Do Otto heat engines operate on a closed system or an open system? Why?

In general Otto heat engine is an open system.

Explanation:

Otto cycle:

Otto cycle is an ideal cycle for all practical use petrol engine. It have four processes in which two are constant volume processes and other two are reversible adiabatic processes.

In Otto heat engine mixture of air and fuel enters inside the cylinder and this mixture burns and produces some amount of work. So for this set of mixture Otto heat engine is a closed system. But we know that engine produces continuous power so mixture of air fuel enters and leaving continuously so it is the case of open system.

In general Otto heat engine is an open system.