Nitromethane - Uses - As An Engine Fuel

As An Engine Fuel

Nitromethane is used as a fuel in motor racing, particularly drag racing, as well as for rockets and radio-controlled models (such as cars, planes and helicopters) and is commonly referred to in this context as "nitro." The oxygen content of nitromethane enables it to burn with much less atmospheric oxygen.

4CH₃NO₂ + 3O₂ → 4CO₂ + 6H₂O + 2N₂

The amount of air required to burn 1 lb (0.45 kg) of gasoline is 14.7 pounds (6.7 kg), but only 1.7 lb (0.77 kg) of air is required for 1 lb of nitromethane. Since an engine's cylinder can only contain a limited amount of air on each stroke, 8.7 times more nitromethane than gasoline can be burned in one stroke. Nitromethane, however, has a lower energy density: Gasoline provides about 42–44 MJ/kg whereas nitromethane provides only 11.3 MJ/kg. This analysis indicates that nitromethane generates about 2.3 times the power of gasoline when combined with a given amount of oxygen.

Nitromethane can also be used as a monopropellant, i.e., a fuel that burns without added oxygen. The following equation describes this process:

2 CH₃NO₂ → 2 CO + 2 H₂O + H₂ + N₂

Nitromethane has a laminar combustion velocity of approximately 0.5 m/s, somewhat higher than gasoline, thus making it suitable for high speed engines. It also has a somewhat higher flame temperature of about 2,400 °C (4,350 °F). The high heat of vaporization of 0.56 MJ/kg together with the high fuel flow provides significant cooling of the incoming charge (about twice that of methanol), resulting in reasonably low temperatures.

Nitromethane is usually used with rich air/fuel mixtures because it provides power even in the absence of atmospheric oxygen. When rich air/fuel mixtures are used, hydrogen and carbon monoxide are two of the combustion products. These gases often ignite, sometimes spectacularly, as the normally very rich mixtures of the still burning fuel exits the exhaust ports. Very rich mixtures are necessary to reduce the temperature of combustion chamber hot parts in order to control pre-ignition and subsequent detonation. Operational details depend on the particular mixture and engine characteristics.

A small amount of hydrazine blended in nitromethane can increase the power output even further. With nitromethane, hydrazine forms an explosive salt that is again a monopropellant. This unstable mixture poses a severe safety hazard, and is forbidden for use in model aircraft fuels.

In model aircraft and car glow fuel, the primary ingredient is generally methanol with some nitromethane (0% to 65%, but rarely over 30%. However, in NHRA Drag racing the mixture is 90% nitromethane and 10% methanol, since nitromethane is expensive compared to methanol) and 10–20% lubricants (usually castor oil and/or synthetic oil). Even moderate amounts of nitromethane tend to increase the power created by the engine (as the limiting factor is often the air intake), making the engine easier to tune (adjust for the proper air/fuel ratio).