Fuel 101

Many racers have used premium fuels or additives in their cars to improve performance, but many don't completely understand exactly why they are improving performance. Let's attempt to explain some of the basic features of racing fuels as they relate to engines, and how those features affect the ability of the engine to generate power;

BASIC ENGINE FUNCTION

Your engine works because fuel explodes and pushes the piston down, exerting force on the crankshaft, which in turn drives the gearbox and the wheels. The more energy exerted, the more force is put to the wheels - but to get that energy at the 'right time' is of critical importance. A piston travels up and down in the cylinder, based on the rotation of the crankshaft - as it approaches the top of its 'up' stroke, the fuel and air mixture is ready to be exploded; on reaching the top, otherwise known as "Top Dead Centre" or TDC, the fuel explodes, exerting force on the piston and pushing it on its 'down' stroke. The faster the piston moves, though, the trickier this process becomes... the spark will always ignite the fuel at the same speed (under similar conditions), so the faster the piston approaches TDC, the sooner the spark needs to be fired in order to ensure the fuel explodes and pushes the piston into the down stroke. Too late, and the energy is lost because the piston is already on its down stroke (it is still attached ot the crankshaft, remember!) - too early, and the fuel explodes while the piston is still on its way UP, creating a force in the wrong direction. This opposing force works against the crankshaft's rotation, creating the 'pink' or 'ping' that is associated with PREDETONATION and a loss of power because the crankshaft is being slowed down.Ideally, the fuel mixture is exploded just as the piston is beginning its down stroke, creating the maximum 'push' and therefore the maximum power output. To control the timing of the spark, the engine management employs 'ignition advance' or in other words it makes the spark happen earlier and earlier in the rotation of the piston depending on the engine speed. At idle speed, an engine may have little or no ignition advance - at 8,000 rpm it may have 40 degrees of advance, meaning the computer is firing the spark when the crankshaft is 40 degrees away from putting the piston at TDC. (There are 360 degrees in one full rotation, so 40 degrees of advance means the piston is only 2/3 of the way UP the cylinder when the spark is fired).

OCTANE

This is the most commonly known (and often misinterpreted) term used to describe the 'quality' of gasoline. The typical misunderstanding is that the higher the octane, the better the fuel - which is not always true. For any given basic fuel in which you can increase the octane (and nothing else) with some kind of additive, then you are also increasing the ability of that fuel to resist pre-detonation, or "pinking". This in turn allows your engine to have more aggressive ignition timing, and higher compression through installation of different pistons and modification of the head or block. It is these modifications, NOT the increased octane, which give your engine more power... modern electronically controlled engines will recognise the higher octane fuel and will automatically improve their settings to give more power and efficiency, but on return to a lower octane fuel will enter a 'safe mode' with less ignition advance.

How does Octane prevent pre-detonation? In very simple terms, higher octane fuels are more stable under high pressure and heat (the conditions inside your engine) and therefore burn more consistently; poor quality fuels carry unburned residues from one explosion cycle ot the next, and these hot particles can themselves cause predetonation of the next fuel/air mixture if the compression (pressure inside the engine) is high enough. Methanol, one of the purest fuels available, will burn completely into carbon dioxide and water vapour, leaving no residue and giving it a high Octane number, but is another perfect example of why octane doesn't mean power (we will discuss that shortly).

How is Octane measured? The ability of any specific fuel to resist pre-detonation is measured by using a test engine and comparing how that test fuel burned versus pure Iso-Octane fuel.

Quick chemistry lesson - carbon based molecules can be 'prefixed' based on the number of carbon atoms in that molecule: Meth = one, Eth = 2 and so on up to our Oct = 8 example. The -OL suffix indicates an alcohol molecule - Methanol = one carbon alcohol. Methane = one carbon gas, Octane = eight carbon liquid (heavier molecule cannot be a gas like Methane at normal atmospheric conditions).

If the test fuel is as good as Iso-Octane, it pre-detonates at exactly the same compression settings, and has an OCTANE NUMBER of 100. Fuels that pre-detonate before Iso-Octane (lower compression) are given a number lower than 100, fuels that resist predetonation for longer (higher compression) have numbers above 100. Additionally, there are two methods of testing, one which is more stringent that the other, using higher revs, higher inlet temperature and so on. This is known as the MOTOR OCTANE test, the easier being known as the RESEARCH OCTANE test. Since the two are performed using different parameters, they also produce different Octane numbers - Motor Octane Number (MON) usually being lower than Research Octane Number (RON) because it is a more demanding test. At the Gas Pump, the average of the two numbers is usually quoted, giving:

(RON + MON) ÷ 2             or                 (R+M)/2

Gasoline with a pump Octane number of 90 can, therefore, be 100 RON and 80 MON, or 91 RON and 89 MON. In racing fuels, higher MON numbers are good because they indicate how the fuel will react under more difficult conditions.

OXYGENATION

This is a measure of additional oxygenating components in the fuel. To burn, fuels need oxygen which the engine provides in the form of AIR. Limit the oxygen, and you limit the amount of fuel that can be burned inside the cylinder. Turbo-chargers force additional air into the engine, but it is the OXYGEN in that air that the engine needs... if you can find a way to get more oxygen to the fuel, then you can add more fuel - making more power with each explosion and therefore generating more power to the wheels. Oxygenated fuels are a simple way of getting more oxygen into the cylinder for each explosion, thereby allowing the fuel settings to be increased to make a correct burn ratio... too much oxygen makes the car run lean, or HOT, possibly damaging pistons and valves. Too little oxygen makes the engine run rich or COOL, but fouling spark plugs, wasting fuel and potentially diluting engine oil over time, reducing your lubrication protection.

Some racers believe that oxygenated fuels are 'only for turbo-charged cars' - the above should tell you why normally aspirated engines can also benefit.

MORE OXYGEN = MORE FUEL = MORE POWER!

ENERGY

The energy contained within a fuel is one of the most important qualities of that fuel; A basic way to think of this is, longer carbon chain molecules require more energy to create, and therefore release more energy when destroyed. Methanol is a single carbon molecule, and although in our earlier example we stated it had a high Octane value (higher than Iso-Octane) it actually produces less energy when burning than Iso-Octane does. (Think of methylated sprits burning at a low temperature, gasoline burning at a much higher temperature). It takes, on average, TWICE AS MUCH methanol in each explosion inside the engine to create the same power as petroleum based racing fuels - volume for volume, Methanol has about half as much energy as most racing fuels - but the Methanol advantage is its purity which gives superb protection against detonation.

The energy released when burned is the primary reason one fuel will generate more power than another under similar conditions, however our Methanol example shows that some fuels can allow an engine to be modified in such a way that it generates as much power even though the fuel itself has less energy to give up.

VP Rally Max, at 100 Octane, will generate more power than VP MS109 at 109 Octane; RallyMax has more energy per explosion, and even though it is 9 Octane points lower, it produces increases of up to 20 horsepower over MS109 in local WRC cars.