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Nitromethane

Nitromethane is a fuel that is used in racing, particularly in drag racing, to provide more power. When you hear the term "nitro-burning funny car" or "top-fuel dragster," that means that the engine burns nitromethane. 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 (14.6 kilograms) of air to burn a 2.2 pounds (kilogram) of gasoline, and only 3.7 pounds (1.7 kilograms) 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 (42 MJ/kg). Nitromethane provides 5,000 BTU/pound (12 MJ/kg). The amount of nitromethane also provides some cooling, making the charge a bit denser and increasing power.

The flamefront 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 is why Funny Cars and Dragsters "spit fire" 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 approximatlly 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 for nitromethane drag racing engine 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.

The Lewis dot structure of nitromethane has a resonance structure, thus the nitrogen-oxygen bonds can be considered to have a bond order between one and two.

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