Implementation of the Partially Premixed Combustion concept in a 2-stroke HSDI diesel engine fueled with gasoline

Abstract

Partially Premixed Combustion (PPC) of fuels in the gasoline octane range has proven to combine low NOX and soot emissions with high indicated efficiencies, while still retaining control over combustion phasing with the injection event. Previous research performed in four-stroke engines, has shown how the operating region where gasoline PPC concept can be successfully implemented is largely linked to the octane number of the fuel, making difficult to cover the entire load range with a fixed fuel.

In this framework, 2-stroke engines arise as a promising solution to extend the load range of gasoline PPC concept, since it intrinsically provides equivalent torque response with only half the IMEP required in a 4-stroke cycle. Moreover, 2-stroke architecture provides high flexibility on the air management parameters to substantially control the cylinder conditions and affect the combustion environment, allowing proper combustion control even in low load conditions.

An experimental investigation has been performed to evaluate the potential of the PPC concept for pollutant control, using a commercial gasoline with Research Octane Number of 95 in a newly-designed 2-stroke poppet valves automotive diesel engine. The experimental results confirm how it is possible to achieve stable gasoline PPC combustion at a low speed medium load point (1200 rpm, 5 bar IMEP); with good combustion stability (σIMEP below 3%), high combustion efficiency (over 98%), and low NOX and zero soot levels; thanks to the wide control of the cylinder gas temperature provided by the air management settings. Nevertheless, in agreement to the results reported in the literature, the indicated cycle efficiency attained at this low load operating condition is lower than the obtained in conventional diesel combustion conditions. Therefore, a dedicated optimization process of the engine hardware and engine settings is required to fully exploit the benefits of gasoline PPC concept in the investigated 2-stroke engine architecture.