Fuel Terminology (maybe some clarification for you guys)
Fuel Terminology & Definitions:
OCTANE
Below is a more detailed explanation of the types of octane. Generally, Octane is rated in Research Octane Numbers, (RON), Motor Octane Numbers, (MON) and Pump Octane Numbers (R+M÷2). Pump Octane numbers are what you see on the yellow decal on your local gasoline pump and represents the average of the (RON) and (MON). (RON) is generally the higher number and is the number many companies choose to use advertising their products, however (MON) is the octane number more relevant to racing as it is one of the factors that represents the fuel's ability to resist preignition. Other important factors related to a fuel's ability to make power and resist preignition are listed below including BURNING SPEED, ENERGY VALUE and COOLING EFFECT. SEE OCTANE REQUIREMENTS BELOW.
Motor Octane (MON):
(a) CFR Tested @ 900 rpms, timing is varied with compression ratio, fuel is preheated to 300 degrees Fahrenheit, intake air is preheated to 100 degrees Fahrenheit.
(b) Motor Octane, measured under varying load is definitely the most representative octane measurement for actual engine octane requirements.
(c) The closer the Motor Octane number to the Research Octane number the more stable the fuel is throughout the RPM range . This is very critical when running higher that 7500+ RPM's engine speed. (See the Octane Differential in the chart above)
R+M÷2 Octane (R+M/2):
RESEARCH Octane numbers
(a) This method of measurement consists of adding together both the MOTOR and and then dividing by two.
(b) This is the number in that yellow box that you see on the gas pumps.
(c) This number should only be used when determining which fuel to use in your street car or tow vehicle. This method is NOT intended for correct use in your racing engine.
Research Octane (RON):
(a) CFR tested @ 600 RPM's, fixed timing at 13 degrees BTDC, fuel temp is not controlled and intake air is varied with the barometric pressure.
(b) This is basically a No-Load test and this number should NEVER be used to determine which fuel to use in your race engine.
Specific Gravity (SG):
Density of the fuel.
(a) This is the weight of fuel compared to water, water being at 1.00. If race fuel is .750 (specific gravity) it would weigh ¾ the weight of water. If water at 60 degrees Fahrenheit weighs 8.125 pounds per gallon (ppg), then race fuel at .750 will weigh 6.09 (ppg).
(b) The less dense the fuel (lower specific gravity) the higher the BTU content and the better the stability of the fuel at higher RPM's.
(c) The lower the specific gravity of the fuel, the more crucial the jetting becomes, and the easier it is to run the engine lean, possible causing internal damage.
(d) The combustion speed is determined primarily by the pressure (density) of the fuel mixture. A lower (SG) or viscosity fuel will flow more through an orifice (jet) and a higher (SG) or viscosity will flow less.
Lead Content (LC):
Amount of Tetraethyl Lead in grams per gallon.
(a) When lead is added to fuel, the knock resistance (anti-knock value) is increased. Lead is also used to increase the octane to a higher number.
Reid Vapor Pressure RVP:
The pressure of the fuel. The tendency of the gasoline to evaporate. Too high of a RVP may cause the fuel to boil or evaporate in the pump, lines or carburetor at a given temperature which can lead to "vapor lock". Too low of a RVP and the fuel will not vaporize and could cause difficulty starting the engine when cold. Most Racing Fuels have an RVP in the range of 5 to 7.
Color
Simply the physical appearance color of the fuel.
Dielectric
The electrical charge of the molecules within the fuel. These are approximated numbers of the fuel dielectric value when using a HDE G-01 Fuel Analyzer. The accuracy of fuel checked should be within +/- 0.4 points. When two stroke oil is added to the fuel the meter reading will be INCREASED by 0.1 to 0.2 points depending on the type and ratio of the two stroke oil. The 2 stroke oil may also significantly change the color. Fuel additives will also change meter reading from that of untreated fuel.
Consistency:
It is very important that the fuel you use maintain the purity and consistency regardless of which brand fuel you use. Try to purchase fuel that has been stored in sealed containers and a high volume dealer. Open or bulk fuel storage tanks may have a tendency to sweat and produce moisture, therefore changing the consistency or purity of the fuel. If at all possible buy your fuel in a factory sealed drum although the cost is generally higher but the quality may well be worth it in the long run.
Burning Speed:
Burning Speed is the speed at which fuel releases its energy. In a high speed internal combustion engine there is very little time (real time - not crank rotation timing) for the fuel to release its energy. Peak cylinder pressure should occur around 20 degrees ATDC. If the fuel is still burning past this point, it will not contribute to the peak cylinder pressure thus it is not contributing to the power output of the engine. Too fast of a burn or too slow of a burn of the fuel will result in less than optimum power.
Energy Value:
Energy value is an expression of the potential energy in the fuel. This energy is measured in BTU's (British Thermal Units) per POUND, not per gallon. This data is important as Air Fuel (AF) ratio is measured in weight and not volume. This value may vary with compression ratio and engine speed.
Cooling Effect:
Cooling effect of the fuel is related to the heat of vaporization. The higher the heat of vaporization, the better its effect on cooling the intake mixture for a denser charge. This is of some benefit in 4-stroke engines, but can be a large source of gain in 2-stroke engines.
Knock - Detonation - Ping
Intense pressure within the cylinder due to inability of fuel to have a controlled burn This could be caused by to low of an octane fuel for the application or incorrect tuning procedures. The sound you hear is from the actual vibration of the cylinder walls or the fuel exploding micro seconds prior to timed ignition firing which may cause two intense high pressure waves to collide or clap together and the colliding energy wave produces the sound you hear. This is hard on an engine. It is just like taking a ball peen hammer to the pistons. Use of proper fuel and tuning techniques will control this situation. Excess oil in the cylinder due to poor ring seal can cause detonation.
Pre - Ignition
Pre-ignition is the premature ignition or lighting of the mixture in the cylinder. This condition takes place prior to the spark plug firing. It is usually caused by a deposit or object glowing in the combustion chamber. A overheated spark plug or incorrect tuning parameters can cause preignition. This condition usually occurs while the piston is traveling up during the compression cycle. Pre-ignition may cause the piston to attempt to change directions which can cause major mechanical damage within the engine. Pre-ignition is much different that detonation although they both can reak havock on your engine.
Octane Requirements -
Fuel Needs
The required octane is one that provides a controlled burn throughout the power cycle of the engine. As noted above octane, is not the only factor within the fuel that controls the burn and there are many factors within the engine that affect the burn rate which then affect the amount of octane or burn control required. These factors include, but may not be limited to, engine compression, camshaft timing, ignition timing, combustion chamber design, spark plug location, valve adjustment, engine operating temperature, fuel mixture, and weather. Extremely high octane is not best for performance.
A higher octane slows the burn rate and conversely a lower octane quickens the burn rate. You desire a burn rate that will match the needs and characteristics of your engine so that you may produce the maximum amount of power, but at the same time retain reliability. It is better to error to the higher side of the octane instead of the lower.
Normally a margin of safety is desired just in case the air gets really dense or the engine temperature climbs a little high to prevent detonation in these instances
