Gasoline & 2 Stroke Engines
Typically karting sanctions specify fuel brand and grade, often by using a Digitron fuel Meter. We had several letters asking various questions, from what fuels are legal to which fuel is best.
Rather then go into a full scientific analysis, we will summarize the characteristics to help you decide on fuel. We do have a forthcoming CD coming based on our 2-Stroke Technology series that will go into more detail, and includes animated images of combustion.
Gasoline: A Chemical Soup
Gasoline is a combination of anywhere from 6-12 different hydrocarbon fluids with small amounts of additives to modify properties. First we deal with the properties, then get down to understanding what will probably work best in your application. These properties affect cost and performance in one way or another. Critical properties include (but not limited to):
Volatility influences how well the fuel will atomize. Atomization is a key component to an efficient combustion and maintains the integrity of Heat of Vaporization, Octane, and Propagation Rate.
Generally measured in Vapor Pressure, this property is regulated for pump fuel by EPA mandate to limit the mass of fuel evaporated on refueling the family sedan or tow vehicle. Its one of the compromising factors about "pump fuel" as the lower the volatility, the less likely it can be formulated to provide a high Heat of Vaporization. As this test is performed under specific conditions it does not conclude that deviation from the test standards will result in a linear reduction of the Heat of Vaporization qualities. For example, gasoline blended with ethanol has a relatively low vapor pressure at room temp, but increases substantially at higher temps, at a rate beyond most other hydrocarbons commonly found in fuel. It raises the Heat of Vaporization dis-proportionately to basic fuel stocks.
Heat of vaporization
This property measures the amount of thermal energy absorbed by the liquid in changing properties to a vapor during vaporization. Normally the Hydrocarbon components that provide greater HOV properties are referred to as "Light Fractions" by the Refiners, as they tend to have lower densities (but not always).
A 2 stroke demands a high HOV to provide internal cooling. It cools the incoming air leading to higher fuel-air charge density, and cools the piston substantially allowing higher combustion temperatures. Pump fuel is not optimized to operate in a lean burning 2 stroke. More about this later. Racing fuel formulated for 2 strokes is optimized to provide a high HOV but with it usually comes higher volatility or vapor pressure beyond the limits allowed for pump fuel.
Octane and Rate of Propagation
Octane is the most misunderstood property of all. Octane is merely a reference to the resistance to detonate. Detonation in simple terms is a spontaneous ignition created in means other then the spark plug, but after the spark plug has already fired. That results in multiple flame fronts radiating out from the point of ignition and when they collide, they cause the piston to rattle. In a 2 stroke, squish is often used to create a turbulent band around the periphery where most detonation is known to occur. Detonation could be caused by the convective heat traveling at the speed of sound or radiant heat traveling at the velocity of light, but in any case it's caused by heat. Lean mixtures, high compression are just a few of the contributors.
As the heat of combustion coalesces it produces two distinct phases; Kernel Ignition, and Flashover Propagation, kind of like where the heat of the Kernel Ignition reaches a critical mass of thermal energy to begin a rapid ignition of the remaining fuel. Keeping with simplicity, imagine about 10% of the fuel consumed takes half the total time of combustion, and the remaining 90% of the fuel mass takes the remaining 50% of the time during Flashover.
To avoid complexity we won't delve into Flash Point.
Rate of Propagation is the velocity in which the combustion expands and occurs. By adding certain hydrocarbons (Isooctane is one of them) we can reduce the spontaneous ignition without severely compromising the Rate of Propagation, but it has limits. Most hydrocarbons that have higher Octane ratings above Isooctane (100) compromise Rate of Propagation. For example a 104 Octane non-leaded fuel almost always has a slower Rate of Propagation. Some of that can be compensated for in timing, but it's not a the ideal way to solve the problem. The additional compression allowed with a higher non-leaded fuel generally results in more power if you can raise the compression to benefit.
However, if the higher octane fuel (up to 100) is achieved through blending Isooctane without the use of slower propagating hydrocarbons, you will loose no power by having "too much Octane".
Tetra-Ethyl Lead (TEL) has always been the foe of detonation (and the EPA) with virtually no compromise in the Rate of Combustion. This works by absorbing significant levels of the radiant heat that would normally heat the hydrocarbon molecules to achieve spontaneous ignition, under conditions ideally controlled by the physical/optical properties.
Obviously the more heat that can be achieved with a given fuel-air mass the more power it makes - as long as we don't compromise other properties. There are various compounds beyond the normal dozen or so hydrocarbons that can yield benefits in energy content, but at the compromise of other properties. Safety is one of them. As we don't need any toxic substances added to our sport, we won't elaborate on what they are. Suffice it to say Energy Content is a factor for power, but generally a secondary consideration in formulation.
Now here is an "explosive" topic. Air contains about 1/5th O2 and as a gas takes up a lot of volume. Many compounds can carry extra oxygen along in a smaller volume. Nitro-methane is a great example of an low volume high concentration bearer of O2, but its not ideal in a high revving 2 stroke. Methyl-Butyl-Tertiary-Ethyl (MBTE) is also a good oxygenate for a typical car, but not beneficial for most typical kart motors. Ethanol is also pretty good for cars, but again not the best in high revving water cooled reed motors. There is one other substance that anyone caught using needs to both be banned permanently from the sport, as well as sentenced to working in a Cancer Ward for a year if they are caught using.
