Combined experimental-numerical analysis of hydrogen as a combustion enhancer applied to wankel engine

Abstract

Hydrogen is regarded as one of the most potential alternatives for high-efficiency and eco-friendly combustion. The influences of hydrogen fractions and equivalence ratios on chamber pressure, combustion phases, wall heat loss, brake thermal efficiency, and cyclic fluctuation of Wankel engines were studied by testbench measurement. The intrinsic mechanisms of these effects were analyzed through numerical investigation. Results indicated that increasing hydrogen fraction increased the mass fraction of high-temperature regions, the timings taken for formations of intermediate products were advanced, and the peak H2O2 and CH2O reduced while OH concentration increased. These factors were responsible for increased chamber pressure and the shortened flame development and propagation periods in experiments. A close inverse relationship can be drawn between the wall heat loss and brake thermal efficiency with respect to hydrogen addition. A larger hydrogen fraction coupled with a smaller equivalence ratio manifested lower wall heat loss and higher thermal efficiency. Compared with hydrogen-free regimes, brake thermal efficiency of 6% hydrogen fraction was increased by 64.6%. There was a positive proportional correspondence between the initial flame development and cycle-to-cycle fluctuation. Increasing the hydrogen content caused the enhanced turbulent intensity during the initial combustion stage, which contributed to the stability improvement of engine operations.