1. Carburettor & Fuel-Injection Systems
ATC AGK Chapter 4

2. Aim
To develop understanding of the carburettor and fuel-injection system, describe its use and threats to its operation

3. Objectives Describe components of the carburettor
Describe components of the fuel-injection system
State threats of the carburettor and fuel-injection systems
State advantages/disadvantages of each system
Explain the proper use of the mixture control and abnormal combustion
Describe threats associated with the fuel-injection system of the C172SP

4. 1. Carburettor Carburation
Carburation is the process of vaporizing liquid fuel and mixing it with air at the correct proportions.
The ideal fuel to air ratio is 1:15, known as the chemically correct mixture.
In internal combustion engines, maximum power output is achieved at a ratio of 1:12 due mostly to thermal inefficiencies.
For reliable ignition the mixture must be kept within a range of 1:9 (rich) and 1:20 (lean)
Using a mixture that is too rich can lead to loss of power, rough running, high fuel consumption, fouling of spark plugs and formation of lead deposits.
Using a mixture that is too lean can lead to excessively high cylinder head temps, detonation.

5. 1. Carburettor Float Type Carburettor
The float type carburettor is used in most older aircraft.
The throat of the carburettor is shaped like a venturi, reducing the pressure inside the throat, sucking fuel and air into the manifold.
The pressure drop is proportional to the airflow through the throat which is controlled by the pilot via the throttle or butterfly valve.

6. 1. Carburettor Float Type Carburettor
As the air flows through the venturi the difference between the lower air pressure in the throat and the relatively higher pressure air above the fuel in the float chamber ensures constant fuel flow.
Mixture control governs the volume of fuel entering the system for a given volume of air to achieve the required mixture ratio by weight. In this case a simple crescent valve is used.

7. 1. Carburettor Float Type Carburettor
The accelerator pump is linked directly to the throttle and supplies additional fuel during increases in power.
The diffuser uses bleed air to aid in vaporisation of the fuel.
For high powered operation, the enrichment valve is opened to allow for additional cooling.

8. 1. Carburettor Float Type Carburettor
During idle operations, the venturi effect is not sufficient to draw fuel from the main jet. The required fuel is metered through a small idling jet with an inlet near the butterfly valve.
With the mixture control in the idle-cut off position, the idle cut off and mixture crescent valves close restricting all fuel from the engine.

9. 2. Fuel injection system Fuel Injection
Most older training aircraft use carburetted engines, however given the many disadvantages of the system most modern training aircraft use fuel injection systems.
The system couples a venturi with a fuel control unit to inject fuel directly into the intake manifold.

10. 3. Threats to Operation Impact Icing
Occurs when supercooled water droplets come in contact with the forward facing surfaces of the aircraft and immediately turn to ice.
Can also occur if the aircraft components are below zero and they come into contact with non-supercooled moisture.
Typically forms on the air filter and in the air intake duct.
Most likely to occur:
When the ambient temperature is below zero
When flying through visible moisture (cloud)

11. 3. Threats to Operation Fuel Icing
Also known as evaporation or refrigeration icing.
Vaporisation of a liquid to a gas requires heat, this heat is extracted from the air entering the carburettor.
Vaporisation accounts for around 70% of the temperature drop in the carburettor.
Ice will form downstream of the main jet on the inlet manifold and butterfly valve.
Can occur in a wide temperature range with a relative humidity as little as 50%.

12. 3. Threats to Operation Throttle Icing
As the fuel air mixture passes through the venturi the pressure reduction results in a temperature drop.
Ice will typically form on the throat of the venturi and on the butterfly valve.
The subsequent power reductions are more apparent at low power settings due to the ice forming on the tips of the butterfly valve restricting air flow.

13. 3. Threats to Operation Icing Symptoms
Regardless of the type of icing the symptoms will present themselves in a similar ways and include:
Gradual reduction in RPM for aircraft fitted with a fixed pitch propeller (such as the C172SP)
Gradual reduction in MAP for aircraft fitted with a variable pitch propeller (such as the C172RG)
Rough running
If remedial action is not taken complete loss of power may occur

14. 3. Threats to Operation Icing Conditions
As the below graph shows icing can occur across a wide range temperatures and humidity's.
Below around -10⁰C any moisture in the atmosphere will have formed into ice so intake icing is highly unlikely.

15. 3. Threats to Operation Carburettor Heat
When carburettor heat is applied warm, unfiltered air from around the exhaust manifold is drawn into the carburettor.
The warm air will melt any ice within the carburettor and will bypass the normal air intake duct allowing an alternate air source in the case of impact icing.

