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Exhaust gas recirculation (EGR) and ignition timing have strong effects on engine performance and exhaust emissions. In the present study, detailed chemical reactions with 29 steps of hydrogen oxidation with additional nitrogen oxidation reactions were coupled with an advanced CFD code to investigate the engine performance and emission characteristics of a SI engine fueled with hydrogen. The NOx formation within the engine was computed using the extended Zeldovich mechanism with parameters adjusted for a carbon-free fuel such as hydrogen. The computational results were validated against experimental results with equivalence ratio of 0.84 and fixed ignition timing at crank angle of 5° BTDC (before top dead center). The simulations were then employed to examine the effects of EGR and ignition timing on the engine performance and NOx formation and emission. The EGR ratio was varied between 5% and 15% while the ignition timings considered were 5°, 10°, 15°, and 20° BTDC. It was found that NOx emission increased with advancing the ignition timing away from TDC while the indicated engine power showed an increasing trend with further advancing the ignition timing. Higher indicated mean effective pressure (IMEP) and indicated thermal efficiency were obtained with an advanced ignition timing of 20° BTDC. The model was also run with three different EGR ratios of 5%, 10% and 15% with fixed ignition timing at 5° BTDC. The simulation results quantified the reduction in NOx and the indicated engine power with the increase in the EGR ratio. The computations were consistent with the hypothesis that the combustion duration increases with the EGR ratio. Finally, the maximization of engine power and minimization of NOx emissions were considered as conflicting objectives. The different data points were plotted in the objective space. Using the concept of “knee”, (5° BTDC, 0% EGR) was selected as the optimal operating point representing the best trade-off between maximum engine power and minimum NOx emissions.

During the past decade, considerable effort has been made to introduce alternative energy sources for use in conventional diesel and gasoline engines. Many researchers have attempted to use hydrogen as a fuel in the diesel engine due to its ability to reduce pollutant emissions, such as carbon monoxide and unburned hydrocarbons. With the rapid increase in computational capabilities, 3D computational fluid dynamics CFD codes become essential tools for practical design, control and optimization of hydrogen engines. In the present study, detailed chemical kinetic reactions with twenty steps of hydrogen oxidation with additional nitrogen oxidation reactions were coupled with AVL FIRE® code to study combustion processes in a diesel engine using hydrogen as the fuel. Moreover, a spark ignition model built by C++ program was incorporated into the AVL FIRE® software to simulate the hydrogen ignition behavior. The model was validated by the experimental results and employed to examine important parameters that have significant effects on the engine performance. The simulation results show that the variations of peak in-cylinder pressure, heat release rate, gas cylinder temperature, ignition delay, combustion duration, and NO emissions reasonably agree with the experimental findings. The exhaust gas recirculation (EGR) was also employed at different levels in the engine model. It was found that both peak cylinder pressure and gas cylinder temperature decrease as EGR level increases due to dilution effect. The computations are consistent with the hypothesis that gas cylinder temperature decreases with increasing EGR level and that the decrease in gas cylinder temperature results in the reduction in NO emissions.

Considerable efforts have been made to introduce alternative fuels for use in conventional diesel and gasoline engines. There is significant interest in adding hydrogen to a diesel engine to reduce emissions and improve efficiency. However, the main challenge associated with the use of hydrogen in diesel engines is high nitrogen oxide (NOX) emissions. In the present study, a reduced chemical kinetics mechanism, consisting of 52 reactions and 29 chemical species for n-heptane fuel combustion, was incorporated with detailed chemical kinetics consisting of 29 reactions for hydrogen as well as additional nitrogen oxidation. This reaction mechanism was coupled with 3-D advanced CFD software to investigate the performance and emission characteristics of a diesel-hydrogen dual-fuel engine. Computational results showed good agreements with the experimental results for brake thermal efficiency, CO2, CO, and NOX emissions. The model was then employed to examine the effects of exhaust gas recirculation (EGR) and N2 dilution on NOX emissions. The computational results quantified the reduction in NOX emissions with EGR and N2 dilution, and a more remarkable reduction was found with 30% N2 dilution. However, in terms of the N2 dilution, a general decreasing trend was observed for both NOX and CO2 emissions, while CO emissions increased. In relation to the EGR, the NOX emissions decreased while CO2 and CO emissions significantly increased. Additionally, the results showed that the indicated mean effective pressure (IMEP) and indicated power decreased as the N2 dilution increased. The same trend was observed for the EGR but the reduction was less compared to that of the N2 dilution.