MA4-5 Carb Economizer Operation

[cliffy]

After looking at a long running thread about cooling and fuel flow and seeing so much confusion on the economizer, I thought this thread on how the carb actually works might be of interest-

http://www.insightavionics.com/pdf files/MA-4 Carb Manual.pdf

This above is the best description of how the carb works from what I can find.

If you'll go to page 1-3   paragraphs d an e you will read how the "economizer" actually works and how extra fuel is drawn into the engine at WOT and how the FF is reduced when the throttle is pulled back slightly. 

Suffice it to say that there is an air bleed in line with the fuel path to the discharge nozzle. Bleeding air into that path "leans" the mixture. How much is governed by how big the "air bleed jet" is. (NOT a field changeable jet nor legal to change because of carb model designation).

Basic carb theory- the more air through the venturi- the more fuel is pushed through the main metering jet into the engine because the "pressure" in the throat of the carb with airflow is lower than the pressure of ambient air on the fuel in the float bowl. WOT means highest fuel flow.

Now, a higher FF is needed to not only produce the rated HP of the engine but also the cool the cylinders at high power down low where 100% HP is achievable.  This "extra" fuel flow is figured into the APPROVED carb design through the economizer system and air bleed jet size.

Now, when the extra fuel flow is not needed and the throttle is pulled back for cruise the economizer air bleed is opened (it was held closed by a pin at WOT) and air is bled into the fuel stream going to the discharge nozzle thereby leaning the mixture by a predetermined amount (WITH A FULL RICH SETTING ON THE MIXTURE CONTROL).

Once altitude is reached where manual leaning with the mixture control can be accomplished (less than 75% HP) then, the mixture is leaned (by the manual mixture control) further down the path to the main discharge nozzle and the economizer is no longer in the picture because the FF is now lower than the leaning provided by the economizer. This is why you can leave the throttle wide open for climb and cruise and still lean the mixture manually. It all flows through the same fuel channel. You don't need to pull the throttle back AND manually lean the mixture. The economizer only works, to lower FF with throttle position, when the manual mixture is full rich. Pulling the throttle back AFTER manually leaning the FF has NO effect on the FF to the engine as the FF is already below that predetermined FF that the economizer was set for. 

Hope this helps

 

[DXB]

15 minutes ago, cliffy said:

After looking at a long running thread about cooling and fuel flow and seeing so much confusion on the economizer, I thought this thread on how the carb actually works might be of interest-

http://www.insightavionics.com/pdf files/MA-4 Carb Manual.pdf

This above is the best description of how the carb works from what I can find.

If you'll go to page 1-3   paragraphs d an e you will read how the "economizer" actually works and how extra fuel is drawn into the engine at WOT and how the FF is reduced when the throttle is pulled back slightly. 

Suffice it to say that there is an air bleed in line with the fuel path to the discharge nozzle. Bleeding air into that path "leans" the mixture. How much is governed by how big the "air bleed jet" is. (NOT a field changeable jet nor legal to change because of carb model designation).

Basic carb theory- the more air through the venturi- the more fuel is pushed through the main metering jet into the engine because the "pressure" in the throat of the carb with airflow is lower than the pressure of ambient air on the fuel in the float bowl. WOT means highest fuel flow.

Now, a higher FF is needed to not only produce the rated HP of the engine but also the cool the cylinders at high power down low where 100% HP is achievable.  This "extra" fuel flow is figured into the APPROVED carb design through the economizer system and air bleed jet size.

Now, when the extra fuel flow is not needed and the throttle is pulled back for cruise the economizer air bleed is opened (it was held closed by a pin at WOT) and air is bled into the fuel stream going to the discharge nozzle thereby leaning the mixture by a predetermined amount (WITH A FULL RICH SETTING ON THE MIXTURE CONTROL).

Once altitude is reached where manual leaning with the mixture control can be accomplished (less than 75% HP) then, the mixture is leaned (by the manual mixture control) further down the path to the main discharge nozzle and the economizer is no longer in the picture because the FF is now lower than the leaning provided by the economizer. This is why you can leave the throttle wide open for climb and cruise and still lean the mixture manually. It all flows through the same fuel channel. You don't need to pull the throttle back AND manually lean the mixture. The economizer only works, to lower FF with throttle position, when the manual mixture is full rich. Pulling the throttle back AFTER manually leaning the FF has NO effect on the FF to the engine as the FF is already below that predetermined FF that the economizer was set for. 

Hope this helps

 

Thanks! This is a clearer explanation than I've heard to date and probably a far better one than most A&Ps could give.  I still am a little confused by what design problem this "economizer circuit" system was originally intended to address. If the purpose of the economizer was simply to provide "economy" at full mixture operations outside of WOT, then the same could be achieved simply by leaning.  Perhaps it prevents an uncombustibly rich mixture if one does not remember to lean at lower power settings under certain conditions?

[Hank]

39 minutes ago, cliffy said:

Once altitude is reached where manual leaning with the mixture control can be accomplished (less than 75% HP) then, the mixture is leaned (by the manual mixture control) further down the path to the main discharge nozzle and the economizer is no longer in the picture because the FF is now lower than the leaning provided by the economizer. This is why you can leave the throttle wide open for climb and cruise and still lean the mixture manually. It all flows through the same fuel channel. You don't need to pull the throttle back AND manually lean the mixture. The economizer only works, to lower FF with throttle position, when the manual mixture is full rich. Pulling the throttle back AFTER manually leaning the FF has NO effect on the FF to the engine as the FF is already below that predetermined FF that the economizer was set for. 

I pull the throttle back slightly not so much to reduce fuel flow but to create turbulence in the carb for better atomization and mixing. This apparently works, as the couple of flights that I used WOT instead of WOT- did not achieve the same fuel economy when measured at the fuel pump.

[Prior owner]

DXB- 

I don’t think the economizer was designed to deal with a design problem.

It’s name is somewhat misleading, although it is actually appropriate. I think It is just a WOT enrichment device that only functions at lower DA’s to keep temps down during a climb. If Marvel had instead designed the carb with yet another jet circuit just for WOT, it probably wouldn’t have worked as simply and elegantly as this solution, which removes itself automatically as you climb?

 

 

[cliffy]

No design problem involved Just the requirement to enrich for cooling (above that FF required to make full power).