Dual Fuel Engine, CFD simulations of combustion and emissions in a diesel-ammonia dual fuel engine

Abstract

This study was conducted to better understand the complex behavior of dual-fuel diesel-ammonia combustion in compression ignition engines, with the aim of developing a simulation model that could replicate experimental results. The work specifically explores how changes in modeling parameters—such as swirl intensity and initial temperature—affect fuel-air mixing, ignition behavior, and emissions formation, in order to calibrate the model against experimental observations.

To investigate this, computational fluid dynamics simulations were carried out using the open-source software OpenFOAM. The simulations included both the gas exchange and the closed-cycle combustion phases, using separate geometries for each part. The gas exchange simulations focused on how in-cylinder flow conditions vary at different crank angles, while the combustion simulations looked at how spray behavior influences ignition timing and pollutant formation.

The results reveal that using simplified assumptions for initial in-cylinder flow—such as modeling it as solid-body rotation—does not accurately capture the real flow behavior during early spray injection, especially when ammonia is introduced well before top dead center. The interaction between flow structures and the fuel spray depends heavily on initial conditions, leading to different mixing patterns that significantly impact the efficiency and completeness of ammonia combustion. Swirl strength was found to play a major role in determining how diesel and ammonia interact and distribute in the cylinder, with clear effects on the amount of unburned fuel and emissions produced.

Further analysis of combustion products showed that nitrogen-based pollutants are closely tied to how ammonia decomposes and how the flame interacts with the fuel-air mixture. Although the general sequence of combustion was similar across different cases, the quality of fuel mixing and the emissions profiles varied noticeably. Comparisons with experimental data showed partial agreement, but also pointed out limitations in the current modeling strategies.

Overall, the findings highlight the importance of accurately capturing flow-spray interactions and initial in-cylinder conditions in dual-fuel simulations. Future studies should incorporate results from gas exchange simulations directly into the combustion phase and seek better experimental validation using time- and space-resolved measurements of critical species.

Popular Abstract

How Computers Help Us Tackle Emissions Challenges in Engines

Even as electric vehicles gain attention, internal combustion engines remain a crucial source of power worldwide. These engines continue to supply a significant portion of the world’s energy and contribute notably to global emissions. While electrification is an important part of the future, internal combustion engines will remain essential for many years to come—especially in heavy-duty transportation, industrial applications, and regions where affordable, reliable energy is needed. Rather than abandoning this technology, it is vital to improve and adapt it to be cleaner, and this is where computer simulations play a key role.

Modern computational tools allow engineers to explore the complex processes inside an engine with unprecedented detail. Combustion, the chemical reaction that generates power, involves turbulent flows, rapid heat release, and complex chemical reactions. These phenomena are difficult to measure directly in experiments, making simulations a powerful alternative. Computational fluid dynamics uses mathematical models and powerful computers to simulate how fuel mixes, ignites, and burns inside the engine. This helps researchers understand how different fuels, engine designs, and operating conditions impact efficiency and emissions without costly and time-consuming physical tests.

One exciting area of research is the use of ammonia as an alternative fuel. Ammonia contains no carbon atoms, so it does not produce carbon dioxide when burned, offering great potential for reducing greenhouse gases. It can be produced sustainably using renewable energy sources, contributing to a cleaner energy future. However, ammonia presents several challenges, such as low reactivity, a high ignition delay, and the formation of nitrogen-based pollutants that require careful control. Diesel fuel, on the other hand, is well known for its easy ignition properties, which makes it an ideal pilot fuel to initiate combustion in dual-fuel systems that include ammonia. This ease of ignition helps overcome ammonia’s reluctance to combust by effectively triggering the combustion process and enabling the sustained burning of ammonia.

By blending ammonia with diesel in dual-fuel engines, researchers seek to combine the advantages of both fuels. Simulations are essential to optimize these systems, helping to design better fuel injection strategies and combustion chambers that improve efficiency and reduce emissions. The interaction between diesel and ammonia is critical: the diesel spray ignites readily and produces the necessary heat to support ammonia combustion, which otherwise suffers from high ignition thresholds. This complex interplay influences flame stability, emission formation, and overall engine performance, making it a key focus in the development of cleaner combustion technologies.

It is important to understand that computer simulations are approximations. The fundamental equations governing fluid flow and combustion, developed over many years, cannot yet be solved analytically. Instead, numerical methods and powerful computing are used to approximate solutions. Turbulence, the chaotic swirling motion of fluids that strongly influences combustion, remains one of the most challenging phenomena to model accurately. Scientists continuously work to improve physical models and validate simulations against experiments to ensure realistic predictions.

The simulation results reveal differences in combustion behavior influenced by initial temperature and swirl number. Higher swirl promotes wider ammonia dispersion and better mixing with diesel, while lower swirl causes ammonia to accumulate near the cylinder walls. Elevated initial temperatures accelerate evaporation and chemical reactions, reducing ignition delay. Species concentration trends show that nitric oxide as the predominant nitrogen oxide species. Diesel combustion appears more efficient under low swirl, and higher temperatures promote faster reaction rates.

These findings highlight the critical role of diesel’s rapid ignition in initiating combustion and ammonia’s distribution in sustaining it. Understanding the interaction between these fuels is essential for advancing dual-fuel engine development. Combining simulations with experimental data deepens knowledge of combustion processes, supporting the design of more efficient, cleaner engines capable of using alternative fuels like ammonia. Computational tools remain central to tackling environmental challenges and advancing sustainable transportation.

• author: Lindberg, Ludvig
• supervisor: Fatehi Hesameddin
• organization: Department of Energy Sciences
• course: MVKM01 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Dual-fuel combustion, Computational fluid dynamics (CFD), OpenFOAM, Gas exchange simulation, Closed-cycle simulation
• report number: ISRN LUTMDN/TMPH-25/5634-SE
• ISSN: 0282-1990
• language: English
• id: 9200473
• date added to LUP: 2025-06-17 08:28:53
• date last changed: 2025-06-17 08:28:53