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Fuel consumption

S

Sdemeyer

Active Member
Hi Folks, it seems like I am burning an excessive amount of fuel and not sure whats going on. Lycoming O320 - 150HP, FP prop, 10-3678-32 carb. Seems that if I don't pull back on the red knob, quite a bit, I'm burning around 11.5 GPH at cruise power ( 2450 RPM @ 1500msl ). I can lean it to around 9 but seems abnormal to have to lean so soon and down low. Just wondering if I should be checking out the carb or if this is normal behavior. At SL WOT takeoff power, its burning 16.5 GPH. At 8500msl and 2500rpm, I can lean to a little over 8 gph. I've heard the -32 can burn a little more fuel but mine just seems excessive.

Scott
 
First question would be are you SURE you fuel flow is calibrated properly? Your numbers do seem high - I also run a 150HP setup with a FP prop. At cruise power down low, if I don't lean I am around 10.5 gph, although I usually lean it down to around 7.5 down low, and 6.5 at 8k or so. (I generally cruise at 60-65% power).

If you have an engine monitor you could do a lean test and see just how ROP you are running. For me, I was too lean and ended up having to drill the jet out two sizes. Now I have enough fuel flow for proper cooling on takeoff. This was after having to rebuild the carb that was flooding.

Chris
 
Seems high for a 150, but the -32 carb does have a WOT enrichment circuit, so I would expect it to be a bit rich. Did you build this plane? Many drill their main jets to overcome a lean condition during some WOT phases. I suspect some may think "if a little is good, more is better." That would leave you very rich. I have a 160 and when it had a carb, I drilled the jet, but never burned more than 14 GPH in any phase of flight.

I would not consider this to be a too big of a deal, though it would be nice to be able to trust it with the red knob full in. The Lycomings are always very rich and you'll want to get used to leaning in most phases of flight. I have FI and an FP prop. I routinely lean a bit at take-off, as it is too rich with the prop spinning 2300. As it moves up to 2500 I have to richening it back a bit. The overly rich condition was impacting my initial climb performance. It is still running 150+ ROP. That gives me much better power and I have no issues with cooling.

These carbs are very old designs and pretty unsophisticated. Further, they are set up to err on the rich side of anything, so you will find some power settings worse than others.

Learn what your peak EGTs are at various RPMs and just lean to 150-200 ROP in high load situations and best power (80-100) in others or LOP.


EDIT: 16.5 at 2500 is at least 3 GPH high for a 150 maybe more, so I would expect something is going on in your carb or with your fuel flow sender. You'll want to address this relatively soon. As the hot weather gets here, engines need less fuel. At some point you run the risk of your engine sputtering at the DE of the runway because its too rich. I would be leaning on take off and getting the flow down to around 13 or so to avoid problems, assuming you have verified the accuracy of your fuel flow device. You should be able to easily index your Mixture shaft with the proper point.

There are several things that can cause this, but high on my list would be a leak somewhere (should be easy to spot that much leakage) or debris in your needle/seat or saturated float. I have seen other report bowl screws that loosened causing problems (an easy visual check).

Larry
 
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Sdemeyer said:
Hi Folks, it seems like I am burning an excessive amount of fuel and not sure whats going on.....At SL WOT takeoff power, its burning 16.5 GPH.
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First calibrate fuel flow. And remember "boost pump on" can skew calibration.

Best power is found across a fairly wide mixture range, from 0.55 BSFC on the fat end to 0.45 BSFC on the lean end. Absolute highest horsepower is found at the lean end, but we're talking about 2% difference more or less. Thus setting up a carb or FI to deliver fuel flow at the fat end of the range is standard, as it adds detonation margin.

On a nice dyno we would calculate details like fuel density based on temperature, but that's unnecessary here, where the task is to determine if the reported fuel flow is in the ballpark. We'll just use 6 lbs per gallon and rated power.

