Engine design as related to airplane power : with particular reference to performance at varying alt

[Zipper730]

Well if you just google "churn" it is obvious what churning is. Basically severe turbulence in a fluid (milk) caused by vanes rotating in a drum to produce butter.

So basically the oil gets churned into foam at lower altitudes?

The "impeller" is pretty much a mirror image of the turbine. A bowl with a lot of fins and some bracing to keep them from bending (aircraft ones might be better made?).

Okay

There are two ways to vary the speed of the "turbine" . . . One is have the distance between the two variable. One of them slides on it's shaft further away from the other and this allows the oil (or working medium) to slip in the empty space between the two and not transmit the full force to the turbine.

As I grasp it, you'd need something to push the two together (or apart)?

The other is to vary the amount of oil in the housing. No moving part but full of oil or nearly so means as the Impeller rotates the oil, moved by centrifugal force moves to the outer rim and crosses over to hit the fins on the turbine, then flows to the center and crosses back to the impeller. Impeller is turing at 24,000rpm on a DB601 and higher on later engines. Less oil in the housing means less force is transmitted.

My understanding is probably just wrong here, but I'd figure more oil would mean more slippery...

However this oil is going to get hot and if you use the engine oil and not a separate supply you are going to get lots and lots of tiny air bubbles in the oil which need to be taken out before the oil is feed back into the engine.

I guess the air bubbles either reduce the oil's slickness, produce turbulence that adds resistance, or both

 

[pbehn]

So basically the oil gets churned into foam at lower altitudes?

I don't know, In the post it mentioned a raise in temperature. I have operated a butter churn in a museum. It is really hard work, all that work is just converted to heat because nothing actually goes anywhere in the churn.

 

[Shortround6]

So basically the oil gets churned into foam at lower altitudes?

Okay
As I grasp it, you'd need something to push the two together (or apart)?
My understanding is probably just wrong here, but I'd figure more oil would mean more slippery...
I guess the air bubbles either reduce the oil's slickness, produce turbulence that adds resistance, or both

The parts never actually touch so the slipperiness of the oil doesn't come into it. The oil is just the medium used to transfer the motion of the impeller to the turbine.

If the little air bubbles turn into fewer but larger air bubbles and then the air bubbles travel along to a bearing where things can get real interesting real quick.

Interesting in the form of the so called old chinese curse "may you live in interesting times".

 

[Zipper730]

The parts never actually touch so the slipperiness of the oil doesn't come into it. The oil is just the medium used to transfer the motion of the impeller to the turbine.

I'm totally confused here... the oil spins the turbine which spins the impeller? Or the engine spins the impeller?

 

[pbehn]

I'm totally confused here... the oil spins the turbine which spins the impeller? Or the engine spins the impeller?

Zipper, you really are special at times, google impeller, google torque converters and read how they work. With no engineering knowledge it is obvious an impeller "impels" it isn't impelled.

 

[Zipper730]

Zipper, you really are special at times

Yeah, I know... I can be smart at times, average at others, and frankly remarkably dumb in other areas.

So the device doesn't use a direct connection but uses hydraulic fluid only which is supplied by the engine and presumably that's varied by some kind of engine power setting?

 

[pbehn]

Yeah, I know... I can be smart at times, average at others, and frankly remarkably dumb in other areas.

So the device doesn't use a direct connection but uses hydraulic fluid only which is supplied by the engine and presumably that's varied by some kind of engine power setting?

I don't know why you ask me, I have no special knowledge, why not use google and find out how they work.

 

[swampyankee]

Yeah, I know... I can be smart at times, average at others, and frankly remarkably dumb in other areas.

So the device doesn't use a direct connection but uses hydraulic fluid only which is supplied by the engine and presumably that's varied by some kind of engine power setting?

The speed ratio can be controlled by changing the angle of a set of stators between the impeller and turbine. There are other methods, but I am far from expert in torque converters.

 

[MiTasol]

Look on the net for a video on how your auto transmission works - the first stage is a torque converter.
The only thing different is the fluid - in an aircraft engine it is engine oil, very thick and viscous compared to Auto Transmission Fluid (ATF)
Some ATFs slip more with increasing pressures and some slip less so you will need to find an auto transmission that uses one that slips less to see how the aero oil one works.

 

[Zipper730]

I don't know why you ask me, I have no special knowledge, why not use google and find out how they work.

It was the one thing that didn't seem to be stated... so I made a guess

The speed ratio can be controlled by changing the angle of a set of stators between the impeller and turbine.

So the amount of oil and stator position

 

[pbehn]

It was the one thing that didn't seem to be stated... so I made a guess

Fluid couplings are a technology all of their own. Between the 1940s and present day they have advanced hugely. Since I have seen "Automatic transmission fluid" on sale in a garage it is clear some have their own special liquid. On some cars it was possible to lock them in some way so it became a mechanical connection. At all times they waste energy, in the days where the same engine was offered with a gear box or auto transmission the auto model always used more fuel. A fluid coupling wastes energy, most of which is turned into heat.

 

[Shortround6]

Any time you change the direction or the speed of a spinning shaft you are going to lose a small fraction of power. It might be on the order of under 1% to 2% percent but it does happen. Gear sets are not frictionless.

fluid couplings date back to about 1905. The WIki article seems pretty good.

Fluid coupling - Wikipedia

Hermann Föttinger actual built a few fluid couplings for motor ships (early diesels) before WW I. Unlike steam engines the early (and later) diesels are not infinitely variable in speed and some means of controlling the ships speed (varying the propeller speed) independently of the speed of the engine was needed. Variable pitch ships propellers were still a long way off.

 

[Zipper730]

This is a good video which illustrates the point very well

 

[Shortround6]

Good find.

 

[Deleted member 68059]

What the DB605 hydraulic coupling looks like:

View: https://youtu.be/irOc9gloabo


How its characteristics affect altitiude performance:

View: https://youtu.be/Pm4SaAnPtYI


The oil properties are critical to the performance, viscosity is one of the primiary characteristics defining how much
of the fluid inside gives what "slip". This is not really occuring via "churning" but torque transfer as the oil "rotates"
in a toriodal cavity (a doughnut shape).

Torque in = Torque Out
Power = Torque x Speed
So if speed varies....but torque is constant...then only power can vary...

Therefore the speed difference between input and output shaft is transferred
directly into power loss, which manifests as heat in the oil, caused by viscous
shear in the oil. Hence with a large supercharger high slip diverts massive
amounts of heat to the oil, requiring a larger oil cooler for the DB engine.

If you wanted you can buy this copy of Race Engine Technology magazine in which
I explain this is more depth:

Race Engine Technology – Issue 098 | High Power Media

I have all of this sitting at my desk in the memiors of the chief designer of Daimler-Benz, so you
could also just ask....