About FuelRacing is all about fuel. It amazes me how little is really known about it. I’m pretty sure that if there is a fuel that has been tried, I’ve tried it. Mostly back in the 60’s. Everything I’m writing today I didn’t know then, I just dumped it in the tank and tried it. This isn’t a technical paper. You need a PHD to understand the chemical and physics of what is going on. I’m going to mention numbers that are reasonable approximations, but will give you a good idea what is going on.
SCTA rules for motorcycles and cars allow:
GasolineAnything other than gasoline runs in the fuel class.
Fuel
Unapproved gasoline.Alcohol, Methanol in this case.
NitromethaneNitrous Oxide is also allowed and will be discussed last. It’s not a fuel, but is classified as one in the rule book.
And for cars only:
Hydrogen. I’m not going to cover hydrogen. If you are using it you already have a PHD and it’s too complex to cover here.
Diesel There is a separate class for diesel.
You can’t talk about fuel without getting a few related items out of the way.
The internal combustion engine is a heat engine. The heat is derived through a chemical reaction between the fuel and the oxygen in the air. The chemical reaction creates heat, and the heat causes the combustion gasses to expand and push the piston down. The chemical reaction is started by a source of ignition, a spark plug. It is important to realize that almost all of the work that is done to make an engine go faster is to get more air in the engine. Cams, valves, porting, polishing, etc. The power is limited by the air. Fuel you can pump in with a fire hose.
AirAir is composed of 78 percent nitrogen and 21 percent oxygen. It’s the 21 percent oxygen that concerns you. The nitrogen just gets in the way. You can increase the percentage with nitrous oxide. We will get to that later.
You can’t change the percentage, but you can stuff more of it in with a supercharger or turbocharger. It doesn’t change the fuel equation and we won’t be discussing it here.
We won’t be discussing the effects of compression ratio either. Well, ok, just a little. The distance between molecules in an uncompressed fuel/air mix makes it very slow to burn. The more you compress the charge, the closer the molecules, the faster the burn, the more power you can get from it. The density of the fuel/air mixture is the subject of a great deal of interest throughout the racing world. The cooler the charge of fuel and air going into the engine, the denser it will be. And the denser it is, the more potential energy there is in each incoming charge.
But this discussion is mostly about fuel.
GasolineThe first thing you have to realize is that saying “gasoline” is like saying “wine” and expect to describe all the different varieties, pinot, cabernet, merlot, etc. Like wine, there are unlimited variations. The formulation of the gasoline you buy for your car, or your race car, is unknown, and can change at any time. You have no way of knowing. Winter gasoline is different from summer gasoline. In cold climates gasoline has to be more volatile to ignite. In summer it has to be less volatile to avoid vapor lock. Gasoline of the exact same brand in one location is different from one 20 miles away. Very different.
Crude oil is a product of the remains of prehistoric plants and animals. After refining you end up with various blends of hydrocarbons, This consists of hydrogen atoms and carbon atoms in various combinations.
Gasoline can contain any number of hydrocarbons. Two examples:
A particular blend of gasoline can have 40 different hydrocarbons blended. And they all consist of hydrogen and carbon, generally nothing else.
To avoid a PHD in chemistry, we can boil it down to a couple of terms that are of value to the land speed racer.
BTU – A measure of heat.
Octane – A measurement of anti-knock properties.
Stochiometric Air/fuel ratio – The correct balance of air and fuel that will consume all of the fuel with all of the oxygen.
Heat of vaporization – How quickly it evaporates, and cools the engine.
BTU’s describe the heat output of a fuel
The heating value of fuel is the entire issue. The more heat you can produce, the faster it will go. (Until it melts.)
Look up British Thermal Unit if you want the specifics, It’s just a reference number here.
Gasoline (varies) 20,943 BTU’s/lb
Methanol 9,770 BTU’s/lb
Nitromethane 5,160 BTU’s/lb
If this was the only criteria, we would only run gasoline. But the fuel is useless without air.
Stochiometric Air/fuel ratioTo have the maximum power output we need to determine the correct amount of fuel to use with the air in the cylinder. Again, the engine can only suck in a given amount of air.
To determine how much fuel is needed for a given amount of air you need to analyze every hydrocarbon in the gasoline and determine the chemical bonds that are broken and reformed with the oxygen. We’re going to shorten the math and say the 14.7:1 by weight is the approximate correct ratio. 14.7 lbs of air to 1 lb of gasoline. Because of the way the fuel ignites in the cylinder under pressure maximum horsepower is generally achieved at ratios closer to 12:1. “Rich” in the fuel nomenclature. The mixture in contact with the relatively cold cylinder walls, the fuel that doesn’t get vaporized and stays as a droplet and other things keeps some of the fuel from being utilized. Your correct ratio can only be roughly determined on a dynamometer with final tuning at Bonneville on the salt.
