Flying over square
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So, do any of the round engine old pharts know how the auto lean works?
(Sorry folks, I wrote that late last night after consuming two bottles of NZ's finest and shouldn't have been anywhere near a computer.)
(Sorry folks, I wrote that late last night after consuming two bottles of NZ's finest and shouldn't have been anywhere near a computer.)
Last edited by osmosis; 21st Oct 2011 at 02:46.
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Even the non geared R985 Pratt is same, don't let the prop drive the engine, the counter weights get really unhappy.
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The Auto lean is a balance of pressure input fuel to metered fuel across a modulated diaphragm that is fed sense static pressure as well as the dynamic pressure in the carby throat. 2 diaphragms with a balance beam that ultimately modulates fuel flow to the "injector"
Auto lean only works on those round engines that have pressure carburettors ( also known as throttle bodies) which are actually something between a float carby and inlet injection as practiced by Continental and Lycoming. Older float carburettor engines are a lot triclier to lean.
The series of balanced pressure diaphragms are set up before the supercharger which is important, and the system is really sensitive to backfires which can tear the dynamic air pressure diaphragm and then the mixture control becomes U/S so no overfuelling on start.
Auto lean only works on those round engines that have pressure carburettors ( also known as throttle bodies) which are actually something between a float carby and inlet injection as practiced by Continental and Lycoming. Older float carburettor engines are a lot triclier to lean.
The series of balanced pressure diaphragms are set up before the supercharger which is important, and the system is really sensitive to backfires which can tear the dynamic air pressure diaphragm and then the mixture control becomes U/S so no overfuelling on start.
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Any radial if the prop drives the engine the whole engine gets unhappy, factors such as Keystone rings, even in non geared engines are an issue, not to understate the need to maintain positive pressure on the master bearing, the engines were never designed to be wind turbines.
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Theory of operation
Fuel regulator air diaphragm dividing chambers A and B
There are four chambers in the fuel regulator portion of the carburetor. They are referred to by letters A, B, C, and D, with the A chamber closest to the throttle body. The fuel metering servo valve responds to pressure differentials across the diaphragms. The resulting diaphragm movement controls fuel flow into the engine under all flight conditions.[9]
Chamber A The diaphragm located closest the carburetor body is the air metering diaphragm. It measures the difference in air pressure taken from two locations within the carburetor. Chambers A and B are on opposite sides of the air metering diaphragm. The velocity of the air flow entering the carburetor is measured by placing one or more venturi directly in the airflow. The venturi creates a lower than atmospheric pressure that changes with the velocity of the air. The negative air pressure from the venturi is connected to "Chamber A" on the side of the air metering diaphragm closest to the carburetor body. As the air pressure in chamber A is decreased, the diaphragm is pulled toward the carburetor body.[9] Chamber B The mass of the air entering the carburetor was measured by placing a number of impact tubes directly in the airflow, generating a pressure higher than atmospheric pressure that represents the real-time air density. The impact tube pressure is connected to "Chamber B" on the side of the air metering diaphragm farthest from the carburetor body. As the air pressure in chamber A is increased, the diaphragm is moved toward the carburetor body toward the fuel metering valve. Chamber A also contains a spring that creates a force toward the fuel metering valve when the air flow is absent.[9] The difference in pressure between the two air chambers creates what is known as the air metering force, which moves the fuel metering valve open when it is greater than the opposing force or closed when it is less than the opposing force.[9]
Fuel regulator fuel diaphragm dividing chambers C and D
The second diaphragm is the fuel metering portion of the regulator, and is located farthest from the carburetor body. It measures the difference in fuel pressure taken from two locations within the regulator itself. Chambers C and D are on opposite sides of the fuel metering diaphragm.[9]
Chamber C Chamber C contains unmetered fuel, that is the pressure of the fuel as it enters the carburetor. The pressure in this chamber moves the metering valve outward when the fuel pressure is higher than the pressure in chamber D, on the opposite side of the diaphragm.[9] Chamber D Chamber D contains metered fuel, that is fuel that has already passed through the metering valve, but not yet injected into the air stream. The pressure in this chamber moves the metering valve inward when the fuel pressure is higher than the pressure in chamber C, on the opposite side of the diaphragm.[9] The difference in pressure between the two fuel chambers creates the fuel metering force, which acts to close the servo valve.
