Manifold Pressure Question

[DC8 Flyer]

Explain the fluid coupling. Im an engine mechanic idiot.

 

[Donsa320]

Explain the fluid coupling. Im an engine mechanic idiot.

Well, the engine was of course able to change rpm rather rapidly, as all recips can, and so it was not possible to mechanically connect the Power Recovery Turbines (3 per engine BTW) directly to the crankshaft. The PRT's could not possibly change rpm as quickly as would be required for that arrangement. So the output of the PRT ended in a quill shaft with a paddle-like wheel in the fluid coupling which spun another wheel in the coupling using the oil to transmit the torque. That second wheel was shafted back to the engine crankshaft and turned with it. They looked like minature Fluid Drive units from a 1941 Dodge. This allowed the engine rpm to change rapidly and the PRT rpm to do whatever it needed to do, independantly. At Take-Off power the PRTs supposedly contributed several hundred horsepower to the engine output.
I hope that helps.

~DC

 

[DC8 Flyer]

So it almost works like a free turbine on a turboprop, except it uses engine oil instead of low pressure air to turn the "turbine"?

 

[Donsa320]

So it almost works like a free turbine on a turboprop, except it uses engine oil instead of low pressure air to turn the "turbine"?

Yes, that would be a reasonable statement. Except of course only a small percentage of the total power output was recovered by the PRT as opposed to a free turbine turbo-prop engine.

~DC

 

[A Squared]

These systems were used on many WW2 aircraft, mainly bombers.

I can't think of a single WWII aircraft which had Turbocompound (PRT) engines. The Turbocompound engines were a later development. I believe that the only production Turbocompound engine was the R-3350 (the R-3350's on the B-29 were not turbocompound) The R-3350TC powered post war, pre-jet era aircraft such as the C-119, Super constellation, DC-7, P2V Neptune. Allison developed a turbocompound version of the V-1710 The V-1710 TC was intended to be used in a version of the Bell Kingcobra, which was cancelled. Pratt & Whitney developed aversions of the R-4360 which was sometimes referred to as a turbocompound, although it was actually using a varaable nozzle on the turbocharger to maximize jet thrust from the exhaust. THis enginewas to be used in the B-50C and b-36C. Both models were cancelled before production. Avbug can chime in here if I'm wrong, but I don't think that the KC-97 ever had the Turbocompound version of the 4360 installed.

 

[Donsa320]

I can't think of a single WWII aircraft which had Turbocompound (PRT) engines. The Turbocompound engines were a later development. I believe that the only production Turbocompound engine was the R-3350 (the R-3350's on the B-29 were not turbocompound) The R-3350TC powered post war, pre-jet era aircraft such as the C-119, Super constellation, DC-7, P2V Neptune. Allison developed a turbocompound version of the V-1710 The V-1710 TC was intended to be used in a version of the Bell Kingcobra, which was cancelled. Pratt & Whitney developed aversions of the R-4360 which was sometimes referred to as a turbocompound, although it was actually using a varaable nozzle on the turbocharger to maximize jet thrust from the exhaust. THis enginewas to be used in the B-50C and b-36C. Both models were cancelled before production. Avbug can chime in here if I'm wrong, but I don't think that the KC-97 ever had the Turbocompound version of the 4360 installed.

Yes, the C-97/B-377 and B-50 had the R-4360. It had a straightforward single speed geared supercharger AND a turbo-supercharger which we did not bring on line until we had climbed to the critical altitude for the geared supercharger...about 7500 feet. Since the turbo supercharger also supplied cabin pressueization, we were unpressurized below 7500 ft. There was no aircycle machine or vapor cycle cooling so it got warm in the cabin at low altitude in warm ambient conditions...like the South Pacific.

The R-3350 was the only turbocompunded engine I was ever aware of and was used on the airplanes you mentioned plus a large Martin twin flying boat, the name of which just slips my mind at the moment

I think the mention of WW2 machines may be confused with the turbo-superchargers of that era, maybe.

~DC

 

[USMCmech]

I may be wrong, but I could have sworn that I saw a B-17 and a B-24 wiht PRTs. Thunderbird and Dimond Lil to be precise.

The B-29 and Connie definately did.

