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What is pre-ignition and/or detonation?

Probably the most common (or at least commonly blamed) catastrophic failure in a gasoline powered engine is pre-ignition and/or detonation. They are often used interchangably by mechanics and laymen alike, but they are two very distinct problems that I will attempt to explain as follows:

By definition, pre-ignition is ignition of the fuel/air mixture before the desired time (i.e. before the spark plug fires). If you recall your high school physics, the 'flash point' of a flammable fuel is variable on a number of factors, including temperature and pressure. There is a direct relationship between temperature and pressure such that as pressure increases, so does temperature (conversely, as pressure is reduced, temperature drops). When the fuel/air mixture is introduced into the combustion chamber of an engine, it is subsequently compressed to a specified ratio (called the compression ratio) and that means it also experiences a temperature increase. Without dwelling too much on what determines the optimal ratio, suffice to say that the manufacturer bases the final ratio based on a number of factors - combustion chamber design and fuel octane are probably most important. If you squeeze this mixture beyond it's operating design (by planing the heads or modifying the combustion chamber design), the fuel-air mixture will actually ignite itself without the aid of a spark plug. In some engines this is actually a designed desirable outcome - a. diesel engine operates on this very principal. To a gasoline engine however, it means certain death and it's called pre-ignition. Pre-ignition can be caused by a number of things since it is essentially a spontaneous combustion caused by 'hot spots' in the combustion chamber. These hot spots can be caused by air leaks, carbon build-up, improper or inadequte oil, too high compression ratio, too low a grade of fuel, improper carburetor calibration, inadequate cooling etc. An engine experiencing pre-ignition will see combustion temperatures rise dramatically in a very short period of time resulting in a melting of the aluminum piston. This condition usually results in piston failure either through a hole being blown in the top of the piston (as the source of the pre-ignition cuts through like a torch), or the edges melting and sticking to the cylinder walls and pinching the rings into the ring lands. Pre-ignition can lead to detonation, but as you'll see it's not the same.

Detonation occurs when two flame fronts collide. It is similar to pre-ignition in that a hot spot ignites the fuel BUT, at the same time (or close to it), the spark plug also ignites the fuel mixture and these two 'flame fronts' move towards each other and collide. This noise can be heard as 'pinging' or 'knocking' when accelerating in a car up a hill or passing (it can't be heard in an outboard motor because normal operational noises are louder and hide it). In fact, the octane rating was specifically derived as a measurement of a fuels ability to resist or prevent detonation and it's called the Anti-Knock Index (AKI). The higher the AKI (or octane rating), the more stable the fuel thus preventing detonation. While this pinging or knocking sounds harmless enough, you have to remember you're hearing this noise outside the engine. Given that you can't even hear the violent ‘normal' explosion occurring inside a gasoline engine, imagine how much more violent the explosion is if you can hear it as knocking behind the dash of your car. When these two flame fronts collide, it's equivalent to a sledge hammer being dropped onto the top of the piston, and it's easy to see how this situation might manifest itself into engine failure. Like pre-ignition, detonation raises the combustion temperature considerably, but more notably often cracks the piston, breaks rings and ring lands, and can bend rods.

So how do we prevent these conditions from happening? First and foremost, ALWAYS use the recommended grade of fuel for your engine. Most older outboard motors were designed to run on 89 Octane fuel, which used to be called ‘regular gas'. Nowadays, regular gas is 87 octane (and even 86 in some cases) and if it's used in an engine designed to run on 89, you have set yourself up for certain failure under the right conditions (hot days, water skiing, prolonged high speed operation etc). Your next best defense against pre-ignition/detonation is FRESH GAS. Today's fuels have a ‘shelf life' of about 3 months in summertime heat. Given that some of that time was used up in transport and storage, the means you don't have long to use it before some of the additives begin to ‘gas off' which results in - you guessed it, a lower octane rating. Also, avoid RFG (alcholol blended) fuels at all costs (click here for an explanation). Next, keep your engine properly tuned and maintained with the correct spark plug heat range installed (spark plugs are not ‘all the same'). Finally, the correct propeller pitch is imperative to ensure your engine isn't labouring and self destructing (picture a car attempting to accelerate in 4th gear from a stopped position).

While it is often very difficult to determine what the source of pre-ignition or detonation was after a failure, a competent mechanic will ensure that possible sources are examined and corrected to ensure it does not re-occur. This includes meticulously rebuilding the engine and strictly adhering to the manufacturers specifications for clearances, tolerances, timing, and synchronization. It also means ensuring any recalls or service bulletins which have not been performed on the engine are done, as well as a visually inspection and verification that the engine is rigged properly and the correct propeller pitch is being used.

Happy boatin'

Dave Brown
Brown's Marina Ltd.


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Brown's Marina Ltd.
1641 Chaffey's Lock Road
Elgin, Ontario, K0G 1E0 CANADA

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Tel: 1-613-359-5466 - Fax: 1-613-359-6376
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