Increasing concern about the impact of internal combustion engines on the environment has led to ever more stringent emission legislation, and the introduction of more sophisticated equipment to enable the requirements to be achieved. One way of improving the emissions from direct injection (DI) diesel engines is to use multi-stage fuel injection, and an investigation performed on such a system is reported in this paper. In this case, the multi-stage fuel injector caused an increase in the exhaust smoke at low load, and an in-cylinder photographic technique was used to examine why this occurred.A multi-stage fuel injector with a VCO nozzle was fitted to a small, high-speed, direct injection diesel engine fitted with a transparent piston for optical access. The combustion process was filmed using a high-speed 16 mm cine camera, and the fuel injection process was illuminated by a high power, copper-vapour laser. Combustion photographs were obtained at different engine operating conditions, and the following parameters were varied: type of fuel injector, i.e. single- or two-stage fuel injector, fuel injection quantity and timing. The photographs showed that the development of all the fuel sprays was similar with the single-stage fuel injector, and this resulted in a good distribution of fuel throughout the combustion chamber. However, it was found that with the two-stage fuel injector the fuel sprays were not even, and there was a significant difference between the dominant spray and the weakest: this resulted in an uneven distribution of fuel in the combustion chamber, and led to a high level of soot formation. The photographic technique clearly showed the reason why the two-stage fuel injector was producing inferior performance, and the observations of fuel injection and combustion processes confirmed the reasons for poor combustion performance.