Could Nitromethane Save on Gas?

You have probably heard about how racers will mix nitromethane or nitroethane with their fuel for a power boost. Could mixing it with your street car fuel give a gas mileage boost? The answer is no, but let us see why.

Your car's engine is fundamentally an air breather, just like people. It gets energy from the chemical reaction of fuel with oxygen (from the air) and both of those ingredients have to be present. The proportions matter. Thinking simplistically, imagine that one part oxygen reacts with one part fuel. The end products of the reaction are carbon dioxide (the greenhouse gas) and water. Gasoline is mainly made of short chain alkanes, such as octane. The complete combustion of octane with oxygen is described by the chemical equation

2 C₈H₁₈ + 25 O₂ --> 16 CO₂ + 18 H₂O

We can see that we need 25 oxygen molecules from the air to burn two octane molecules. This oxygen is what makes your engine an air breather. Without air there would be no oxygen and the engine cannot run. Air is also the limiting factor. When the intake stroke fills the cylinder with fuel air mixture there will be a certain amount of octane and oxygen. Reacting all of it gives the maximum possible energy output of that explosion. If the balance is not perfect, the component in excess goes to waste. A rich mixture has too much fuel and a lean mixture has too little fuel for the amount of oxygen.

Air is very thin, much less dense than fuel. That means the oxygen is the limiting factor. It is very easy to produce a cylinder full of fuel, but without oxygen it can't be burned. This is where turbochargers and superchargers come in. They pressurize the air to increase the density and pack more of it into the cylinder. This allows more fuel to be burned per cycle. Looking at the picture of a nitromethane molecule above, you can see two red atoms. Those are oxygen. Nitromethane has the chemical formula

CH₃NO₂

Those two oxygen atoms mean that the nitromethane is bringing oxygen into the cylinder. And because nitromethane is a liquid and not a gas it has a density a thousand times greater than air. Nitromethane allows you to greatly increase the amount of oxygen in the cylinder, much more than a turbocharger or supercharger could. And that in turn means you can now add more fuel and thus get a bigger energy output on the power stroke of the cycle.

So nitromethane does not increase the efficiency at which an engine burns fuel. It allows the engine to burn more fuel per cycle which increases the power output. Running at 2000 RPM a six cylinder engine will experience 12000 combustion events in its cylinders. By combusting more fuel each of those 12000 times we have a larger power output. But we also burned more fuel, so no increase in efficiency.

Nitromethane lets you burn more fuel in less time for better power but does not get you more mileage. Good for racers, but not for saving on gas.