Academic Spotlight: Investigating Hydrogen-Diesel Dual-Fuel Engines

Co-Author:
Annabelle Evans Undergraduate Researcher, University of New South Wales

Hydrogen is a promising alternative fuel for internal combustion engines. It is a carbon-free energy carrier and has the potential to be produced renewably. In addition, hydrogen has a superior energy density compared to batteries, making it an attractive option for heavy-duty applications.

However, if you just put hydrogen into a conventional compression ignition engine, you’re going to run into trouble. Hydrogen has a higher autoignition temperature than diesel, so it’s difficult to ignite hydrogen by compression alone. My research group is working on an engine that will use a little bit of diesel, which will ignite under compression, to act a bit like a match for the hydrogen.

Hydrogen-diesel dual-fuel engines are capable of being significantly cleaner and more efficient than traditional diesel engines, but careful consideration must go into their design. Hydrogen has a higher adiabatic flame temperature than diesel, which can lead to higher NOx emissions, and extreme temperatures can cause substantial heat losses, reducing the efficiency of the engine.

In order to design an optimal hydrogen-diesel dual-fuel engine, you need to understand the mechanisms driving the NOx emissions and heat losses. I set out to investigate these phenomena using computational fluid dynamics simulations. Simulations are cheaper and easier to run than experiments, and they give you more data than you can get experimentally. With my CONVERGE simulations, I could track the temperature, pressure, and mixture composition at many locations inside the engine.

I used CONVERGE’s detailed chemistry solver and high-fidelity emissions models to simulate a dual-fuel engine with different combinations of hydrogen and diesel [1]. I varied the hydrogen fraction from 0% to 90% and assessed the NOx emissions and heat losses for each condition. In my simulations, I assumed the hydrogen was evenly mixed with the air inside the cylinder when the diesel was injected at the top of the compression stroke.

I found that NOx emissions varied with hydrogen fraction, as shown in Figure 1. After initially increasing, NOx emissions began to decrease as the hydrogen fraction grew to more than 50%.

We believe the reason that NOx emissions rise initially is because hydrogen’s high flame temperature increases the combustion temperature, which leads to more NOx formation. When you reach a certain level of hydrogen addition, however, you see the fuel and air mixing more evenly before combustion, which reduces the NOx emissions.

Our research group is also running experiments on the dual-fuel hydrogen engine concept, but they have not yet tested hydrogen fractions above 50%. However, these simulation results are a promising indicator that future experiments may show a decrease in NOx emissions with greater hydrogen fractions.

In terms of heat losses, I found that there were three main contributing factors: combustion phasing, the equivalence ratio near the cylinder walls, and turbulence. The heat transfer was concentrated primarily in a specific region around the edge of the piston, as you can see in Figure 2. Much of the injected diesel ends up in these regions, which causes high temperatures. In addition, the narrow “squish zone” above the piston rim generates turbulence, which promotes heat transfer.