Altitude, Mixture and Fuel Flow
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Whilst that's undoubtedly interesting, skiingman, what would be your answer to the ATPL question posed, assuming the most basic carburettor without any additional mixture management features? Do you believe any of the four possible answers are correct for such a system? If not, why not?
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As altitude increases, if the mixture is not leaned:
A the volume of air entering the carburettor remains constant and the fuel flow decreases
B the volume of air entering the carburettor decreases and the fuel flow decreases
C both the density of air entering the carburettor and the fuel flow decrease
D the volume of air entering the carburettor decreases and the fuel flow increases
Knowns
-density decreases
-mixture will enrich
-power will decrease
Unknowns:
-aircraft speed
-propeller control type
-ergo, engine speed and load
-engine VE as a function of speed and load
Potential answers and their problems:
a) Volume flow at the carburetor depends on engine speed and volumetric efficiency. Neither are known. Fuel flow should decrease for a simple carb, as Keith Williams correctly explained. If we assume no changes in engine speed or VE due to the enrichment like someone writing an exam might, this answer could be construed as correct.
b)VE probably goes down as mixture gets progressively richer and load falls. An aircraft with a fixed pitch prop and less power will probably have a slower engine speed too. This is a likely real world answer, but I bet it is wrong on the answer key.
c)The density of air decreases, and as per above the fuel flow likely decreases. I would choose this as the "most correct" answer.
d)This is the least likely answer.
As a test taker I'd choose c), but while cognizant that there is a good chance a) is correct, and with an annoyed knowledge thab b) is almost certainly true, yet "incorrect". My earlier comments about this being an ill-conceived question still stand. Maybe you can tell me which answer is correct.
A the volume of air entering the carburettor remains constant and the fuel flow decreases
B the volume of air entering the carburettor decreases and the fuel flow decreases
C both the density of air entering the carburettor and the fuel flow decrease
D the volume of air entering the carburettor decreases and the fuel flow increases
Knowns
-density decreases
-mixture will enrich
-power will decrease
Unknowns:
-aircraft speed
-propeller control type
-ergo, engine speed and load
-engine VE as a function of speed and load
Potential answers and their problems:
a) Volume flow at the carburetor depends on engine speed and volumetric efficiency. Neither are known. Fuel flow should decrease for a simple carb, as Keith Williams correctly explained. If we assume no changes in engine speed or VE due to the enrichment like someone writing an exam might, this answer could be construed as correct.
b)VE probably goes down as mixture gets progressively richer and load falls. An aircraft with a fixed pitch prop and less power will probably have a slower engine speed too. This is a likely real world answer, but I bet it is wrong on the answer key.
c)The density of air decreases, and as per above the fuel flow likely decreases. I would choose this as the "most correct" answer.
d)This is the least likely answer.
As a test taker I'd choose c), but while cognizant that there is a good chance a) is correct, and with an annoyed knowledge thab b) is almost certainly true, yet "incorrect". My earlier comments about this being an ill-conceived question still stand. Maybe you can tell me which answer is correct.
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Bizarre, really, all this talk of compressible flows and gas laws, yet a basic failure to visualise a simple physical fact.
At least in my postulated simple engine an induction stroke takes in a fixed volume of gas. Say 1ltr, This has come in through the intake duct at temperatures, pressures and velocities determined by the intake conditions, but in the end, when the intake valve shuts there is 1ltr of inducted gas in the engine. Later, at TDC on the exhaust stroke 1ltr of gas has been expelled. The exhaust gases have departed down the pipe with temperatures, pressures and velocities determined largely by the combustion conditions and the dimensions of the exhaust duct.
But the clear fact remains. The engine took in 1ltr of assorted gases and discharged 1ltr of assorted gases. This and RPM determines the volume flow through the engine. This would be confirmed by measurements of flow factors in the intake and exhaust ducts, but volume flow would only be available from calculation from velocity, temperature and pressure measurement.
And I do not dispute that this will not apply absolutely to more developed engines, but swept volume and RPM will remain the dominant factors in determining VOLUME flow
Dick
At least in my postulated simple engine an induction stroke takes in a fixed volume of gas. Say 1ltr, This has come in through the intake duct at temperatures, pressures and velocities determined by the intake conditions, but in the end, when the intake valve shuts there is 1ltr of inducted gas in the engine. Later, at TDC on the exhaust stroke 1ltr of gas has been expelled. The exhaust gases have departed down the pipe with temperatures, pressures and velocities determined largely by the combustion conditions and the dimensions of the exhaust duct.
But the clear fact remains. The engine took in 1ltr of assorted gases and discharged 1ltr of assorted gases. This and RPM determines the volume flow through the engine. This would be confirmed by measurements of flow factors in the intake and exhaust ducts, but volume flow would only be available from calculation from velocity, temperature and pressure measurement.
And I do not dispute that this will not apply absolutely to more developed engines, but swept volume and RPM will remain the dominant factors in determining VOLUME flow
Dick
