Combustion Control Using Two-Stage Diesel Fuel Injection in a Single-Cylinder PCCI Engine

A diesel-fueled premixed charged compression ignition (PCCI) combustion technique using a two-stage injection strategy has been investigated in a single cylinder optical engine equipped with a common-rail fuel system. Although PCCI combustion has the advantages of reducing NOx and PM emissions, difficulties in vaporization of a diesel fuel and control of the combustion phase hinder the development of the PCCI engine. A two-stage injection strategy was applied to relieve these problems. The first injection, named as main injection, was an early direct injection of diesel fuel into the cylinder to achieve premixing with air. The second injection was a diesel injection of a small quantity (1.5 mm³) as an ignition promoter and combustion phase controller near TDC. Effects of injection pressure, injected fuel quantity and compression ratio were studied with variation of an intake air temperature. The experimental results showed that the two-stage injection could be used as a combustion phase controller only in the case of a low intake air temperature but maintained the effect of ignition promotion while the ignition timing was advanced by the high intake air temperature. The main injection timing should be advanced earlier than BTDC 100°CA for the homogeneous and non-luminous combustion. Results of the PCCI combustion showed reduced NOx by more than 90% but increased fuel consumption and HC, CO emissions compared to direct injection (DI) diesel engine. The base injection angle 150° was modified to 100° for the reduction of smoke and unburned fuel made from the over-penetration of fuel spray, which implies the way of optimized injector selection to achieve the successful early injection strategy.