THE QUESTION OF FUEL                                                     Printer-friendly format
Thinking Of Mixing Your Own Fuel? Better Think Again.
Mixing Fuel Is Dangerous Business

By John Copeland

The question of fuel tampering has plagued karting for decades, and for all the efforts by sanctioning bodies and tech officials, we're no closer to a workable solution than we were in the 1960s. Those with the knowledge and the resources to circumvent the testing procedures have enjoyed an unfair advantage over their competitors. But fooling around with fuel isn't just unfair, it's dangerous. Few karters have the background in fuel chemistry to approach the task of getting more power in a logical, educated manner. Most just try a little of this and a little of that, mostly additives that they've heard about somebody else using, and hope for the best. The guy who said "A little knowledge is a dangerous thing" must have been talking about mixing up fuel. Maybe the best way to begin is to clear up some of the misinformation about fuel and additives and how they work.

We have to begin by understanding that all the horsepower our engines are ever going to make are stored in the fuel. It's that simple. The specific energy content of the fuel/air mixture is the key. The more fuel energy your engine can EFFICIENTLY burn, the more power it will produce. A lot of factors influence this fuel energy: gross volume, air/fuel ratio, density of the mixture, completeness of vaporization, and flame speed. You'll notice we didn't mention octane rating. That's because, in and of itself, octane rating does nothing to improve power output. All octane rating does is measure the ability of a fuel to resist pre-ignition (also called detonation).

Higher octane fuels allow the use of higher compression ratios, and THAT produces more power. However, Yamahas, Piston Ports and Controlleds that have cc limits called out in tech that can't even begin to approach the compression ratios to take advantage of high octane fuel, While an octane rating does influence flame speed, so do other factors. Let's look at each of the other factors individually.

Vaporization is just what it sounds like: how well is the fuel/air mixture dispersed at the point of ignition. Incompletely atomized fuel burns more slowly and may not burn completely. It doesn't do you any good if it isn't completely consumed by the time the exhaust port opens.

The better a job you can do in getting a uniform dispersion of fuel in the incoming air, the more completely it will burn. There are a number of additives that act to reduce the surface tension of gasoline and aid its vaporization. Unfortunately, most oils in common use have relatively high surface tension in solution with gasoline and so inhibit the vaporization process. Most gasoline manufacturers already add detergent-like additives to their fuel, so this ground has already been covered.

Flame speed is also pretty self-explanatory, but there are two sides to this coin. On one hand, the faster the fuel/air mixture burns, the higher expanding gas pressure will be and the longer the pressure will have to work on the piston before the exhaust port opens. However, since the ignition system is timed to fire before the piston reaches top-dead-center, some of that gas pressure will actually work AGAINST the piston as it completes the compression stroke. They call it "knock" in your family car, but it's really pre-ignition and it can be really destructive. It can literally chew the top of a piston away a little bit at a time. In less then a minute, at the RPM that today's two cycles run the top of the piston is gone and you're done. In extreme cases, pre-ignition can break pistons, and the damage that it can do is impressive (and expensive).

Higher octane fuels, in general, burn more slowly than low octane fuels, but other additives that have little or no bearing on octane rating can affect flame speed.

The density of the fuel/air mixture is the subject of a great deal of interest throughout the racing world. The cooler the charge of fuel and air going into the engine, the denser it will be. And the denser it is, the more potential energy there is in each incoming charge.

Remember that all the horsepower you're going to get is stored in that fuel and air, so the denser a charge you can get into the engine, the better. Superchargers and turbochargers increase the charge density mechanically by compressing it, but that generates a lot of heat in the mixture before it ever gets into the cylinder.

Consequently, more and more "boosted" engines use intercoolers, radiators that cool the mixture and make it denser before it gets to the combustion chamber. Chilling the fuel in the tank has some merit, but with the simple carburetors that karts use, maintaining that desired fuel/air ratio becomes extremely difficult if you begin to fiddle with the temperature of the incoming fuel. It's much more efficient to use fuel additives that have high heat of vaporization to cool the charge. All liquids absorb heat energy when they change from a liquid to a gaseous state, that's how the freon in a refrigerator works. When a liquid is vaporized, like your fuel going through the carb and into the air stream, it gets colder. That cools the air it mixes with and the resulting fuel/air charge gets denser. Neat, huh? Well, different liquids have different heats of vaporization. Alcohols have a substantially higher heat vaporization than gasoline (ever notice how cold it gets if you get some on your hand?), but other highly volatile options exist. In general, hydrocarbons with lower boiling points will have higher heats of vaporization.

