Assessment of air management strategies on particulate number and size distributions from a 2-stroke compression-ignition engine operating with gasoline Partially Premixed Combustion concept

The newly designed partially premixed combustion concept has demonstrated its potential to reduce nitrogen oxides and particulate matter emissions combined with highly indicated efficiencies. However, it is highly dependent of the ignition characteristics of the fuel and the air/fuel mixture preparation. Therefore, the proper selection of an injection strategy, of the combustion chamber design and of the air management strategy are critical to ensuring successful partially premixed combustion operation in the full engine map. The objective of the present investigation is to evaluate the use of multiple air management strategies over the air/fuel effective equivalence ratio (phi(eff)) and cylinder charge reactivity and its consequent impact on particle number emissions and particle size distribution. Tests were carried out in a newly designed 2-stroke high-speed direct-injection compression-ignition engine operating with partially premixed combustion concept using 95-research-octane-number gasoline fuel. A scanning mobility particle sizer was used to measure the size distribution of engine-exhaust particles in the range from 6.3 to 237 nm. Three different steady-state operation modes in terms of indicated mean effective pressure and engine speed were investigated. The experiments showed an increase in the particle number emissions and a progressive shift in the particle size toward larger sizes, increasing the accumulation-mode particles and reducing the nucleation-mode particles with the decrease in the differential pressure between intake and exhaust (Delta P) and the valve overlap period. Finally, the particle formation process was limited by the increase in the exhaust gas recirculation rate.