Free-piston engine combustion and emissions formation

The operating characteristics of the free-piston engine differs significantly from those of conventional engine. Particularly, the very high piston acceleration around top dead centre has been noted by many authors. This has been found to enhance in-cylinder gas motion (squish), benefiting fuel-air mixing and the combustion process, as well as reducing emissions formation. Further, high piston acceleration leads to a faster power stroke expansion in the free-piston engine compared with crankshaft engines. This reduces the time spent in the high-temperature parts of the cycle, reducing heat transfer losses to the combustion chamber walls and the formation of temperature-dependent emissions.

The group has studied the in-cylinder processes of free-piston engines on a fundamental level, through among other things computational fluid dynamics simulation. Advantages compared with conventional engines have been identified, including reduced nitrogen oxides emissions formation (typically in the order of 15%; see above figure) and reduced heat flux to the combustion chamber surface (see below).