This answer mentions the proposed use of an internal combustion engine (ICE) in ULA's ACES upper stage to extract energy from LH2/LOX in multiple forms for multiple uses, which is described further in Evolving to a Depot-Based Space Transportation Architecture linked there as well. For example, as described in the linked paper An Integrated Vehicle Propulsion and Power System for Long Duration Cryogenic Spaceflight an electric generator would convert mechanical energy to electricity, heat would potentially be captured by the Integrated Vehicle Fluids system, and the exhaust would be used for "milli-G level settling thrust" which would keep the cryogenic propellants at one end of their cryostats (continuous, low-level ullage for days or weeks), significantly lowering the heat loading.
In a conventional internal combustion engine most of the expansion happens (by design) inside the engine itself, in a controlled way, in order to extract the power. But the quantity of thrust necessary for something on the order of milli-G level settling is quite substantial, and it seems would have to take place within an externally mounted nozzle rather than inside an engine.
How might an internal combustion engine generate substantial external thrust?