Ok, so 16.5 GPH x 6 lbs per gallon is 99 lbs. 99 lbs / 150 rated HP is 0.66 BSFC. Waaaay fat, and getting worse as the aircraft climbs and HP is reduced.
 
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DanH said:
Ok, so 16.5 GPH x 6 lbs per gallon is 99 lbs. 99 lbs / 150 rated HP is 0.66 BSFC. Waaaay fat, and getting worse as the aircraft climbs and HP is reduced.
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This is interesting Dan but I have a related question. The OP mentioned 16.5GPH at takeoff. He has a fixed pitch prop which means he is probably only turning 2100-2200 RPM. Does this reduced RPM have any effect on the "proper" fuel flow given he is way below rated HP? My uneducated guess would be that since he is way below rated HP his fuel flow should be much less than full rated fuel flow.
 
16.5 gph is my typical fuel flow rate on IO360 at full power, full rich, 2700 rpm, making 180hp. That is way fat for 150hp.
 
Zuldarin said:
This is interesting Dan but I have a related question. The OP mentioned 16.5GPH at takeoff. He has a fixed pitch prop which means he is probably only turning 2100-2200 RPM. Does this reduced RPM have any effect on the "proper" fuel flow given he is way below rated HP? My uneducated guess would be that since he is way below rated HP his fuel flow should be much less than full rated fuel flow.
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You are correct. Dan's figures are based on 150 HP and the OP is probably in the 135-140 HP range at take off, depending upon his prop pitch. 2100 is a static RPM. Most FP props will be turning 2300 or so once rolling at a decent speed and 2400 or so at flying speed and approaching Vx/Vy.

Dan's calc's are based on HP. You could use the Lyc chart to determine the HP at any RPM/MAP configuration to get the HP and replace the 150 in his calc with that figure.

Larry
 
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My Lyc chart for a 150 HP -A model shows a FF of 13.0 at 2500 RPM and max MAP. This would be lower if your RPMs are less than 2500.

The key is to first verify your indicated fuel flow readings to avoid chasing a ghost.

Larry
 
Thanks for all the replies. I did verify my EI ff is accurate. Over the last 100hrs, it has been right on for how much used and I just did another flow test by disconnecting the fuel line from the carb and filling 25 gals into gas cans. The FF totalizer was spot on. There are no leaks in the fuel system. I purchased the carb from acs about 6 months ago. Turning on the boost pump increases FF by a gallon or two. My takeoff rpm is 2400 and wot s&l is 2800. I have a new 10-5009 carb and I will swap it out with my -32 , and see if that helps.
 
Zuldarin said:
This is interesting Dan but I have a related question. The OP mentioned 16.5GPH at takeoff. He has a fixed pitch prop which means he is probably only turning 2100-2200 RPM. Does this reduced RPM have any effect on the "proper" fuel flow given he is way below rated HP?
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Yes. 6 lbs per gallon and rated power were assumptions made because I was too lazy to locate a 320 power chart at 7:30 AM. If actual power is (for example..I remain lazy) 135 HP, then 99 / 135 = 0.73 BSFC...rich as 6 feet up a bull's butt.

Note that 0.50 BFSC is the middle of the best power range, so for a quick mental reference to reasonable fuel flow, lbs of fuel should be equal to, or a little more than, HP/2.
 
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Happy to report that swapping the carb fixed the fuel flow. I think its a lot more efficient now. 12.5 GPH WOT, takeoff. 8.7GPH down low cruise at 2450RPM - no leaning.

Now the next question is why my new -32 that I just paid $880 for is burning so much fuel, and what can I do to fix it?
 
Sdemeyer said:
Now the next question is why my new -32 that I just paid $880 for is burning so much fuel, and what can I do to fix it?
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As I mentioned in my last post, I would start with the float. Defective needle/seat, debris in needle/seat interface or saturated float. Also, if it did this since getting it I would look at the float level (it is adjustable and is sensitive to even very small inaccuracy). There are other things that can cause rich running, but these are the most likely.

