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Full rich vs leaned: Difference in fuel consumption

Hi,

I just wondered if anyone had any practical experience (ie done any physical tests) regarding the difference in fuel consumption between running their aircraft engine full rich vs leaned (lets say LOP). In absolute terms it would clearly be engine dependent, so a difference expressed as a %age would be most appropriate

I have heard a few stories, but I just wondered what the forum’s experience is.

Our TIO-540 will consume about 50% more in cruise if run full rich and delivers less power. We can argue a lot on where to run the engine but running it full rich all the time is no option. You will not even get close to book values on consumption, endurance, performance, spark plug life etc. Running full rich all the time is like never retracting the gear on a retract plane because someone said you might get into trouble with the gear in…

www.ing-golze.de
EDAZ

On the Mirage depending on altitude full rich to a properly leaned ROP would save about 30%. Full rich to LOP would save about 60%.

EGTK Oxford

I meant to test this today but I spent most of the flight at FL110-130 which is not a good place to try going full rich because the engine might just stop

From memory, and carefully testing it under constant-thrust conditions (i.e. same RPM and same IAS), full rich is about 1.3 x less MPG than peak EGT.

Peak EGT is very close to stochiometric which occurs around 25F LOP and the curve is very flat there anyway. Deep LOP gives little in the way of MPG improvement and what you do get is via second order effects combined with a low RPM. I have some stuff here

Administrator
Shoreham EGKA, United Kingdom

The Cirrus SR22 you can run full rich up to 4000 ft in the climb with full power and it will have a fuel flow of about 29 GPH initially. From 4000 ft the FF shouldn’t be above 24 GPH… and so on. In Cruise (FL80-120 normally) the FF will be about 12.5-13.5 GPH and (if I remember correctly) about 11.5 GPH at FL160. Full Rich is no option in the cruise – because, as Peter has said, the engine will stop!

In the DA40 I believe you would at some altitude get something like 11-12 GPH full rich at 65% and 8 GPH at peak EGT.

LFPT, LFPN

Asked the question re a PA28/140 I am looking at. 35L per hour Rich and 24L per hour leaned at approx. 60%. He did say he wouldn’t lean more than 50%.

EGBJ, EGBP, EGTW, EGVN, EGBS

PA31 Cheiftain (TIO-540s) – 24USG per engine full rich, 17USG at peak EGT.

(clearly dependant upon altitude and power setting).

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

It makes a huge difference. On the Aerostar I see about 45-50gal/hr at full rich, both engines. I lean that back to 25gal/hr (in total) and only lose about 5-10kts in speed. She’ll do 200kts on 25gal/hr all day up high. Or 185kts on 21gal/hr up high. Very economical for a twin and shows you that a twin doesn’t burn twice as much fuel as is often claimed. It’s about 30% more only to a comparably capable single.

Last Edited by AdamFrisch at 10 Nov 22:52

Using GAMI injectors and a multi-probe analyzer on normally aspirated -540s, approx. 25/50 oF LoP would reduce fuel flow from 25-26 USGPH total, to around 21-22 USGPH.

This would typically be at full throttle altitudes for a normally aspirated engine. Speed loss would be around 5%. Compared to a Saratoga, the Aztec would be using around 55% more fuel, or conversely the Saratoga would be using 35% less fuel than the Aztec – all a matter of perspective.

Not such a massive saving, however the CHTs were nicely below 380 oF, and the engines seemed happy on the LoP diet.

The Super Cub has a Stromberg carb, and while it has a mixture control, it operates in compliance with Rule 5 at 75% power, so mixture is always rich. Perhaps LoP, not always useful on a carb engine, might save 1/2 USGPH.

Hopefully the SC will need to fly higher one day, and I will see if leaning improves power at altitude. i.e. I would use it for peak RPM in a high density altitude environment.

Enstone (EGTN), Oxford (EGTK)
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