Experimental and Numerical Investigation of the Engine Operational Conditions’ Influences on a Small Un-Scavenged Pre-Chamber’s Behavior

Despite significant benefits in terms of the ignition enhancement, the strength and timing of the turbulent flame jets subsequently issuing into the main chamber strongly depend on the pre-chamber combustion process and, thus, are sensitive to the specific engine operating conditions it experienced. This poses considerable difficulties in optimizing engine operating conditions as well as controlling engine performance. This paper investigates the influence of engine operating conditions on the pre-chamber combustion event using both experimental and numerical methods. A miniaturized piezo-electric pressure transducer was designed to be placed inside the engine cylinder head to record the pre-chamber inner volume pressure, in addition to conventional pressure indication inside the main chamber. The pressure difference between the main chamber and the pre-chamber (Δp = p_pre-chamber - p_{main chamber}) served as an indicator of the pre-chamber combustion event, through the study of the crank angle resolved Δp under different engine operating conditions. The variations include spark timing, air-fuel ratio and engine speed, as well as engine intake air temperature and boost pressure, for which a single-parameter sensitivity approach was carried out. 3D-CFD simulation in the RANS context and 0D modeling was further employed to estimate the turbulence level and thermo-physical conditions inside the pre- and main chambers in dependence of the engine operating conditions. The results indicate that the Δp peak value and its timing show the highest sensitivity to the air-fuel ratio variation, followed by the intake air temperature, while the sensitivity of Δp peak value and its timing to the pressure difference and the absolute pressure level at spark timing were observed to be considerably lower. Based on the observations, two correlations were proposed to predict the Δp peak value and its timing for which successful validation against a large number of engine operating conditions was demonstrated.