This study investigates the combustion and emissions of a compression ignition (CI) engine operating with mixtures of hydrogen (H₂) and carbon monoxide (CO) injected with the intake air. Hydrogen and carbon monoxide were chosen as the gaseous fuels, because they represent the main fuel component of synthesis gas, which can be produced by a variety of methods and feed stocks. However, due to varying feed stock and production mechanisms, syngas composition can vary significantly. It is currently unknown how a varying H₂/CO (syngas) ratio affects the cycle efficiency and gaseous emissions. The experiments were performed on an air-cooled, naturally aspirated, direct injection diesel engine. The engine was operated at 1800 RPM with a compression ratio of 21.2:1. Two load conditions were tested; 2 bar and 4 bar net indicated mean effective pressure (IMEPⁿ).For all test conditions the added syngas demonstrated lower cycle efficiency than the diesel fuel baseline. The lower cycle efficiency is thought to directly come from the amount of unburned syngas escaping with the exhaust gases. For the 2 bar IMEPn condition the NOx emissions remained relatively constant for all conditions tested, however for the 4 bar IMEPⁿ condition, the NOx emissions increased with diesel fuel substitution for all H₂/CO proportions. The NO₂/NOx ratio was found to significantly increase for all conditions tested, compared to the diesel base case. It is speculated that this increase is caused by the increase in HO₂ radicals which increases the NO to NO₂ conversion.