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The Explosive Facts About Racing Fuels

Nitromethane Racing Fuel

Nitromethane is a fuel that is used in drag racing to provide maximum power. When you hear the term "Nitro-Burning Funny Car" or "Top Fuel Dragster", that means the engine burns nitromethane fuel. Model aircraft fuel contains about 10% nitromethane.

Nitromethane's chemical formula is CH3NO2. For comparison, gasoline is typically C8H18. The oxygen in nitromethane's molecular structure means that nitromethane does not need as much atmospheric oxygen to burn—part of the oxygen needed to burn nitromethane is carried in the fuel itself.

You need 32.1 pounds of air to burn a 2.2 pounds of gasoline, and only 3.7 pounds of air for the same amount of nitromethane to burn. A cylinder can only hold so much air on each stoke, and with that amount of air you can burn 8.7 times more nitromethane than gasoline. By pumping in 8.7 times as much nitromethane per stroke, you get about 2.4 times more power per stroke. Gasoline provides 18,000 BTU/pound. Nitromethane provides 5,000 BTU/pound. The amount of nitromethane also provides some cooling, making the charge a bit denser and increasing power.

The flame front does not move as quickly in nitromethane as it does in gasoline, meaning that there is not enough time to burn all the nitromethane in the cylinder when the engine is running at high RPM. When the exhaust valve opens, burning nitromethane flows out through the exhaust pipe. That’s why Nitro Motors spit flames from their exhaust pipes.

Nitro Facts

Nitromethane (CH3NO2) is a fuel commonly used in drag racing in both cars and boats.

Nitro fuel burns yellow, the white flames you see at night from the headers is raw burning hydrogen. The flame front of nitro in the combustion chamber is approximately 7050 degrees F. If a cylinder loses spark for a second, nitro fuel accumulates in the cylinder and can cause a massive engine explosion (hydraulic a cylinder), nitromethane does not compress too well. A nitro-burning dragster will use approximately 14-15 gallons of nitro for start-up, staging and a 1/4 mile run.

It is usually use as a mixture with methanol to reduce peak flame temperatures. Its high heat of vaporization results in significant cooling of the air fuel mixture entering an internal combustion engine. The fuel energy of nitromethane is 2.3 times that of iso-octane gasoline for the same mass of air.

The combustion reaction of nitromethane is:

4CH3NO2 +3O2 -> 4CO2 + 6H2O + 2N2

This exothermic reaction results from the strong nitrogen-oxygen bonds. The presence of oxygen in the structure leads to more efficient combustion.

Nitromethane isn't a widely used fuel source since the combustion yields mixtures containing relatively large amounts of nitric acid vapor. The drivers of vehicles fueled by nitromethane (or nitro as the racers refer to it), as well as workers in the starting area must wear gas masks to protect themselves from inhaling the nitric acid vapor.

Nitromethane (MSDS) Material Safety Data Sheet

Methanol Racing Fuel

Methanol, also known as methyl alcohol or wood alcohol, is a chemical compound with chemical formula CH3OH. It is the simplest alcohol, and is a light, volatile, colorless, flammable, poisonous liquid that is used as an antifreeze, solvent, fuel, and as a denaturant for ethyl alcohol.

Methanol is produced naturally in the anaerobic metabolism of many varieties of bacteria. As a result, there is a small fraction of methanol vapor in the atmosphere. Over the course of several days, atmospheric methanol is oxidized by oxygen by the help of sunlight to carbon dioxide and water.

Methanol burns in air forming carbon dioxide and water:

The combustion reaction of methanol is:

2CH3OH + 3 O2 --> 2CO2 + 4H2O

A methanol flame is almost colorless. Care should be exercised around burning methanol to avoid being burned.

Racing Gasoline Basics

The most important characteristic of gasoline is its ''Research Octane Number'' (RON) or octane rating, which is a measure of how resistant gasoline is to premature detonation (engine knocking). It is measured relative to a mixture of (octane/isooctane), (2,2,4-trimethylpentane) and (n-heptane). So an 87-octane gasoline has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane.

There is another type of octane, called "Motor Octane Number" (MON), which is a better measure of how the fuel behaves when under load. Its definition is also based on the mixture of isooctane and n-heptane that has the same performance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.

In most countries  (including all of Europe and Australia) the 'headline' octane that would be shown on the pump is the RON, but in the United States and some other countries the headline number is in fact the average of the RON and the MON, sometimes called the "Research Octane Number" or RON. Because of the 10 point difference noted above this means that the octane in the United States will be about 5 points lower than the same fuel elsewhere. 87 octane fuel in the United States would be 92 in Europe.

It is possible for a fuel to have a RON greater than 100. This reflects the fact that isooctane is not the most knock-resistant substance available.  Racing fuels, (Avgas) and (liquefied petroleum gas [LPG]) typically have octane ratings of 110 or significantly higher.

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful.  Using a fuel with a higher octane allows an engine to be run at a higher compression ratio without having problems with knock. Compression is directly related to power, so engines that require higher octane usually deliver more power. Some high-performance engines are designed to operate with a compression ratio associated with high octane numbers, and thus demand high-octane gasoline. It should be noted that the power output of an engine also depends on the energy content of its fuel, which bears no simple relationship to the octane rating.  Some people believe that adding a higher octane fuel to their engine will increase its performance or lessen its fuel consumption.  This is false - engines perform best when using fuel with the octane rating they were designed for.

The octane rating was developed by the chemist Russell Marker. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, unmixed with other isomers of heptane or octane, distilled from the resin of Jeffrey Pine. Other sources of heptane produced from crude oil contain a mixture of different isomers with greatly differing ratings, which would not give a precise zero point.

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