4. A Review on Combustion Rate Control, Spray-Wall Impingement, and CO/UHC Formation of the Gasoline Compression Ignition Engines

Gasoline compression ignition (GCI) is a promising engine combustion concept achieving high efficiency and low nitric oxide (NO_x) and soot emissions. However, major challenges arise from the excessive pressure rise rate or even knocking combustion at high loads and combustion instability at low engine loads. Internal hot exhaust gas recirculation, low-octane fuel, multiple split injections, and spark assistance are the measures to control the GCI combustion rate. Besides, the early fuel injection employed in GCI to enhance fuel premixing may result in potential spray-wall impingement, wall wetting, and the increase of unburned hydrocarbons (UHC), as well as carbon monoxide (CO) emissions. A detailed understanding of the ignition mechanism and the factors that control the combustion rate of GCI is the key to address these issues. Great efforts have been made to gain insights into these in-cylinder physical phenomena and the links among them. Planar laser-induced fluorescence (PLIF) techniques were applied extensively to visualize the fuel distribution and combustion in the piston bowl and squish region. Most recently, the fuel trapping effect and UHC formation process in the piston crevice of GCI engines was investigated using PLIF and three-dimensional simulation, and the distinct effects of injector dribbling on UHC spatial distribution and emissions of GCI were highlighted. The current study reviews the literature on the ignition, combustion rate control, spray-wall impingement, and CO/UHC formation of GCI engines using emissions measurement and laser diagnostics. Some vital suggestions are proposed for GCI combustion and CO/UHC emissions control.