Carburation And Fuel Injection Flashcards
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
What is ram air?
When air intakes are usually facing forward to utilise the increase in pressure created by the air speed and prop wash.
- Increases the density of the air resulting in better performance.
What is filtered air?
- Air that passes through an air filter before entering the carburettor
- Removes debris, dust, or other contaminants from air before is reaches the engine.
- Slightly reduces performance, but offers better protection for the engine.
What is carburettor heat air?
Carb heat air is unfiltered, avoid using on the ground.
What is alternate air?
In a fuel injected engine, there is a different air intake called “alternate air” situated in an engine bay, protected from impact icing.
What is a fuel injection system?
A high pressure pump (injection pump) forces fuel through a calibrated nozzle (injector) directly into the combustion chamber or by the inlet valve.
- Fuel-air ratio is controlled by a “fuel control unit”, a lot more precise and efficient compared to a carburettor.
- Eliminates the need for a primer or choke and ensures consistent performance across a range of temperatures
What is direct flow injection?
Where fuel is metered by fuel control before being fed to the injection pump.
- Pressure and fuel will only be provided to each cylinder during the induction stroke.
- Sprayed directly into the combustion chamber - instant vaporisation
Advantages of fuel injection
- Better fuel efficiency
- Reduced risk of icing
- Fewer maintenance problems
- Increased engine efficiency
Disadvantages of fuel injection
- Vapour lock - bubbles around injector line, difficult to restart engine
- Fuel line blockage - contaminant in fuel
- Fuel balance - happens if pilot is not monitoring the fuel tank
