Particle formation, growth, coagulation and combustion in the cylinder of an indirect injection passenger car type diesel engine have been studied using a system which allows the cylinder contents to be rapidly expelled through a blowdown port, diluted, and collected in a sample bag for subsequent analysis. Characteristic blowdown times were about 0.5 ms. Samples were analyzed using a condensation nuclei counter to determine particle number concentrations and an electrical aerosol analyzer to determine particle volume concentrations in the 0.01 to 1.0 μm diameter range.Measurements were made with the engine operating at 1000 rpm and an equivalence ratio of 0.32. Peak particle number concentration in the cylinder 13 times the exhaust level, and peak particle volume (or mass) concentration in the cylinder 3 times the exhaust level were observed. These results suggest that significant particle coagulation and oxidation occur during the expansion stroke.