Carburation And Fuel Injection Flashcards
(32 cards)
What is a simple float carburettor?
Device used in internal combustion engines to mix air and fuel in the appropriate proportion for efficient combustion.
- Uses float and needle valve system to regulate the fuel
- Created by the Venturi effect.
What is a venturi?
Narrow constriction in the air passage that accelerates the airflow, creating a region of low pressure.
- Pressure drop draws fuel from the float chamber into the airstream - undergoes atomization.
As the fuel droplets disperse into the airflow, they mix and distribute evening.
What is the idling system?
While the engine is idling the throttle butterfly is closed.
- Reduces Venturi effect to the point that no more fuel is being drawn from the float chamber.
What is the acceleration system?
- Due to rapid throttle changes, the demand for fuel increases quickly. However, the drop in pressure in the Venturi may not respond fast enough.
- The acceleration system AKA. An accelerator pump compensates for the lag by proving
What is the power enrichment system?
During high power operations, the engine requires a richer mixture to achieve optimal performance and prevent issues such as detonation or over heating.
- Power enrichment system compensates for this by adding more fuel to the mixture when needed.
- Consists of a seperate fuel circuit, controlled by a metering valve or jet. Additional fuel supply enriches the mixture, providing the necessary power and cooling during high-power operations.
Disadvantages of a float-type carburettor.
- Carb ice
- Fuel metering imprecision
- Difficulty in delivering consistent and evenly distributed fuel-air mixture to all cylinders in multi-cylinder engines. Results in uneven combustion and reduced engine efficiency.
What is the theoretical best fuel-air ratio?
1:14.7 (1:15)
- AKA chemically correct mixture (CCM) or stoichiomentric ratio.
When are rich mixtures most used?
- Take-off
- Climb
They provide better cooling for the engine and maximise power output.
What does a lean mixture do?
- Achieves better fuel economy
- Can reduce emissions while maintaining engine temperature within safe limits
What is a lean mixture ratio?
1:18
What is a rich mixtures ratio?
1:9
What is the maximum power output ratio?
1:12
What does an over-rich mixture do?
- Loss of power
- Rough running
- Fouling of spark plugs
What does an over lean mixture do?
- Increase of cylinder head temperature (CHT). If not rectified, will lead to detonation, loss of power, and possibly engine failure.
What does carb ice do?
- Reduction in engine performance
- Potential engine failure
What are the three main types of carb ice?
- Refrigeration (or fuel) ice.
- Throttle ice
- Impact ice
What is refrigeration (or fuel) ice?
- Forms due to the temperature drop in the carburettor.
- Cause by the vaporisation of fuel
- Ice forms on the walls of the carburetor
- Restricts airflow
- Reduces engine performance
- Occurs when outside air temperature is as high as +25 degrees and air humidity is above 50%
Most likely to form when fuel-air mixture is 0 to -10 degrees in carburettor.
What is throttle ice?
- Forms at or near the throttle plate in the carburettor.
- Forms when the throttle is partially closed, where there is a pressure drop and a decrease in temperature.
- Restricts airflow
- Reduces engine performance
- Can form when outside air temperature is as high as +25 degrees and if the relative humidity is above 50%
Most likely to form when fuel-air mixture is 0 to -10 degrees in carburettor.
What is impact ice?
- Forms when the aircraft flies through visible moisture e,g, rain, snow, or cloud droplets
- Can freeze upon contact with cold surfaces of the air intake and carburettor
- Causes blockage
- Restricts airflow
What are symptoms of carb ice?
- Fixed pitch propeller = RPM will slowly decrease
- Constant speed propeller = drop in manifold pressure
- Both cases, engine performance is affected
When is carb ice most likely to occur?
- Low power setting
- During descent
- Slow flight
How do you reduce risk of carb ice?
- Increase power for a short period of time
- Carb heat on below 2000RPM or during the approach to land.
What does carb heat do?
- Directs warm air from around the exhaust manifold into the carburettor to increase temperature and melt any ice.
- Temporary drop in engine performance
What does excessive carburettor heat do?
- Air entering engine = less dense
- Results in a reduced mass of fuel-air mixture
- Decrease in engine power output
- Reduced cooling efficiency - higher cylinder head and oil temperatures potentially causing over heating
- Higher temps - increases risk of detonation
