Top Fuel dragsters produce some of the highest engine power levels in motorsport, and their fuel is a major reason why. Instead of gasoline, they burn nitromethane, a molecule with the formula CH3NO2. The key engineering idea is that nitromethane carries oxygen inside each molecule, so the engine does not have to get all combustion oxygen from incoming air.
This lets the engine burn a much larger mass of fuel during each cycle.
Key Facts
- Nitromethane formula: CH3NO2.
- Balanced combustion: 4 CH3NO2 + 3 O2 -> 4 CO2 + 6 H2O + 2 N2.
- Stoichiometric air fuel ratio for gasoline is about 14.7:1 by mass.
- Stoichiometric air fuel ratio for nitromethane is about 1.7:1 by mass.
- Energy density of gasoline is about 44 MJ/kg, while nitromethane is about 11.3 MJ/kg.
- Power = work / time, so burning more fuel per second can greatly increase engine power.
Vocabulary
- Nitromethane
- Nitromethane is a liquid fuel with formula CH3NO2 that contains oxygen within its own molecules.
- Stoichiometric ratio
- The stoichiometric ratio is the chemically ideal air to fuel ratio for complete combustion.
- Combustion
- Combustion is a chemical reaction in which fuel reacts with oxygen and releases thermal energy.
- Supercharger
- A supercharger is a compressor driven by the engine that forces more air into the cylinders.
- Power
- Power is the rate at which work is done or energy is transferred.
Common Mistakes to Avoid
- Thinking nitromethane has more energy per kilogram than gasoline. This is wrong because nitromethane has lower energy density, but it allows much more fuel mass to burn for the same amount of air.
- Ignoring the oxygen atoms inside CH3NO2. This is wrong because the built in oxygen is what lowers the amount of outside air needed for combustion.
- Using the gasoline air fuel ratio for nitromethane calculations. This is wrong because nitromethane burns near a much richer stoichiometric ratio of about 1.7 kg air per 1 kg fuel.
- Assuming the supercharger alone explains Top Fuel power. This is wrong because the supercharger increases air flow, but nitromethane chemistry is what allows the engine to burn an extreme amount of fuel with that air.
Practice Questions
- 1 A gasoline engine uses 14.7 kg of air per 1 kg of fuel, while a nitromethane engine uses 1.7 kg of air per 1 kg of fuel. For 10 kg of air, how many kilograms of each fuel can be burned at stoichiometric conditions?
- 2 Use energy densities of 44 MJ/kg for gasoline and 11.3 MJ/kg for nitromethane. Using your fuel masses from the first question, estimate the chemical energy released for each fuel with 10 kg of air.
- 3 Explain why a fuel with lower energy per kilogram can still help a Top Fuel dragster engine produce more power than gasoline.