 

[Bernoulli]

OK... it took a long time to read through this thread over a couple days due to a lot of ahhh...let call it disagreement. But God help us...lets get back to the original question and better yet lets expand on it a wee bit. (If Av-bug and DC-8 could both provide a short simple explanation...and hopefully they'll be in agreement)...I'll copy and paste it into my CFI notes.

#1. During a run-up, why does manifold pressure increases when RPM is dropped...
and (let see if yall agree )

#2 Why at cruise does manifold pressure rise when you increase the throttle but don't touch the prop lever and...

#3. At crusie power (not low power like in a run-up)... if you reduce rpm by pulling back on the prop lever does manifold pressure still rise?

#4. And I can't help it but I'd like to know if you agree on why during low rpms on the ground does the throttle control RPM and manifold pressure. With props full forward we are able to add the throttle and increase the RPM to approx 17-18 hundred RPM... but at higher throttle settings RPM is controlled by the prop / governor.

MUCOS GRACIAS

 

[gern_blanston]

If I may: (Flame suit on)

#1. During a run-up, why does manifold pressure increases when RPM is dropped...

-Less 'sucking' by the engine because RPMs have decreased so the engine can't take as much air IN.

#2 Why at cruise does manifold pressure rise when you increase the throttle but don't touch the prop lever?

-Less restriction to 'sucking' by the engine because the butterfly's opening up and allowing more air IN.

#3. At crusie power (not low power like in a run-up)... if you reduce rpm by pulling back on the prop lever does manifold pressure still rise?

-Yes. (IF the throttle plate is not all the way open already)

#4. And I can't help it but I'd like to know if you agree on why during low rpms on the ground does the throttle control RPM and manifold pressure. With props full forward we are able to add the throttle and increase the RPM to approx 17-18 hundred RPM... but at higher throttle settings RPM is controlled by the prop / governor.

-The governor can only do so much. After it's reached its low-pitch (high-RPM) stop, it can't control the RPM anymore. The amount of power the engine's putting out does. As to manifold pressure, the answer's the same as #2. The throttle still varies how much air is available to the engine, and how hard the engine has to 'suck' to get it. (throttle plate position in the intake tract)

(Flame suit off)

 

[USMCmech]

First a brief review.

The downgoing piston creats a vacum and sucks in air.

The closed throttle plate closes most of the intake off, and only allows a small amount of air to pass.

This results in a low pressure inside the intake manifold.

MAP is basicly a throttle position indicator. "your throttle is 1/2 open because you have 21" MAP" This information combined with how fast the engine is turning allows you to determine how much power is being produced. Example: 2500 RPM and 25" MAP equals 85% power.

#1. During a run-up, why does manifold pressure increases when RPM is dropped...

The drop in RPMs results in less suction, so the pressure risses inside the intake manifold.

#2 Why at cruise does manifold pressure rise when you increase the throttle but don't touch the prop lever and...

By opening the throttle you are allowing more air to enter the engine. At wide open throttle (WOT) there is effectively no restriction, therefore MAP is the same as atsmopheric pressure.

#3. At crusie power (not low power like in a run-up)... if you reduce rpm by pulling back on the prop lever does manifold pressure still rise?

At WOT there would not be any change in MAP, BUT actual power produced by the engine would drop with RPMs. The engine is takeing the same amount of air/fuel for each revolution, but it is takeing fewer "breaths".

#4. And I can't help it but I'd like to know if you agree on why during low rpms on the ground does the throttle control RPM and manifold pressure. With props full forward we are able to add the throttle and increase the RPM to approx 17-18 hundred RPM... but at higher throttle settings RPM is controlled by the prop / governor.

When taxing at low power settings, the governer is well underspeed and the prop baldes are at their flat pitch limit. Therfore if you add some throttle (say 1500 RPM) the engine speeds up untill the governer comes into play (at 2500 RPM).

If you want, you can experiment with this.

AT LOW POWER ONLY!!!! Proceed at your own risk.

Try taxing with the prop controll all the way out to the low RPM/coarse pitch limit. When you increase throttle you may see the governor come into play at 1500 RPM or so.

Be VERY!!! carefull doing this as it can put a huge load on the prop. The governor will maintain 1500 RPM even at full throttle, which can be very bad for the prop.