Why nitro? The fuel that makes your eyes water

Stand anywhere near the start line when a fuel bike fires up and you will understand the first thing about nitromethane before anyone explains it: it stings. Within a few seconds your eyes are watering and your nose is telling you to step back. Regulars barely notice. Newcomers usually end up laughing and wiping their faces.

The second thing about nitro takes a little more explaining. Why would anyone run an engine on a fuel that is expensive, unpleasant to be around and capable of destroying the engine it powers? The answer is oxygen.

Fuel needs air

Every internal combustion engine works the same way at heart: fuel and oxygen burn together and the expanding gases push a piston. The limit on how much power you can make is usually not how much fuel you can squirt in. It is how much air you can get into the cylinder to burn it with.

Petrol needs a lot of air. The Wikipedia article on nitromethane puts it at roughly 14.7 kilograms of air to burn one kilogram of petrol. That ratio is why road cars have big air filters, why turbochargers exist, and why engine tuners obsess about intake design.

Nitro brings its own

Nitromethane is a small molecule with oxygen already built into it. The same article gives the air needed to burn a kilogram of nitromethane as about 1.7 kilograms, a fraction of what petrol needs. That changes the arithmetic completely.

A kilogram of nitro actually contains less energy than a kilogram of petrol, roughly a quarter as much by the article's figures. But because each breath of air can burn so much more of it, an engine can pour in many times more fuel per cycle. The Top Fuel article puts it this way: for a given amount of air, an engine can burn around 7.6 times as much nitromethane as petrol, and an engine burning nitro can make up to about 2.4 times the power.

In other words, nitro is not a "stronger" fuel in the way people often assume. It is a fuel that lets you burn far more of it in the same space.

What that does to an engine

Put enormous quantities of fuel into a cylinder and some odd things happen:

• The engine runs rich, deliberately. Nitro engines are usually run with more fuel than can be fully burned, because the excess helps cool the engine and protect it from uncontrolled detonation.
• Fire comes out of the pipes. The Top Fuel article notes that unburned nitro can escape the exhaust and ignite in the open air, which is where the long flames you see at night meetings come from.
• Parts wear fast. Pressures and temperatures inside a nitro engine are extreme. Inspection and part replacement between rounds is normal, not exceptional.
• Things can let go. A cylinder that fills with more liquid fuel than it can burn can lock the engine or damage it badly. Superchargers can fail violently. This is why fuel classes require blankets, restraints and other safety equipment that slower classes do not.

Why your eyes water

The sting comes from the exhaust. Burning nitromethane in a rich mixture produces a cocktail of combustion products, including nitrogen compounds that irritate eyes and airways. The irritation is mostly a nuisance for spectators at a distance, but crews who work close to fuel engines for long periods take it more seriously, and many wear eye protection and keep their exposure short.

If you find the fumes uncomfortable at a meeting, the easy answer is to move back from the start line or watch from further down the track. It is not worth toughing out.

A hazardous substance, not a party trick

Nitromethane is a hazardous substance as well as a fuel. It is flammable, it can be sensitive to shock and contamination in some conditions, and it needs careful storage and handling. In New Zealand, the storage and handling of hazardous substances at workplaces falls under rules administered by WorkSafe New Zealand, and tracks and sanctioning bodies set their own requirements on top for fuel handling in the pits.

None of which is a how-to. Nothing in this piece is advice on buying, storing or using nitro. If you are curious about fuel classes, the people to talk to are the sanctioning body and experienced crews at the track.

Why bikes use it

For a motorcycle, the attraction is the same as for a car, and in some ways greater. A bike engine is small and light. Running it on nitro, usually with a supercharger, lets that small engine make power on a scale that would otherwise need something far bigger. The trade-offs are the same too: cost, complexity, wear and a constant need for careful tuning.

The tuning is where much of the craft lies. A crew adjusting fuel delivery and ignition timing for the day's weather and track is balancing as much power as possible against the point where the engine hurts itself. Get it right and the bike runs hard and straight. Get it wrong and you find out quickly.

The smell of the sport

For many people who grew up around drag racing, nitro is part of the memory: the sting in the eyes, the sweet chemical smell, the shake in the ground. It is not for everyone. But it explains why the quickest bikes in the sport are as quick as they are, and why the people who run them treat the fuel with so much respect.