Assessment of Dilution Options on a Hydrogen Internal Combustion Engine 2023-24-0066

The hydrogen internal combustion engine is a promising alternative to fossil fuel-based engines, which, in a short time, can reduce the carbon footprint of the ground transport sector. However, the high heat release rates associated with hydrogen combustion results in higher NOx emissions. The NOx production can be mitigated by diluting the in-cylinder mixture with air, Exhaust Gas Recirculation (EGR) or water injected in the intake manifold. This study aims at assessing these dilution options on the emissions, efficiency, combustion performance and boosting effort. These dilution modes are, at first, compared on a single cylinder engine (SCE) with direct injection of hydrogen in steady state conditions. Air and EGR dilutions are then evaluated on a corresponding 4-cylinder engine by 0D simulation on a complete map under NOx emission constraint.

On the SCE at 3000rpm and 10.7bar IMEP, air and EGR dilutions allow a high dilution rate, leading to a significant NOx reduction: from 2.8g/kWh to less than 0.05g/kWh. The indicated efficiency goes through a maximum of about 45.5% with NOx emissions around 0.4g/kWh without affecting the lubricant consumption, calculated from carbon-bearing gas emissions. The water injection doesn’t affect the efficiency but reduces NOx emissions moderately without inducing excessive lubricant consumption. For the three dilution options, a common guide curve of NOx emissions versus a thermal dilution rate, based on the fuel chemical energy, diluent heat capacity and water vaporization latent heat, is presented. The best indicated efficiency for a tolerable NOx level of 0.4g/kWh is achieved for lambda of 2.4 with air dilution path and EGR rate of 10% at lambda of 2. After these dilution tests involving hydrogen and possibly high content of water in the chamber, only a thin layer of corrosion is observed on the iron cast liner without any structural or mechanical damage. The 0D simulation shows the interesting combination of air and EGR dilution allowing higher load under NOx emission constraint. EGR gases containing water with high heat capacity allows to reduce the quantity of diluent at high load, therefore the filling work and turbocharger effort.