The amount of air available to the engine varies with changes to altitude, temperature and humidity. The higher the altitude, the thinner the air, the hotter the air the thinner, and humidity displaces the air making less available. The air/fuel ratio stays the same. You have to determine how much air is available to you. This is where mass air flow sensors are used on the intake to determine how much air is going in, and oxygen sensors on the exhaust to determine if excess oxygen is going out. Don’t have them? Spark plug color is the key. The oil mixed with the gasoline in two strokes ruins oxygen sensors.
If you have more fuel than air, the mixture is considered “rich” and more oxygen than fuel would be “lean”.
So when we consider the correct fuel/air ratio:
Gasoline at 20,943 BTU’s/lb at 14.7:1 air/fuel ratio by mass equals 0.068 lbs of fuel/lb of air or
1415 BTU’s/lb of air.
For comparison:
Methanol 9,770 BTU’s/lb at 6.4 air/fuel ratio by mass equals
1527 BTU’s/lb of air. Methanol gets you about 22% more bang and runs cooler.
Nitromethane 5,160 BTU’s/lb at 1.7:1 air/fuel ratio by mass equals
3035 BTU’s/lb of air. Nitromethane gets you 123% more heat. It’s no wonder the top fuel drag racers use it.
Someone asked recently about the difference between MR8 and U2. These are VP racing fuels, and they make fine racing fuels, but other than octane rating, you can’t tell one from the other by the specifications. The heating value of fuel is what makes horsepower, not octane! Higher octane fuels may have a lower heating value and produce less horsepower.
ERC Racing Fuels does publish those figures and they are listed below. If you look at ERC 110K versus ERC A-8C, the octane rating is higher, but the heating value is lower. The higher heating value doesn’t do you any good if you can’t avoid detonation, or knock, so it’s a trade off. The low compression ratios of a two stroke work to your advantage in running a higher heating value fuel.
OctaneGasoline engines prior to the 1930’s produced very low power due to the fact that gasoline has a high volatility and can ignite during the combustion process on its own. Diesel engines rely on this, gasoline engines don’t tolerate it. Various compounds are added to gasoline to prevent premature ignition. Leaded gasoline uses tetra-ethyl lead and other lead compounds. Unleaded gasoline uses methyl tertiary-butyl ether (MtBE) among others.
Octane is a fuel's ability to resist detonation and/or preignition. Octane is rated in Research Octane Numbers, (RON); Motor Octane Numbers, (MON); and Pump Octane Numbers (RON+MON/2 or R+M/2). Pump Octane Numbers are what you see on the yellow decal at gas stations, representing the average of the fuel's MON and RON. VP Racing Fuels uses MON because this test method more accurately simulates racing conditions. The conditions under which fuels are tested using the RON method are not as demanding, thus the number is normally higher than the MON rating. This leads many other fuel companies to rate their fuels using the RON in an effort to make them appear more resistant to detonation. Don't be fooled by high RON numbers or an average -- MONs are the most relevant ratings for a racing application. Be aware, however, the ability of fuel to resist detonation is a function of more than just octane.
Does higher octane mean more horsepower? No! The heating power of the fuel determines the horsepower, and higher octane fuels can have a lower heat value. Drat! More heat, more knock. Less knock, less power. Not necessarily. You do have to look at the heat value of the fuel, because if you don’t, the guy sailing past your record did! Read on for ways to deal with knock.
Combustion comes in three forms: Normal combustion, Pre-ignition, and detonation.
Under ideal conditions the internal combustion engine burns its fuel air mix in the cylinder in an orderly and controlled fashion. The combustion is started by the spark plug some 15–40 crankshaft degrees prior to TDC (top dead center) at the point of maximum compression. This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases. This point is typically 14–18 crankshaft degrees ATDC (after top dead center).
The spark plug produces an electrical spark that jumps a small gap from its center electrode to its ground electrode. This spark, if the air/fuel mix is within the flammable range for the fuel, initiates combustion. The initial phase forms a small kernel of flame approximately the size of the spark plug gap. For the first few milliseconds of the combustion process, this flame kernel is struggling to survive, producing only slightly more heat than is necessary to continue the combustion process. As it grows in size its heat output increases allowing it to grow even faster.
After this early slow burn phase passes, the flame kernel grows much faster expanding rapidly across the combustion chamber. This growth is due to the travel of the flame front through the combustible fuel air mix itself and due to turbulence rapidly stretching the burning zone into a complex of fingers of burning fuel air that have a much greater surface area than a simple spherical ball of flame would have. This greatly accelerates the combustion process.
In normal combustion, this flame front moves throughout the fuel air mix at a rate characteristic for the fuel-air mixture. Pressure rises smoothly to a peak, burning nearly all the available fuel then falls as the piston descends. In normal combustion this produces a rapid increase in cylinder pressure as the piston passes TDC and begins to move down the cylinder. As mentioned above in a properly tuned engine the maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC, so that the increasing pressure can give the piston a hard push when its speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases.
