Optimization of operating conditions in the early direct injection premixed charge compression ignition regime

Early Direct Injection Premixed Charge Compression Ignition (EDI PCCI) is a widely researched combustion concept, which promises soot and CO2 emission levels of a spark-ignition (SI) and compression-ignition (CI) engine, respectively. Application of this concept to a conventional CI engine using a conventional CI fuel faces a number of challenges. First, EDI has the intrinsic risk of wall-wetting, i.e. collision of fuel against the combustion chamber periphery. Second, engine operation in the EDI regime is difficult to control as auto-ignition timing is largely decoupled from fuel injection timing. In dual-mode PCCI engines (i.e. conventional DI at high loads) wall-wetting should be prevented by selecting appropriate (most favorable) operating conditions (EGR level, intake temperature, injection timing-strategy etc.) rather than by redesign of the engine (combustion chamber shape, injector replacement etc.). This paper presents the effects of EGR concentration, intake temperature, intake pressure, injection timing, injection pressure and fuel temperature on engine performance and emission behavior in EDI PCCI mode. In addition, several minor adjustments to the conventional injector nozzle are investigated. Wall-wetting and engine performance are characterized by the measured emissions (smoke and unburned hydrocarbons) and in-cylinder pressure (CA50 and IMEP). The main contribution of this paper is to investigate the cumulative effects on engine performance and emissions, unburnt hydrocarbons (HC) in particular, of various known measures designed to address wall-wetting. All experiments have been performed at low load (~ 3-4 bar IMEP) and at an engine speed of 1200 RPM, using a modified 6-cylinder 12.6 liter heavy-duty DI DAF XE 355 C engine. Experiments are conducted in one dedicated cylinder, which is equipped with a stand-alone fuel injection system, EGR circuit and air compressor, fuelled with commercial diesel fuel (EN590)