Larry
 
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Also check your fuel pressure. My carb was apparently overpressurized from a bad engine driven fuel pump (>7-8 psi at times), causing it to run very rich. I replaced the pump but still had to rebuild the carb. You don't want a bad fuel pump overpressurizing your new carb.

Chris
 
"Now the next question is why my new -32 that I just paid $880 for is burning so much fuel, and what can I do to fix it?[/QUOTE]


Is this a brand new carb from the manufacture? If it was rebuilt it could be possible that someone resized or replaced the jet with larger?? They are out there.
 
Back to square one

Seems my problems are not over. The engine quit on takeoff and the only way I was able to get full power again, without it dying was with the mixture halfway out. I let it sit for 15-20 minutes and then it ran fine. I'm 80% sure that heat is not the problem. The only gauges I didn't install are fuel pressure and manifold pressure. I'm guessing too high or sporadic fuel pressure, but not sure until I get a gauge installed.
 
That probably rules out the carb, especially given that it worked well the other day. Certainly intermittent excessive fuel pressure should be the next thing to rule out given these events. However, this is not likely given that you have seen flow increase by 2 GPH when turning on the boost pump. This basically confirms that the boost pump has a higher pressure than the mechanical pump. The likelihood of both failing on the high pressure side, during periods of high flow, (i.e. WOT), at the same time is very low. These pumps can fail high at low volumes, but it is uncommon for them to fail high when delivering close to half their rated volume - 16.5 GPH.

Do you have a fuel filter? The needle/seat only opens a few thousands during normal operation. debris doesn't have to be significant in size to create a blockage here. If the needle can't fully close on the seat, you will overfill the carb bowl and be rich. The tip of the needle is rubber, so it is easy for trash to get stuck on it. Debris here seems probable given that it happened to a brand new carb, then happened again to a second presumably good carb. Also a decomposing rubber hose can generate a good amount of debris. Suggest you pull the mesh filter in both carbs and look for evidence of this. You should also pull the gascolator and examine the mesh screen and bottom of the sump for collecting debris.

Larry
 
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New engine or used? It's not unheard of for a new engine to ship with a high pressure fuel pump instead of low. For a used engine, as in my case, a pump can fail high (despite what anybody says, I seen it for myself) resulting in over pressure and flooding. I'd guess there's a strong chance your fuel pressure is to blame, or like suggested above, debris in the system.

Chris
 
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Sdemeyer said:
The only gauges I didn't install are fuel pressure and manifold pressure. I'm guessing too high or sporadic fuel pressure, but not sure until I get a gauge installed.
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How do you do a pre-flight check for proper operation of both fuel pumps without a fuel pressure gauge???? Proper fuel pressure has more priority for go-no-go decision for me than the mag check......
 
Sam Buchanan said:
How do you do a pre-flight check for proper operation of both fuel pumps without a fuel pressure gauge???? Proper fuel pressure has more priority for go-no-go decision for me than the mag check......
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Yes, I am changing my procedures and monitoring capabilities. The plane started out life with a gravity feed system, hence no need for fuel pressure gauge. ( Its a Mustang II with 25 gal header tank ). The gravity feed system worked well until a steep, full power, climb test in phase 1, proved otherwise.

I installed a new (overhauled) mechanical fuel pump as well as an electric backup and life was good again, until recently. It still gravity feeds about 8.5 GPH even through the fuel pump so I never bothered to install a pressure gauge. Bad logic on my part.

I've removed and examined the tank screen, gascoaltor screen and carb inlet screen and see absolutely no debris so its got to be something else. I have a new pressure gauge and related parts on the way and will reply with my findings one I get them installed.

Really appreciate all the help and advice here on VAF.

Oh, I'm nearly complete with an RV6A build and I think I am going to like the RV6 a lot more than the Mustang. Lots more room in the RV :)
 
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