Fuel consumption

[Sdemeyer]

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

[YellowJacket RV9]

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

[lr172]

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

[DanH]

Originally Posted by Sdemeyer

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.

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.

[Zuldarin]

Originally Posted by DanH

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.

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.

[airguy]

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.

[lr172]

Originally Posted by Zuldarin

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.

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

[lr172]

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

[Sdemeyer]

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.

[lr172]

Originally Posted by Sdemeyer

Turning on the boost pump increases FF by a gallon or two.

This would increase the probability of an issue with the needle/seat. Let us know if the carb swap addresses the issue.

Larry