Oxygenates take time to decompose during the combustion cycle and most result in slowing the Rate of Propagation. Not really a problem at speeds under 8-9000 RPM, but above that they slow down the Rate of Propagation. While that could be considered as a possible benefit to extending the heating of the pipe gases for overrev, it doesn't work that way at least on most motors. It does tend to benefit some of the Yamaha Can and Pipe classes, but not much, and depends on the oxygenate.
VP and Phillips both make 2 stroke racing fuels that contain oxygenates. Testing them, we found gains in the 4000-8000 RPM range, with losses at the 9,000+ RPM range that were not significant, but not easy to compensate for. They work great in a typical MX bike or water sled, but not very well in a kart. Higher RPM requires a high Rate of Propagation and the safer oxygenates just don't cut it.
There are other oxygenates like ether that can increase the Rate of Propagation while severely reducing octane.
Except for one, these oxygenates send Digitron Di-Electric test meters large sums positive in reading.
One of them does work very well but has a huge carcinogen problem. This compound contains a highly active free radical of oxygen with a high affinity for absorption into the skin. The consequences are simply deadly. The reason it works at high RPM is because it decomposes under compression so its ready to burn when the plug fires. It leaves a distinct acrid odor from the exhaust and is believed to be found in one or two off-beat racing fuels. Not VP or Phillips.
Purity and Cost
Pump fuels are blended under constraints of EPA, Cost, and performance, and in that order. Most local retailers are actually buying whatever fuel the depot can ship at the lowest cost that meets the rated octane. A few major brands still control their distribution, but it's becoming less common. When we ran Yamaha KT 100's we found Exxon 92 Octane produced comparable power to Phillips B32 at least at the time. B32 has some superior formulation in terms of cooling, but the Exxon was oxygenated with Ethanol, also a good cooling compound.
Pump fuels also contain minerals like sulphates that leave traces of ceramic solids behind. Ceramic solids are not exactly what you want building up in the exhaust ports and head to break loose and score cylinders. What can be saved in a fuel cost is spent on added maintenance in classes where high revs and close tolerances are involved.
Racing Fuels are usually formulated without the minerals. VP, Howell, and Phillips have exceptional quality in their formulation and consistency between batches. It's the retailer you need to learn to trust. We have access to a mass spectrometer and routinely test fuel. We have seen instances where Racing Fuel has been "cut" with Pump Fuel probably or higher profit. This is the exception among retailers but they do exist.
Where would we be without it? The most effective fuel tech is a Parce Ferme control of the fuel tank. Not always practical. The most common Fuel Tech is with a Digitron Fuel Test. This device measures the Di-Electric constant of the fuel. That relates to the ability of the fuel to absorb and store positive and negative ions and provides about 90% confidence in the legality of the fuel.
Oxygenates tend to send the meter positive in a hurry. Typical leaded racing fuels spec'd for Kart Racing do not contain oxygenates and generally measure very close to each other on the Digitron. We tested Phillips B32, VP C12, and Howell "Track Fuel" with the Digitron (no oil) and they all came out within 10 points. We also looked at spectrographic analysis and found remarkable similarities. Not exactly the same, but very similar.
Fuel Storage and Care
Racing fuel being formulated with light fractions tends to loose the volatiles while sitting in a plastic fuel jug. With a highly tuned shifter motor fuel stored in a cool dark area in the typical plastic jug can loose performance in just a week or so. Generally its safe for a few weeks, but it all depends on how well sealed the jug is. Synthetic oils when mixed with fuel have better shelf life's then many say they do. Just the same, we wont race on fuel that's been out of the drum for over a week.
Castor oil is good stuff, but decomposes somewhat if left mixed for a while.
Sunlight or any UV light (Fluorescent) decomposes the volatiles (fumes) in the fuel jug.
Pump-around carbs that recycle fuel back to the tank drive the volatiles off pretty quickly. Add fuel between heats rather then running the tank down. Keeping the fuel tank about 1/2 - 3/4 full minimizes the evaporation of the light fractions. I can't count the number of times I have seen folks re-jet when the air didn't change, but the fuel did.
Applications - What works best?
Obviously you want to use whatever the promoter says to use. If you have a reputable fuel retailer at your track (we do!) then it certainly helps to buy fuel from them. It keeps the stock rotating and assures a steady supply of good fuel.
Test the track fuel in practice if you race in a class that allows high compression or open ignition. Most folks in these classes tune to run on the edge of detonation, and you never know if some significant changes may be in order to run the track fuel. More often then not this is an exception, but finding out at the last minute can be costly.
If you can select a fuel, here are some recommendations in order of preference;
These are non-shifter classes with non-corrected compression ratios below 11:1.
ICC/ICA Water cooled Reed Controlled Compression
80cc Shifter/SKUSA FMX Controlled Compression & Ignition
Open 80cc 125cc Shifters
This Search Engine will target Race Fuel Websites