1. Lesson 7: Fuels And Fuel Systems

2. Fuels And Fuel Systems
Fuel: The energy source for the combustion process
Combustion occurs when fuel comes into contact with oxygen, and the temperature of the mixture is raised to its kindling point.
The fuel and oxygen mix, and oxidation, or burning, occurs.

3. Air : Fuel Ratio
Stoichiometric is a chemically correct mixture in which all of the chemical elements are used and none are left over. (15:1)
Fifteen pounds of air to one pound of gasoline.
15:1 = 0.067

4. Air : Fuel Ratio
What air - fuel mixture would be used to produce the most power?

5. Air : Fuel Ratio
The design of the engine induction system and the valve timing requires a mixture that is slightly richer than chemically perfect in order to produce the maximum power.
This also runs cooler and prevents overheating and detonation under high engine loads.
Maximum power is normally considered to be produced with a mixture of approximately 12:1 or

6. Exhaust Gas Temperature
There is a direct relationship between the temperature of the exhaust gas and the mixture ratio being burned.
As mixture ratio is leaned, the EGT rises until peak temperature is reached, and then it drops off.
This peak EGT will always be reached with the same air : fuel ratio regardless of the power.
Used as a reference for adjusting the mixture.

7. Exhaust Gas Temperature

8. Specific Fuel Consumption
The number of pounds of fuel burned per hour for each horsepower developed.
Pounds of fuel burned per hour
Brake horsepower produced
Used to rate or to compare the performance of aircraft engines.
Used rather than thermal efficiency.

9. Thermal Efficiency
The ratio of useful work done by an engine to the heat energy of the fuel it uses, expressed in work or heat units.

10. Reciprocating Engine Fuels

11. Reciprocating Engine Fuels
Composition
Aviation gasoline is a hydrocarbon fuel refined from crude oil.
Straight-run gasoline
All gasolines are blends of different hydrocarbons and additives.
Annual US usage of avgas was approximately 0.14% of motor gasoline consumption in 2008.
The annual US usage of avgas was 186 million US gallons (700,000 m3) in 2008, and was approximately 0.14% of the motor gasoline consumption. From 1983 through 2008, US usage of avgas declined consistently by approximately 7.5 million US gallons (28,000 m3) each year.[6

12. Reciprocating Engine Fuels
Fuel Grades
(grade = octane)
Grade-80 RED
Grade-100 Green
Grade-100LL (Low Lead) Blue
Grade-115/145 Purple
The required grade of fuel must be placarded on the filler cap of the aircraft fuel tanks.
80/87 – Used in engines with low in engines with low compression ratio – phased out at this point/very limited availability.
100/130 – Mostly replaced by 100-LL
100LL – Most commonly used aviation gasoline, produced worldwide.
115/145 - Originally used as primary fuel for the largest, boost-supercharged radial engines needing this fuel's anti-detonation properties.[23] Now, limited batches are produced for special events such as unlimited air races.
Many grades of avgas are identified by two numbers associated with its Motor Octane Number (MON).[7] The first number indicates the octane rating of the fuel tested to "aviation lean" standards, which is similar to the anti-knock index or "pump rating" given to automotive gasoline in the US. The second number indicates the octane rating of the fuel tested to the "aviation rich" standard, which tries to simulate a supercharged condition with a rich mixture, elevated temperatures, and a high manifold pressure. For example, 100/130 avgas has an octane rating of 100 at the lean settings usually used for cruising and 130 at the rich settings used for take-off and other full-power conditions.[8]

