In this work, the operating strategy for diesel injection methods and a way to control the exhaust gas recirculation (EGR) rate under dual-fuel PCCI combustion with an appropriate ratio of low-reactivity fuel (propane) to achieve high combustion stability and low emissions is introduced. The standards of combustion stability were carbon monoxide (CO) emissions below 5,000 ppm and a CoV of the indicated mean effective pressure (IMEP) below 5 %. Additionally, the NOx emissions was controlled to not exceed 50 ppm, which is the standard of conventional diesel combustion, and PM emissions was kept below 0.2 FSN, which is a tenth of the conventional diesel value without a diesel particulate filter (DPF).
The operating condition was a low speed and load condition (1,500 rpm/ near gIMEP of 0.55 MPa). Under the conventional EGR rate condition (30 %), although dual-fuel combustion with a conventional diesel injection strategy results in lower PM emissions than that of conventional diesel combustion, NOx emissions did not decrease, whereas CO and hydrocarbon (HC) emissions increased (premixed ratio of 30 %). To further reduce the NOx emissions, a higher EGR rate was used (from 30 to 35 %). Additionally, diesel injection timing was advanced to the PCCI region, which meant that the ignition delay was longer than the injection duration. In particular, early split diesel injection was effective in reducing NOx emissions and improving combustion stability due to the improved reactivity stratification. The results emphasize that NOx emissions can be reduced to half that of conventional diesel combustion, and near zero PM emissions can be achieved (i.e., 0.1 FSN level). Therefore, a suitable operation strategy exists, which includes diesel injection and EGR control, for dual-fuel PCCI combustion.