Effects of different injection strategies and EGR on partially premixed combustion

Premixed Charge Compression Ignition concepts are promising to reduce NOx and soot simultaneously and keeping a high thermal efficiency. Partially premixed combustion is a single fuel variant of this new combustion concepts applying a fuel with a low cetane number to achieve the necessary long ignition delay. In this study, multiple injection strategies are studied in the partially premixed combustion approach to reach stable combustion and ultra-low NOx and soot emission at 15.5 bar gross indicated mean effective pressure. Three different injection strategies (single injection, pilot-main injection, main-post injection) are experimentally investigated on a heavy duty compression ignition engine. A fuel blend (70 vol% n-butanol and 30 vol% n-heptane) was tested. The effects of different pilot and post-injection timing, as well as Exhaust-gas Recirculation rate on different injection strategies investigated. All the measurements were performed at the same load, combustion phasing, lambda and engine speed. The results show that all three injection strategies produced ultra-low soot emission, while less NOx emission was noticed for pilot-main injection because of less diffusion combustion mode. Pilot-main injection strategy decreases the maximum pressure rise rate effectively compared to single injection. For pilot-main injection at 15.5 bar gross indicated mean effective pressure, when 24.3% (pilot/total fuel mass ratio) of fuel injected at -30 crank angle after top dead center in the pilot and the rest injected in the main with 45% EGR rate, 48.97% gross indicated efficiency is achieved. In addition, ultra-low soot (0.19 ppm) and NOx (0.327 g/kWh) emissions are achieved respectively without using after treatment.