Bottled gas was used to simulate the output of an onboard reformer (21% H₂, 24% CO, 55% N₂). Measurements were carried out on a 4- stroke, 2-cylinder, 0.5-liter engine, with EGR, in order to calculate the heat release rate through a detailed two-zone model.
A quasi-dimensional model of the flame was developed: it consists of a geometrical estimate of the flame surface, which is then coupled with the heat release rate. The turbulent flame speed can then be inferred. The model was then applied to blends of gasoline with hydrogen-rich gas, showing the effect on the flame speed and transition from laminar to turbulent combustion.
Comparison between the quasi-dimensional model and the conventional Metgalchi-Keck + Damköhler model gave a general validation for gasoline operation and suggested a modification of the usual time-delay function for transition from laminar to turbulent flame.
Results give new insight in previous findings from the heat release calculation: the effect of hydrogen-rich gas addition on flame speed is predominant in the early phase of the flame propagation, and the effect of the high curvature of the flame at the onset of combustion, compensated by the high mass diffusivity of hydrogen, is believed to be the physical reason to such behavior.