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To obtain a better understanding of the effect of torch nozzle area on the turbulent flame propagation in a spark ignition engine with an unscavenged prechamber, the combustion characteristics are analyzed from ensemble-averaged pressure diagrams, and the turbulent flame propagation in both the pre- and main chambers are observed using a high-speed camera with image-processing. Also, a numerical simulation is attempted by applying a k-ε model of turbulence to the two-dimensional unsteady flow field and an one-step irreversible reaction model to the combustion process. These engine experiments and numerical calculations are performed under the same operating conditions, that is, optimum spark advance for best torque (MBT), volumetric efficiency of ηv=80±2%, air-fuel ratio of A/F≃15 and engine speed of N ≃1000 rpm, with various torch nozzle areas (An). Consequently, the effect of torch nozzle on the flame propagation pattern become more clear.

Two-dimensional combustion processes in a spark ignition engine with and without an unscavenged horizontal prechamber are calculated numerically using a k-e turbulence model, a flame kernel ignition model and an irreversible reaction model to obtain a better understanding of the spatial and temporal distributions of flow and combustion. The simulation results are compared with the measured results under the same operating conditions of experiments, that is, the minimum spark advance for best torque (MBT), volumetric efficiency of 80±2 %, air-fuel ratio of 15 and engine speed of 1000 rpm, with various torch nozzle areas and an open chamber. Consequently, the flow and combustion characteristics calculated for the S.I. engine with and without prechamber are discussed to examine the effect of torch jet on the velocity vectors, contour maps of turbulence and gas temperature.

To examine the effect of torch jet direction on the combustion characteristics and engine performances, a spark-ignition engine with each divided chamber having a torch nozzle of different flow direction is used by changing the torch nozzle area, prechamber volume and air-fuel ratio, while keeping the engine speed of 1000 rpm and the volumetric efficiency of ην ≃ (80 ± 2) % constant. Typical pressure diagrams for different torch jet directions are analyzed to obtain such combustion characteristics as the crank angles of combustion start and finish, heat release rate and mass burned fraction. The engine performances, e.g. mean effective pressure and specific fuel consumption, are also measured. As a result, it can be made clear not only the effect of torch jet direction on the combustion characteristics, but also the relationship between the combustion characteristics and the engine performances for different torch jet directions.

To obtain a more reasonable model of torch combustion in a spark ignition engine with a vertical or horizontal prechamber, the instantaneous temperatures of combustion gas are measured by an infrared absorption-emission pyrometer with a narrow band pass filter for CO2 gas, while changing the torch nozzle area and air-fuel ratio. The gas temperature diagrams indicate that the ignition timing, flame propagation and combustion duration in the main chamber with vertical prechamber differ entirely from those with horizontal one. The fact is verified by comparing them with the heat release rates obtained from the pressure diagrams and with the flame propagation taken by means of high-speed photography. The measured gas temperature diagrams are, therefore, found to provide a lot of useful and local information concerning the combustion process and the engine performance in the prechamber engines.