A microwave-enhanced plasma generation technique was combined with laser-induced ignition to improve ignition characteristics. A locally intensified microwave field was formed near the laser-induced breakdown plasma. As the plasma absorbed the microwaves, the plasma emission intensity increased. The plasma lifetime could be controlled by changing the microwave oscillation duration. Furthermore, the microwave-enhanced laser-induced breakdown plasma improved the minimum ignition energy of the methane/air pre-mixture with just a small amount of absorbed microwave energy.

The stringent regulations that were placed on gasoline vehicles demand significant improvement of the powertrain unit, not only to become cleaner but also more efficient. Therefore, there is a strong need to understand the complex in-cylinder processes that will have a direct effect on the combustion quality. This study applied multiple high-speed optical imaging to investigate the interaction between the in-cylinder flow, the spark, the flame, and combustion performance. These individual elements have been studied closely in the literature but the combined effect is not well understood. Simultaneous imaging of in-cylinder flow and flame tomography using high-speed Particle Image Velocimetry (PIV), as well as simultaneous high-speed spark imaging, were applied to port-injected optical gasoline imaging. The captured images were processed using in-house MATLAB algorithms and the deduced data shows a trend that higher in-cylinder flow velocity near the spark will increase the stretch distance of the spark and decrease the ignition delay. However, these do not have much effect on the combustion duration, and it is the flow-field in the entire area surrounding the flame development that will influence how fast the combustion and flame growth will occur.

Among multiple factors that affect the quality of combustion, the intricate and complex interaction between in-cylinder flow/turbulent field and flame propagation is one of the most important. In this study, true simultaneous, crank-angle resolved imaging of the flame front propagation and the measurement of flow-field was achieved by the application of high-speed Particle Image Velocimetry (PIV). The technique was successfully implemented to avoid problems commonly associated with PIV in a combustion environment, such as interferences and reflections, avoided thanks to a number of adjustments and arrangements. All experiments were carried out inside a single-cylinder optical gasoline engine operated at 1200 rpm, using port fuel injection (PFI) with stoichiometric mixtures. It was found that the global vortex location of the tumble motion heavily influences the flame growth direction as well as the flame shape, mainly due to the tumble-induced flow across the ignition source. The flame propagation also influences the flow-field such that the pre-ignition flow can be maintained and the flow of unburned region surrounding the flame front will be enhanced.