Optical Measurement of Spark Deflection Inside a Pre-chamber for Spark-Ignition Engines

The start of combustion in a spark-ignited engine is highly dependent upon the conditions between the two spark plug electrodes at ignition. In addition to the air-to-fuel ratio in this gap, the gas flow is seen as most critical. In a combustion engine with a standard spark plug that protrudes into the combustion chamber, this gas flow is mainly dependent upon the tumble, swirl, or squish that is developed by the cylinder head and the piston movement. However, the air movement in the pre-chamber depends on the orientation of the orifices towards the main combustion chamber (MCC). This implies a less complex manipulation of local velocity in the electrode gap. This paper focuses on the effect of different pre-chamber designs on spark deflection by the inflowing gas. Therefore, a test rig was developed using the spark plug thread in the cylinder head of a motored engine. The oscillating piston of the turning engine generates the necessary air movement in the test chamber in which the pre-chamber spark plug is mounted. A camera detects the deflection of the spark between the electrodes inside the pre-chamber. Different spark plugs were tested in combination with eight different hole designs. This work relates the spark deflection to different spark plug designs, e.g., hole number, diameter of and position in the cap, diameter of the spark plug, distance between the electrodes, or position of the electrode gap. The measured spark length is also compared to the high voltage of the spark and the velocity of the spark deflection.