Aviation Fuels: AVGAS and Jet Fuel

• Aviation fuel comes in two main types: AVGAS (gasoline) and Jet Fuel (kerosene type).
• Both are distilled from crude oil and composed of hydrocarbons.
• The distillation process involves heavier components burning off later to become various fuels like kerosene and diesel.
• Baffles are used in fuel tanks and other systems to manage the flow of liquids and gases.

AVGAS

• Primarily used for reciprocating (piston) engines.
• Highly volatile and extremely flammable due to its low flash point.

Jet Fuel

• Possesses a higher flash point, making it less flammable than AVGAS.

Volatility

• Refers to a liquid's tendency to turn into vapor.
• Low volatility means slow vaporization, which can lead to hard engine starting, slow warm-up, and poor acceleration.
• Vaporization is affected by ambient pressure decreases and ambient temperature increases.
• Excessive fuel volatility can cause detonation and vapor lock.
• AVGAS is blended to limit vaporization, especially at high altitudes, ensuring smooth performance.

Reid Vapour Pressure Tester

• Measures fuel volatility.
• AVGAS has a vapor pressure between 5.5 and 7.0 PSI at 100°F.

Problems with AVGAS

• Vapour Lock
• Carburetor Icing
• Detonation

Vapour Lock

• Occurs when AVGAS vaporizes in the fuel line or other components between the fuel tank and the carburetor.
• Caused by excessively hot fuel, low pressure, or excessive turbulence.
• Vaporized fuel blocks the flow to the carburetor.
• Three main causes include low atmospheric pressure, high fuel temperature, and excessive fuel turbulence.
• High altitude causes lower atmospheric pressure, reducing the boiling point of the fuel and forming vapour bubbles.
• Heat from the engine can boil the fuel in the lines.
• Sloshing of fuel in tanks mixes air into the fuel, increasing turbulence.

Preventing Vapor Lock

• Keep fuel lines away from heat sources, sharp bends, and steep rises.
• Maintain boost pumps under pressure.
• Vapour eliminates may be installed ahead of the metering device.

Carburetor Icing

• Occurs when fuel vaporizes, extracting heat from its surroundings.
• The more volatile the fuel, the more rapid the heat extraction.
• Vaporization primarily occurs in the venture throat or throttle valve of the carburetor.
• Rapid temperature drops can freeze moisture in the incoming air.
• Carburetor icing typically occurs between 30°F to 40°F (-1.1°C to 4.4°C) OAT (outside air temperature).

Detonation

• A rapid, uncontrolled explosion of fuel due to high pressure and temperature.
• Sounds like an engine knock or ping.
• Increasing the octane rating of fuel can reduce detonation.

Octane Rating

• Indicates how much compression a fuel/air mixture can withstand without detonating.
• Rating of anti-detonation or anti-knock qualities
• Tetraethyl lead is added to increase the octane rating.

AVGAS Grades

• 80/87 (RED)
• 82UL (unleaded, PURPLE, newest)
• 100/130 (GREEN)
• 100LL (Blue)

Fuel Mixture

• Lean mixture = more air
• Rich mixture = more fuel
• Lean mixtures burn hotter than rich mixtures.
• Lower octane ratings suit lean mixture settings.
• 80/87 grades are used in low compression engines, while 100/130 grades are used in higher compression engines.
• 100LL means low lead, indicating reduced tetraethyl lead content.
• Numbers indicate performance; the first number is for lean mixture rating, and the second is for rich mixture rating.

Flash Point

• The lowest temperature at which a volatile liquid can vaporize.
• Flammable fuels have a flash point <38°C
• Combustible fuels have a flash point >38°C
  • AVGAS flashpoint -27°C
  • Jet A flashpoint 38°C
  • Jet A-1 flashpoint 38°C
  • Jet B flashpoint <38°C

Turbine Fuel

• Contains more Sulphur
• Made from heavier hydrocarbons than AVGAS
• Jet A/Jet A-1 consists of a kerosene blend
• Jet B is a kerosene/gasoline blend for cold weather operations (primarily military)

Turbine Fuel Volatility

• Higher volatility fuels are needed for cold weather starts and aerial restarts, but can create vapour lock if too volatile
• Turbine fuels are less volatile than AVGAS
• JET A has a freezing point of -40°C, and is only used in the USA and Canada
• JET A-1 is more volatile and has a Freezing point of -47°C
• Jet B is most volatile and is jet fuel with gasoline. Freezing point is -50°C, and is used in Alaska, Russia, and Northern Canada.

Turbine fuel problems

• Contains water and micro-organisms

Water in Jet Fuel

• Condenses out of air within storage tanks, fuel trucks and aircraft fuel tanks.
• Has two forms: dissolved water and free water.
  • Dissolved water; cannot be removed from fuel, remains in the humidity of air.
  • Free water; appear as slugs or droplets in fuel. Entrained water has a cloudy appearance and can result in potential engine failure, and restricted flow of fuel as ice can plug fuel filters.
• In low temperatures, the fuel may need to be preheated.
• Hot "bleed air" from the engine or engine oil can be used as heat sources.
• "Prist” is added during fueling is to act as an anti-icing additive and lower the water freezing point

Contamination from Micro-organisms

• Water in fuel encourages growth of micro-organisms such as algae.
• Additives and water elimination can resolve this issue

Turbine Fuel Characteristics

• Able to be pumped and flows easily under all operating conditions.
• Very efficient burn under all conditions
• Permits engine start under all conditions
• High calorific value
• Has a minimal corrosive effect on fuel system components
• Provides lubrication for the moving parts of the fuel system and other aircraft systems
• Minimal fire hazard
• Produces minimal harmful effects on the combustion system or turbine blades when burned

Calorific Value

• Amount of heat released during combustion
• Heating value in turbine fuel is controlled by specific gravity and volatility factors

Viscosity

• Resistance of a fluid to flow
• "Stiffness" of a fluid or its internal friction
• Factors in combustion characteristics

Specific Gravity

• The ratio of turbine fuel density to water density at a specified temperature
• As turbine fuel warms, it expands and therefore shifts the specific gravity.

Other fuels

• MOGAS (automotive gas).
  • Has looser manufacturing specifications
  • Differences in physical and chemical properties can lead to poor fuel distribution, poor anti-knock properties, and excessive motor oil dilution.
  • More volatile than AVGAS
  • More prone to vapour lock and carburetor icing.
  • Ethanol is added to MOGAS.
  • Ethanol free MOGAS exists
• Diesel
  • Starting to become available
  • Diamond uses diesel engines in aircraft using jet fuel for efficiency

This aviation lesson describes two main types of aviation fuel: AVGAS and Jet Fuel. AVGAS (aviation gasoline) is primarily used for piston engines, while Jet Fuel (kerosene type) is used for turbine engines. Fuel volatility, or a liquid's tendency to vaporize, is a key consideration.