Fuel/air ratio is really two subjects. First of all, and familiar to all of us, is the ratio that we can adjust at the carburetor. There is, of course, an optimum ratio of gasoline to air for most efficient combustion. This ratio is generally agreed to be approximately 7:1, 7 pounds of fuel to 1 pound of air. Unfortunately, for most of us, the restrictions of running air-cooled engines make it impossible to approach that ratio. Instead, we pour substantially more fuel through our engines as a COOLANT.

That's right, some detailed computer modeling in the '60s indicated that almost 50 percent of the fuel passed through an air cooled two-cycle engine was not consumed in combustion, but rather its vaporization within the cylinder leeched heat away. (See heat of vaporization above) That's why all the factory motorcycle racers are water-cooled now, and why those water-cooled gearbox karts are so much quicker than their air-cooled cousins. They can simply run leaner fuel/air ratios that more closely approach the optimum ratio. The goal at the carburetor is to get the leanest ratio that the engine's cooling capability will tolerate.

T h e larger, and substantially more complex side of the fuel/air ratio, issue is how that ratio changes with different fuels and additives. The theoretical optimal fuel/air ratio for methanol, for example, is approximately 18:1, as opposed to 7:1 for gasoline described above.

Nitroparaffins like nitromethane, nitropropane, and others are more like 70:1. That's 10 TIMES MORE FUEL for a given amount of air to burn correctly. What this means to the fuel mixer is this; as you adjust the chemical content of the fuel, you may substantially change the volume of fuel that the carb is required to mix for the engine to perform correctly. Legality not withstanding, adding enough of these sorts of additives to generate a noticeable improvement in performance will likely require substantial modifications of the passages in the carburetor to accommodate the volume of fuel required. Percentages small enough not to exceed the carb's ability to deliver fuel will not provide enough improvement in power to be measurable.

Most of the commonly used fuel additives actually have a lower specific energy content per unit volume than racing gasoline. But their higher optimal fuel/air ratio (called the stoichiometric ratio) more than makes up for the lower energy per volume with a lot more volume. Gasoline could be accurately described as a chemical "vegetable soup," containing dozens of chemical compounds. Fuel chemists at every major oil company are constantly fiddling with the composition of their products in effort to stay on top of the market. Cleaner burning, lower emissions, and better fuel mileage are at the top of their priority list, but they're also interested in things like stability for long-term storage, low VOC emission for handling, and other factors. Primarily, gasoline is a blend of several chemical families, including, but not limited to, Alkanes, also known as paraffins, Iso-Alkanes like iso-octane and triptane Cycloalkanes (napthenes), Alkenes (olefins), and Aromatics like benzene and toluene.

Varying the ratios of these ingredients will modify the characteristics of the gasoline. To these basic building blocks the fuel chemists add an ever-increasing variety of modifiers to minimize gum formation to prevent metal iron migration from handling equipment, and to accomplish lots of other goals. Many pump gasolines are now "seasonally adjusted" with alcohols, ethers, and other products to improve their performance in varying weather conditions.

Racing gasoline is, in general, not subject to as much chemical monkeying around. Pump gasolines, with their constantly changing composition, is risky business, legality-wise. Just because a certain grade of pump gas from a certain pump passed tech for the last 10 years, is no guarantee that it will pass tomorrow. This is not necessarily because the test has changed, but because the gas may have.

In upcoming months we'll take a look at several of the more commonly used fuel additives; what they're supposed to do for you, and what the pluses and minuses are. We'll also look at how they react to different methods of fuel tech, both the old standards like the Digatron meter and water test, and some of the newer, more sophisticated testing procedures. We'll hopefully unravel some of the mysteries and myths about fuel additives. And when its over, we'll look down the road a bit and see if we can see where this whole fuel thing is going.

One word of caution, if you think this series is going to be a cheater's handbook, think again. With every illegal additive, we'll discuss how tech can uncover it. But if you think this series will help you run faster, you're probably right. Hopefully, once you learn how fuel really works and what all those additives do and don't do, if you're paying attention, you'll probably go faster.

A repeat National Champion once told me that, after years of getting around the fuel rules, trying to pour power into the tank, he discovered that he went faster, with fewer failures, when he ran straight gas and oil. "Every time I tried to juice the fuel and go faster, I hurt myself. And I expect 99 percent of the racers out there do the same." I expect he's right and, in upcoming months, we'll look at why.

Until then, play it straight and concentrate on the things that will really make you faster.

See you next month.

The Question of Fuel - Part 2

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