16. 3. Threats to Operation Carburettor Heat
A drop in power when carb heat is applied is expected due to the decreased density of the warm air. If rough running then occurs it is an indication that carb icing is present and that the melted ice is now passing through the cylinders. Once the icing has cleared power should increase again
Carb heat should be applied:
When flying in known or suspected icing conditions
When icing is suspected
On approach
When operating outside of the green range of the RPM indication
In accordance with the flight manual
Icing can occur on the ground so a carb heat check prior to take-off is advisable, turn carb heat on, you should get a small drop in RPM. If RPM increases before carb heat is switched off, it indicates there was ice present.
Avoid using carb heat for extended periods on the ground as the air source is unfiltered.

17. 4. Advantages/Disadvantages
Fuel Injection vs Carburetion
Advantages of the fuel injection system include
Induction icing is almost eliminated
Uniform delivery of fuel to each cylinder
Improved control of fuel-air ratio
Increased response to changes in power setting
Increased efficiency
Disadvantages of the fuel injection system include
Vapour lock in the fuel lines can make hot starts difficult
Greater susceptibility to contamination
Greater awareness of fuel distribution required
For the updraft FCU, such as found on the C172SP, high mixture settings when taxiing and over priming can lead to fuel residue clogging the injection system
Aircraft engines do not need to be primed before start when warm, in aircraft with fuel injection systems doing so may mean the FCU has to be overhauled, at great expense

18. 5. Mixture Control Leaning the Mixture
Powered aircraft operate in an environment of varied air density. Therefore, such aircraft require a facility to manage the fuel flow relative to air density for safe and efficient power generation. The mixture control achieves this goal. When the mixture control is moved rearward in a carburetted engine, it rotates the mixture crescent valve. This modifies the airflow available to the diffuser and effectively reduces the fuel entering the engine. This process is termed leaning.
The same is achieved in the fuel-injection system via the fuel control unit.

19. 5. Mixture Control Leaning the Mixture
Once the desired power setting is set, leaning the mixture involves retarding the mixture control lever. In a fixed pitch propeller aircraft, an RPM increase will be noticed passing the region of best power. With further leaning, the RPM will decrease and show signs of rough running. Return the mixture control to achieve maximum indicated RPM and then enrichen slightly. If the aircraft is equipped with an EGT gauge, lean for peak EGT and then adjust for best economy/power.
For a constant speed propeller, the leaning is done with reference to a fuel-flow/EGT gauge. Always refer to the AFM/POH for aircraft specific leaning procedures and beware of company Operations Manual requirements.

20. 5. Mixture Control Mixture settings Recommended mixture settings:
Take-Off: full-rich unless high density altitude generally above 5000AMSL
Climb: usually full-rich unless in accordance with the AFM.
Cruise: leaned for best power/economy as desired
Descent: enrichen as required for best power/economy
Landing: full-rich
The idle-cut is the normal means of shutting down the engine. It is accomplished by fully retarding the mixture control. This leaves no combustible fuel/air mixture in the system with the potential to cause injury.

21. 5. Abnormal Combustion Abnormal Combustion
Normal combustion can be defined as the process in which the compressed fuel/air charge in the cylinder is ignited at the intended time by the spark plugs with the resulting flame front spreading across the cylinder head in a smooth, uniform manner until the charge has been consumed.
Generally speaking there are two types of abnormal combustion:
Detonation
Pre-ignition

22. 5. Abnormal Combustion Detonation
Typically occurs when the pressure or temperature of the charge is too high resulting in spontaneous explosive combustion.
Indicated by rough running, high CHT or possible engine failure
Contributory factors include:
Expired fuel
Operation of carb heat at high power, low airspeed
Using fuel with a lower than recommended octane
Over lean mixture
Excessive manifold pressure
Over heated engine
If detonation is suspected:
Enrichen the mixture
Reduce the throttle
Increase airspeed
Open cowls

23. 5. Abnormal Combustion Pre-ignition
Occurs when the charge prematurely ignites
Typically caused by a hot spot on the cylinder igniting the charge
Indicated by:
Rough running
Back fire
Sudden rise in CHT
Engine failure
Can be caused by:
Hot spots on the cylinder from carbon deposits
High power settings with a lean mixture
Overheated spark plugs

24. 6. C172SP Threats Threats specific to C172SP - FCU
Insufficient Air
Fuel from strainer
Fuel to injectors
Air inlets
Throttle body
Diaphragm
Fuel Traveling back into the throttle body
During priming and rich mixture operations at low power settings, non-vaporised fuel
from the fuel injectors is drawn down by gravity and enters the air inlet.
Fuel residue accumulates on the diaphragm and hinders correct fuel metering.
Consequences range from rough running to complete engine failure.

25. Questions?