Diesel combustion and the formation of pollutants are directly influenced by the spatial and temporal distributions of the fuel within the combustion chamber of an internal combustion engine. The requirements for more efficient and responsive diesel engines have led to the introduction and implementation of multiple injection strategies. However, the effects of such injection modes on the hydraulic systems, such as the high pressure pipes and fuel injectors, must be thoroughly examined and compensated for. The objective of this study was to investigate the effects of fuel injection equipment characterisation and optimisation on diesel combustion and emissions with two-stage fuel injection. The fuel injection system was characterised and optimised through the measurement of the fuel injection rate and quantity, in particular, the interaction between the two injection events was quantified and compensated for. The effects of twin and variable split two-stage injection and dwell angle on diesel combustion and emissions were investigated in a high-speed direct injection single-cylinder optical diesel engine using heat release analysis and high-speed fuel spray and combustion visualisation technique. The results indicated that two-stage injection has the potential for simultaneous reduction of NO and soot emissions. Nevertheless, the studied two-stage strategies resulted in higher soot emission, mainly due to the interaction between two consecutive fuel injection events, whereby the fuel sprays during the second injection were injected into burning regions, generating fuel-rich combustion. In addition, the variable two-stage strategies produced high levels of uHC emission in comparison to single and twin split injection cases. This was mainly attributed to firstly greater fuel quantity injected during the second injection and secondly poor mixing and air utilisation during the second fuel injection event.