This thesis opens with a review of present knowledge about pollution from internal combustion engine exhaust, covering the formation of pollutants in the engine, some of the effects of emission on man, and the influence of various engine parameters on exhaust emission. The thesis describes the development and application of ultrasonic atomisers operating at approximately 100 kHz, the investigation of the formation of droplets in a liquid layer on a vibrating surface and the measurement of droplet size using liquid wax and microphotography. Chapter 4.3 presents the results of calculations of eigenvalues and vibration amplitudes in an ultrasonic atomiser, using a three dimensional finite element programme. Chapter 6.2 and 6.3 describes experiments on a two-stroke and a four-stroke engine which demonstrate the improvement in engine emissions which can be achieved using ultrasonic atomisers, particularly at low load. Experiments were carried out with ultrasonic atomisers mounted in a novel carburettor unit, and the result were compared with the engine performance using a normal carburettor. A special manifold intended to achieve the best possible utilisation of the atomiser was designed and tested.