Oxygenators are the 'Hottest' Topic in Fuel Chemistry

[rsss396]

[TD=width: 50%]link http://www.foxvalleykart.com/print/prt_fuel4.html





THE QUESTION OF FUEL
PART 4 - Oxygenators are the 'Hottest' Topic in Fuel Chemistry [/TD]
[TD=width: 50%]

[/TD]
[/TR]
[TR]
[TD=width: 100%, colspan: 2]
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.

[/TD]
[/TR]
[/TABLE]

[rsss396]

EXPENSIVE ALTERNATIVE

Nitropropane ((CH3)2CHN02) is a rather expensive nitroparaffin that is, in the right form, about 70 percent as potent as Nitromethane (CH3 N02). I say "the right form" because Nitropropane comes in two forms, called Nitropropane I and Nitropropane 2. Nitropropane I is the most readily available, because it is a sometimesused cleaning solvent. Unfortunately, it is completely worthless as a combustion reactant for our purposes. Nitropropane 2, however, contributes significantly to oxygenation of the combustion process when used in concentrations of 10 percent or more by volume. Its primary hazard is that it is extremely volatile, sensitive to even ignition by static electricity. And, at over $50 per gallon on the open market, few racers will be tempted to mess with it.
The ethers are a family of oxygen-bearing hydrocarbons that have drawn increasing attention from the fuel industry. With a relatively high percentage of oxygen per volume (15 to 18 percent), they bring considerable free oxygen to the combustion process. But unlike the alcohols, they can actually improve vaporization over straight gasoline, while reducing exhaust emission in passenger cars and trucks. The result is what the industry calls "improved drivability" and relates primarily to cold weather starting and cold engine running. Of more interest to us is the higher heat of vaporization and its resulting colder inlet charge and heat leeching, as mentioned above.

The most widely known ether is Ethyl Ether (C2 H5 OC2 H5) and it is the primary ingredient in automotive "starting fluid" sprays. Incredibly volatile, it will vaporize even at sub-zero temperatures and is just the ticket for getting your Chevy started on a bitter cold morning. Thankfully, we don't race in such conditions. For our purposes, Ethyl Ether simply is too volatile; it evaporates too quickly and at too low of a temperature to render it as a useful additive in karting. Besides, Ethyl Ether's telltale odor makes it very hard to hide. One other serious problem with some ethers is their tendency to form unstable, explosive compounds called
Peroxides. These dangerous compounds can develop when ethers are exposed to either heat and/or sunlight, even in closed containers. For the most part, ethers are relatively safe, healthwise. Like any other hydrocarbon, of course, they are harmful or fatal if
swallowed, but most members of this chemical that we are likely to encounter in fuels are relatively safe.
A very important exception to the previous comment is a material called Diethylene Ether H8 02), or more commonly referred to as Dioxane. With twice the oxygen per molecule of Ethyl Ether, it would seem to be an attractive oxygenator. PLEASE READ THIS! DIOXANE IS A VERY POWERFUL SKIN-ABSORBED POISON AND KNOWN CARCINOGEN. It is neither safe to handle, nor to breathe, nor to be around in any way. Its combustion products, in the form most likely to be emitted behind a kart, are also poisonous and carcinogenic. This is nothing to fool around with! Anyone foolish enough to monkey with this material has no business on the racetrack and no business in the sport! There is also a compound called Dioxine, but it is of no value whatsoever as a combustion additive, although someone may accidentally refer to Dioxane as Dioxine and vice-versa.

NEW FUEL ADDITIVES

On a happier note, you may have heard about some new fuel additives from the oil companies called MTBE, ETBE and TAME. These are ethers too and the letters stand for Methyl Tertiary-butyl Ether, Ethyl Tertiarybutyl Ether, and Triamyl Methyl Ether. The first two compounds are made by reacting Methanol or Ethanol with isobutylene and all three have found considerable success as gasoline additives, yielding significant oxygen to the combustion process.

The oil companies have seized on MTBE and TAME, and more recently on ETBE, as environmentally friendly ways to enhance octane rating, improve drivability, and "stretch" gasoline through the use of renewable resources. It is unlikely that you would see any significant improvement in engine performance by adding these compounds in quantities beyond what the oil companies are already putting in the fuel, between 15 and 19 percent by volume. The only way to be sure that these compounds are not in the gas you take to the track is, as we've said before, to purchase racing gasoline from a reputable dealer of racing gasoline, from the drum. However, if you wish to experiment with them, and if you can find a resource to provide them, they are reasonably safe to use. Again, 15 to 19 percent is the industry standard. At this point we have not finished the research to determine how these additives will affect the digatron meter or how much will send it over the magic "0.00" mark.

So let's summarize the subject of oxygenators. Given that we can't ever get enough oxygen from the limited amount of air the engine can suck down the carb throat to affect really efficient, complete combustion of the fuel, some oxygenators can provide additional "free" oxygen to enhance the combustion process. The most common of these are the alcohols, but, because of their dielectric properties, they won't get past the digatron meter test at tech. Propylene Oxide and some of the other Nitro-Paraffins are good sources of oxygen, but are corrosive to engine and carb parts, and some of them are very dangerous, health-wise. Ketones are, for the most part, worthless as oxygen sources, as they consume all their own oxygen during combustion, leaving none to improve the combustion of the other fuel. And, finally, ethers can improve combustion and liberate additional heat energy from the fuel, but require such large percentages to achieve the desired results as to be hard to conceal. And Dioxane, an ether, is way too dangerous to monkey around with.

One more thing; the use of oxygenators in air-cooled engines is a particularly awkward juggling act. Remember, we said in the first installment of this series that tests had shown that we use almost 50 percent of the fuel that goes into the engine as a COOLANT. Well, when you bump up the oxygen level of the fuel mixture, whether by adding more air or by adding oxygenators, the heat of combustion and the temperature in the engine will go up accordingly. Oxygenators, in effect, lean out the engine. Given the cooling limitations of the air-cooled engine, the only option is to richen the mixture and there goes any hope of a significant performance advantage. What we're saying is this: There is, most likely, one or more oxygenators already present in any gasoline you can buy at the pump these days. These compounds may cause your fuel to fail the digatron fuel meter at tech. The only way to avoid it is to buy race gas from a barrel. Adding oxygenators to fuel on your own is dangerous, to you and to your fellow competitors. Any performance gain that you might have achieved from the addition of oxygenators to your fuel is, if you use an air-cooled engine, most likely negated by having to run the mixture richer to compensate for the additional heat that the oxygen-enriched fuel generates when it burns.