Combustion Mode Switching Characteristics of a Medium-Duty Engine Operated in Compression Ignition/PCCI Combustion Modes

Premixed charge compression ignition (PCCI) combustion is a novel combustion concept, which reduces oxides of nitrogen (NO_x) and particulate matter (PM) emissions simultaneously. However, PCCI combustion cannot be implemented in commercial engines due to its handicap in operating at high engine loads. This study is focused on the development of hybrid combustion engine in which engine can be operated in both combustion modes, namely, PCCI and compression ignition (CI). Up to medium loads, engine was operated in PCCI combustion and at higher loads, the engine control unit (ECU) automatically switched the engine operation to CI combustion mode. These combustion modes can be automatically switched by varying the fuel injection parameters and exhaust gas recirculation (EGR) by an open ECU. The experiments were carried out at constant engine speed (1500 rpm) and the load was varied from idling to full load (5.5 bar brake mean effective pressure (BMEP)). To investigate the emission and particulate characteristics during different combustion modes and mode switching, continuous sampling of the exhaust gas was done for a 300 s cycle, which was specifically designed for this study. Results showed that PCCI combustion resulted in significantly lower NO_x and PM emissions compared to the CI combustion. Lower exhaust gas temperature (EGT) in the PCCI combustion mode resulted in slightly inferior engine performance. Slightly higher concentration of unregulated emission species such as sulfur dioxide (SO₂) and formaldehyde (HCHO) in PCCI combustion mode was another important observation from this study. Lower concentration of aromatic compounds in PCCI combustion compared to CI combustion reflected relatively lower toxicity of the exhaust gas. Particulate number-size distribution showed that most particulates emitted in PCCI combustion mode were in the accumulation mode particle (AMP) size range, however, CI combustion emitted relatively smaller sized particles, which were more harmful to the human health. Overall, this study indicated that mode switching has significant potential for application of PCCI combustion mode in production grade engines for automotive sector, which would result in relatively cleaner engine exhaust compared to CI combustion mode engines.