Current four stroke engine research objectives focus on increasing part load thermal efficiency while reducing exhaust emissions. Methods currently employed to achieve this are homogeneous charge lean burn technology, stratified charge lean burn technology and the use of alternative fuels. Of all possible alternative fuels hydrogen is viewed by many as being, ultimately, the fuel of the future. Methanol is regarded as being a likely transitional fuel between the current petroleum transport fuels and hydrogen. Hydrogen is known to be effective in extending the operating lean limit of an engine when used in small amounts in conjunction with conventional fuels. This thesis presents the results of an investigation into the use of hydrogen as a supplementary fuel in a methanol fuelled Ricardo E6 engine. Three methods of introducing supplementary hydrogen into the engine were investigated: Untimed manifold injection, early direct injection and injection through a modified spark-plug. Suitable injection systems were designed and tested on the Ricardo E6 engine. An engine management system, data acquisition system and post processing software were developed to enable data to be acquired and analyzed. The direct injection systems were the subject of schlieren visualization investigations of injected gas distribution to improve performance and elucidate engine related phenomena. Results detailing the effects of hydrogen supplementation on the engine performance, combustion and exhaust gas emissions are presented and discussed. The early direct injection of hydrogen through a dedicated cylinder head tapping proved to be the least effective method of introducing supplementary hydrogen in the Ricardo E6 engine. The position of the injector diametrically opposite the spark-plug coupled with low levels of bulk mixture motion in the cylinder resulted in the formation of an inversely stratified charge (lean near spark-plug) even when injection commenced very early in the compression stroke. The inversely stratified charge resulted in a given level of hydrogen supplementation not being as effective as was the case in the other two fuelling systems. The homogeneous air/fuel mixture formed by the untimed manifold injection of hydrogen was found to have a beneficial effect on both the combustion and emissions of the engine especially in lean air/fuel mixtures. Introducing supplementary hydrogen into the inlet manifold of the engine was however found to reduce the volumetric efficiency of the engine, reducing the level of output power produced by the engine. Injecting hydrogen through a modified spark-plug was found to be the most effective way of introducing supplementary hydrogen. The improved performance of the modified spark-plug system over the other two systems was found to be due to the presence of a localized hydrogen/air mixture at the spark-plug electrodes at the time of ignition. Extension of the lean limit past that possible with methanol alone was demonstrated whilst maintaining both a high thermal efficiency and low indicated specific unburned hydrocarbon emissions. Direct injection of supplementary hydrogen into the combustion chamber, either with the aim of forming a homogeneous charge, or forming a richer air/fuel mixture in the vicinity of the spark-plug without the use of a pre-chamber, has not been reported previously in the published literature. The investigation of both of these fuelling methods is presented in this thesis and constitutes an original contribution to the field of engine research.