Carburettor & Fuel-Injection Systems

ATC AGK Chapter 4

Aim

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

Objectives1. Describe components of the carburettor2. Describe components of the fuel-injection

system3. State threats of the carburettor and fuel-

injection systems4. State advantages/disadvantages of each system5. Explain the proper use of the mixture control

and abnormal combustion6. Describe threats associated with the fuel-

injection system of the C172SP

1. CarburettorCarburation

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.

Float Type CarburettorThe 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.

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.

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.

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.

1. Carburettor

2. Fuel injection systemFuel 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.

3. Threats to OperationImpact 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)

3. Threats to OperationFuel 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%.

3. Threats to OperationThrottle 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.

3. Threats to OperationIcing 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

3. Threats to OperationIcing 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.

3. Threats to OperationCarburettor 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.

3. Threats to OperationCarburettor 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 againCarb 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 manualIcing 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.

4. Advantages/Disadvantages

Fuel Injection vs CarburetionAdvantages 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 efficiencyDisadvantages 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

5. Mixture ControlLeaning 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.

5. Mixture ControlLeaning 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.

5. Mixture ControlMixture 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.

5. Abnormal CombustionAbnormal 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

5. Abnormal CombustionDetonation

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 failureContributory 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 engineIf detonation is suspected:• Enrichen the mixture• Reduce the throttle• Increase airspeed• Open cowls

5. Abnormal CombustionPre-ignition

Occurs when the charge prematurely ignitesTypically caused by a hot spot on the cylinder igniting the chargeIndicated by:• Rough running• Back fire• Sudden rise in CHT• Engine failureCan be caused by:• Hot spots on the cylinder from carbon deposits• High power settings with a lean mixture• Overheated spark plugs

6. C172SP ThreatsThreats specific to C172SP - FCU

During priming and rich mixture operations at low power settings, non-vaporised fuel

Insufficient Air

Fuel from strainer

Fuel to injectors

Air inlets

Throttle body

Diaphragm

Fuel Traveling back into the throttle body

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.

Questions?