Setting a tight piston-to-head clearance of around 0.040 inch is a great way to suppress detonation by creating more turbulence within the chamber.
More Octane
Paul Weggenmann, Santa Rosa, CA; I was hoping you could answer a question I had about the dreaded enemy: detonation. I understand the basic principle, that if a particular fuel is too flammable (too low in octane), the fuel mixture tends to ignite inside the combustion chamber while the piston is on its way up during the compression stroke. A higher octane reduces the chance of detonation by being more resistant to burn. What I don't understand is why retarding the timing has any effect on detonation. If the fuel mixture is igniting inside the combustion chamber independently of the spark plug firing, why does advancing or retarding the timing have any effect at all on detonation? Also, will you explain what's happening inside the engine that makes that rattling sound during detonation?
Jeff Smith: Let's take a shot at this with a couple of definitions, Paul. Pre-ignition can be defined as any initiation of combustion ahead of the time when the spark plug should fire. This can be caused by any kind of hot spot inside the combustion area, such as a glowing spark-plug ground strap or perhaps an overheated exhaust valve. Detonation can be defined as uncontrolled combustion, where a severe pressure spike is created in the combustion space as opposed to a more gentle and predictable rise in pressure as the combustion process continues.
Most descriptions of combustion use an analogy of a fire burning across a prairie; the fire moves across the top of the piston in a similar fashion to a burning, grassy field. The time-honored description of detonation is that the end gases trapped toward the outer edges of the combustion space auto-ignite and form a second flame front that collides with the main flame front causing the rattle we all hear. More recent theorists agree that the end gases do auto-ignite but that they create a pressure spike in between the flame fronts, which have been measured on cylinder pressure traces on running engines experiencing detonation. Both theories also agree that this pressure spike hammers the piston very hard and that it is this movement of the piston in the bore that causes that all-too-familiar, audible rattling. Either way, detonation is uncontrolled combustion.
What's interesting is how pre-ignition can create a situation where the negative effects of detonation occur, and because of this, the abuse on the cylinder can enhance the onset of further detonation such that the two situations tend to build on themselves. We had experience with pre-ignition most recently with our E85 engine when we used a way-too-hot heat-range plug that caused all kinds of problems. At least we didn't eat a piston, which usually happens. This is why cold-heat-range plugs are so important on high-compression, supercharged, or nitrous engines with high cylinder temperatures and pressures.
Ignition timing plays the major role in detonation. By definition, detonation demands some kind of properly timed ignition source to occur. Octane merely suppresses the occurrence of detonation, allowing the fuel to continue to burn in a more controlled sequence across the combustion space while the temperature and pressure continue to increase. High-octane fuel allows the engine builder/ tuner to establish an optimal ignition timing curve for the engine. However, octane does not affect the speed of the flame front. This is a common misconception.
We've all heard the overly simplistic ignition-timing recommendation that suggests adding more timing until the engine rattles. It's worth visualizing what occurs with too much ignition timing to understand why this suggestion is not always valid, even if the engine does not detonate. The combustion event is not an explosion. This means the entire oxidation process of the air/fuel mixture requires time, in the form of degrees of engine rotation, for complete combustion. This is why the engine requires lead or ignition timing ahead of the piston achieving the top dead center (TDC) location. This lead time allows combustion pressure to rise to its peak at the proper piston position. Generally, we want this to occur at 15 to 20 degrees after top dead center (ATDC), where the connecting rod has maximum leverage on the crank. If we start the ignition-timing process too soon, cylinder pressure will increase to an excessive level before the piston reaches TDC. This means the engine must expend energy to push the piston up against this cylinder pressure. So, it should make sense that too much ignition timing can hurt power almost as much as too little timing. There's a bunch more to all this that gets into the thermodynamics of the combustion process, but that is way beyond what we understand, and hopefully, this answers your question. I'm sure the engineers reading this magazine are already sharpening their electronic pencils to educate both of us. Hopefully, they will also treat us kindly.