C Model Fuel Burn Rate

[cliffy]

We really don't have 2 fuel jets in our carburetors -

Let's begin with basics. On any carb'd engine the main fuel flow comes from the float bowl, through the main metering jet (this is the main way of limiting maximum fuel flow to the engine) and out the main fuel nozzle located in the throat of the carb. Fuel flows through the nozzle to the engine anytime intake air is flowing through the throat of the carb. It is being "pushed" through the nozzle by the difference between the air pressure (pushing) on/in the top of the float chamber ( higher pressure) and the lower air pressure in the throat of the carb due to the flow of air speeding up (Bernoulli's Principle) through the carb throat. The faster the air (more throttle) the more fuel is "pushed" by the higher pressure air in the float chamber through the fuel nozzle. WOT = max fuel flow, governed by the main metering jet.

On air cooled engines like we have, the air cooling is not enough to keep the engine within heat parameters without the use of "extra" fuel for cooling at WOT and low altitude.  Governed again by the main metering jet size called for by the engine maker. 

When looking at the "mechanical" fuel enrichment system (economizer system) in the 180 HP Lycoming MA4-5 carb, the system really doesn't have 2 fuel jets. What happens is that as the power comes up (with WOT) the throttle arm is pushing on a spring loaded pin valve, holding it closed. Once we pull back off of WOT a little this spring loaded pin valve opens. This valve ports air from the top of the float chamber (higher ambient air pressure) through a precisely sized hole and into the main fuel nozzle after the main metering jet for the carb (a lower pressure area as we said before). More air after the main jet means less fuel so the mixture "leans" a little, as we don't need the extra fuel for cooling like we did at WOT.  All done "automatically by Lycoming design. Thus, as Lycoming calls it, the "economizer" system.

Once we get high enough and want/need to lean the mixture for altitude the leaning of the economizer system has no effect as we take the FF below the amount the economizer system can do by using the use of the mixture control. Remember, the economizer has a very limited effect because of the highly precise hole that it uses for leaning. 

Some who have cylinder head temp problems may want to check another item to try and cure the issue. Every airplane is certified with a very precisely calibrated carb. Each carb for each airframe installation has a very specific fuel flow design even though they are all MA4-5 carbs. These carbs are specified in the TCDS for each airplane. Sometimes 2 or more are allowed, each with maybe a different fuel flow specific (higher or lower).  If you can't seem to get a handle on your temps you might check very carefully to see if you have the correct SPECIFIC carb for your airplane. 

[carusoam]

Great details, Cliffy!

Any idea what the extra fuel flow through the enrichment circuit actually is, or where it might be documented?

A pin type valve is not typically on/off... 0/1...  If the only window we have is watching EGTs that can be pretty slow to react... it might be a real challenge to ‘see’ if the enrichment circuit is on or off...

Best regards,

-a-

[cliffy]

No idea on the amount of enrichment but I am sure it varies from one dash number carb to another by the size of the precise hole. 

It's either open or closed. Nothing in between.

Like I mentioned, unless one is down low and running wide open this system has no meaning to the total operation. Once the mixture control is pulled this system is out of its control range and has no effect. You can leave the throttle wide open and lean it at altitude with this system having no effect on the fuel flow (leaned) what so ever. 

[MIm20c]

We had great weather for flying today so I decided to go out and do some testing.  I’m posting this info for those without an engine monitor or fuel flow gauge.  I get conflicting info when comparing the mapa key numbers and the operating handbook so I ran all the tests at 75-80 degrees ROP at roughly 70 ish percent power. 

One thing I notice is when I’m on the turbo or down low pulled back the egt’s run very close. However, up high at wot or slightly pulled back the back two cylinders run a warmer egt. Also when the o360 cylinders peak they are not crisp like an io550 instead they get kind of mushy and hold the peak values into the lean.  I have a unique monitor setup (for a C) that also shows TIT.  Consistently running 110 degrees ROP via the TIT would give me 75-80 on the leanest cylinder.

Surprisingly, the speed stayed very close throughout the different altitudes and power settings 148-150k TAS.  However, the fuel consumption increased around 1 gph descending from 10.5 to 6.5.  Also going from rich of rough (very smooth) to 75 ROP increased the FF by 0.8 gph.

[MIm20c]

pressure altitude/temp/RPM/MP/FF/TAS

10,500/-2c/2480/19.8/9.2/150

9,500/-1c/2500/20.7/9.6/151

8,500/0c/2400/21.4/9.7/149

7,500/1c/2400/22.3/10.3/150

6,500/3c/2400/22/10/148

Leaning another 0.8 gph from the above values did not noticeably lower TAS or add roughness. 

