This investigation examines the effects of load, speed, exhaust gas recirculation (EGR) level and hydrogen addition level on the exhaust gas emissions, particulate matter size distribution and chemical composition. The experiments were performed on a 2.0 litre, 4 cylinder, direct injection engine. EGR levels were then varied from 0% to 40%. Hydrogen induction was varied between 0 and 10% vol. of the inlet charge. In the case of using hydrogen and EGR, the hydrogen replaced air. The load was varied from 0 to 5.4 bar BMEP at two engine speeds, 1500 rpm and 2500 rpm. For this investigation the carbon monoxide (CO), total unburnt hydrocarbons (THC), nitrogen oxides (NOX) and the filter smoke number (FSN) were all measured. The in-cylinder pressure was also captured to allow the heat release rate to be calculated and, therefore, the combustion to be analysed. A gravimetric analysis of the particulate matter size distribution was conducted using a nano-MOUDI. Finally, a GC-MS was used to determine the chemical composition of the THC emissions. The experimental data showed that although CO, FSN and THC increase with EGR, NOX emissions decrease. Inversely, CO, FSN and THC emissions decrease with hydrogen, but NOX increases. When hydrogen was introduced the peak cylinder pressure was increased, as was the maximum rate of in-cylinder pressure rise. The position of the peak cylinder pressure was delayed as hydrogen addition increased. This together with the obtained heat release patterns shows an increase in ignition delay, and a higher proportion of premixed combustion. The experimental work showed that the particulate matter size distribution was not dramatically altered by the addition of EGR, but the main peak was slightly shifted towards the nucleation mode with the addition of hydrogen. Hydrogen addition does not appear to have a large effect on the chemical composition of the THC, but does dramatically decrease the emissions.