The present disclosure relates to an engine in which compression and expansion is performed in the same cylinder. Also disclosed is a microprocessor for controlling the state of various valves in the cylinder—including an intake valve, a transfer valve, and an exhaust valve—to cause a compression or expansion to occur. A compression tank is provided for receiving, via the transfer valve, compressed air, which may be retrieved during an expansion (combustion) cycle. Compressed air for from multiple consecutive compressions may be stored in the tank and retrieved later, including for multiple consecutive expansions. Compression and expansion are not required to occur in any fixed or predetermined pattern and the microprocessor may evaluate vehicle sensors to determine a power demand, and cause compression or expansion to occur depending on the given power demand.