Flow and combustion in disc and pent-roof SI engines
Reported in this thesis is a study of combustion in a disc-shaped combustion chamber spark ignition engine, and in-cylinder flow and combustion in an idealised pent-roof spark ignition engine. Both engines were skip fired, to remove residuals and ensure a welldefined in-cylinder fuel-air mixture. Other important parameters were also controlled, e.g. inlet temperature, inlet pressure, air mass flow, mixture strength, engine speed and spark timing. With the disc-shaped spark ignition engine, a shadowgraph technique was used to study early flame development. Simultaneous natural light and shadowgraph imaging techniques were adopted to validate the later use of the former method for monitoring flame propagation in the pent-roof engine. The disc-shaped engine flame images were processed to yield mean flame radius, flame centroid and to describe flame 'circularity'. Good agreement was obtained between flame radii obtained from natural light and shadowgraph images. No correlation was found between early flame development, centroid displacement, flame 'shape' and the rate of combustion as defined by the crank angle at which peak pressure was attained. The pent-roof engine was 'mapped' to determine optimum conditions, prior to flow and flame studies on the same engine. Flow was analysed using laser doppler and particle tracking velocimetry techniques. Mean and rms velocities were obtained. Observed flow patterns at the two engine speeds tested (750 and 1500 rpm) differed and were not as expected for the simplified geometry. Similar trends in rms velocity were observed at all locations tested, with similar magnitudes at all points and in all directions tested during the critical combustion period. Simultaneous top and side natural light flame images were generated using two high-speed digital cameras; in-cylinder pressure was also recorded. The top and side images were analysed in terms of top and side successive flame positions and top and side mean flame radius. Centroid displacements, from side flame views, were also determined. No correlation was found between initial flame developments and later flame development viewed from the side.