Combustion Mode Evaluation of a Methanol–Diesel Dual Direct Injection Engine with a Control of Injection Timing and Energy Substitution Ratio

Methanol, as a renewable fuel, is an attractive option for internal combustion engines. The dual direct injection method is one of the most promising strategies for applying methanol fuel in diesel engines as the flexible injection control enables combustion mode switching. In this study, a 1-L single-cylinder common-rail diesel engine with a compression ratio of 17.4 is retrofitted by installing an additional methanol direct injector with 35 MPa injection pressure. The engine is operated at 1400 rpm, intermediate load, and fixed midpoint combustion phasing of 10 °CA aTDC with a fixed total amount of energy while applying an energy substitution principle with up to 70% energy supplied by methanol. From the experiments, three distinct combustion modes were identified. When early methanol injection timings were selected in the range of 180–60 °CA bTDC, the primary combustion mode was premixed burn. Late injection timings of 10 °CA bTDC to TDC led to heat release rate shapes of the diffusion flame mode. In between these injection timings, partially premixed combustion was achieved where the higher methanol substitution ratio achieved carbon dioxide (CO₂) emissions reduction by up to 11% and nitrogen oxides (NO_x) emission suppression by up to 12%. It was also found that with increasing methanol energy substitution ratio, a significant reduction in smoke emissions was achieved. However, the decreased power output and increased emissions of unburnt hydrocarbon (uHC) and carbon monoxide (CO) were measured due to incomplete combustion caused by lower flame temperature of methanol.