Study on optimal combustion strategy to improve combustion performance in a single-cylinder PCCI diesel engine with different combustion chamber geometry

[Display omitted] •Combustion chamber and injection strategy were changed for optimal combustion.•In narrow angle chamber, IMEP was maintained when advancing injection timing.•Combination of combustion chamber and strategy improved combustion performance.•Without deterioration of engine performance, emission can be reduced. To determine the optimal operating conditions in a homogeneous charge compression ignition (HCCI) engine, early and multiple injections were conducted to reduce wall wetting or promote the ignition of the premixed charge in a single-cylinder diesel engine. The baseline engine was modified to prevent wall wetting as follows: (1) Reduction of the geometric compression ratio (from 17.8:1 to 15:1) to expand the ignition delay, by modifying the piston (2) Reduction of the spray-cone angle (from 156° to 60°) to reduce fuel deposition on the cylinder wall and top of the piston. The experimental results show that a high combustion pressure was caused by rapid premix combustion in the conventional engine. The modified engine configuration maintained a high thermal efficiency, whereas the injection timing became too early. The two-stage injection strategy significantly increased the combustion pressure and advanced the ignition compared to single injection because of reduced spray penetration and improved combustion efficiency. The induction of cooled-exhaust gas recirculation increased the ignition delay of the mixture and decreased the maximum combustion pressure. A comprehensive analysis of experimental results, it was founded that changes in combustion chamber geometry and application of two-stage injection and exhaust gas recirculation resulted in significant reductions in NOx and improved combustion performance.