Study on HCCI combustion improvement by using dual assisted compression ignition (DACI) on a hydraulic free piston engine fueled with methanol fuel

•A dual assisted compression ignition combustion mode is realized.•This combustion mode has more controllability on the HCCI combustion.•This combustion mode has wide operating conditions with high efficiency.•Typical spark ignition combustion characteristics are showed at lower CR.•Two-stage heat release characteristics are showed at higher CR. A dual assisted compression ignition combustion mode is researched on a hydraulic free piston engine. The dual assisted compression ignition combustion mode is realized by combining a glow plug with a spark plug in the ignition system to assist the homogeneous charge compression ignition combustion. The effects of control parameters and operating parameters on dual assisted compression ignition combustion mode are revealed by experiments. It is found that as the compression ratio increases, the combustion characteristics of dual assisted compression ignition change from spark ignition to two-stage heat release. The dual assisted compression ignition combustion has more controllability on the combustion phase and the heat release rate than glow plug assisted compression ignition due to the direct and stable spark ignition. When the temperature of the glow plug is lower, the heat release phase is dominated by the spark plug. With the temperature further increases, the heat release process is mainly controlled by the glow plug. The dual assisted compression ignition combustion has a wider compression ratio range than spark assisted compression ignition to guarantee the high indicated thermal efficiency. The suitable methanol/air equivalence ratio range for dual assisted compression ignition combustion mode is between 0.6 and 0.8.