Tech. Tip: Fuel Terminology

Air/Fuel Ratio  - The ratio of pounds of Air to pounds of Fuel needed for combustion in an engine. Air/Fuel ratio is based on pounds of AIR to pounds of FUEL but carbs are metered (jetted) by volume so changes in fuel can change A/F ratios. A/F Ratios range from about 2:1 for NitroMethane to about 16:1 for gasoline, with 14.7:1 considered the stoichiometric or chemically correct ratio under perfect conditions with normal (non-oxygenated) gasoline.  Gasoline A/F ratios for best power tend to be in the 13.25:1 - 13.75:1 range.

Anti Knock Index or AKI - The octane number you see at the pumps in the US equal to (R+M) /2 . This is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON) .

ASTM - "American Society for Testing and Materials" This organization is widely recognized as the authority on specifications for petroleum products.

Auto Ignition Temperature - The temperature at which a combustible mixture will ignite on its own. This is sometimes called the "Spontaneous Combustion Point". This is closely associated with detonation.

British Thermal Unit - BTUs are used to measure the energy content of a fuel, usually in BTU/lb. The higher the BTU value the greater the potential energy available. Gasoline is in the range of 20,200 BTUs while Methanol has an energy content of only 9,700 BTUs.

Detonation - An uncontrolled pressure rise and the associated heat rise caused by the reacting of the fuel in the combustion chamber. This is usually associated with the pinging sound you here from engines as multiple flame fronts collide in the combustion chamber.

Flame Front - The advancing of fuel reacting from a source of ignition. During normal combustion there is only one flame front which advances from the spark plug to the outer edges of the combustion chamber. During abnormal combustion multiple flame fronts can start, which leads to detonation.

Flame Speed - Flame speed or burn time is the time in milliseconds from 10% to 90% fuel burned. Flame Speed is a function of fuel chemistry, and engine design . The component  make up of a racing fuel will influence the burn time whether it's a high octane fuel or not. Racing fuels usually have the Flame Speed adjusted based on the intended application (i.e. engine rpm, bore size , etc.)

Hydrocarbons - The building blocks of gasoline made up of chemical compounds consisting of hydrogen and carbon atoms only . Gasoline is made up of a combination hydrocarbons with different molecular weights and structure. The various components that make up gasoline are classified according to the number of carbon atoms in the molecules. These components form chemical groups called Aromatics, Paraffins, and Olefins. Aromatics are the components that most people are familiar with. They include compounds like Toluene, Benzene, Aniline, Benzene, and Xylene among others. They are commonly used to increase octane and to raise BTU content. They can often times be found in commercial octane boosters. The amount of energy a gallon of gas will produce is a function of the proper combination of Aromatics, Paraffins, and Olefins. These combinations are designed to produce a specific distillation curve depending on the application.

They fall into three basic groups:

Light Fractions - which vaporize from 85 to 130 degrees(F)

Medium Fractions - which vaporize from 130 to 250 degrees(F)

Heavy Fractions - which vaporize from 250 to 400 degrees(F).

Octane Number - A measure of the anti-knock characteristics of a given fuel. The octane number you see at the pumps is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON). This number is sometimes referred to as the Anti Knock Index or AKI. Each of these octane numbers is determined by ASTM laboratory tests. Low-speed and low load knock characteristics are determined by the RON test method. The MON method tests high-speed, high load, high temperature situations, in practice these conditions exist during periods of high speed power accelerations, hill climbing, or any period of high power output(sounds like racing to me). Obviously the MON number will be lower, but it's the one racers should concern themselves with.

Oxygenates - Oxygen bearing chemicals that can be added to fuel that bring additional oxygen to the combustion process. Currently two types of oxygenated compounds - ethers and alcohols - are used in gasoline at levels higher than 2 percent.

The ethers generally consist of :

TAME - Tertiary Amyl Methyl Ether

MTBE – Methyl Tertiary Butyl Ether

ETBE – Ethyl Tertiary Butyl Ether

While the alcohols are generally :

Methanol - (MeOH)

Ethanol – (EtOH)

Isopropanol – (IPA)

t-Butanol – (TBA)

Mixed C1 to C5 alcohols

Nitromethane, Nitropropane #2, Propylene Oxide, Piric Acid are also used for their power producing qualities. All of these compounds vary in the oxygen content from a high of 50% for methanol to a low of about 15% for TAME. Some of these compounds are better than others at producing additional power and are very often used as gasoline extenders. Oxygenates are also being used in an attempt to reduce exhaust emissions.

Pre-Ignition - The starting of ignition by any source other than the spark plug before normal firing of the spark plug.

Reformulated Gas – RFG - The 1990 Clean Air Act (Act) requires the Environmental Protection Agency (EPA) to issue regulations that would require gasoline to be "reformulated" so as to result in significant reductions in vehicle emissions of ozone-forming and toxic air pollutants. This cleaner gasoline is called reformulated gasoline (RFG). RFG is required to be used in nine major metropolitan areas of the United States with the worst ozone air pollution problems. In addition, several other areas with ozone levels exceeding the public health standard have voluntarily chosen to use RFG . The major differences between RFG and conventional gasolines are :

RFG has lower levels of certain compounds that contribute to air pollution - notably Benzene.

