The Danger of the Wrong Octane Fuel

Another day and another holed piston. These are not melted through, they are blasted through by pre-ignition. Now-and-then we see motorcycle owners with problems like this. I have told that everything would be fine if motorcycle owners would stick to the manufacturer advised octane level, even the engine would 'ping' (detonate) sometimes.

What most motorcycle owners are hearing is detonation but far worse; it is pre-ignition caused by running too high an octane fuel that is still burning long after the exhaust valve opens, and so overheats the exhaust valve. Detonation and pre-ignition are not the same thing although they can be connected. The calorific value of all petrol is about the same but the higher the octane, the slower it burns,and so it has to run in a high compression engine to speed up the burn.

The burn should be completed when the exhaust valve opens. The compression ratio governs the speed of burn; the higher – the faster.
The easiest time to light the fuel/air mixture is at the start of the compression stroke where it can be lit by an overheated exhaust valve. When this happens the flywheel insists that the compression stroke be completed and the resultant explosion has nowhere to go but into the crankcase via the piston crown. The ideal octane fuel to use is one that will just detonate under gross abuse; too much throttle at too low revs.

Once I accidentally filled my high compression ratio sportsbike with RON91 fuel and on little throttle I rode it home, but it went so wll that I carefully used it up until at the end I used too much throttle for a gap in the traffic when it indicated its distress. When I removed a cylinder head I found no damage at all. It has a compression ratio of around 10.:1.Tag: Maintenance Octane Piston Parts Mechanical Technology Ignition Ethanol Exhaust-Valve Fuel
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Wednesday, 20 January 2016 @ 03:46 PM ICT
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