Pre-ignition, Detonation Different in Many Ways

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link http://www.foxvalleykart.com/print/prt_fuel3.html

THE QUESTION OF FUEL PART 3 - Pre-ignition, Detonation Different in Many Ways

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By John Copeland

Welcome to the third installment in our series about fuel. As 1 said at the beginning, the whole point of this series is to try to demystify the subject of fuel and fuel additives. Of course, this really applies primarily to the gasoline classes, although I'm told that some Briggs racers have been rumored to add a drop or two to their methanol when the pump-around wasn't being used. The fact is, there are a list of things that will burn as fuel in your engine as long as this page, or longer. I'm sure you've heard of a lot of them, and some you haven't. But all you amateur fuel chemists out there can't hold a candle to the high-powered research going on at oil companies all over the world, both for improved pump gas and for better racing fuel.
True confession time: This project is turning out to be a lot more complex than I ever dreamed. A lot of you have written or called to offer encouragement and information. I've also received dozens of reprinted articles and faxes with additional information to work into upcoming articles. This is a subject of global proportions and I've spent the last few weeks reviewing literature from petroleum industry publications, racing magazines, all sorts of resources. To all of you who have taken the time to read the first two articles and sent more material for me to review, my greatest thanks. Keep it coming! As long as there is more to report on the subject, I'll keep after it.

Whoever said, "No good deed goes unpunished" must have been a writer. I know 1 always chuckled at the letters to car magazine editors pointing out errors in print. Now the shoe is on the other foot. In the first installment (November '94) 1 referred to pre-ignition as another term for detonation. Boy did I hear about that! Let me make this very clear. While they are similar in some respects, in many important ways, detonation and pre-ignition are very different. Both are very destructive conditions and both are the result of combustion initiated by some force or condition other than the firing of the spark plug.

Pre-ignition is the ignition of the fuel/air mixture in the cylinder, prior to the firing of the plug, most often by a "hot spot" in the cylinder, the head, on the piston assembly or on the plug itself. It can be a carbon buildup, an overheated spark plug electrode, or a sharp bit of metal, like you might get in the head if the engine "swallowed" something and dinged up the top of the piston and the head.

This premature ignition generates intense heat, not only from the combustion itself, but because the combustion happens earlier in the compression stroke than it should and the rapidly expanding hot gasses are subjected to additional compression, generating more heat. The only good news here is that pre-ignition generally shows up pretty quickly as rapidly climbing cylinder head temperature. You may not always be able to hear pre-ignition, but, if you run a cylinder head temp it's hard to miss the warning signs.

Detonation, as so many of you correctly pointed out, is an entirely different animal. Detonation occurs when the fuel/air mixture ignites from the combination of heat and pressure within the cylinder during compression. Like pre-ignition, this ignition occurs independent of the spark plug firing, but, unlike pre-ignition, it can and does occur after the plug has fired. The existing heat in the cylinder and head, combined with the rapidly rising pressure as the fuel/air charge expands from the spark ignition, exceeds the fuel's ability to resist spontaneous combustion, and it explodes. The result is the collision of two independent flame fronts and the results are violent!

You've probably heard your car or truck "ping" on a hard pull when its in too high gear. Well, that's detonation. Just as thunder is the collision of two air masses after a bolt of lightning separates them, detonation is the collision of two expanding gas masses. This is thunder in your cylinder, so to speak, but much more destructive. If you can't hear it over the noise of your engine, and it goes on for very long, the results will be expensive. Often there is no significant rise in head temperature, but if you use an exhaust gauge, you may see a drop in EGT. Detonation exerts tremendous physical forces, as well as thermal ones, and it can break pistons, destroy ring lands, break rings and even lead to bearing failure.

Detonation is of particular interest here because it is directly related to fuel quality. As we discussed before, octane rating is a measure of how well a particular gasoline will resist detonation. It is measured as a comparative figure to iso-octane, which is defined as having a 100 octane rating. While most engines in popular use in karting have relatively low compression, there are still conditions under which detonation may occur and that means you'd better fuel up with enough octane to resist that detonation. However, remember from our earlier discussion, higher octane generally means slower flame speed, and that's not particularly good in our application.

