Lean of peak operation.
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The GA guys can answer this better, but here’s my thoughts:
Typical piston aircraft engines run at very low RPM, with relatively large engine displacement making modest power. Two sparkplugs per cylinder. They can ignite, and run, on a wide variety of mixtures, including well lean of stochiometric.
Lean of peak EGT may not be that lean of stochiometric ideal. I don’t know the relationship between peak EGT air/fuel ratio and ideal air/fuel ratio. Peak EGT might actually be fairly rich, for example. I’m not certain that they correlate.
Further, when leaning out, the engine is running at something much less than full throttle. When you push full throttle for take off or climb, you’re also going full rich. So, the lean of peak happens only at modest load.
I’ve got very little time in GA airplanes, so my understanding of this is rudimentary. I’m hoping some GA guys can chime in.
Cheers,
Typical piston aircraft engines run at very low RPM, with relatively large engine displacement making modest power. Two sparkplugs per cylinder. They can ignite, and run, on a wide variety of mixtures, including well lean of stochiometric.
Lean of peak EGT may not be that lean of stochiometric ideal. I don’t know the relationship between peak EGT air/fuel ratio and ideal air/fuel ratio. Peak EGT might actually be fairly rich, for example. I’m not certain that they correlate.
Further, when leaning out, the engine is running at something much less than full throttle. When you push full throttle for take off or climb, you’re also going full rich. So, the lean of peak happens only at modest load.
I’ve got very little time in GA airplanes, so my understanding of this is rudimentary. I’m hoping some GA guys can chime in.
Cheers,
the chrysler lean burn engines ran lean of peak, without damage, the proble is the electronics and algorithms along with the carburetor limits could not cope with engine aging.
During the first gas crisis (1978) I tuned my 350 lean, it had to have carb heat to run in the winter, all the time. I made modifications, dual exhaust manifold stoves feeding dual snorklesdual exhaust. 69 impala would get 19 to 21 MPG
No damage
During the first gas crisis (1978) I tuned my 350 lean, it had to have carb heat to run in the winter, all the time. I made modifications, dual exhaust manifold stoves feeding dual snorklesdual exhaust. 69 impala would get 19 to 21 MPG
No damage
As was mentioned, Chrysler Lean Burn engines ran very lean under light loading during the ~15 years they were in production. There were more cases of (relatively) benign head cracks between the valves (not into water) in the smallblocks than in the years before and after Lean Burn, but for the most part there was no damage that affected performance unless the control system failed and stayed lean under load (ever seen an exhaust valve with a pie-shaped burn from rim to stem? I have!) The system was pretty clunky because of 1970s electronics and the fact that active mixture control was being applied to carburetors, but it worked a lot better than its reputation, frankly. Even the first generation hardened valve seat heads held up fine under the added abuse.
My own speculation: I think the real reason you don't see extensive use of lean combustion in modern car engines boils down to one word: Emissions. Lean combustion sends the NOx through the roof. It cleans up HC and CO emissions nicely, but there's always a see-saw between HC/CO and NOx. With the increasing emphasis on NOx as a ground-level ozone contributor in the past 30 years, its just not a regime that carmakers can really use. Sure, the reduction phase of catcons can clean up some NOx, but the less you start with the less clean-up is needed.
The picture below can help to answer your question. (link to informative webpage https://www.kitplanes.com/adding-direct-air-fuel-ratio-monitoring/ ) During operations lean of 14.7 to 1, and during modest power settings (less than 70% for example) there is very little risk of detonation. Hence the safety of LOP operations.
This is because a mixture lean of stoichiometric burns more slowly. For maximum efficiency during lean ops, more timing advance is very helpful. The use of electronic ignition can really improve efficiency during LOP and low output operations. Such as flying a normally aspirated aircraft at 12,500 feet. Aircraft engines can achieve thermal efficiencies above 40% when properly configured.
The other part was answered above. Many automotive engines can operate very well at air/fuel mixtures lean of stoichiometric. Ford's relatively inefficient 5.4L V8 shows a stunning 25% efficiency gain when operated at a lean air/fuel ratio. It's an unusual case, as the engine can be smoothly operated as lean as 21 or 22 to 1 air/fuel. This requires more throttle opening to maintain power, reducing pumping losses.
This is because a mixture lean of stoichiometric burns more slowly. For maximum efficiency during lean ops, more timing advance is very helpful. The use of electronic ignition can really improve efficiency during LOP and low output operations. Such as flying a normally aspirated aircraft at 12,500 feet. Aircraft engines can achieve thermal efficiencies above 40% when properly configured.
