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THE QUESTION OF FUEL
PART 4 - Oxygenators are the 'Hottest' Topic in Fuel Chemistry [/TD]
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By John Copeland
By far the hottest (if you'll pardon the expression) topic in fuel chemistry these days is the subject of oxygenators. In their never-ending quest to formulate cleaner burning gasoline for the general motoring public, the major oil companies are using compounds to improve the combustion efficiency of the gas you can buy at the pump. Unfortunately, the real villain here isn't the gasoline, it's the poor efficiency of today's production automobile and truck engines. Better designed engines, operating at much higher temperatures, would go a long way toward cleaning tail pipe emissions, but, the truth is, it's much cheaper for them to try to fix it in the gas tank.
Oxygenators are, pretty much, just what they sound like: compounds that increase the amount of oxygen available for fuel combustion. You remember from high school chemistry that a fire can't burn without oxygen? Well, it's the same inside your engine's combustion chamber. It needs adequate oxygen to burn the fuel. Unfortunately, in most circumstances, we just aren't getting enough, either because the volume of air coming through the carb (remember, air is only about 20 percent oxygen) isn't sufficient, or because the engine can't manage the heat load that a leaner mixture (one containing more air per unit of fuel volume) would generate. (Refer back to Part 1 of this series for more about fuel/air ratios). Oxygenators are, in general terms, flammable compounds that contain at least a portion of the oxygen they need for combustion as part of their own composition. Gasoline, in its basic, unaltered form, contains absolutely no oxygen. It must rely totally on airborne oxygen for combustion. Oxygenators can enhance combustion by assuming some of the burden of providing combustion oxygen
These compounds have been around for a long time but, for the most part, their use has been limited to applications where their ability to furnish most, or all, of the oxygen for their own combustion meant that they could burn explosively. A good example is Tri-Nitro-Toluene, better known as TNT. But for the purposes of improving the quality of fuel combustion, significantly slower-burning, less unstable oxygenators are the focus of interest.
By far the most widely known oxygenators, and the most widely used in commercial gasolines, are alcohols and alcohol-related derivatives. But the fuel chemists at the oil companies have developed a whole new crop of these compounds in hopes of creating a leaner-burning, cleaner fuel/air reaction. This leaner, cleaner combustion translates, in your car or truck, to better fuel mileage and cleaner fuel chemistry tail pipe emissions. We've all heard about gasohol as a catch-all name for gasoline/alcohol blends. For our purposes, we can pretty well dismiss all these alcohol blended gasolines because we already know that they won't pass the standard digatron meter test. But let's look at some of the other oxygen bearing fuel additives that are finding their way into gasoline. Some of these are being added by the gasoline manufacturers and some are ..., well, let's just say that some are finding their way into kart fuel by other means.
`THE RACER'S CHOICE'
Propylene Oxide (CH3 CHCH2 O) has seen considerable use as a performance enhancer over the years. Even when we didn't know what it was doing, we knew it was doing something good. The fact is, Propylene Oxide does several things that racers like. It is highly volatile, boiling at only 93 degrees Fahrenheit, and has a correspondingly high heat of vaporization. That means that it helps cool the incoming fuel charge, thus improving charge density and improving power output. That helps leech some of the latent heat out of the engine as well. It also brings along some of its own oxygen to the party in the combustion chamber. That means that it helps the rest of the fuel components burn more completely, improving the efficiency. Unfortunately, that additional oxygen tends to make the fuel charge burn with a somewhat higher heat of combustion, releasing more heat into the engine. This can more or less negate the positive heat-leeching effect. And it also puts the higher heat exactly where you don't want it; in the head and on the piston crown. In your car or truck, that higher heat and improved efficiency means fewer tail pipe emissions. On the kart track, it means more bang out of every drop of fuel going through the carb. Here's the downside. Propylene Oxide is bad for you, real bad. It is corrosive in contact with skin, just like battery acid. It is a skin-absorbable poison, fatal at 1,500 milligrams per two kilograms of body weight. And it had been determined to be a Class 3 carcinogen. Even if you are willing to assume the risks of using this material yourself, you are also exposing any competitor behind you to risk from incompletely combusted Propylene Oxide. Don't do it. Anything less than about 8 percent added to gasoline (by volume) has no measurable effect, but any more than about 3 percent will send the digatron meter sailing.
There are a couple of chemicals in the Nitroparaffin family that are of some interest as oxygenators. Methyl Ethyl Ketone (C2
H5 COCH3), often referred to as MEK, appears on the surface to be an attractive oxygenator. A commonly used industrial solvent, MEK has the unfortunate property of consuming all its own oxygen during its own combustion, leaving none to benefit the remaining combustion process. Coupled with its relatively low specific energy, it's basically a waste of time.
The same goes for Acetone (C3 H6 O), whose relatively meager supply of oxygen isn't even sufficient to support its own combustion, much less lend any to the gasoline reaction. Acetone does have one attractive property, however. It is extremely hydroscopic, meaning that it attracts and absorbs water. In the old days, the McCulloch racers knew this and used to mix acetone with their alcohol to help suspend the moisture that the alcohol attracted and put it in a more combustible form. It will do this in gasoline as well and, since water is not soluble in gasoline at all, but acetone, even acetone that has absorbed some water, is soluble in gasoline, it's a good way to deal with water-contaminated gasoline. But there's no power advantage to be had here and, if you're having a problem with water in your gasoline, you don't need a chemical to fix it. You need a better gasoline supplier. By the way, ketones like MEK and Acetone are also really hard on rubber and plastic parts, like carb diaphragms, etc. In concentrations of less than about 15 percent by volume, it is impossible to see any change in the combustion process, while anything over 10 percent may dissolve your metering diaphragm before the day is done. Sounds like a bad bargain.
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