Some visual of air cooled engines fuel cooling at rich
 

You can jump to the 2:30 mark.




More cooling fuel on the Merlin




The 985 even has a rich mixture.




The design of the aircraft engines in most piston aircraft is a rich mixture for cooling the engine, the cylinder in particular - the valve overlap/lead and lag also help with cooling & make for an impressive light show at night! More so if no exhaust is fitted.


Anyone wish to rip the exhaust of a IO-520 and run it at night?

Like the one of the Merlin, Bend a lot.:ok:

I have seen a Merlin without exhaust stubs, on a stand, running at night.

From memory, the flames from the exhaust ports were well over two feet long!

And the noise was....:ok::ok::ok::ok:

Two feet long is a very cool cylinder temp cooling factor.

Glad you love the noise.

I have lots of time between 985s with short stacks, mostly at night. We used to lean them by the colour of the exhaust...looking for the slightest blue in the ghostly white plume. Everything else I flew in those days had auto-lean and no egt or cht indicators.

I think the R-1830 and R-2000 manuals (pamphlets, really) spoke of rich-of-peak operation, but that assumes 80/87 octane fuel for cruise. (We used 100/130 for take-off...and 115/145 on the R-2800)

I am glad to have had the chance to really learn my craft alone at night droning along with those honest engines, red post lights shining on primitive instruments and with my future all ahead. Can 19 year old pilots still get jobs like that?

You are right about that. The Mustang and Lincolns had Rolls Royce Merlins, too. I flew both types and their exhaust fires were spectacular. Sitting in the Mustang heading into the prevailing wind, the exhaust flames at idle or if over-priming, would come perilously close to entering the cockpit in the Mustang. Or at least they seemed to, anyway.
I used to cower behind the windscreen while keeping the starter motor engaged praying the engine would start.
For the Lincoln the inboard engines were about ten feet from the pilots and if over-priming and wind from the wrong direction the flames would blow towards you and the other pilot would yell "Keep the bloody prop turning" in the hope it would blow out the flames.

Had a job in 1948 when I was 16 at Camden NSW, working with the Herald Flying Services as a general hand. They had a couple of Lockheed Hudson freighters (VH-SMK and VH-SML) and two DC3 freighters VH-SMH and SMI.

At idle RPM, the P&W 1850 (?) of the Hudsons really put out long gouts of flame from the exhaust. After engine start my job was to disconnect the battery cart. The pilot would throttle back the engine on the side where the battery cart was plugged in. The electrical lead from the cart was plugged into a receptacle about one foot from the exhaust pipe outlet.

For a 16 year old it was quite frightening at night to have the rich mixture flames licking close to my head as I tried to heave the battery lead out of the receptacle especially if the tarmac was wet and one slipped trying to get a grip on the wire. All the while the propeller was turning at 700RPM and its slipstream would blow the flames nearer to my face.

Worse still was if the engine caught fire in the throat of the carburettor. Usually following massive back firing during start. There would a glow deep inside the carb inlet and flames could be seen at night reflecting on the engine top cowl. The pilot would yell to whoever was standing by and tell him to put out the carb fire as there was no engine extinguisher to cover that area.

I hated that job because it meant pushing a ladder against the wing leading edge and climbing on to the wing with a C02 bottle then lean on top of the engine and squirt the Co2 into the carb inlet. I burnt my hand sometimes while grasping the carb inlet to prevent myself from slipping off the top of the engine. Never had carb fires in the DC3 as they had Bendix-Stromberg carby's while the Hudsons had Ceco carby's. The Ceco's were prone to back firing and carb fires during engine start.

The day carb fires didn't worry me as much as the midnight fires where the flames were more visible and thus more scary.:{

I guess Centaurus a good reason why there were no mullet hair styles then.

I think the flames from the exhaust stubs have more to do with camshaft profile & ignition timing than mixture.

In cruise would hope there would be no flames to see. I would hope that all the fuel had been burnt inside the cylinder while working it's butt off to drive the piston down. Burning fuel outside the cylinder seems to me to be a waste of fuel. Maybe I'm overly optimistic.:{

Mmmmmkay. I seems like you believe that posting video of exhaust flames at night somehow proves that engines shouldn't be run LOP. Is that about right, or do you have some other point?

Close to 7000 hours running R-2800's LOP, so my experiences lead to a different conclusion.

Guys these are ground runs - all knobs fully forward (actually the throttle is moving), so take off and climb.

How it is leaned in cruise is up to you, due to the valve timing not being adjustable. I am not sure if fuel would or would not still be burning at a Lean Of Peak setting. It is possible that all fuel/air is burned but also that some is not and still burns in the exhaust - but far less.

This whole deal of running engines on the ground in a test stand in front of applauding punters is well......a little silly.
Then deducting all sorts of semi scientific ‘facts’ is even sillier.
This isn’t

Refer my first post - valve lead and lag.


That is the control factor of the flame.


A lean mixture on cruise in a manifold exhaust could re ignite fuel with un burnt air on a hot exhaust or part of one (a hot weld dag) , But I expect the mixture is still in flame when exhaust is opening - on most occasions.

To borrow from another post:- Fuel? What fuel?

By definition running LOP all the fuel should be burned and there is a surplus of unburnt air at the end of the power stroke. If properly timed there should be no fuel left to burn by about 12 to15 degrees after top dead centre. As the exhaust valve opens long after that the exhaust should be nothing other than hot smoke.

Folks
A lean mixture burns SLOWER than a rich mixture. This is something that few of you are unable or willing to understand. This slower burn rate allows the mixture to still be burning as the exhaust valve opens and exiting via the engine exhaust system thus giving a lower preformance.
This is what causes a burnt exhaust valve but then again they dont exist do they that just a myth.
Whislt running a richer mixture the mixture is burnt within the chamber giving a higher preformance.
Tootie toot toot.

Yes but not much. It's about the same speed as a mixture rich enough to prevent cylinder heads overheating. If it was so lean it was still burning when the exhaust valve opened there wouldn't be enough heat there to hurt anything.

With peak EGT as the reference temperature, leaning immediately causes the temp to drop. Richening at first causes a temperature rise and then the temperature begins to drop. Go far enough either LOP or ROP and the mixture won't ignite. You can do this anytime LOP but ROP you need to be at a very high altitude with a normally aspirated engine.

In correct. Damage shown proves your statement incorrect. Heat is not your problem but burning past your valve is. To say it is not hot enough to burn your valve is you would not see damage at an inspection but this is diect opposite to what we see.

I would bet those valves are burnt by running too lean but on the rich side side of peak EGT. ie about 25 degrees ROP where the peak pressure is greatest. If your customers must run ROP insist they run far enough ROP to not damage their aircraft.

It’s amazing that after running piston engines on aircraft for over 100 years opinions and alledged ‘facts’ are still this stoichoimetrically opposed.

Tish boom!

B2N2

I don't think that much evidence of the burn has actually happened on LOP, to be honest it is only a cruise thing and that type of video tech is new compared to the motors we are talking about.


And nothing really new in this piston market, so why spend the coin?


From my old appy days I do recall a benefit from the burning exhaust gas exiting the exhaust value also creating a vacuum - that sucks in extra inlet fuel/air mixture.

It is on this old memory that I think even LOP has some unburnt mixture as the exhaust opens.

Not saying any of this post is fact just what I recall being told and the old motor stuff and actual testing.