The other part was answered above. Many automotive engines can operate very well at air/fuel mixtures lean of stoichiometric. Ford's relatively inefficient 5.4L V8 shows a stunning 25% efficiency gain when operated at a lean air/fuel ratio. It's an unusual case, as the engine can be smoothly operated as lean as 21 or 22 to 1 air/fuel. This requires more throttle opening to maintain power, reducing pumping losses.
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As usual Cujet provides great info. Can I add that todays computer controlled engines can perform all the fuel and timing control for with in programing pretty much any operating conditions? Look at the engine power out put ,increased mpgs and low emissions as compared to engines 20 years ago.
I own and fly a Cessna Cardinal RG (177RG). It's powered by a 200HP (real world power is prob 185HP) Lycoming angle valve engine, which is considered 'high compression' at 8.7 to 1. The airframe is relatively sleek with it's gear up and cantilevered wings (no strut). However, it's a generally underpowered package in cruise flight (at 8500 feet, my typ crz alt) and operation 'lean of peak' is a great way to go a lot slower. While other people may achieve better results, my bird achieves 143Kts in cruise flight, slightly rich of peak EGT, or peak power available. Operating at 50 degrees lean of peak drops speed to 125Kts, fuel flow goes from 10.5gph to 8.7gph. It also adds almost a half hour to my typical (one way) trip of just over 400 miles.
58.8 gal vs 55.7 gal (round trip) . Almost an additional hour's airframe and engine time operating LOP, per round trip.
To make matters worse, operating at lower power like that results in difficulty maintaining altitude when there are downdrafts, such as when flying around the small but thermally active Florida cumulus clouds. (lots of lift in the clouds and sink outside the clouds). As the airspeed drops WAY off trying to maintain altitude during areas of sink. Aircraft with more power to weight do far better during LOP operations. Heck, even the upgrade to the IO390 engine solves this problem for most Cardinal owners. (not available for my year airframe, otherwise I'd do it)
In my case, I find LOP operations so annoying that I simply can't bring myself to do it anymore.
58.8 gal vs 55.7 gal (round trip) . Almost an additional hour's airframe and engine time operating LOP, per round trip.
To make matters worse, operating at lower power like that results in difficulty maintaining altitude when there are downdrafts, such as when flying around the small but thermally active Florida cumulus clouds. (lots of lift in the clouds and sink outside the clouds). As the airspeed drops WAY off trying to maintain altitude during areas of sink. Aircraft with more power to weight do far better during LOP operations. Heck, even the upgrade to the IO390 engine solves this problem for most Cardinal owners. (not available for my year airframe, otherwise I'd do it)
In my case, I find LOP operations so annoying that I simply can't bring myself to do it anymore.
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I'm assuming that's at full throttle (max available manifold pressure) with the prop used to adjust rpm to get the percentage power you want?
Yes, full throttle and 2550 or 2600 RPM. Anything less is simply less power. 2700 RPM achieves nothing.
Cujet - I really appreciate your detailed explanations. Thanks!
Honda had lean burn 1975-2006
My Insight runs 21:1 AFR
My Insight runs 21:1 AFR
Yes, many light aircraft engines now have individual cylinder head and individual exhaust temperature indications. This does help pilots to manage their engines and avoid limits. Well designed planes don’t generally need all that much with regard to In flight engine adjustments. My old plane simply goes to full throttle and stays there. During climb, I adjust the mixture to the placarded fuel flow numbers, ie. 15GPH at 4000, then 13GPH at 8000. (it’s pretty close even if I touch nothing) I only reduce the RPM a little at top of climb. Amazingly, it will be really close to the peak power cruise fuel flow, so I’ll only have to adjust a little. It might be 11GPH, and I’ll adjust to 10.5.
cylinder head temps don’t climb too high because cowl flaps are open and I maintain enough forward speed.
Yes, the engineers do try to get things right for easy operation. But LOP operation requires the pilot to do some serious adjustment.
I like to cruise-climb my plane, and leave the cowl flaps open, which does help to manage temps. Some pilots will climb to cruise altitude at the slowest airspeed possible and it's no surprise they run into cylinder head temperature issues.
Some old piston engine airliners, super leaned in the day. Simple answer is just like with fat mixtures having a cooling effect
same with an extreme lean condition.
same with an extreme lean condition.