Effect of the diesel injection strategy on the combustion and emissions of propane/diesel dual fuel premixed charge compression ignition engines

• Dual fuel combustion as different diesel injection strategies were studied.
• The effect of propane gas on dual fuel combustion was assessed.
• Different engine operating strategies to the dual fuel combustion were suggested.
• Emission reduction potential by the dual fuel PCCI combustion was verified.

In this research, the effects of the propane ratio and diesel direct injection strategy on the emissions and combustion characteristics of a propane-diesel dual fuel PCCI (premixed charge compression ignition) engine were evaluated. Under low speed and low engine load, the propane ratio was varied from 30 to 70% to investigate the effect of the LHV (low heating value) fuel content in the supplied fuel (propane and diesel mixture). The EGR rate was adjusted to suit each propane ratio to ensure combustibility and restricted emissions. Early diesel injection strategies have an excellent effect on the dual fuel combustion in terms of simultaneously reducing NOx (nitrogen oxides) and PM (particulate matter) emissions. This concept is based on PCCI combustion, in which the ignition delay is longer than the diesel injection. Meanwhile, although early single diesel injection has been an effective strategy for reducing NOx and PM emissions simultaneously, it was possible to further reduce NOx emissions using an early split injection strategy with a 30% propane ratio. Additionally, when the propane ratio was 70%, the ignitibility deteriorated due to early single diesel injection, which led to a much leaner air–fuel mixture condition locally prior to auto-ignition, causing unstable combustion. As a result, a multiple injection strategy with earlier main injection and a small diesel post-injection as a triggering source was adopted to stabilize dual fuel PCCI combustion with a high propane ratio. The results emphasized that the diesel injection strategy could be adjusted to suit various propane ratios under dual fuel PCCI combustion to reduce NOx and PM emissions while maintaining thermal efficiency.