Do any of you guys lean for best MPG?

Reason for the question comes after I had spoken with Don at Airflow Performance this morning. I had methodically leaned by .2 GPH until all the cylinders had peaked and faxed the results to Don, so we could tweak the injector inserts....some cylinders were 130 degrees cooler than the peaked # 3 and 5.

I asked him about how far LOP to run once we had done the tweaking and he asked if the Dynon 120 had a MPG calculator and I replied that it did. He said that the most efficient LOP position would be the highest MPG number...and that when it starts dropping off, I'd leaned too far.

I wondered whether that might be in the 200-300 LOP range or what but he said that it didn't matter, just the best MPG...Hmmmm...hadn't ocurred to me but I do see an airspeed loss, running lean of peak and obviously, that's because we're making less power. So, it depends on whether you want the fastest speed or the most bang for your avgas dollar...your call.

BTW, even at 23" and 2400 RPM, 3500', the engine ran all the way down to 10.8 GPH but was getting rough and rather slow. Lyc IO-540.

What say y'all?

Pierre,

Not a -10 (or 540), but I typically lean (using D120) to about 30 degrees LOP on the last cylinder past peak. Much more than that and I get more roughness than I like. I have not tried to lean based on MPG, but may see where that point is next time the weather lets me into the air.

greg

Same here

I guess I just never thought to try it..Its a good idea though.

Knowing that I have a wide margin between where the engine runs lean rough (can't be the most efficient place to run it) and peak then I just set it by habit.

In my case 7GPH (premium mogas) at 24 squared on an IO360...or 7.5 GPH or so with E10.

I will see where max MPG happens next time too.

Frank

Pierre - I dont lean to the gauge because it bounces about too much I find, but I do download the post flight data and calculate the MPG (or at least the miles per litre ) from that.

It turns out to be a very subtle, but repeatable point. If you look at the grey line on this plot, at the top, its the miles per litre. (Multiply by 3.785 tp get MPG) so 7.3 mpl = 27.6 MPG.

I say subtle because if you look at the first of the two flights shown you will see that at about 09:30 I reduced the fuel flow by about .5lph (the blue line). The MPL jumps noticeably from about 7mpl to about 7.3. So for a 2% fuel reduction an improvement of MPG of about 4%.

The other interesting things to notice are the EGT all close up very tight and no drop in speed. Having found this empirically I now set up to fly at 2000rpm, 21lph and 22" which makes for a very repeatable operating regime. Its just hard to fine control the flow without a vernier. On the return (2nd) flight I set it right off with the obvious characteristic trace.

The grey line cuts out if the air is rough, hence the vertical lines. Its calculated and the calculation discards the data if their is a height loss. The return was quite turbulent.

Its a carb'd O-320, P-mags c/s MT prop.

Best MPG no-wind will be slightly higher than best glide speed (L/D Max) at minimum LOP power setting necessary to maintain. You are correct that more leaning = less hp = reduced steady-state cruise speed.

Most folks don't have the patience for best mpg, choosing instead max available power LOP at some chosen RPM. Pulling the rpm back a few hundred may slow you a few mph and gain you several mpg.

Im all for LOP operations, but a quote from one of the Deakin articles has stuck in my head and is relevant here: "I didnt buy (build?) a fast plane so I could go slow."

erich

That's pretty close to my train of thought too!

erich weaver said:

Im all for LOP operations, but a quote from one of the Deakin articles has stuck in my head and is relevant here: "I didnt buy (build?) a fast plane so I could go slow."

erich

Click to expand...

However, there are times when you have 30+ MPH tailwinds and you can then capitalize on high MPG numbers and still run over 200.

Best,

Use carb heat

If MPG is your goal, try the use of carb heat. It will cut your power without pulling back on the throttle, but will also increase the engine's efficiency. Each 3% decrease in engine power will increase mpg 2% and the speed will go down 1%. An engine's efficiency goes up with the square-root of the absolute temperature rise. On a recent flight to and from Carson City for the Reno Air races, I had a TAS of 180 mph and the FF was 4.8-5.0 gph. My GPS said I covered 341 miles each way in 2.0 hours moving time for a trip speed of 170 mph, and I used 10 gal. of gas each way for a trip average of 34 mpg.

LOP or ROP... what to do...

Paul, you getting those numbers with that O-235 in your Lancair?