[carusoam]

MIm,

you may want to include the TN in your avatar data field.  It might be helpful for the few other Cs with your additional hardware.

Do you have a CDT gauge? Or carb temp?  You might be seeing the effect of adding carb heat from the compressor?

Nice collection of data!

Thanks for sharing all that.

Best regards,

-a-

[MIm20c]

I do have a IAT sensor on the JPI that will show the air temp going into the carb but I forgot to lock it on that option today.  I also did not play with the carb heat at all although in the past I’ve found a little bit of turbo helped even things out and make the peaks much more abrupt. 

I was going to test with the turbo at the higher altitudes but it was not making proper MP today in the climb. I’m going to have my mechanic out this week to remove the exhaust system and check the turbo. Kind of bummed as I was planning on flying to Texas next weekend and I’m worried the bird will be in pieces. 

[Oscar Avalle]

6 hours ago, cliffy said:

We really don't have 2 fuel jets in our carburetors -

Let's begin with basics. On any carb'd engine the main fuel flow comes from the float bowl, through the main metering jet (this is the main way of limiting maximum fuel flow to the engine) and out the main fuel nozzle located in the throat of the carb. Fuel flows through the nozzle to the engine anytime intake air is flowing through the throat of the carb. It is being "pushed" through the nozzle by the difference between the air pressure (pushing) on/in the top of the float chamber ( higher pressure) and the lower air pressure in the throat of the carb due to the flow of air speeding up (Bernoulli's Principle) through the carb throat. The faster the air (more throttle) the more fuel is "pushed" by the higher pressure air in the float chamber through the fuel nozzle. WOT = max fuel flow, governed by the main metering jet.

On air cooled engines like we have, the air cooling is not enough to keep the engine within heat parameters without the use of "extra" fuel for cooling at WOT and low altitude.  Governed again by the main metering jet size called for by the engine maker. 

When looking at the "mechanical" fuel enrichment system (economizer system) in the 180 HP Lycoming MA4-5 carb, the system really doesn't have 2 fuel jets. What happens is that as the power comes up (with WOT) the throttle arm is pushing on a spring loaded pin valve, holding it closed. Once we pull back off of WOT a little this spring loaded pin valve opens. This valve ports air from the top of the float chamber (higher ambient air pressure) through a precisely sized hole and into the main fuel nozzle after the main metering jet for the carb (a lower pressure area as we said before). More air after the main jet means less fuel so the mixture "leans" a little, as we don't need the extra fuel for cooling like we did at WOT.  All done "automatically by Lycoming design. Thus, as Lycoming calls it, the "economizer" system.

Once we get high enough and want/need to lean the mixture for altitude the leaning of the economizer system has no effect as we take the FF below the amount the economizer system can do by using the use of the mixture control. Remember, the economizer has a very limited effect because of the highly precise hole that it uses for leaning. 

Some who have cylinder head temp problems may want to check another item to try and cure the issue. Every airplane is certified with a very precisely calibrated carb. Each carb for each airframe installation has a very specific fuel flow design even though they are all MA4-5 carbs. These carbs are specified in the TCDS for each airplane. Sometimes 2 or more are allowed, each with maybe a different fuel flow specific (higher or lower).  If you can't seem to get a handle on your temps you might check very carefully to see if you have the correct SPECIFIC carb for your airplane. 

Thank you! Really interesting. Now where can I find the exact information about what Carb goes into my plane?

Oscar

[MIm20c]

Some pics of the EM. 

First- low power 6.8gph low altitude run

Second-TIT leaned to 40 ROP 8.4gph

Third/Fourth- leaned per the chart above

[Hank]

5 hours ago, cliffy said:

No idea on the amount of enrichment but I am sure it varies from one dash number carb to another by the size of the precise hole. 

It's either open or closed. Nothing in between.

Like I mentioned, unless one is down low and running wide open this system has no meaning to the total operation. Once the mixture control is pulled this system is out of its control range and has no effect. You can leave the throttle wide open and lean it at altitude with this system having no effect on the fuel flow (leaned) what so ever. 

So why, when I made a 2hr trip (central Alabama to the SC coast), I thought I would try WOT since everyone including Bob Kromer said it would work the same. Then I flew home using my standard WOT minus approach above, and had measurably better fuel economy. The return trip was when I recorded Time to Climb data, departing well under 50msl and climbing to 10,000 (Note:  this is best done with a 2nd person writing numbers on a pre-made table).

