Gain in Thermal Efficiency from Additional Expander Unit

[abdulqadir]

If a Wankel gasoline engine was to be fitted with a compact and efficient positive displacement additional expander unit such that the normal exhaust from the engine was further expanded down to atmospheric pressure before being released to atmosphere, what gain in thermal efficiency could be expected ?
In principle an improved Atkinson Cycle would be achieved.
Assume that the adiabatic efficiency of the expander device is 75% and that no gas transfer losses are incurred; that the exhaust gas condition at the time when the exhaust port of the engine opens is 950 C and 5.5 bar absolute pressure; and that the engine operates only at a single speed and at full throttle as could be used, for example, to drive a generator in a Series Hybrid electric vehicle.

 

[Navin A S]

The improved Atkinson cycle would result in an increase in the thermal efficiency of approximately 5-10%. This is because the additional expansion of the exhaust gas allows for more of the energy in the exhaust to be converted into useful work, rather than being wasted as heat. Additionally, the increased compression ratio would also increase efficiency by reducing the amount of fuel needed to create the same power output.