Numerical Optimization of Compression Ratio for a PPC Engine running on Methanol

Partially premixed combustion (PPC) has shown to produce high gross indicated efficiencies while yielding lower pollutant emissions, such as oxides of nitrogen and soot, than conventional diesel combustion. Gasoline fuels with a research octane number (RON) of 60-70 have been proposed as optimal for PPC as they balance the trade-off between ensuring good combustion stability at low engine loads and avoiding excessive peak pressure rise rates at high loads. However, measures have to be taken when optimizing the engine operating parameters to avoid soot emissions. In contrast, methanol has a much lower propensity for soot formation. However, due to a higher RON of methanol the required intake temperature is higher for the same engine compression ratio to ensure auto-ignition at an appropriate timing. Increasing the compression ratio allows a lower intake temperature and improves combustion stability as well as engine brake efficiency. Nevertheless, a higher compression ratio generally increases in-cylinder heat losses and peak pressure. These effects were investigated in a simulation study, which combined 0-D and 1-D models, of a multi-cylinder heavy-duty Scania D13 engine operated in PPC mode and running on methanol. Engine experiments from a single-cylinder engine at different compression ratios were used to validate the simulation models. The optimal compression ratio from a brake efficiency perspective was found for four operating conditions from the 12 mode non-idle European stationary cycle supplemental emissions test points. This compression ratio was then used for optimizing key engine parameters. The results showed that a 21.6:1 compression ratio was optimal instead of the original 17.3:1 compression ratio. Especially at lower engine loads, a significant increase in brake efficiency was found. The main reason was a lower intake temperature which increased the average ratio of specific heats and allowed for a lower boost pressure.