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4x4 Truck Piston Detonation & Preignition - Willie's Workbench

Posted in How To: Tech Qa on March 1, 2009 Comment (0)
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I recently got asked about what "pinging" is, and what the difference is between pre-ignition and detonation, what causes it, and why it is bad for an engine. Basically, both pre-ignition and detonation problems relate to uncontrolled combustion within the engine's combustion chamber. But the differences between the two, both in causes and effects, are vast.

It's not a subject most people are now familiar with. Why? Well, that's because electronic ignition systems, fuel injection, and the mass amount of feedback from engine sensors to the computer system that include air temperature, engine temperature, and engine load, along with better combustion chamber designs, have pretty much eliminated the causes. If either should take place, the computer adjusts for it by retarding the ignition timing curve and richening the fuel mixture. If your owner's manual says to use 91-octane fuel, and you use 87-octane, will the engine "ping"? Probably not, in most cases. The computer will compensate for it; however, fuel mileage and performance will suffer.

Let's talk about detonation first. As a fuel mixture is compressed when the piston is on its upward stroke, the mixture's temperature rises rapidly. If this temperature reaches the flash point (or the ignition temperature of the fuel), then spontaneous combustion can take place with no induced spark from the ignition system. Flash point and ignition point differ slightly: the first is when the liquid or gas vapor "flashes" and then goes out; the latter is when it flashes and maintains enough temperature to remain burning until all the available fuel is used up. Basically, this is how a "compression ignition" or diesel engine operates.

However, gasoline, as opposed to diesel, is a much more volatile, or rapid-burning, fuel. Detonation will usually happen just about the same time or a bit before normal ignition combustion is to take place. It is usually caused by the use of a lower octane-rated fuel than what is required. Poor cylinder head design and spark plug location will also allow detonation to take place, even with proper ignition timing. Milling the head for more compression is a leading cause. Even if the compression ratio is still within the range of the fuel used, the combustion chamber design may be changed just enough to cause problems.

Under normal ignition, the fuel is ignited in the proper location, and the flame front moves in a smooth, even path of travel outward from the spark plug. With detonation occurring at another location, two or more different flame fronts collide and cause an irregular, uncontrolled explosion. We no longer have a smooth and gradual pressure rise in the combustion chamber, but sharp hammer-like explosions. The pinging noise you hear is the sound echoing off the cylinder walls, just as if you were ringing a bell. This irregular hammering can also crack pistons and flatten both rod and main bearings. The ringing of the cylinder walls can also set up a vibration that can rattle the moly coating right off the piston rings. Detonation results in hot spots, which then lead to pre-ignition-our next subject.

Pre-ignition is just what it sounds like: the early ignition of the combustible gases. It has several causes. It can be as simple as cross-firing through spark-plug wiring, or as complex as overheating because of detonation. A hot piece of carbon, a heated metal flashing edge of a poor-fitting head gasket, an overheated exhaust valve, or spark-plug tip all can cause pre-ignition. Again, as in detonation, a sudden rise in cylinder temperature can cause excessive pressure and heat buildup. Head gaskets can blow, valves warp, and pistons develop holes from actual meltdown. One of the biggest causes of pre-ignition is early (or too much) ignition timing.

The question is, how do we control both detonation and pre-ignition? The easiest method is to match fuel octane rating to the engine's requirements. This can be determined by combustion-chamber design, compression ratio, ignition timing, operating temperature, outside air temperature, humidity, air density, altitude, throttle position, fuel mixture, vehicle weight, gear ratio, induction type, manifold design, and literally 50 other things.

Proper octane, as you can see, is kind of a catch-all fix for engine pinging problems. Another way to look at octane is to refer to it as the flash point of the fuel. The higher the flash point (octane rating) of the fuel, the greater the compression it can withstand. What we're after is a smooth, even burn rate, so if we can't obtain it through fuel, then we need to control combustion-chamber temperature. On newer engines, this is done through exhaust gas recycling (EGR), combustion-chamber design, camshaft profile, and computer control of ignition timing and fuel mixture.

On a non-computer-controlled engine, pinging is controlled by adjusting the overall ignition curve, initial ignition advance, as well as total advance within the distributor. Fuel mixture is adjusted by changing the size of the carburetor's jets or metering rods.

On a final note, don't think that a little bit of pinging won't hurt an engine. If the engine is pinging, damage is being done. What you don't know is how much detonation or pre-ignition is taking place that you're not hearing. While computer-controlled engines are supposed to have a fail-safe mode, it doesn't always happen. So if you hear pinging, find out why as soon as possible, and in the meantime, back off the throttle to lessen the engine load and reduce the pinging.

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