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george(at)gesoco.com
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 Posted: Sat Jan 26, 2008 7:38 am Post subject: Matronics List Fund Raiser - 2007 List of Contributors |
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M14P users, please note the following is an explanation that I give to
people about the need for a vapor return on M14P powered aircraft. There
have been several instances where trapped air of vapor has caused engine
interruption at critical times.
The M14P engine has a pressure carburetor. It is similar to a throttle
body injection system. There is almost no fuel stored in the carburetor as
you would find in a float carburetor. The pressure carburetor has a spray
bar located down stream of the venturi. The venturi acts as a mass flow
meter by developing a pressure on a diaphragm. The other side of the
diaphragm is at atmospheric pressure. This differential pressure drives a
needle and at a given fuel pressure will determine the amount of fuel going
to the spray bar. The needle is also temperature compensated.
The fuel air mix then goes to the supercharger where it is then
thoroughly mixed and fed to the intake of each cylinder.
Because the carburetor fuel flow depends on the differential pressure, a
mixture control is not necessary as the amount of fuel is only dependant on
the differential pressure. Thus as you go up in altitude and the outside
pressure decreases, the differential pressure changes so will the amount of
fuel to the spray bar will decrease. It is an automatic leaning system.
As an aside, because fuel is NOT being evaporated in the venturi, there is
less temperature drop in the venturi and that is why there is almost never
carb icing in a pressure carb.
The down side is that if you introduce a small bubble of air in the line
to the carb, then for whatever time it takes for that air to work through
the system the engine will have no fuel. The effect is the same as shutting
off the magnetos. Unlike the Lycoming or Continental continuous flow fuel
injection systems, all the cylinders will experience a lean or no fuel at
the same time.
This scenario also applies to a loss of fuel pressure. As the fuel goes
to the spray bar under pressure, if there is not enough pressure, then there
will be no fuel. A normal suction carb only requires enough fuel pressure to
keep the float chamber full and the low pressure in the venturi will suck
the fuel in from the float chamber due to the reduced pressure in the
venturi. Air bubbles in a float carburetor will be vented out the top of the
float chamber with no ill effect. If an air bubble reaches the mechanical
pump, it will cavitate and the pressure carb will loose the necessary
pressure to operate. It requires a minimum of about 3 psi to operate. This
again has the same effect as momentarily turning off the magnetos.
An additional problem arises when there are vapor bubbles in the fuel.
These small bubbles are carried along with the fuel. These will cause
momentary fluctuations in pressure as the fuel mixture becomes too lean.
Let us imagine a fuel line with a small leak. The leak is such that fuel
will not come out under normal head pressure from a tank., but when the
engine driven pump starts to suck it may suck a little air with it. This
could also be a loose or not tight gasket in the gascolator. It could be any
of the "B" nut fittings with a small scratch or not tightened enough. It
could be a pipe thread not properly sealed. Any of these could manifest
themselves as a power loss, particularly at max power when the fuel pump is
sucking the hardest. It would be hard to detect on the ground as it is
usually impossible to make a full power run up on the ground in most of the
aircraft equipped with the M14P.
A second scenario is that you have a fuel line with a loop in it. You
drain the fuel during maintenance of the gascolator. There remains a slug of
air in the gascolator and/or fuel line. At low power settings it does not
work its way out of the system. Now you take off. The angle of the aircraft
changes, the engine is at max fuel flow now and the bubble of air works it
way to the carburetor. You have a power loss at the most critical time.
I would certainly include a vapor return system as part of my fuel system.
The yaks, Sukhoi, I3s etc all use a vapor return system.
The eastern airplanes I have seen that operate without a vapor return
(Wilga, PZL etc) use only a short aluminum hard line directly between the
fuel pump and the carb to eliminate the possibility of trapped air.
Additionally the gascolator and lines to the fuel pump are below the level
of the fuel pump when the aircraft is on the ground.
Many builders today want to install fuel flow meters. This complicates a
system without a vapor return system and can be a source of trapped air.
Be careful with your fuel system design.
George S. Coy
President
Gesoco Industries Inc.
629 Airport Rd.
Swanton, VT 05488 USA
George(at)gesoco.com
george.coy(at) gmail.com
Phone +1-802-868-5633
Fax +1-802-868-4465
SKYPE: GEORGE.COY
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culpspecial(at)yahoo.com
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| Posted: Sat Jan 26, 2008 8:19 am Post subject: Matronics List Fund Raiser - 2007 List of Contributors |
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Brave man.
