Numerical Investigation of Combustion Mechanisms and Emission Trade-Offs in a Two-Stroke Direct-Injection Ammonia/Diesel Dual-Fuel Marine Engine
(2026) MVKM05 20261Department of Energy Sciences
- Abstract
- This study investigates the combustion and emission characteristics of a two-stroke, direct-injection ammonia/diesel dual-fuel compression-ignition engine under low-load conditions relevant to large marine engines. Although ammonia is carbon-free and can reduce direct CO2 emissions, its low reactivity, strong evaporation cooling, and nitrogen chemistry may cause incomplete combustion, NH3 slip, and N2O formation. A closed-cycle CFD framework was therefore applied using an Eulerian–Lagrangian spray formulation, a modified gRNG k-ε turbulence model, detailed spray sub-models, and a 69-species/389-reaction ammonia/n-heptane kinetic mechanism. One diesel-only case and three ammonia/diesel cases with ammonia injection durations of 3 ms, 4 ms,... (More)
- This study investigates the combustion and emission characteristics of a two-stroke, direct-injection ammonia/diesel dual-fuel compression-ignition engine under low-load conditions relevant to large marine engines. Although ammonia is carbon-free and can reduce direct CO2 emissions, its low reactivity, strong evaporation cooling, and nitrogen chemistry may cause incomplete combustion, NH3 slip, and N2O formation. A closed-cycle CFD framework was therefore applied using an Eulerian–Lagrangian spray formulation, a modified gRNG k-ε turbulence model, detailed spray sub-models, and a 69-species/389-reaction ammonia/n-heptane kinetic mechanism. One diesel-only case and three ammonia/diesel cases with ammonia injection durations of 3 ms, 4 ms, and 5 ms were simulated, corresponding to ammonia energy ratios up to 49.4%. The model was assessed against experimental data and reference simulations, followed by analyses of grid independence, engine performance, emissions, combustion processes, and swirl ratio sensitivity.
The simulations reproduce the main combustion and emission trends with acceptable accuracy. Increasing ammonia injection duration and ammonia energy ratio increases the second-stage heat release from ammonia oxidation and raises indicated mean effective pressure from 1.84 bar to 3.70 bar. Carbon-related emissions decrease per energy input, but incomplete ammonia oxidation increases NH3 slip and N2O emission, causing total greenhouse-gas-equivalent emissions to rise from 79.0 g/MJ to 192.9 g/MJ. In-cylinder analysis shows that ammonia combustion depends on its engulfment by the diesel flame, while colder or weakly connected hot regions promote unburned ammonia and N2O. The swirl ratio analysis indicates that moderate swirl improves spray–flame interaction, whereas excessive swirl over-disperses the fuel, resulting in incomplete combustion. Among the tested cases, a swirl ratio of 15 gives the best overall emission balance. This study concludes that cleaner ammonia/diesel operation requires not only a higher ammonia energy ratio, but careful control of spray interaction, turbulent mixing, ignition structure, and nitrogen chemistry. (Less) - Popular Abstract
- Large ships need powerful engines, but they also need cleaner fuels. Ammonia is considered promising because it contains no carbon and can reduce direct CO2 emissions. However, ammonia is difficult to burn: it ignites poorly, burns slowly, cools the surrounding gas during evaporation, and may form harmful emissions if combustion is incomplete. This study looks into a two-stroke marine engine where diesel and liquid ammonia are directly injected into the cylinder, asking whether replacing part of the diesel with ammonia truly makes the engine cleaner.
The answer is not simply yes. Adding ammonia increases engine load and reduces carbon-based emissions such as CO and CO2 for a fixed engine power output. However, more ammonia also makes... (More) - Large ships need powerful engines, but they also need cleaner fuels. Ammonia is considered promising because it contains no carbon and can reduce direct CO2 emissions. However, ammonia is difficult to burn: it ignites poorly, burns slowly, cools the surrounding gas during evaporation, and may form harmful emissions if combustion is incomplete. This study looks into a two-stroke marine engine where diesel and liquid ammonia are directly injected into the cylinder, asking whether replacing part of the diesel with ammonia truly makes the engine cleaner.