I no longer have the fuel receipts or my notes, but comparing my SC receipt and logged time against my AL receipt and logged time convinced me that WOT operations may be good for my injected brethren, but don't work too well for my carbed Mooney. I've not felt the need to repeat the experiment, but it's easy enough for someone to do:  just make two flights, starting with full fuel on both, and use flight time and fuel receipts to calculate actual economy. The trip with best actual fuel economy had the more even fuel distribution.

[cliffy]

Oscar

Here it is copied from the Mooney C TCDS-

IV. Model M20C, 4 PCLM (Normal Category); Approved October 20, 1961 Engine Textron-Lycoming O-360-A1D or O-360-A1A (Carburetor MA4-5, Flow Setting P/N 10-3878, 10-3878-M, or 10-4164-1).

[cliffy]

Hank-

Winds? Was the flight time the same? Temp the same? Altitude was different? Route exactly the same? Winds at lower altitudes during climb or decent? Your flying technique ? Turbulence? Weight? Any number of things can cause it. 

[hook_dupin]

I’ve got a 1970 M20C

 

I fly 26 squared per POH on climb-out, then set 23” and 2400 for cruise with mixture leaned to ROP at 1450 EGT. Peak is usually 1550. We have all original gauges except perhaps an analog EGT.

Consistantly, I see 8.9 GPH at the 4K-7K altitude blocks.

[DXB]

4 hours ago, cliffy said:

Oscar

Here it is copied from the Mooney C TCDS-

IV. Model M20C, 4 PCLM (Normal Category); Approved October 20, 1961 Engine Textron-Lycoming O-360-A1D or O-360-A1A (Carburetor MA4-5, Flow Setting P/N 10-3878, 10-3878-M, or 10-4164-1).

I got into the weeds on these part numbers a while back when trying to sort my temp issues, though Cliffy clearly knows carbs infinitely better than me. 4164 -1 is the higher flow model, with 3878-M being a 3878 that has been modified to match 4164-1’s fuel flow.

[Hank]

5 hours ago, cliffy said:

Hank-

Winds? Was the flight time the same? Temp the same? Altitude was different? Route exactly the same? Winds at lower altitudes during climb or decent? Your flying technique ? Turbulence? Weight? Any number of things can cause it. 

Fuel burned divided by hours flown makes winds irrelevant. It was a weekend trip, out and back, same course different directions, 1000' altitude difference (9000 msl east, 10,000 msl west). I agree thaf simply compring fuel burn would tell nothing, that's why I divide by flight time to get an hourly rate. Both fields uncontrolled, VFR departure and opened flight plan in the air.

The only deviations were to avoid the ATL Bravo both ways, sacred airspace that I have yet to enter . . . .

Temp alone has never made a significant difference in my fuel burn, but memory says the WOT trip was more than 1 gph higher.

[Urs_Wildermuth]

I always found the POH's from this time lacking, my 1965 edition is pretty messy to go into in flight to e.g. check performance figures and then they are all in mph, not straightforward really to use in the present day.

What I found is that mostly the fuel flow data are quite accurate, obviously only if the power settings are done as in the POH. Where mostly the figures are optimistic is TAS.

Depending whether you are after maximum cruise performance or max range, our C models have very different cruise settings. As I said, it is not really straightforward to extract them to make sense, but I found the following:

For high speed cruise, 2600 RPM yield best speed but also a massive fuel flow. Mooney claims the C model can fly 160 kts from SL to 7500 ft flat out, with 14.4 gph at 2500 ft, 12.9 gph at 5000 ft and  11.8 gph at 7500 ft. Well, I have not really tried it but the best I ever saw was 150 kt at roughly 10.5 GPH, which is again not far off.

If Range is the issue, 2300 RPM are more useful. I found the best range I could get out of the manual to be 769 NM still air (plus 45 min reserve) at 10'000 ft, 7.8 GPH which yields 144 kts TAS. The power setting for that would be 17" and 2300 RPM. This would also mean around 5-18 hrs flight time (plus reserve).

I still think the C Mooneys are very economical, if I compare them to the other Mooneys it comes 2nd only after the J and leaves everything else wey behind in terms of NM per USG used. High speed cruise however is not necessarily where it shines, but below 8 gph and still 145 KTAS is a very neat figure for a 180 hp airplane.

[gsxrpilot]

7 minutes ago, Urs_Wildermuth said:

I still think the C Mooneys are very economical, if I compare them to the other Mooneys it comes 2nd only after the J and leaves everything else wey behind in terms of NM per USG used. High speed cruise however is not necessarily where it shines, but below 8 gph and still 145 KTAS is a very neat figure for a 180 hp airplane.

Well said.