Long tracks, both enduro and some longer sprint tracks, call for numerically short gear ratios, sometimes as low as 4:1 . That means your engine is going to be lugging a lot more than if you piled on gear and over-wound it down the chute. Add to this the fact that most drivers set their carb settings on the straightaway when the engine is approaching top end and engine loading is actually decreasing, and you end up with a lean condition in the midrange, right where the engine is doing all that lugging. Now factor in the increasing trend to shorter and shorter pipe lengths and the resulting scavenging of the cylinder by the exhaust pulse, and you have a great recipe for detonation (Note: to those of you using a slippery pipe, pulling it too soon or too fast has the same effect and wins you a quick ride on the detonation express). The key factor here is the lean condition in the mid-range. Unfortunately, most head temp sensors simply can't respond quickly enough to see this lean condition as you make the transition through it. Exhaust temp can. One side effect of detonation is a rapid drop in exhaust temperature. If the head temp is lower than you'd like and you tweak it in to bring the temperature up, you may inadvertently induce detonation.

With exhaust temp you can see the EGT start to fall and quickly richen up the mixture. There is a lot more to the detonation issue, including head configuration, pipe design, ignition timing and squish band configuration. Al Nunley of Mayko Karting in California has written extensively on the subject and is, perhaps, as knowledgeable as anyone I know. If you want to know more about the mechanics of detonation, specifically as they relate to kart racing, spin back through your old karting publications (I'm not the only one who keeps them forever, am I?), or give Nunley a call.

To combat detonation with fuel octane, you can either use a gasoline that has a higher octane rating by itself, like race gas, or you can doctor up lower octane gas with any number of additives. There are several commercially available add-in octane boosters on the market, and most contain some Tetra-Ethyl Lead or a substitute for it. The problem is, Tetra-Ethyl Lead is the stuff the government made them take out of gas to make unleaded gas in the first place! They keep a pretty tight rein on it and you can't really get what you want from the over-the-counter additives.

Another alternative is to blend your own gasoline with additives that will raise the net octane rating of the fuel. The trick here is to find additives that will do the job without coming up bogus at fuel tech. Alcohols, including methanol, ethanol, isopropyl, tertiary butyl alcohol, toluol and xylol all have octane ratings over 100, but they absorb water, are to some degree corrosive and, here's the kicker, they set the Digatron dielectric meter off, big time.

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On the plus side, they have relatively good potential power and combust completely. While they are, of course, poisonous in unburned form (as is gasoline), the combustion products are relatively clean and nontoxic.

Aromatics like toluene, xylene and benzene all have high octane potential, and all are present in some racing gasolines in varying concentrations. They are also poisonous before combustion, but their combustion is not nearly as complete as the alcohols and their combustion products coming out the pipe aren't all that healthy either.

Toluene and xylene are readily available at paint and hardware stores, while benzene (the best of the bunch, octane-wise) is virtually impossible to get in reasonable quantities because of government regulations. Benzene is high on the government's list of carcinogens, as well as being a vital component in manufacturing some illegal drugs, so steer clear of this one!

Analine is another octane booster on the government's "hit list." Like benzine, it's used to manufacture illegal drugs but, maybe more importantly, it's a skin-absorbed poison and is very toxic. As 1 mentioned in an earlier article, the chemicals that you may have heard about somebody using in their fuel are, in most cases, very dangerous. As these articles continue, we'll be sure to note the critical health and safety hazards of each potential additive we talk about.

In upcoming articles we'll be discussing these and a lot of other additives with regard to their potential for improving performance in a karting application. But from the point of view of detonation and the damage that it can cause, alcohols and aromatics have been the traditional routes to try to "jack up" the octane rating of pump gas. All this supposes that you can't get your hands on good old Tetra Ethyl Lead, the "real thing" octane booster-wise. Well, that's not necessarily so. Racing gas has Tetra Ethyl Lead in it, and so does aviation gas. There are even some leaded fuels still available in some areas for agricultural use.

The point is, nothing currently available is as effective at controlling detonation as leaded fuel. It is readily available in a variety of octane ratings, from ratings just above pump gas to ratings over 115. Bearing in mind that you only want enough octane to prevent detonation. you should be able to accomplish that with a well-considered choice of commercially available gasoline. Or, if you choose, you might consider mixing a Proportion of leaded racing gas or aviation gas with a lower octane pump gas to get the performance you need for your particular application.

Just remember, octane does nothing to improve performance in and of itself. All it does is measure the ability of the fuel to resist detonation. And there is some evidence that it inhibits flame propagation (flame speed) across the combustion chamber and, thus, fuel with too high an octane rating may actually reduce engine performance