The burned-gas propagation process has been characterized in two bi-fuel engines by means of a combustion diagnostic tool resulting from the integration of an original multizone heat-release model with a CAD procedure for the burned-gas front geometry simulation.
Burned-gas mean expansion speed ub, mean gas speed ug and burning velocity Sb were computed as functions of crank angle and burned-gas radius for a wide range of engine speeds (n = 2000-5500 rpm), loads (bmep = 200-790 kPa), relative air-fuel ratios (RAFR = 0.80-1.60) and spark advances (SA ranging from 8 deg retard to 8 deg advance from MBT), under both gasoline and CNG operations. Finally, the influence of intake runner and combustion chamber geometries on flame propagation process was investigated.
Main results show that Sb is generally comparable for the engine running on both gasoline and CNG, at the same engine speed and load, under stoichiometric and MBT operations. In fact, higher temperatures and pressures of the unburned-gas ahead of the flame front under CNG fuelling compensate for natural gas lower laminar-burning speed SL at reference conditions. The tested intake runner sets showed to exert a minor effect on burned-gas propagation. On the contrary, combustion chamber shape and spark plug positioning strongly influenced combustion process. Finally, the ratio of Sb to SL was analyzed as a function of engine operating variables during the rapid-burning interval.