The combustion propagation and burned-gas expansion processes in a bi-fuel CNG SI engine were characterized by applying a newly developed diagnostic tool, in order to better understand how these processes are related to the fuel composition, to the engine operating variables as well as to the exhaust emissions. The diagnostic tool is based on an original multizone heat-release model that is coupled with a CADmodel of the burned-gas containing surface for the computation of the burning speed and the burned-gas mean expansion velocity. Furthermore, the thermal and prompt NO sub-models, embedded in the diagnostic code, were employed to study the effects ofNO formation mechanisms and thermodynamic parameters on nitric oxide emissions.
A previously developed multivalve bi-fuel SI engine has been upgraded to take a wide experimental database of in-cylinder pressure time-histories, engine performance and pollutant emissions throughout an extended air-fuel ratio interval for both fuels, i.e., gasoline and CNG (up to the lean-combustion stability limit) and an enlarged speed range for CNG (up to 6500 rpm). In particular, the CNG pressure reducer upstream of the injection system was replaced to deliver the gaseous fuel at higher pressures and the CNG injectors were replaced with new injectors purposely designed and realized with larger flow nozzle areas. Experimental tests have thus been carried out in a broad interval of 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 with respect to MBT timing).
One of the main findings was that the ratio between the burning speed Sb and the laminar flame speed SL, at the point of the engine cycle where Sb peaks, scales with n. The scale factors were worked out for both gasoline and CNGoperations. The effects of operating engine variables on flame propagation parameters were analyzed.