Examining trade-offs between NOx emissions and hydrogen slip for hydrogen combustion engines

Hydrogen combustion engines could provide a timely, robust and cost effective way to decarbonise hard to electrify transport applications like heavy duty trucks. However, near zero harmful emissions and vehicle range, and therefore high engine efficiency, are key to market and legislative acceptance. This paper will present the results of new experimental work undertaken in a collaboration between Ri-cardo and University of Brighton to examine hydrogen combustion in a single cylinder engine repre-sentative of Euro VI heavy duty hardware with medium pressure hydrogen direct injection. Test work has been carried out to investigate the performance of measures to reduce NOx (EGR and lean com-bustion) to understand more deeply trade-offs between NOx emissions and efficiency. Research to date has shown that lean conditions can result in near zero NOx emissions and measurement of hy-drogen slip at these conditions is needed to understand whether hydrogen slip is increased. In this work, hydrogen exhaust emissions are measured using a newly developed analyser from HORIBA which uses mass spectrometer technology to measure concentrations of hydrogen up to 5% in exhaust gas. Hydrogen emissions data measured using this analyser shows that hydrogen slip increases with air fuel ratio and EGR rate, with a clear trade-off between NOx emissions and slip. Indicated efficiency over the test conditions investigated was relatively constant, deteriorating as high EGR rates led to less stable combustion.