I found with my -9A that running 30-40 Deg LOP (~6.1 gph; 165 mph) Vs 30-40 ROP (~8.3 gph, 180 mph). So LOP cost about 15 mph in TAS, but with a payoff of about 6 mpg more.

So... a 700 mile trip LOP would take 18 min longer and burn 7 gal less ($31).

elippse said:

If MPG is your goal, try the use of carb heat. It will cut your power without pulling back on the throttle, but will also increase the engine's efficiency. Each 3% decrease in engine power will increase mpg 2% and the speed will go down 1%. An engine's efficiency goes up with the square-root of the absolute temperature rise. On a recent flight to and from Carson City for the Reno Air races, I had a TAS of 180 mph and the FF was 4.8-5.0 gph. My GPS said I covered 341 miles each way in 2.0 hours moving time for a trip speed of 170 mph, and I used 10 gal. of gas each way for a trip average of 34 mpg.

Click to expand...

Hi Elippse, you have mentioned carb heat before and it always interests me when you do. I certainly have used it a little in the cruise, though not enough to draw my own empirical conclusions though it clearly has some positive effects. The poor fuel distribution of a carb'd 320 improves dramatically, I assume because the engine is ingesting vapour rather than droplets.

But I have two questions:

1. I half remember from mech. eng. classes that the max effficiency required the highest temp spread between the ingoing and outgoing air/fuel mix. It was also suggested that colder denser air aided this. Are you saying that in fact that cold/dense is a minor effect and with warmer air in you will get warmer exhaust out? Why would the absolute temp rise change except for the fact that by leaning you are getting to an optimium mix of fuel air. I can see the heat helps vapourisation of the fuel and therefore distribution, but how does it help the absolute temp. rise?

2. Am I correct in thinking that at anything but WOT the engine efficiency does not care if MP is achieved through ram effect or adjustment of the throttle?

If that is true, the way we were taught to use the carb heat is all wrong for RV, since the air is filtered with/without carb heat. We would be better only to open the the carb heat door for WOT operations. ie takeoff and high altitude cruising. Is there a flaw in that logic?

Steve Sampson said:

Hi Elippse, you have mentioned carb heat before and it always interests me when you do. I certainly have used it a little in the cruise, though not enough to draw my own empirical conclusions though it clearly has some positive effects. The poor fuel distribution of a carb'd 320 improves dramatically, I assume because the engine is ingesting vapour rather than droplets.

But I have two questions:

1. I half remember from mech. eng. classes that the max effficiency required the highest temp spread between the ingoing and outgoing air/fuel mix. It was also suggested that colder denser air aided this. Are you saying that in fact that cold/dense is a minor effect and with warmer air in you will get warmer exhaust out? Why would the absolute temp rise change except for the fact that by leaning you are getting to an optimium mix of fuel air. I can see the heat helps vapourisation of the fuel and therefore distribution, but how does it help the absolute temp. rise?

2. Am I correct in thinking that at anything but WOT the engine efficiency does not care if MP is achieved through ram effect or adjustment of the throttle?

If that is true, the way we were taught to use the carb heat is all wrong for RV, since the air is filtered with/without carb heat. We would be better only to open the the carb heat door for WOT operations. ie takeoff and high altitude cruising. Is there a flaw in that logic?

Click to expand...

I don't know the reason for this, but I got it out of the motor bible, C.F. Taylor. If you have the data sheet for an engine's power vs rpm and altitude, you'll see a little formula in the upper left that says you should correct for non-standard temps by the square-root of the ratio of (460 + 59) / (460 + OAT). Since the density is proportional directly to this ratio, not the square-root, the factor that comes in is that the engine's efficiency goes up with the square-root of the absolute temperature rise. Density increases 1% for each 5?F temperature drop, but the efficiency increases 1% for every 10?F temperature rise, so that power decreases 1% for 10?F temperature rise.
'Got to go with the experts!
And yes, Mike, those numbers are with the O-235 that you got from a project that got it from a wreck and had it in you EZ and sold it to Oscar for his Lancair and he sold it to me for my plane when he changed to an O-320! Whew! But I put in 9.7:1 pistons and dual Plasma EIs.
I recently installed a true curved-divergent-submerged NACA duct, not one of those shortened kind, for my induction air. It is 8" long for a 7? ramp and has a 1"H by 4"W inlet and fans out to go through two 4" X 5" K&N filters on either side of the carb. I used to have a bypass around my previous duct and filter, but with the 40 sq. in. then as now, there was no discernable drop through the filter, so it's in place all the time.