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High speed, time resolved Particle Image Velocimetry (PIV) diagnostics was applied to an optical SI engine to study the interactions between in-cylinder flow field and flame development. Optimisation and certain adaptations have been made to the diagnostic setup to enable time-resolved, simultaneous measurements of both PIV data and flame tomography imaging from the same original captured image set. In this particular study, interactions between flow and flame during lean-burn operating conditions at various tumble strength have been investigated and compared to a standard stoichiometric operation. Diagnostics were performed for both the vertical plane (x-y) and the horizontal plane (r-⊖) of the combustion chamber with a particular focus in the pent-roof area. Some major differences in the tumble flow-field prior to ignition has been observed between the lean and stoichiometric conditions.

Exhaust gas recirculation (EGR) has proven to be beneficial for not only fuel economy improvement but also knock and emissions reduction. Combined with lean burning, it can assist gasoline engines to become cleaner, more efficient and to meet the stringent emissions limit. However, there is a practical limit for EGR percentage in current engines due to many constraints, one of which being the ignition source. The Microwave Discharge Igniter (MDI), which generates, enhances and sustains plasma discharge using microwave (MW) resonance was tested to assess its ability in extending the dilution limit. A combination of high-speed Schlieren imaging and pressure measurements were performed for propane-air mixture combustion inside a constant volume chamber to compare the dilution limits between MDI and conventional spark plug. Carbon dioxide addition was carried out during mixture preparation to simulate the dilution condition of EGR and limit the oxygen fraction.