Partially Premixed Combustion (PPC) of fuels in the gasoline octane range has proven its potential to achieve simultaneous reduction in soot and NOX emissions, combined with high indicated efficiencies, while still retaining control over combustion phasing with the injection event. However, the octane range where the ignition properties of a given fuel are optimum depends on the engine running conditions. Thus, low octane fuels present problems for extending the ignition delay at medium to high engine loads; while too high octane fuels have ignition problems at low engine loads.
Two-stroke engines arise as a promising solution to extend the load range of the PPC concept, since it intrinsically provides equivalent torque response with only half the IMEP required in a four-stroke engine. In this framework, the present research aims to evaluate the performance of the PPC concept for pollutant control using a commercial RON95 gasoline at different load conditions in a single-cylinder two-stroke diesel engine with poppet valves in the cylinder head.
Experimental results confirm how it is possible to implement the PPC concept in this engine from low loads (3 bar IMEP) to medium loads (5 bar to 10 bar IMEP) at low engine speed (1200 rpm); keeping competitive combustion stability (COVIMEP below 3%), high combustion efficiency (over 98%), lower NOX and soot levels compared to conventional diesel combustion (CDC) and moderate CO and HC emission levels; thanks to the wide control over the in-cylinder gas temperature provided by the air management settings. In agreement to results reported in the literature, the indicated efficiency increases with the load, but the expected benefits compared to CDC were not observed along these tests. Therefore, the suitability of the proposed two-stroke architecture combined with the PPC concept has been proven, but a dedicated optimization of both engine hardware and settings is necessary to fully exploit its benefits.