Laseroptische Untersuchungen des Verbrennungsprozesses in einem PKW-Dieselmotor

The process of the conventional diesel combustion in small-size passenger car engines is an object of research. The combustion process can be divided in the sub-processes injection, mixture formation, auto-ignition and combustion. In the present work different laser-optical measurement technologies were used to receive information about the combustion in a passenger car diesel engine. The investigations were carried out in a cylinder of a passenger car diesel engine, modified for the optical access. Diesel as the fuel was used. The combustion of the pre-injection spray, which serves mainly for the reduction of combustion noise, was qualitatively visualized for an 8 hole nozzle. Obviously, during pre-injection combustion soot is formed. Some of this soot remains up to the beginning of the main injection event. This is confirmed by flame luminescence imaging of the auto-ignition of the main injection with the standard 6 hole nozzle. Soot formation and soot oxidation of the main injection were examined in detail under different conditions by Laser-Induced Incandescence (LII) as well as the Laser Extinction Method (LEM). Employing both measurement techniques the temporal development of the local soot distribution and the average soot volume fraction were determined till the late mixture-controlled combustion phase. In general, a lower injection pressure and the use of the 8 hole nozzle showed significant differences in the soot distribution in comparison to the reference operating point. The influence of exhaust gas recirculation (EGR) on the soot oxidation was examined by a single-cycle analysis. This revealed that by application of EGR a raised number of soot pockets appears without soot luminescence. With EGR combustion reactions cease more often in the presence of soot. Therefore, soot is only slowly further oxidised in these regions. At the end of the late mixture-controlled combustion phase the Spontaneous Raman Scattering (SRS) could be applied successfully for determination of temperature and mole fraction distribution of oxygen and carbon dioxide above the piston bowl. From the results it can be reasoned, that soot emissions must predominantly result from the slowing down of the oxidation reactions at low temperatures. Local oxygen concentrations are high, and hence should not be the primary cause of soot oxidation limitation. Soot deposition on the bowl surface was estimated by determination of the transmission reduction of the glass piston bowl based on LII signal. It was shown by a comparison with exhaust soot emissions of different operating conditions that soot on the piston bowl does not contribute directly to the exhaust soot emissions