Carburettor Flashcards
What component controls airflow in a carburettor?
Butterfly
What is the air pressure like in a carburettor’s float chamber?
It is atmospheric
Where is atmospheric pressure found in a carburettor?
Float chamber
How is fuel made to flow into the venturi airflow?
High pressure in float chamber and lower pressure in venturi
What factor influences the mass of fuel flowing in a carburettor?
Speed of airflow in venturi
Faster airflow in a carburettor venturi means what is also greater?
Differential pressure between flloat chamber and venturi
And thus the greater the fuel flow
How is fuel flow increased in a carburettor?
Differential pressure increases causes more fuel to flow into the venturi
What happens to fuel/air ratio when throttle is set to maximum power?
Airflow increases at a greater rate than fuel flow
What is used to overcome airflow increasing at greater rate than fuel flow when throttle is fully open?
Accelerator pump
What component allows engine to run smoothly at idle power?
Idling jet
When an engine is idling, what happens to differential pressure in a carburettor?
It is insufficient to force fuel into the venturi chamber
When a carburettor’s differential pressure is insufficient to prompt fuel flow, what can be said about the engine?
It is idling
What are the two types of mixture control in a carburettor?
Needle and back suction
Why is it good practice to have Mixture fully rich in a climb?
The excess fuel helps cool the engine
What does excess fuel do with regards to engine operation?
Acts as a cooling agent
What assists in engine cooling in a climb in relation to the carburettor?
Excess fuel when Mixture is fully rich
What carburettor setting will result in higher CHT?
Lean mixture
Over-rich mixture symptoms
Loss of power
High fuel consumption
Spark plug fouling
What is the major outcome of an excessively lean mixture setting?
Detonation
Why is use of idle cut-off a safe way of shutting down an engine?
No combustible fuel/air mixture is left in the carburettor
Detonation is likely with what power setting?
Excessively lean mixture
Why is detonation an outcome of an excessively lean mixture?
Excessively lean means very high cylinder head temperatures
What happens if pressure and temp rise in a cylinder is too high?
Detonation
What happens to temperature of a gas when it is compressed?
It rises
What items should be performed if detonation has occured?
Enrichen the mixture
Reduce cylinder pressure (throttle back)
Increase airspeed to assist cooling
In pre-ignition, when does combustion occur?
Before the spark plug operates
Difference between detonation and pre-ignition combustion
Detonation is explosive combustion, pre-ignition is progressive
Difference between detonation and pre-ignition in engine conditions
Detonation applies to whole engine, pre-ignition is unique to one cylinder
What often causes pre-ignition?
Carbon deposit in a cylinder
What happens to temperature as air accelerates through a carburettor venturi?
It drops
What kind of ice occurs in a carburettor if super-cooled water droplets come into contact with the carburettor’s components?
Impact ice
How can impact ice form in a carburettor in moist, warm temperatures?
The carburettor’s components are sub-zero from higher altitudes
When fuel vaporises, what happens to its temperature?
It falls substantially
Why does temperature fall substantially when fuel vaporises?
It absorbs latent heat
The absorption of latent heat by fuel in vaporisation has what effect?
Substantial temperature reduction
How does fuel ice form?
Fuel vaporisation reduces air temperature to below 0 degrees
Moisture in fuel/air mixture precipitates and forms ice
Why does temperature decrease after air/fuel passes the butterfly (throttle valve)?
Static pressure decreases, thus reducing temperature
When is the acceleration and temperature drop of fuel/air greatest in a carburettor?
At low throttle settings
Why is the acceleration and temperature drop of fuel/air greatest at low throttle settings?
Butterfly restricts airflow the most at these settings, creating greater pressure drops
When does a butterfly’s pressure changes occur most drastically?
At lower throttle settings because airflow is restricted the most
Visible moisture is not necessary for what kind of carburettor icing?
Throttle ice
What is throttle ice?
Fuel/air mixture accelerates, pressure drops and temeprature drops
Ice forms on throttle valve
3 types of carburettor icing?
Impact ice
Fuel ice
Throttle ice
What types of carburettor icing occurs with high OATs?
Fuel and throttle ice
Carb heat appplication involves an initial reduction of engine power how?
Heated air is less dense
What replaces carb heat in a fuel-injection system?
Alternate air
In a fuel-injection system, why is a venturi still present?
It senses the pressure differential
Approximate fuel/air ratio for a rich mixture?
1:8
Approximate fuel/air ratio for a lean mixture?
1:20
