Mixing
possible
IAV has investigated the potential of hydrogen combustion engines with low-pressure direct injection and presented the results at the Aachen Colloquium for Sustainable Mobility. Key to the standard use of the technology is homogenization of the mixture.
Christian Buck, October 2022
In addition to purely battery-electric vehicles (BEV), the hydrogen drive also offers a possible alternative to the combustion of fossil fuels. As an alternative to BEV, fuel cell vehicles (FCEV) are often at the center of the discussion, but the hydrogen-powered combustion engine (H2-ICE) should not be overlooked either – it can score points, at least in the short to medium term, with a longer service life, greater robustness against poor fuel quality and lower costs compared to fuel cell.
“Especially in the light and heavy commercial vehicle sector, the hydrogen combustion engine as a bridge technology enables CO2-free mobility,” says Marc Sens, Senior Vice President Powertrain Research and Technology at IAV.
40 percent more power density
IAV has therefore investigated the potential of hydrogen combustion engines with low-pressure direct injection (LP-DI) – and thus again looked at a problem that has not yet been solved: Previous hydrogen combustion engines rely on intake manifold injection (PFI), which, however, has a poor power density. Direct injection does not have this problem: It delivers around 40 percent more power per volume compared to PFI. It also avoids the risk of reignition in the inlet duct. “Thanks to direct injection, we are currently experiencing a real renaissance of the hydrogen combustion engine,” says Maximilian Brauer, Senior Technical Consultant Powertrain Calibration and Technology at IAV.
The low-pressure direct injection offers a high full load potential, but also requires a sophisticated mixture formation. To achieve progress in this area, IAV experts have visualized direct injection using the Schlieren technique on the one hand and have also carried out extensive CFD simulations on the behavior of the hydrogen in the combustion chamber on the other.
Among other things, different Z positions of the injection nozzle were investigated. If it is flush with the cylinder head, the hydrogen is partially deposited at the top edge of the combustion chamber (Coandă effect), which leads to a bad mixture formation. Even a slightly recessed nozzle is not optimal, because this variant results in a low spray angle and an unfavorable spatial spray distribution. If, on the other hand, the nozzle protrudes slightly into the combustion chamber, the result is a better mixing ratio.
New nozzle geometries required
Another important finding is that conventional nozzle geometries are not able to homogenize the mixture sufficiently. Hydrogen mixes with air significantly less than liquid fuels, which penetrate the combustion chamber charge during evaporation and have a high local homogenization level. Instead, hydrogen injection tends to displace the charge and a large part of the hydrogen cannot mix sufficiently with charge air. As a countermeasure, a jet cap over the injection valve is recommended, which has several holes and is thus able to spread hydrogen better in the combustion chamber. In preliminary studies, caps with up to four (experimental) or up to 12 (simulative) holes were investigated.
With a 3D CFD simulation, IAV experts finally calculated the homogeneity of the mixture in the engine, with a jet cap with four holes providing the best results. Overall, it was found that the homogeneity depends very much on the position of the injector, the cap design and the twist level. “The homogenization of the mixture is the key to a successful development to production readiness,” says Dr. Jochen Maass, Technical Consultant Fuel Systems at IAV. In this way, the raw nitrogen oxide emissions and the tendency to pinking can be noticeably reduced. This is why the tech solution provider continues to work to achieve its own homogeneity goals.
Our experts on the topic
Marc Sens
marc.sens@iav.de
Maximilian Brauer
maximilian.brauer@iav.de
Jochen Maass
jochen.maass@iav.de