To understand the striking difference in heat release rate between the vertical and horizontal torch nozzle in a prechamber spark ignition engine, the torch flow patterns remained on the piston and both the valves heads are visualized by the oil-film technique, changing the number, sectional area and direction of torch nozzle, respectively.
As a result, the jet velocity of torch nozzle is confirmed to decrease proportionally as the nozzle increases in diameter or number. Furthermore, it is revealed that combustion in the main chamber with vertical torch nozzle is accelerated by strong turbulence due to the torch jet impingement, and then it shows not only a sharp peak in heat release rate, but also earlier completion in combustion than that of horizontal torch nozzle.