Analysis of the Combustion and Emissions of a Diesel Engine in Early-Injection, Partially-Premixed Charge Compression Ignition Regimes

Early-injection, partially-premixed charge compression ignition yields low levels of nitrogen oxides and soot with premixed fuel-ambient mixtures and low-temperature combustion. However, hydrocarbons and carbon monoxide, which mainly stem from over-lean mixtures and wall wetting, are drawbacks. Engine tests were conducted in early-injection, partially-premixed charge compression ignition regimes using a single-cylinder light-duty direct-injection diesel engine. To prevent wall wetting under early-injection conditions, a wide and shallow two-step piston bowl was applied. To enable sufficiently premixed conditions to occur before the start of combustion, the engine operating conditions were controlled in order to use an ignition dwell of longer than 2° crank angle after the end of injection. The injection timing, the injection pressure, the exhaust gas recirculation rate and the in-cylinder swirl were varied, and the relationships between those parameters and combustion and emissions were analyzed. The results showed that the ignition dwell between the end of injection and the start of combustion affects the emissions of carbon monoxide, hydrocarbons and soot significantly by changing the premixed level, and an ignition dwell of between 3° crank angle and 5° crank angle produces fairly low levels of nitrogen oxides and soot together with acceptable levels of hydrocarbons and carbon monoxide under non-wetting high-dilution conditions. Wall wetting significantly increases the amount of hydrocarbons and, when the ignition dwell is prolonged to longer than 7° crank angle, excessive carbon monoxide is produced.