OCTANE AND ENGINE PERFORMANCE
To grasp the link between the octane number and engine performance, it is important to understand maximum brake torque (MBT), which is realised at a specific speed and load condition of the engine when the spark timing is optimal. Engine-calibration engineers spend hours on an engine dynamometer to tune the ignition-timing map of the entire grid of speed and load points to realise MBT.
During day-to-day driving, an engine spends most of its time in a part-load condition, where the driver does not depress the throttle all the way to the stop. With a fixed airfuel ratio at 14,7:1 to optimise fuel economy and emissions, the MBT curve during part load is illustrated below. Advancing ignition timing any further than MBT timing simply sees a reduction in engine torque. The fuel-octane number has little influence in this condition because in-cylinder pressures are relatively low.
When maximum performance is demanded from an engine, the driver depresses the accelerator fully (WOT) to a full-load engine condition. As the in-cylinder pressures and temperatures are now at their highest, it is usually impossible to reach MBT by advancing the ignition timing because auto-ignition (knock) sets in (explained in the graphic below). This is called knock-limited performance. By using a fuel with a higher-octane number, the knock limit is pushed out and more timing advance is allowed, resulting in a gain in performance. In this condition, many manufacturers enrich the fuel mixture to further combat the onset of knock and help cool the exhaust valves, but this obviously has a negative effect on fuel economy.
HIGHER-OCTANE FUEL PUSHES OUT THE KNOCK LIMIT