The 5-stroke internal combustion engine developed by Ilmor utilizes two firing cylinders that exhaust alternately into a central expansion cylinder, extracting extra work. This allows the engine to run with an expansion ratio of 14.5:1 like a diesel while achieving fuel consumption of only 226 g/kWh. The expansion and compression processes are decoupled, enabling independent optimization. Initial running of the prototype produced impressive fuel efficiency over a wide operating range due to greater work extraction in the low pressure cylinder upon knock onset, providing self-compensation.