The answer is not simply yes. Adding ammonia increases engine load and reduces carbon-based emissions such as CO and CO2 for a fixed engine power output. However, more ammonia also makes complete combustion harder. Some ammonia remains unburned as ammonia slip, and some forms nitrous oxide, i.e., N2O, a much stronger greenhouse gas than CO2. In the ammonia/diesel cases, total greenhouse-gas-equivalent emissions increase even though direct carbon emissions decrease. This means that a carbon-free fuel does not automatically lead to a climate-friendly engine.
The simulations show that diesel works as the ignition source for ammonia. Ammonia burns effectively only when its spray meets the hot diesel flame in the right region and at the right time. If ammonia enters colder parts of the cylinder or misses the most reactive flame zone, it burns incompletely, producing ammonia slip and N2O. This study also shows that air swirl inside the cylinder must be carefully controlled. Moderate swirl improves mixing and helps ammonia burn, while excessive swirl spreads the fuel into colder regions and worsens combustion. Among the tested cases, a swirl ratio of 15 gives the best balance.
Overall, ammonia can help decarbonize marine engines, but the path forward is not simply to inject more ammonia, while it is to make engine design ensure more complete ammonia combustion so that the whole system will become cleaner in reality, not just on paper. (Less)
Please use this url to cite or link to this publication:
https://lup.lub.lu.se/student-papers/record/9238047
- author
- Qiu, Zichen LU
- supervisor
- organization
- course
- MVKM05 20261
- year
- 2026
- type
- H2 - Master's Degree (Two Years)
- subject
- keywords
- Marine engine, Ammonia/diesel dual fuel, Direct dual-fuel stratification, Computational fluid dynamics, Greenhouse-gas emissions, Ammonia slip, Swirl ratio
- report number
- ISRN LUTMDN/TMHP-26/5692-SE
- ISSN
- 0282-1990
- language
- English
- id
- 9238047
- date added to LUP
- 2026-06-16 13:47:58
- date last changed
- 2026-06-16 13:47:58
@misc{9238047,
abstract = {{This study investigates the combustion and emission characteristics of a two-stroke, direct-injection ammonia/diesel dual-fuel compression-ignition engine under low-load conditions relevant to large marine engines. Although ammonia is carbon-free and can reduce direct CO2 emissions, its low reactivity, strong evaporation cooling, and nitrogen chemistry may cause incomplete combustion, NH3 slip, and N2O formation. A closed-cycle CFD framework was therefore applied using an Eulerian–Lagrangian spray formulation, a modified gRNG k-ε turbulence model, detailed spray sub-models, and a 69-species/389-reaction ammonia/n-heptane kinetic mechanism. One diesel-only case and three ammonia/diesel cases with ammonia injection durations of 3 ms, 4 ms, and 5 ms were simulated, corresponding to ammonia energy ratios up to 49.4%. The model was assessed against experimental data and reference simulations, followed by analyses of grid independence, engine performance, emissions, combustion processes, and swirl ratio sensitivity.
The simulations reproduce the main combustion and emission trends with acceptable accuracy. Increasing ammonia injection duration and ammonia energy ratio increases the second-stage heat release from ammonia oxidation and raises indicated mean effective pressure from 1.84 bar to 3.70 bar. Carbon-related emissions decrease per energy input, but incomplete ammonia oxidation increases NH3 slip and N2O emission, causing total greenhouse-gas-equivalent emissions to rise from 79.0 g/MJ to 192.9 g/MJ. In-cylinder analysis shows that ammonia combustion depends on its engulfment by the diesel flame, while colder or weakly connected hot regions promote unburned ammonia and N2O. The swirl ratio analysis indicates that moderate swirl improves spray–flame interaction, whereas excessive swirl over-disperses the fuel, resulting in incomplete combustion. Among the tested cases, a swirl ratio of 15 gives the best overall emission balance. This study concludes that cleaner ammonia/diesel operation requires not only a higher ammonia energy ratio, but careful control of spray interaction, turbulent mixing, ignition structure, and nitrogen chemistry.}},
author = {{Qiu, Zichen}},
issn = {{0282-1990}},
language = {{eng}},
note = {{Student Paper}},
title = {{Numerical Investigation of Combustion Mechanisms and Emission Trade-Offs in a Two-Stroke Direct-Injection Ammonia/Diesel Dual-Fuel Marine Engine}},
year = {{2026}},
}