Fuel and Octane Boosters

Regardless of whether the fuel supply on your car is a carburetor fed or controlled by an electronic fuel injection system, the better the quality and the more volatile the air/fuel mixture is, the better the power and throttle response you will get from your car. We've discussed the air/fuel mixture elsewhere; here we'll look at fuel quality, which comes down to the octane rating.

Octane rating

In our section of the 4-stroke engine we mentioned that the air/fuel mixture gets drawn into the engine cylinder and is compressed into a small space during the compression stroke, at the end of which the mixture is ignited when the spark plug fires. When the air/fuel mixture ignites spontaneously, i.e., before the spark plug fires, it is called preignition, knocking or detonation, which can quickly reduce the engine to a pile of scrap metal! One of the causes of preignition or detonation is tendency for the air/fuel mixture to ignite spontaneously when it is compressed too much.

However, one of the goals of engine modification is to increase the amount of air/fuel mixture in the cylinder and then to compress the mixture as much as possible and allows for the advancing of ignition timing without causing spontaneous combustion. The octane rating of the fuel is one area where we can suppress spontaneous ignition of the air/fuel mixture as the octane rating of a fuel indicates the extent to which the fuel can be compressed before it ignites spontaneously.

Before 1999, the octane rating of fuel was boosted by the addition of a chemical by the name of tetraethyl lead, allowing cars to run with higher compression ratios without the side effects of detonation and engine knock. However, leaded fuel had a negative effect on the environment and most refineries have replaced tetraethyl lead with methyl tertiary butyl ether (MTBE), which is created from methanol, and also increases the octane rating.

Alternative Fuels

There are also other alternative fuels, such as nitromethane, which is used in the top fuel dragsters. The chemical formula for nitromethane is CH3NO2, which is similar to gasoline with the addition of nitrous oxide (NO2). As you now know, nitrous oxide has its own oxygen atoms that are released during combustion. Thus, a lower air to fuel ratio is required when using nitromethane. With gasoline it is typically 14,7 mass of air to 1 mass of gasoline but in a nitromethane the ratio drops to 1,7 mass air to 1 mass of nitromethane. This translates to more power per stroke. However, nitromethane is not as dense as gasoline. Furthermore, nitromethane does not burn as fast as gasoline; as a result burning nitromethane is often expelled during the exhaust stroke resulting in flames shooting out the exhaust. Thus the increase in power is not directly related to the increase is the amount of fuel in the cylinder. Nevertheless, an increase of around twice the amount of power is achievable by switching to nitromethane.

Alcohol fuels like methanol are another alternative fuel that is extremely volatile. Alcohol fuel is used in the American IRL series and CART series. One of the disadvantages of alcohol fuels is the high scouring effects on tanks and pipes and it has very high consumption figures as the air to fuel ratio of alcohol is about 7.1 to 9.1 mass of air to 1 pound of alcohol.

Alcohol fuels also have a higher latent heat value, allowing a higher density of charge entering the engine, and has exceptional cooling properties, which counteracting the effect of high internal temperatures. Thus, the compression ratios of cars running on alcohol based fuels can be as high as 19:1!

Octane Boosters

Octane boosters are one ways of increasing the octane rating of stock fuel. Toluene is one such octane booster that is very popular. If was used during the turbo-era of Formula 1 in the early 1980's when the turbocharged 1500cc engines running at around 5 bar of boost pressure were in use. As you know, this amount of boost pressure is quite high, but Toluene allowed these engines to operate at these extremely high boost pressures by suppressing detonation.

Toluene is both denser than gasoline and it has a higher heat rating. As a result it releases more energy per unit volume during combustion. The higher heat rating means that exhaust gasses are able to carry a higher kinetic energy which is advantageous for turbocharged engines as it has more force to turn the vanes on the turbine wheels, reducing turbo lag.