Flame Kernel Growth and Propagation in an Optical Direct Injection Engine Using Laser Ignition

The demand for more efficient and clean engines have prompted the research and development of new engine technologies. Automotive engines expected to run with leaner mixtures and higher compression ratios. Lean burn is effective to increase fuel economy whilst reducing emissions but unreliable ignition of the lean mixtures by the conventional spark plug is one of the problems which causes concerns to the engine designers. Laser ignition is a promising technology and holds many benefits over the spark ignition because it can extend the ignitability of lean mixtures with flexibility of the ignition location and absence of electrode degradation for improved engine performance with lean burn. In this study, high-speed photography is used to investigate the flame kernel growth and propagation in an optical direct injection engine using laser ignition by an Nd:YAG laser. Coupled with heat release analysis, the results demonstrate that the variations in air-fuel equivalent ratio have considerable influence on the flame kernel growth rate, but compared with spark ignition, the combustion process has reduced initiation time with an overall faster flame propagation speed associated much reduced cyclic variability. It is also revealed that engine performance benefits more in the leaner conditions by using laser ignition compared to stoichiometric or rich conditions. Besides, the influence of laser ignition position is discussed, and the advantage of highly flexible ignition position for laser ignition could be applied to improve engine efficiency.