George Coy <george(at)gesoco.com> wrote: [quote]--> M14PEngines-List message posted by: "George Coy"
M14P users, please note the following is an explanation that I give to
people about the need for a vapor return on M14P powered aircraft. There
have been several instances where trapped air of vapor has caused engine
interruption at critical times.
The M14P engine has a pressure carburetor. It is similar to a throttle
body injection system. There is almost no fuel stored in the carburetor as
you would find in a float carburetor. The pressure carburetor has a spray
bar located down stream of the venturi. The venturi acts as a mass flow
meter by developing a pressure on a diaphragm. The other side of the
diaphragm is at atmospheric pressure. This differential pressure drives a
needle and at a given fuel pressure will determine the amount of fuel going
to the spray bar. The needle is also temperature compensated.
The fuel air mix then goes to the supercharger where it is then
thoroughly mixed and fed to the intake of each cylinder.
Because the carburetor fuel flow depends on the differential pressure, a
mixture control is not necessary as the amount of fuel is only dependant on
the differential pressure. Thus as you go up in altitude and the outside
pressure decreases, the differential pressure changes so will the amount of
fuel to the spray bar will decrease. It is an automatic leaning system.
As an aside, because fuel is NOT being evaporated in the venturi, there is
less temperature drop in the venturi and that is why there is almost never
carb icing in a pressure carb.
The down side is that if you introduce a small bubble of air in the line
to the carb, then for whatever time it takes for that air to work through
the system the engine will have no fuel. The effect is the same as shutting
off the magnetos. Unlike the Lycoming or Continental continuous flow fuel
injection systems, all the cylinders will experience a lean or no fuel at
the same time.
This scenario also applies to a loss of fuel pressure. As the fuel goes
to the spray bar under pressure, if there is not enough pressure, then there
will be no fuel. A normal suction carb only requires enough fuel pressure to
keep the float chamber full and the low pressure in the venturi will suck
the fuel in from the float chamber due to the reduced pressure in the
venturi. Air bubbles in a float carburetor will be vented out the top of the
float chamber with no ill effect. If an air bubble reaches the mechanical
pump, it will cavitate and the pressure carb will loose the necessary
pressure to operate. It requires a minimum of about 3 psi to operate. This
again has the same effect as momentarily turning off the magnetos.
An additional problem arises when there are vapor bubbles in the fuel.
These small bubbles are carried along with the fuel. These will cause
momentary fluctuations in pressure as the fuel mixture becomes too lean.
Let us imagine a fuel line with a small leak. The leak is such that fuel
will not come out under normal head pressure from a tank., but when the
engine driven pump starts to suck it may suck a little air with it. This
could also be a loose or not tight gasket in the gascolator. It could be any
of the "B" nut fittings with a small scratch or not tightened enough. It
could be a pipe thread not properly sealed. Any of these could manifest
themselves as a power loss, particularly at max power when the fuel pump is
sucking the hardest. It would be hard to detect on the ground as it is
usually impossible to make a full power run up on the ground in most of the
aircraft equipped with the M14P.
A second scenario is that you have a fuel line with a loop in it. You
drain the fuel during maintenance of the gascolator. There remains a slug of
air in the gascolator and/or fuel line. At low power settings it does not
work its way out of the system. Now you take off. The angle of the aircraft
changes, the engine is at max fuel flow now and the bubble of air works it
way to the carburetor. You have a power loss at the most critical time.
I would certainly include a vapor return system as part of my fuel system.
The yaks, Sukhoi, I3s etc all use a vapor return system.
The eastern airplanes I have seen that operate without a vapor return
(Wilga, PZL etc) use only a short aluminum hard line directly between the
fuel pump and the carb to eliminate the possibility of trapped air.
Additionally the gascolator and lines to the fuel pump are below the level
of the fuel pump when the aircraft is on the ground.
Many builders today want to install fuel flow meters. This complicates a
system without a vapor return system and can be a source of trapped air.
Be careful with your fuel system design.
George S. Coy
President
Gesoco Industries Inc.
629 Airport Rd.
Swanton, VT 05488 USA
George(at)gesoco.com
george.coy(at) gmail.com
Phone +1-802-868-5633
Fax +1-802-868-4465
SKYPE: GEORGE.COY
--
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