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Full rich vs leaned: Difference in fuel consumption

I have some data from a flight done today:

Conditions: Socata TB20 IO540-C4D5D 3B prop, 4800ft +4C QNH 1000 2400rpm

Peak EGT: IAS 146kt 12.5 USG/hr 23" MP
130F ROP: IAS 146kt 14.8 USG/hr 22.5" MP
Full rich: IAS 146kt 17.5 USG/hr 22" MP

To get meaningful results, the RPM and the IAS were held constant i.e. constant power / constant thrust, and the MP was varied slightly to achieve the constant IAS.

So, taking the Peak EGT value as the base, 130F ROP (“best power”) burns 18% more fuel, and Full Rich burns 40% more fuel!

For a comparison, I normally fly (low level) slightly LOP at 23" IAS 139kt 11.5 USG/hr. It is about 50F LOP.

I forgot to try LOP for an IAS of 146kt; clearly the MP would have to be above 23" to achieve that. And the MPG should be exactly the same as for peak EGT, because once you are stochiometric (close to peak EGT) there is no MPG gain if thrust (power) is kept constant. Obviously, you get better MPG if you fly slower and that will be true anytime you are flying above Vbg, which for a TB20 is about 95kt for the airframe (i.e. gliding), though the best-MPG speed is more like 110kt due to the engine efficiency not being constant versus power.

At the above conditions, 146 IAS is 156kt TAS, so we have the following MPG:

50F LOP (with reduced IAS) 13.0 MPG
Peak EGT 12.5 MPG
130F ROP 10.5 MPG
Full rich 8.9 MPG

You might ask: why is Best Power relevant?

  • if I was in a hurry to get somewhere and fuel was not relevant
  • it is needed to reach the operating ceiling (though obviously the fuel flow and speed etc figures will all be different at FL200)
  • a lot of people why 100F ROP or so out of mistaken belief that it is “right”

Full Rich is a complete waste of time and money unless you want to achieve the highest possible speed.

These results will be approximately valid for other piston GA so PPL training industry take note

Administrator
Shoreham EGKA, United Kingdom

A very simple rule of thumb is that you get 1nm/litre from a Cheiftain.

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

Very economical for a twin and shows you that a twin doesn’t burn twice as much fuel as is often claimed

Or just shows that some twins don’t burn twice the fuel of a similar single! (e.g. a Baron seems to be like flying two Bonanzas in formation in terms of fuel burn).

Andreas IOM

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.

Oxford (EGTK), 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

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

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

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

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!

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
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