slow flame propagation was a key challenge.
Ammonia as a sustainable fuel: Review and novel strategies
A review identifies ammonia's high ignition temperature and slow flame speed as barriers, and highlights dual-fuel strategies and advanced ignition (e.g., turbulent jet ignition) as solutions.
Optical Study on the Effects of Wall and Intake Temperatures on Ammonia Combustion and Emissions
In an optical spark-ignition engine, raising wall temperature improved combustion stability and reduced N2O and NH3 slip, but increased NOx; intake temperature mainly affected early flame development.
Effects of using ammonia as a primary fuel on engine performance and emissions in an ammonia/biodiesel dual‐fuel CI engine
In a dual-fuel CI engine, ammonia replaced 69.4% of biodiesel energy, cutting CO2, CO, and HC, but increasing NO; combustion duration shortened by 19 crank angle degrees.
Future Ship Emission Scenarios with a Focus on Ammonia Fuel
A shipping scenario for 2050 with widespread ammonia use projected 40% CO2 reduction but only 22% CO2-equivalent reduction due to N2O; NH3 slip was estimated at 930 Gg in the North and Baltic Seas.
Impact of Post-Injection Strategies on Combustion and Emissions in a CTL–Ammonia Dual-Fuel Engine
In a CTL-ammonia dual-fuel engine, post-injection (8-12 mg at 10-15°CA) with 5-10% ammonia energy fraction reduced CO, NOx, and soot while improving combustion stability.
Overview of Autoignition and Flame Propagation Properties for Ammonia Combustion
A technical review of ammonia autoignition and flame propagation properties identifies poor ignition and slow flame speed as key challenges, and discusses enhancement strategies like hydrogen blending and detonation engines.