13. Reciprocating Engine Fuels
Alternate Fuels
STC’s which permit the use of autogas or mogas in engines.
Lower price
No changes or adjustments to the engine are required
May be used interchangeably with avgas.
Supplemental Type Certificate for automotive gasoline.
Automotive gasoline — known as mogas or autogas among aviators — that does not contain ethanol may be used in certified aircraft that have a Supplemental Type Certificate for automotive gasoline as well as in experimental aircraft and ultralight aircraft. Some oxygenates other than ethanol are approved. Most of these applicable aircraft have low-compression engines which were originally certified to run on 80/87 avgas and require only "regular" 87 anti-knock index automotive gasoline. Examples include the popular Cessna 172 Skyhawk or Piper Cherokee with the 150 hp (110 kW) variant of the Lycoming O-320.[citation needed]
Some aircraft engines were originally certified using a 91/96 avgas and have STCs available to run "premium" 91 anti-knock index (AKI) automotive gasoline. Examples include some Cherokees with the 160 hp (120 kW) Lycoming O-320 or 180 hp (130 kW) O-360, or the Cessna 152 with the O-235. The AKI rating of typical automotive fuel does not directly correspond to the 91/96 avgas used to certify engines. Sensitivity is roughly 8-10 points meaning that a 91 AKI fuel might have a MON of as low as 86. The extensive testing process required to obtain an STC for the engine/airframe combination helps ensure that for those eligible aircraft, 91 AKI fuel provides sufficient detonation margin under normal conditions.[citation needed]

14. Reciprocating Engine Fuels
Fuel Contamination
Solids
Water
Ice
Microorganisms

15. Water Water is one of the major sources of contamination.
At altitude the temperature is low enough to cause the water to condense out of the fuel and form free water.
The freed water can freeze and clog the fuel lines.
Water is slightly soluble in gasoline.
Fuel will hold more water in solution if it is warm than it will if it is cold.

16. Fuel Metering Systems

17. Fuel Metering Systems
Principal Function is to sense the amount of air entering the engine at any moment and meter into that air an amount of fuel that will provide a uniform air : fuel ratio.
System will provide a uniform air : fuel ratio as the airflow varies.

18. The Aircraft Float Carburetor
Airflow Sensing
The air measuring unit is the venturi.
Makes use of a basic law a physics:
As the velocity of a gas or liquid increases, the pressure decreases.

19. The Aircraft Float Carburetor
Simple Venturi

20. The Aircraft Float Carburetor
Fuel Metering Force
Fuel from the aircraft’s tank is delivered to the float bowl of the carburetor.
The main fuel nozzle is located in the center of the venturi.
When air is flowing in the venturi a pressure differential between the venturi and the float chamber exist (Fuel Metering Force).

21. Fuel Metering Force
HIGH
LOW

22. The Aircraft Float Carburetor
Air Bleed
Air bled into the main metering system decreases the fuel density and destroys surface tension.
This results in better vaporization and control of fuel discharge, especially at lower engine speeds.

23. Air Bleed

24. Air Bleed

25. The Aircraft Float Carburetor
Air Flow Limiter
Throttle Butterfly
Venturi size

26. The Aircraft Float Carburetor
Mixture Control System
Back Suction Mixture Control
Varies the pressure in the float chamber between atmospheric and a pressure slightly below atmospheric.
Variable Orifice Mixture Control
Changes the size of the opening between the float bowl and the discharge nozzle.

27. Back Suction Mixture Control

28. Variable Orifice Mixture Control

29. The Aircraft Float Carburetor
Mixture Control System (Idle System)
Pressure of the air at edge
of the throttle valve and
above the valve is low.
Fuel rises from the bowl
due to the low pressure
above the throttle valve.

30. The Aircraft Float Carburetor
Acceleration System
Picks up fuel from
bowl at idle and
discharges it through
the pump discharge
when the throttle is
opened.

31. The Aircraft Float Carburetor
Power Enrichment System
Removes some of the heat by enriching the fuel-air mixture at full throttle.
Some only provide full power enrichment when the throttle is all the way open.
When takeoff power is required, throttle should be opened fully.

32. The Aircraft Float Carburetor
Float Carburetor Preflight Inspection
No fuel leaking
Sump all drain points

33. The Aircraft Float Carburetor
Carburetor Icing And Heat Use
Carburetor ice means ice at any location in the induction system.
Impact ice
Fuel ice
Throttle ice

34. Carburetor Ice Impact ice
Formed by the impingement of moisture-laded air at temperatures below freezing onto the elements of the induction system which are at temperatures below freezing.
Air scoop, heat valve, carburetor air screen, throttle valve and metering elements.

