In-Cylinder Fluid Flow, Fuel Preparation and Combustion in SI Engines - Application of Optical Diagnostics

Sammanfattning: In this thesis laser-based measurements of in-cylinder flow and fuel concentrations in two stroke and direct injected four-stroke engines are presented. The major problem with the carburetted two-stroke engine is the short-circuiting of fuel that occurs during the scavenging phase. This leads to large emissions of unburned hydrocarbons. The object of this thesis has been to map the flow behaviour in two-stroke engine cylinders during the scavenging phase, and to detect differences between different cylinder designs. The measurement techniques which have been utilised are Laser Doppler Velocimetry (LDV), Laser Induced Fluorescence (LIF) and Mie scattering from fuel droplets, LSDI. Of these measurement methods, LDV and LSDI have been used inside the cylinder. LIF was used outside the exhaust port. The second object of this thesis has been to map fuel distribution, in cylinder flow and combustion in an air assist direct injected four-stroke engine. For the fuel distribution measurements, PLIF was used. The flow measurements were performed with PIV, and flame photography was used to map flame development / combustion. All two-stroke measurements were performed in engines running at their rated speeds, 9000 rpm for three of the designs and 5800 rpm for one design. All engines were run at full load with combustion. For the two-stroke engine measurements, it was found that: „h The LDV measurements inside the cylinders show that cylinders with cup handle transfer channels have a flow pattern inside the cylinder that gives less short-circuiting, and hence less emissions of hydrocarbons, than the cylinder with open transfer channels. „h The LIF measurements outside the exhaust port show that the HC emissions that are caused by short-circuiting comes earlier in the scavenging phase for the cylinder with open transfer channels than is the case for the cylinders with cup handle transfer channels. „h The LSDI measurements in the cylinder provide the transfer channel flow angle, for the cylinders with cup handle transfer channels. For the cylinder with open transfer channels, the results are not as useful; fuel droplet vaporisation close to the exhaust port ruins the results. Measurements in the four-stroke engine showed: „h For the direct injected four-stroke engine, in-cylinder fuel distribution was measured with PLIF. Results show that stratification of fuel is feasible with late injection timings in combination with bowl-in-piston. „h PIV measurements of the flow show that late injection increases the level of in-cylinder flow velocities, that inevitably break down into turbulence. „h Flame photographs of the combustion in a stratified charge mode show two different kinds of combustion; one primary, premixed combustion and one diffusion combustion.