An optically accessible high-speed direct-injection diesel engine was used to study the effects of injection angles on low-sooting combustion. A digital high-speed camera was employed to capture the entire cycle combustion and spray evolution processes under seven operating conditions including post-top-dead centre (TDC) injection and pre-TDC injection strategies. The nitrogen oxide (NO x) emissions were also measured in the exhaust pipe. In-cylinder pressure data and heat release rate calculations were conducted. All the cases show premixed combustion features. For post-TDC injection cases, a large amount of fuel deposition is seen for a narrower-injection-angle tip, i.e. the 70° tip, and ignition is observed near the injector tip in the centre of the bowl, while for a wider-injection-angle tip, namely a 110° tip, ignition occurs near the spray tip in the vicinity of the bowl wall. The combustion flame is near the bowl wall and at the central region of the bowl for the 70° tip. However, the flame is more distributed and centralized for the 110° tip. Longer spray penetration is found for the pre-TDC injection timing cases. Liquid fuel impinges on the bowl wall or on the piston top and a fuel film is formed. Ignition for all the pre-TDC injection cases occur in a distributed way in the piston bowl. Two different combustion modes are observed for the pre-TDC injection cases including a homogeneous bulky combustion flame at earlier crank angles and a heterogeneous film combustion mode with luminous sooting flame at later crank angles. In terms of soot emissions, NO x emissions, and fuel efficiency, results show that the late post-TDC injection strategy gives the best performance.
