Fluid motion: Effect on cycle-to-cycle combustion variation, flame development, and spark delivery in spark-ignition engines.

Cycle-resolved Laser Doppler Velocimeter measurements are taken at the spark gap of a firing optical engine. These measurements are correlated with peak pressure and spark energy. The purpose is to gain a deeper understanding on how mean flow fluctuations at the spark gap affect spark delivery, flame development, and cycle-to-cycle combustion variation. The engine intake port is modified to vary the level of swirl within the combustion chamber. Spark voltage and current are measured to characterize ignition system performance, and cylinder pressure measurements give information about the combustion process. The spark plug electrodes are rotated to determine how their orientation affects spark delivery. A conditional sampling analysis of the velocity-pressure data is also performed. Engine combustion cycles which have a higher mean flow velocity at the spark gap before ignition will have a higher peak cylinder pressure. This result does not depend on flow direction and appears to be due to reduced heat transfer to spark plug electrodes and an increased spark energy release rate. The correlation between cycle resolved mean flow velocity and peak pressure increases if the absolute value of the velocity measurements is used. Also, the greater the number of velocity measurements used in the determination of the cycle-resolved mean velocity, the greater the correlation. Engine cycles which have a higher mean flow velocity will also cause the ignition system to deliver more spark energy. This result was obtained on a cycle-by-cycle basis and under steady state conditions. For the steady state conditions, the mean flow velocity was changed by modifying the port configuration.