Detonation and preignition are two subjects that can create a lot of headaches and cost a person lots of money. An engine that suffer from these ills can be destroyed in minutes. These are also two subjects that are not well understood by most, I've seen a lot of pros that were confused in this area.
Part of the confusion stems from the fact that detonation and preignition are very similar in the way they occur and in the damage they create. To add to the mess, when one of the problems starts it can induce the other to start. If left unchecked for any length of time the engine is reduced to a rebuild job, possibly to something that cannot be rebuilt.
To start an understanding of all this mess, it's best to first understand what is desirable during engine combustion. In this part of engine operation a compressed mixture of gas and air is ignited by a spark. The fuel mixture burns, starting at the spark plug and moves smoothly outward toward the piston dome and cylinder walls. The key thought here is that the burn is controlled and it happens at a desired point in time. The engine is designed with this event in mind, any heat and pressure that the burning fuel creates is readily accepted without damage. This would be normal combustion.
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Preignition is an example of abnormal combustion. The burn is no longer controlled. The defining event of preignition is that the air/ fuel mixture ignites before the spark occurs. This is basically caused by a hot spot in the combustion chamber, such as a carbon deposit that has hyper-heated. As compression of the fuel mixture is occurring, the hot spot ignites the fuel mixture. |
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A few thousandths of a second later the expected spark occurs and also ignites the fuel. The two flame fronts collide with a violence (the noise produced by this collision is commonly known as pinging or knocking). The resulting shock wave is more than the engine was designed for. There is also an abnormal rise in combustion chamber temperatures. Preignition results in melted piston crowns, holes in piston tops, melted spark plug electrodes and detonation. Many times you'll find the damage towards the center of the piston though the damage can occur in other areas. |
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Detonation is also an abnormal combustion. The difference is that it occurs after the spark. As the flame front is moving from the spark plug outward, the pressures in the combustion chamber increase. Under certain conditions (when something isn't right) this pressure combined with combustion chamber temperatures can ignite the remaining unburnt fuel and create a second flame front. Just as with preignition the colliding fronts create a sonic shock that damages the engine.
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I once saw a demonstration of detonation. The "combustion chamber" was a glass tube, about 2 inches in diameter and 5 feet long. One end of the tube was closed off. A small bit of low grade gas was added to the tube and the open end was fitted with a stopper, the stopper having a small hole through it. The tube was swung for a moment to allow the fuel to evaporate and for the resulting fumes to mix with the air (a carburetor type action). A flame from a lighter was held to the opening in the stopper and the fuel was ignited. A flame front traveled rapidly down the tube producing a whistling sound (prior to this demonstration the same actions were done but with a quality grade gas in a larger quantity. This produced a slower moving flame front with a very quiet sound). About a foot before the flame reached the end of the tube a second flame front developed and collided with the first front. The resulting "boom" caused every last person in the room to jump. The demonstration was repeated and although we knew what to expect every last person jumped a second time. The sonic shock produced by colliding wave fronts is a force to be greatly respected. It is a force that can tear up an engine without reservation.
Detonation results in eroded pistons, ring grooves and rings. You might also find broken spark plug insulators at the tips. It can also create preignition. With detonation the main destructive force is the sonic shock. Also as the second flame front occurs towards the outer edge of the cylinder the damage tends to be concentrated in that area (outer edges of piston).
With both preignition and detonation an indirect result is the scoring of pistons and cylinder walls. The increase in temperature and pressure can overcome the lubricating property of the engine oil.
There are several reasons for preignition and detonation. It is usually difficult, sometimes impossible, to determine what factors caused the problem to occur. It is best to check all possibilities of cause when correcting this situation.
Over advanced timing can create preignition or detonation. Advancing the timing over recommended specifications increases combustion chamber temperature, creating hot spots for preignition and creating conditions favorable to detonation. A second reason, high engine temperature, can create the same favorable conditions. When working at correcting a preignition problem you need to ensure that the timing specs are per factory recommendations. Thoroughly test the ignition system for proper operation throughout the engine's RPM range. Ensure that the engines cooling system is functioning properly.
Another possibility is a high compression ratio. As stated earlier, high pressure and high temperature alone can ignite fuel. A higher than normal compression can contribute to preignition and detonation. Modifying the cylinder heads can cause this to happen. As well carbon buildup on the piston domes or cylinder heads can push the compression up (as well as breed hot spots). Be sure the engine hasn't been modified. Check to see if there is carbon buildup and remove it as necessary.
A lean air / fuel ratio can create preignition or detonation. A fuel mixture with too much air will burn much more readily than a proper mixture. It is easier to ignite with a hot spot or with high pressures. The cure for this is to ensure the fuel system is clean, well repaired and working properly. Included is the boats fuel system (gas tank, withdraw tube, gas hose, primer bulb, fuel filter and fittings) and the engines fuel system (fittings, hoses, filter, fuel pump and carburetors). Test the system for restsricitons. Test the system for air leaks.
High air inlet pressure is given as another cause of preignition and detonation. A clearer way of saying this would be "low vacuum". The engine isn't pulling enough air through the carburetor to make enough fuel mixture to survive. This in effect creates a lean fuel condition. Be sure the intake portion of the engine is properly assembled. Be sure there are no air leaks in the manifolds or case halves.
There are an additional two reasons that contribute to preignition (and not necessarily to detonation). One, given above, is hot spots. The other is wrong spark plugs. A spark plug of the wrong heat range can get too hot for the engine and will itself become the hot spot causing preignition. Keep the carbon deposits removed. Use the correct plugs.
Keep in mind that when searching for reasons causing these problems you will usually find more than one of the reasons present. A combination of advanced timing and combustion deposits may be one situation. It could be any other combination. If one reason is found and corrected, keep on searching until all possibilities have been checked.
There is one last reason for preignition or detonation. It is fuel with too low of an octane rating. The octane number given to gas is an indication of it's resistance to ignition from heat and pressure (it has nothing to do with the amount of "power" gas can produce). The higher the rating number the more resistant to detonation or preignition.
The octane requirement of a given engine is determined from that engine's design. It is engineered with a specific compression ratio in mind, a specific range of operating temperature, a specific ignition timing and a specific RPM range at wide open throttle. All of these factors (and others) are designed into the engine to work together in producing a specific amount of horse power. It is these factors that determine what octane rating is required to prevent detonation and preignition in that engine.
Be sure to follow the manufacture's recommendation on octane rating. Do not modify the compression ratio of the engine. Be sure you are propped to the correct RPM at WOT. Keep the cooling system in good shape, keep the timing to factory specs.
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