RFG will not evaporate as easily as conventional gasoline – lower RVP.

RFG will contain "chemical oxygen" (oxygenates) – causing lower stoichiometric Air/Fuel ratio

Reid Vapor Pressure (RVP) - A standard indicator of gasoline volatility or how quickly a fuel evaporates. Oil companies vary RVP seasonally to correspond to the weather. Gasolines have an RVP range of 5 to 15. High RVP fuels are used in cold climates for easier starting, because they will tend to evaporate easily. But a high RVP fuel can easily vapor lock in hot weather, or under most racing conditions, regardless of the air temperature. Most racing fuels have low RVP ratings that are fixed year round, usually in the 5.5 to 8.0 range although some special purpose fuels have RVP set as low as 1.5.

Relative Air Density - RAD - A working number based on barometric pressure, air temperature, altitude and humidity. RAD is used by tuners to help determine the jetting changes required by changes in weather or air temperature. Although usually measured with a RAD gauge, it is possible to calculate relative air density using a chart or calculator.

Sensitivity – The difference in octane numbers between RON and MON. A high sensitivity fuel would have an MON much lower than it’s RON. This is a general indicator of the sensitivity of the fuel to changes in severity of the engine operating conditions. Standard US pump fuel usually has a sensitivity of 10, so a 92 octane US unleaded pump fuel will usually have an RON of 97 but an MON of only 87.

Spark Lead Time - The time before TDC needed to fire the spark plug to ensure complete combustion of the trapped mixture. Different fuels and engine setups will require different spark lead times. As a general rule, the more efficient the combustion the less lead time (timing) the engine will require. This is especially true of high compression four stroke engines.

Specific Gravity - A measure of the density of a liquid relative to the density of water, with water having a specific gravity of 1.0. Given the fact that Air/Fuel ratio is based on pounds of AIR to pounds of FUEL, but carbs are metered (jetted) by volume changing the specific gravity of your fuel can have a profound affect on the A/F ratio of your engine. In short you've changed the jetting by changing the density of the fuel. Octane boosters and alcohol based fuels (gasahol) can change the density of the fuel a large amount.

Surface-Ignition – The igniting of the fuel air mix by any source other than the spark plug before or after normal firing of the spark plug. This is usually caused by hot spots in the chamber like, carbon deposits, too hot a plug.

Volatility - The tendency of a fuel to evaporate {to change from a liquid to a gas state}. This is one of the most fundamentally important qualities of fuel in carburated engines because it has a major influence on the vapor-air ratio in the cylinders at the time of ignition. The greater (higher) the volatility of a fuel the greater the tendency to evaporate. In a normal engine nearly all the fuel needs to be evaporated before ignition. So for any Engine/Air Temperature combination there is a minimum volatility that is required for proper running.

Volatility Curve - Also know as the "Distillation Curve". The ASTM Distillation test provides a measure, in terms of volatility, of the relative proportions of all the hydrocarbon components of a gasoline. The ASTM distillation curve designates the maximum temperatures at which 10%, 50%, and 90% of the fuel will be evaporated as well as the maximum end point temperature. These distillation characteristics, define and control, starting, warm-up, acceleration, vapor lock, and crankcase dilution. The significance of any point on the ASTM volatility curve depends on the temperature range in question. Under low temperature conditions as in cold weather starting, when only a small portion of the fuel is evaporated, the low temperature end of the curve is the most. While for higher temp ranges like those in the intake of a hot engine the shape of

the low temp end of the curve is much less important since all of this part of the fuel is evaporated anyway. Oil companies vary the 10% distillation point seasonally to correspond to the weather. Different parts of the country will have their fuels blended to compensate for regional elevations as well. Most Racing fuels have Distillation curves that fall into a much more narrow temperature range that is held consistent year-round.

Vapor - Liquid (V/L) Ratio - The ratio of volume of vapor formed at atmospheric pressure to the volume of gasoline . The V/L ratio increases with temperature for any given gasoline. The temperature at which the maximum V/L ratio is specified for each ASTM volatility class is based on the air temperatures and the altitude associated with the use of the class. V/L ratio for Avgas will be much different than the ratio used for Racing or Pump gas.

Vapor Lock - Rapid formation of vapor in the fuel lines or carb, that causes a restriction in flow. Vapor formation begins to occur in fuel lines, pumps, etc. when the fuel reaches a temperature where the vapor pressure of the fuel is equal to the pressure in the system. Gravity feed fuel systems (positive pressure) tend to be less prone to vapor lock than snowmobile type vacuum pump systems where negative pressures exist.

References and further reading :

Harold H. Schobert  -  The Chemistry of Hydrocarbon Fuels - Butterworth-Heinemann Ltd.

Keith Owen, Trevor Coley - Automotive Fuels Reference Book - SAE - R151

H.P. Lenz - Mixture Formation in Spark-Ignition Engines - Springer-Verlag

Jeff Hartman - Fuel Injection - Motorbooks International

Germane, Wood, Hess - Lean Combustion in Spark-Ignited Internal Combustion Engines - A Review - SAE  paper 831694

Z. Warhaft - An Introduction to Thermal Fluid Engineering - Cambridge University Press