[DXB]

On 3/18/2018 at 3:30 PM, cliffy said:

When looking at the "mechanical" fuel enrichment system (economizer system) in the 180 HP Lycoming MA4-5 carb, the system really doesn't have 2 fuel jets. What happens is that as the power comes up (with WOT) the throttle arm is pushing on a spring loaded pin valve, holding it closed. Once we pull back off of WOT a little this spring loaded pin valve opens. This valve ports air from the top of the float chamber (higher ambient air pressure) through a precisely sized hole and into the main fuel nozzle after the main metering jet for the carb (a lower pressure area as we said before). More air after the main jet means less fuel so the mixture "leans" a little, as we don't need the extra fuel for cooling like we did at WOT.  All done "automatically by Lycoming design. Thus, as Lycoming calls it, the "economizer" system.

Once we get high enough and want/need to lean the mixture for altitude the leaning of the economizer system has no effect as we take the FF below the amount the economizer system can do by using the use of the mixture control. Remember, the economizer has a very limited effect because of the highly precise hole that it uses for leaning. 

@cliffy  Discussion with @Oscar Avalle reminded me of the document below, which indicates that the economizer has an adjustable setting on the MA4-5 (pages 2-1 and 2-2):

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

Do you have a sense of when it would be appropriate to adjust this setting and how one might do so?  I'm familiar with the idle mixture vs idle speed adjustment procedure but this seems like a totally different animal.

 

[carusoam]

Dev,

I’m not seeing the page numbers the way you describe... I have electric page numbers 24 or 25...

But, I do see fig 9 that details the adjustment for air metered mechanical economizer.... air metering pin, air metering pin jet.

The picture seems to show a lead seal twisted over a possible adjustment...? Figure 7. Has the detail of the lead seal...

Overall, the drawing wasn’t meant for anyone that wants a quick answer...  

Best regards,

-a-

[cliffy]

DXB   I don't think there is any valid reason to "try" to adjust this item except on a flow bench during overhaul to set it to its called for setting per its flow number on its ID tag. This is not a field adjustable parameter as far as I know. I can't imagine a time I would want to mess with the flow settings of any particular carb except for idle mixture and idle speed. 

There is reference somewhere I saw that has the settings of the pin (turns in or out) to match the flow rating but again its not something we want to do in the field. Its set for the model carb and doesn't need fiddling with. 

[carusoam]

The drawings show some interesting things.... the carbs are clearly a work of mechanical art!

1) fuel and air mixing before entering the Venturi... the tube is centered in the Venturi, but on an angle.  The fuel gets air drawn/bubbling into it before it gets to the Venturi.  Quirky distribution of fuel under all conditions, with a lot of randomness, as apposed to being evenly distributed....

2) Can’t quite determine how the economizer fuel gets to the Venturi... one picture has an undetailed item offset from the main fuel nozzle.  The economizer and the accelerator pump seem to be related in the drawing.

3) Can’t quite determine how the accelerator pump fuel gets there either... where it enters the Venturi...

4) there are two screens worth noting... one for the fuel entering the carb. Another, the bowl air vent screen.

5) If one over pumps the accelerator pump on the ground... it appears the excess fuel may drain out the bowl air vent screen...

Cliffy, Thanks for the additional technical details.  

PP reporting what he sees in the pseudo tech drawings, not a mechanic.

Best regards,

-a-

 

[cliffy]

The economizer doesn't carry any additional fuel t all. What it does is "inject" air from above the float  in the float chamber into the fuel nozzle after the main metering jet and before the nozzle outlet in the throat of the carb. It only does this when the throttle is pulled back from WOT and the spring pushes the needle open allowing the air to flow. The "extra " fuel used at WOT comes from the very low pressure in the throat of the carb "drawing" fuel from the higher pressure float chamber. Once the WOT is pulled and the economizer allows air to flow, the air "leans" out the amount of fuel being drawn from the float bowl in the main nozzle. In affect, it allows a higher bowl pressure to be put against the lower venturi pressure thereby lessening the fuel pushed through the main nozzle from the float chamber. 

The accelerator pump has a separate nozzle in the throat of the carb for discharge. If too many pumps are pushed without cranking the engine (thus not sucking the fuel UP into the cylinders) the extra fuel will drain out of the throat of the carb and drop into the carb heat box right behind the air filter.  

The only screen needing attention is the fuel inlet screen. The bowl vent screen is never pulled. If you check the inlet fuel screen, one should have another special lock washer available to use when it is replaced. We all remember that we don't reuse lock washers, safety wire, locking tabs, etc. This really is a big item (lock washers) especially on our mags. NEVER LOOSEN A MAG MOUNTING NUT WITHOUT REPLACING THE LOCK WASHER.