Hydrogen as a combustion enhancer for highly efficient ultra-lean spark-ignition engines
Résumé
Performance of lean burn gasoline spark-ignition engines can be enhanced through hydrogen supplementation. Thanks to its physicochemical properties, hydrogen supports the flame propagation and extends the dilution limits with improved combustion stability. These interesting features usually result in decreased emissions and improved efficiencies which is of the utmost importance for future SI engines targeting ultra-lean conditions at λ ≥ 2 and brake thermal efficiencies above 50%. Compared to previous studies of hydrogen supplementation, this article aims at demonstrating how hydrogen can support the combustion process with a modern combustion system optimized for extreme dilution rates and high efficiency. Experimental investigations performed with a single cylinder engine are reported and show that the minimal amount of hydrogen required to reach λ = 2 is in the range of 2 to 4% of the total intake volume flow rate. At low load, NOx emissions can be lowered down to 33 ppm at λ = 2 and results also show that a 10-fold decrease in NOx emissions is possible when the dilution rate increases from the lean limit without hydrogen up to λ = 2. In those ultra-lean conditions, particle emissions are also significantly lowered. Unburned energy is around 5% in low load conditions at λ = 2 but the engine-out unburned hydrocarbon concentration is maintained at an acceptable level. At high load, combustion timings can be improved thanks to the increase in the maximal dilution rate and to the better auto-ignition resistance of hydrogen. Consequently, the indicated efficiency is increased by more than 6% abs. compared to the reference stoichiometric conditions. Finally, a maximal indicated efficiency of 47.0% is obtained at λ = 2 with 3% of hydrogen at 3000 rpm-13 bar IMEP. For this operating point, similar performance are obtained with a dual air/EGR dilution.
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