What makes ammonia so hard to burn cleanly in engines?
Ammonia's core problem is that it's stubborn to ignite and slow to burn. It has a high auto-ignition temperature, a slow flame speed, and a narrow range of fuel-air mixtures that will actually catch fire [1]. This means that in a standard engine, pure ammonia often won't burn reliably, especially during cold starts or at high speeds. One study using a modern gasoline direct-injection engine found that to get stable operation across most conditions, they had to add at least 10% hydrogen (by volume) to the ammonia fuel [2]. That hydrogen acts as a 'combustion promoter,' helping the flame spread faster and more completely.
Researchers are testing several workarounds. Besides blending with hydrogen or other hydrocarbon fuels, advanced ignition systems like turbulent jet ignition, laser ignition, and low-temperature plasma ignition are being explored to force ammonia to burn [1]. A promising new approach is the 'intelligent liquid-gas twin-fluid co-injection system' (iTFI), which mixes ammonia and air more thoroughly before combustion to improve stability and efficiency [1]. However, these technologies are still in the lab or early prototype stage—there's no clear winner yet, and no large-scale engine data proving they work reliably over thousands of hours.
What are the real-world safety and supply-chain hurdles?
Ammonia is toxic and corrosive, which creates serious safety challenges that aren't fully solved. A 2024 SWOT analysis of ammonia as a marine fuel identified the need for stricter safety protocols, specialized crew training on handling and emergency response, and potential engine modifications for existing ships as major weaknesses [5]. The same analysis flagged that public acceptance of ammonia's safety is still an open question, and competing zero-carbon fuels (like methanol or hydrogen) could overtake it if these issues aren't resolved [5].
On the supply side, the biggest gap is that almost all ammonia today is 'grey'—made from natural gas with large CO2 emissions. A green ammonia supply chain, produced using renewable energy, barely exists at scale [5][6]. One economic study of an ammonia-fueled ammonia carrier ship found that installing separate fuel tanks (to avoid losing cargo space) was more profitable, but the business case was highly sensitive to fuel price: a 25% change in ammonia fuel price caused a 15% swing in profits [3]. This means the economics of ammonia as a fuel are fragile until green ammonia production becomes cheap and widespread.
Can ammonia really be 'clean' if it produces nitrogen oxides (NOx)?
Burning ammonia doesn't emit CO2, but it can produce significant amounts of nitrogen oxides (NOx)—a harmful air pollutant. The same engine study that showed stable operation with 10% hydrogen also found that adding exhaust gas recirculation (diluting the intake air) could cut NOx by up to 40% while keeping the engine running smoothly [2]. That's promising, but it's only one operating condition; we don't yet have comprehensive data on NOx emissions across the full range of engine speeds, loads, and fuel blends.
Another layer of uncertainty is how ammonia sprays behave inside the engine. A 2026 numerical study of liquid ammonia sprays under 'flash-boiling' conditions (where the fuel rapidly vaporizes) found that droplet temperatures could drop to about 235 K (-38°C) within a few millimeters of the injector [4]. This extreme cooling affects how the fuel mixes with air and burns, and current computer models struggle to predict it accurately. The study noted that experimental validation is still ongoing [4], meaning we don't yet have reliable models to design engines that minimize both NOx and unburned ammonia.
About These Sources
This answer is built on 6 peer-reviewed studies — published from 2021 to 2026, 3 from 2024 or later, 2 in Q1 journals, collectively cited 372 times — selected as the most relevant from 8 studies that passed quality screening, drawn from 59 papers retrieved from a database of over 500 million.
Sources used in this answer
Ammonia as a sustainable fuel: Review and novel strategies
This 2024 review identifies ammonia's high auto-ignition temperature, slow flame speed, and narrow flammability range as key combustion barriers, and surveys strategies like hydrogen blending, advanced ignition (plasma, laser, turbulent jet), and the novel iTFI injection system to overcome them.
Operating Limits for Ammonia Fuel Spark-Ignition Engine
In a 2021 experimental study on a modern gasoline direct-injection engine, stable operation with ammonia required less than 10% hydrogen (by volume) as a combustion promoter; adding exhaust gas dilution reduced NOx by up to 40% while maintaining stability.
Economic Evaluation of an Ammonia-Fueled Ammonia Carrier Depending on Methods of Ammonia Fuel Storage
A 2021 economic analysis of an ammonia-fueled ammonia carrier (84,000 m³ VLGC) found that installing separate fuel tanks (avoiding cargo loss) yielded $53.1 million higher profit than using cargo tanks for fuel; profits were highly sensitive to ammonia fuel price (±25% price change caused ±15% profit change).
Evaluation of Anisotropic Turbulence Models for Flash-Boiling Ammonia Sprays for Clean Fuel and Conceptual Electric Vehicle Cooling Systems
A 2026 numerical study of liquid ammonia sprays under flash-boiling conditions showed droplet temperatures dropping to ~235 K within millimeters of the nozzle; the V2F turbulence model best balanced accuracy and efficiency, but experimental validation is still ongoing.
Ammonia: a clean fuel for a cleaner future
A 2024 SWOT analysis of ammonia as a marine fuel highlights zero CO2 emissions and higher energy density than hydrogen as strengths, but identifies reliance on fossil fuels for production, need for safety protocols and crew training, and lack of green supply chain as key weaknesses.
Pathways to a Green Ammonia Future
A 2022 perspective on green ammonia pathways emphasizes that current ammonia production is fossil-fuel-based and that a global green ammonia supply chain using renewable energy is needed but not yet established.