A combined experimental and numerical study of thermal processes, performance and nitric oxide emissions in a hydrogen-fueled spark-ignition engine

This work concerns the study of a spark-ignition engine fueled with hydrogen, using both measured and numerical data at various conditions, focusing on the combustion efficiency, the heat transfer phenomena and heat loss to the cylinder walls, the performance, as well as the nitric oxide (NO) emissions formed, when the fuel/air and compression ratio are varied. For the investigation of the heat transfer mechanism, the local wall temperatures and heat flux rates were measured at three locations of the cylinder liner in a CFR engine. These fluxes can provide a reliable estimation of the total heat loss through the cylinder walls and of the hydrogen flame arrival at specific locations. Together with the experimental analysis, the numerical results obtained from a validated in-house CFD code were utilized for gaining a more complete view of the heat transfer mechanism and the hydrogen combustion efficiency for the various cases examined. The performance of the CFR engine is then identified, since the calculated cylinder pressures are compared with the measured ones, from which performance and heat release rates are calculated and discussed. Further, NO emission studies have been accomplished, with the calculated results not only being compared with the measured exhaust NO ones, but also further processed for conducting an in-depth investigation of the dependence of NO production on the spatial distribution of in-cylinder gas temperature. It is revealed that for lower fuel/air ratio the burned gas temperature is held at low level and the heat loss ratio is quite low. As the load increases and stoichiometric mixtures are used, the wall and in-cylinder gas temperatures increase substantially, together with the heat loss and the NO emissions, owing to the high hydrogen combustion velocity and the consequent high rate of temperature rise. The combustion efficiency is slightly increased, but the indicated efficiency is decreased due to higher heat loss.