When we refer to mixture, we don’t mean air on one side and a cup of gasoline on the other. If you ignited a cup of gasoline it will burn a long time. The only gasoline that will burn is the surface that is in contact with the air. For the engine to achieve maximum power the liquid fuel has to be vaporized. The smaller the drop size, the more in contact with the air. Poor vaporization means that a lot of unburned fuel, and power, is going to go out the exhaust. Even if we had a perfectly vaporized mixture at the correct ratio, but uncompressed, we would have a hard time igniting it because of the distance between molecules. As we compress the mixture we put the molecules closer, and the heat of compression makes them more active and more readily able to combine. Higher compression generally leads to more horsepower, but other factors like detonation lead to limits on compression ratios.
Detonation “Knock”
The fuel/air mixture is normally ignited slightly before the point of maximum compression to allow a small time for the flame-front of the burning fuel to expand throughout the mixture so that maximum pressure occurs at the optimum point. The flame-front moves at roughly 33.5 m/second (110 feet/second) during normal combustion.
It is only when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time that the detonation occurs. It is caused by an instantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinder pressure rises dramatically beyond its design limits and if allowed to persist detonation will damage or destroy engine parts.
Detonation can be prevented by:
The use of a fuel with higher octane rating
The addition of octane-increasing "lead", methylcyclopentadienyl manganese tricarbonyl (MMT), isooctane, or other antiknock agents.
Increasing the amount of fuel injected/inducted (resulting in lower Air to Fuel Ratio)
Reduction of cylinder pressure by increasing the engine revolutions (lower gear), decreasing the manifold pressure (throttle opening) or reducing the load on the engine, or any combination.
Reduction of charge (in-cylinder) temperatures (such as through cooling, water injection or compression ratio reduction).
Retardation of spark plug ignition.
Improved combustion chamber design that concentrates mixture near the spark plug and generates high turbulence to promote fast even burning.
Use of a spark plug of colder heat range in cases where the spark plug insulator has become a source of pre-ignition leading to detonation.
Correct ignition timing is essential for optimum engine performance and fuel efficiency. Modern automotive and small-boat engines have sensors that can detect knock and retard (delay) the ignition (spark plug firing) to prevent it, allowing engines to safely use gasoline of below-design octane rating, with the consequence of reduced power and efficiency.
A knock sensor consists of a small piezoelectric microphone, on the engine block, connected to the engine's ECU. Spectral analysis is used to detect the trademark frequency produced by detonation at various RPM. When detonation is detected the ignition timing is retarded, reducing the knocking and protecting the engine.
Pre-ignitionPre-ignition is a different phenomenon from detonation, explained above, and occurs when the air/fuel mixture in the cylinder (or even just entering the cylinder) ignites before the spark plug fires. Pre-ignition is caused by an ignition source other than the spark. Heat or hot spots can buildup in engine intake or cylinder components due to improper design, for example, spark plugs with a heat range too hot for the conditions, or due to carbon deposits in the combustion chamber. Spark plugs with a high heat range will run hot enough to burn off deposits that lead to plug fouling in a worn engine, but the electrode of the plug itself can occasionally heat soak, and begin glowing hot enough to become an uncontrolled ignition source on its own. Bits of carbon that build up in a combustion chamber can also heat soak to the point where they also are glowing hot and ignite the air-fuel mixture before the proper time.
Pre-ignition and "dieseling" or "run on" are the same phenomenon, except in the latter case the engine continues to run after the ignition is shut off with a hot spot as an ignition source. Pre-ignition might cause rough running due to the advanced and erratic effective ignition timing and may cause noise if it leads to detonation. It may also cause "rumble" which is fast and premature but not detonating combustion.
This heat buildup can only be prevented by eliminating the overheating (through redesign or cleaning) or the compression effects (by reducing the load on the engine or temperature of intake air). As such, if pre-ignition is allowed to continue for any length of time, power output and fuel economy is reduced and engine damage may result. The engine might be slightly harder to get running at once after pre-ignition.
Pre-ignition may lead to detonation and detonation may lead to pre-ignition or either may exist separately.
Fuel heatingThe proper air fuel ratio can auto-ignite if the fuel reaches a high enough temperature and pressure as described above. The opposite of this is a mixture that is too cold to ignite. The desired point is just short of auto-ignition. The inlet air needs to be cool to allow the maximum amount of air into the engine, but has to heat up rapidly to be effective. The fuel doesn’t need to be cold. Honda in the past heated the fuel in their F1 engine to 180 degrees F to maximize horsepower.
Fuel StorageHow long will the gasoline last if I don’t use it all? Gasoline will last a long time if stored properly, but it’s losing energy the entire time. Gasoline forms gum or varnish from the acids in the gasoline and from contact with the oxygen in the air and from the metal in the gas can. What is the scientific name for gum? Gum! I guess they couldn’t come up with something fancy.
