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

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 degrees to 60 degrees) 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.