The air metering force from chambers A and B apply a force to open the servo valve, and is opposed by the fuel metering force from chambers C and D which apply a force to close the servo valve. These two forces combine into movement of the servo valve to adjust the fuel flow to the precise amount required for the needs of the engine, and the needs of the pilot.[9]
Fuel regulator air diaphragm dividing chambers A and B
There are four chambers in the fuel regulator portion of the carburetor. They are referred to by letters A, B, C, and D, with the A chamber closest to the throttle body. The fuel metering servo valve responds to pressure differentials across the diaphragms. The resulting diaphragm movement controls fuel flow into the engine under all flight conditions.[9]
Chamber A The diaphragm located closest the carburetor body is the air metering diaphragm. It measures the difference in air pressure taken from two locations within the carburetor. Chambers A and B are on opposite sides of the air metering diaphragm. The velocity of the air flow entering the carburetor is measured by placing one or more venturi directly in the airflow. The venturi creates a lower than atmospheric pressure that changes with the velocity of the air. The negative air pressure from the venturi is connected to "Chamber A" on the side of the air metering diaphragm closest to the carburetor body. As the air pressure in chamber A is decreased, the diaphragm is pulled toward the carburetor body.[9] Chamber B The mass of the air entering the carburetor was measured by placing a number of impact tubes directly in the airflow, generating a pressure higher than atmospheric pressure that represents the real-time air density. The impact tube pressure is connected to "Chamber B" on the side of the air metering diaphragm farthest from the carburetor body. As the air pressure in chamber A is increased, the diaphragm is moved toward the carburetor body toward the fuel metering valve. Chamber A also contains a spring that creates a force toward the fuel metering valve when the air flow is absent.[9] The difference in pressure between the two air chambers creates what is known as the air metering force, which moves the fuel metering valve open when it is greater than the opposing force or closed when it is less than the opposing force.[9]
Fuel regulator fuel diaphragm dividing chambers C and D
The second diaphragm is the fuel metering portion of the regulator, and is located farthest from the carburetor body. It measures the difference in fuel pressure taken from two locations within the regulator itself. Chambers C and D are on opposite sides of the fuel metering diaphragm.[9]
Chamber C Chamber C contains unmetered fuel, that is the pressure of the fuel as it enters the carburetor. The pressure in this chamber moves the metering valve outward when the fuel pressure is higher than the pressure in chamber D, on the opposite side of the diaphragm.[9] Chamber D Chamber D contains metered fuel, that is fuel that has already passed through the metering valve, but not yet injected into the air stream. The pressure in this chamber moves the metering valve inward when the fuel pressure is higher than the pressure in chamber C, on the opposite side of the diaphragm.[9] The difference in pressure between the two fuel chambers creates the fuel metering force, which acts to close the servo valve.
The air metering force from chambers A and B apply a force to open the servo valve, and is opposed by the fuel metering force from chambers C and D which apply a force to close the servo valve. These two forces combine into movement of the servo valve to adjust the fuel flow to the precise amount required for the needs of the engine, and the needs of the pilot.[9]
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Thank you, 28. So the auto lean is driven soley by the pressure differentials and does not take into account any engine temperatures. Is it in-flight adjustable at all? I'm trying to link it back to this thread's main (but not original) topic of various engine operating temperatures.
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It would appear that I may once again have to refer to the discombobulator ....
Last edited by Avgas172; 21st Oct 2011 at 10:24. Reason: bloody poor spelimg ...
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No temp inputs to Auto Mixture it is set in one position by detent, in the case of a Wright 1820 as an example it is good to 51.5 inches before you must go to full rich.
