Emissions results are presented from an experimental study of an indirect injection diesel engine. The mass measurements of particulate emission are correlated with the measurements of smoke and HC emission. The correlation provides a less expensive way of carrying out preliminary combustion optimisation work. It is concluded that practically all of the particulate mass emission is accounted for by: (a) the black smoke or soot formed in the high temperature fuel-rich regions of the diffusion phase of burning, (b) that fraction (about 50%) of the total HC mass emission which condenses at the particulate sampling filter. The total HC mass emission is itself a function of three distinct sources in the combustion process.The understanding gained is then used to define three combustion ideals for optimising diesel combustion to minimise fuel consumption and emissions of smoke, NOX, particulates, HC, CO, odor and noise.