35. Carburetor Ice Fuel Ice
Forms when any air or fuel entrained moisture reaches a freezing temperature as a result of cooling of the mixture by fuel vaporization.
Cooler air holds less water vapor and the excess water is precipitated in the form of condensation.
Condensate freezes.
Can occur at ambient temperatures well above freezing.

36. Carburetor Ice Throttle ice
Formed at or near a partly closed throttle when water vapor in the induction air condenses and freezes due to the expansion cooling and lower pressure at the throttle.
Temperature drop normally does not exceed
5° F.
How is carburetor ice formation prevented?

37. Fuel Injection Systems

38. Advantages
Even fuel/air mixture distribution
More power
Less fuel
Less problems with carburetor ice

39. Differences from float carburetors
Fuel Injection: Deposits a continuous flow of fuel into the induction system near the intake valve just outside of the cylinder.
Carburetor: The correct amount of fuel is metered into the airflow.

40. Two Types
Bendix RSA
Teledyne-Continental

41. Bendix Fuel Injection System
Uses a venturi and air diaphragm to develop a fuel metering force.
Impact tubes sense total pressure of air entering the engine. (Dynamic + Static)
Venturi senses its velocity.
Both combine to move the air diaphragm proportionally to the amount of air ingested into the engine.

42. Fuel Metering Force
Pressure drop across the orifice in the fuel injector nozzles.
Position of the ball valve in its seat.

43. Idle System
Constant head spring pushes against the air diaphragm and forces the ball valve off its seat. (at low air flow)
As air flow increases the air diaphragm moves over.

44. Idle RPM/Mixture Control
Limit the amount of air allowed to pass the throttle valve.
Limit the amount of fuel to flow to the discharge nozzles.

45. Flow Divider
At idle a spring holds the flow divider valve closed to oppose fuel flow until fuel pressure off-seats valve.
Creating down stream pressure for the fuel control.
Provides cut off of fuel at idle cut off.

46. The Teledyne-Continental Fuel Injection System
Meters fuel as a function of engine RPM.
No Venturi
Special engine driven pump produces the fuel metering pressure. (constant displacement pump)

47. Mixture control
Manual mixture control valve
Variable selector
Fuel is bypassed back to the tank.

48. Throttle control
Controls air valve and fuel valve.
Fuel valve is variable orifice

49. Fuel Manifold Valve
“Spider”
Similar to the flow divider of Bendix
Distributes fuel evenly
Provides positive shut off at idle cut-off position.

50. Starting Procedures (Bendix)
Mixture idle cut-off
Open throttle 1/8 inch
Master on
Boost pump on
Mixture full rich until indication of fuel flow
Return mixture to idle cut-off
Starter engage
At engine start move mixture to full rich

51. Starting (Continental)
Fuel on
Crack throttle 1/8 inch
Mixture full rich
Boost pump on high
Fuel flow indicated engage starter
Boost pump off

52. Starting HOT Engine Mixture idle cut-off Throttle open wide
Boost pump on high
Allow fuel to circulate seconds
Boost pump off
Mixture full rich
Throttle 1/8
Engage starter
Continue normal start

53. Airflow Sensing/Air Metering Force
Review
Airflow Sensing/Air Metering Force
Float Carburetor: Venturi
Bendix: Impact Tubes and Venturi
Teledyne-Continental: N/A

54. Fuel Metering force
Float: Pressure Diff. between venturi and float chamber
Bendix: Balance between the air and fuel forces holds valve off its seat a stabilized amount for and given air flow.
Teledyne-Continental: Engine RPM

55. Mixture Control
Float: Back suction, Variable orifice, Needle valve at idle.
Bendix: Valve in the fuel control regulates the amount of fuel that can flow to main metering jet.
Teledyne-Continental: Variable Selector.