Manifold Pressure Question

[VNugget]

DC8 Flyer: they're right. Remember how you told me that the mass flow changes? Well if it does, then we're no longer dealing with a closed system, and therefore we can't use Bernoulli's equations. There must be something else causing the change.

 

[DC8 Flyer]

..NVM

 

[VNugget]

DC8Flyer:

I may have mispoken with the term MASS FLOW. If the throttle is constant and you pull the props back, your not changing the MASS of the airflow you are chaning the rate of the airflow.

What do you mean by rate? Rate of what?

To change the MASS we would have to turbocharge or in someway ram more air in than is being drawn in.

No we wouldn't. You control mass flow with the throttle, by varying the constriction. Remember that mass flow determines power.

The piston goes down and displaces X amount of volume that is filled with Y amount of air. Y is usually less than X because of throttle plate, filter and other restrictions in the manifold. Close the throttle and the piston goes down but can only be filled with say 80% of its volume,

Um, what? The displacement volume is the same no matter what. Maybe if you suffer a catastrophic engine failure things will change...

the air has the same MASS but just not as much of it.

What in the world is that supposed to mean?

One cubic meter of 29.92 air still weighs the same but since only 80% of the cylinders volume came in, it has to expand and its pressure goes down. Lower pressure, greater volume, same mass. This is why your power goes down (as well as MP) as you pull the throttle back only.

Lower pressure? OK. Greater volume... WHAT?! Same mass? I don't think so.

Remember that the air mixes with fuel at a constant mass ratio, and the more of that mixture that you combust per time, the more energy is released per time (i.e., power)

 

[DC8 Flyer]

..NVM

 

[VNugget]

Sorry, but you're throwing quantifications around like candy at Mardi Gras. Mass, volume, density, eh what's the difference. I don't wanna sit here and try to decipher what you mean based on what you're saying.

The bottom line is that mass flow (mass per time) = power, and the throttle plate controls that mass flow, thereby controlling power.

 

[DC8 Flyer]

AHHHHHHH CRAAAAAAAAAAAAAAAAAAAAP! I just figured out where I was going wrong with my assumption. First off big apologies to AvBug, USMC, Devona etc.

The piston isnt sucking air into the cylinder the higher outside air pressure is pushing it into the cylinder, hence the ram recovery you get when you roll down the runway. Now when you do a runup you dont open the throttle all the way, only to 2200 RPM as I recall, so the pressure in the manifold is below atmospheric (ambient). When you slow the pump down (piston) you give that ambient pressure more time to push into the manifold before the valve closes and voila, pressure rise.

So AvBug, I here by eat my words.

Vnugget got me thinking with the mass thing, and I realized if the mass was changing then so where the denstiy properties of the air and it was no longer an ideal gas that could use Bernoullis law, well without some serious Diffy Q type equations but I DO NOT wanna go there.

Sorry guys, but you gotta admit the math worked out pretty close to showing it worked.

 

[USMCmech]

In the large engines you mentioned, I recall Power Recovery Turbines or some such from my A&P books. Only having seen them in museums, I forget how they work, Maybe you guys can explain them...they were/are pretty neat...

PRTs were an effort to extract power from the exaust gasses on radial engines. If you think carefully you will see how they aided in the development in the turbine engine that we know today.


Basicly take the turbine half of a turbo charger, and conect it to the crankshaft via a fluid coupling (think torque convertor in an automatic tranny). The turbine extracts the power from the exaust gas and sends it back to the engine. What you get is a 5-10% increase in HP.

Since the big radials already had internal superchargers, enginers started thinking about how they could get some power out of the exaust. These systems were used on many WW2 aircraft, mainly bombers. They were very complex, and were often refered to "parts recovery turbines" due to the fact that they often suffered FOD damage.

Most warbirds flying today have disabled their PRTs due to maintence costs.

 

[avbug]

The main user of the PRT, and perhaps the ultimate development of the radial engine, was the R-3350. Known as a turbo-compound engine, it was called so because of it's supercharger as part of the induction system, and it's power recovery turbine which worked exactly as USMCmech described.

Changing out a PRT at 70 lbs, over your head, is just about as much fun as changing a cylinder on the R3350.

Folks wire the blower clutches on warbirds today (remove the control and wire it to low blower on the supercharger), but removing the PRT from the system isn't really practical. Nor a particularly good idea.

 

[Denizen]

Thanks, I remeber looking at a turbo compound engine on a stand at the grand canyon airport(not the one one the south rim, this one is a bit south)

It was leaking a lot and the old-timer associated with the Connie they have chatted about them for a while.

A real interesting bit of aviation.

sorry bout the thread hi-jack

 

[Donsa320]

Donsa, I know you bowed out of this conversation, However a quick ??

In the large engines you mentioned, I recall Power Recovery Turbines or some such from my A&P books. Only having seen them in museums, I forget how they work, Maybe you guys can explain them...they were/are pretty neat...

Thanks

First, my apologies to DC-8, my Dago temper got to me.

RE; the PRT. What others mentioned is correct. I would add that the big advanatge as it was explained to me, in 1953 when we got the "G" model C-119, is; power from the exhaust flow could be extracted with out increasing back-pressure in the exhaust system. So there was no penalty associated with having the PRTs on the engine. The energy was obtained by a decrease in the exhaust system velocity.
But there were problems. The PRTs spun at high rpm and, since they were restrained by the fluid coupling, they could overspeed if the fluid coupling went dry. The coupling was filled with engine oil and was kept filled by the engine oil system. If the inlet clogged, the oil level dropped and the PRT could self destruct throwing parts all over. There were no gauges to tell us what the PRTs were doing so armouring was the answer.
I spent many happy hours in the C-119 with the PRTs. The airplane with R4360 engines on it would burn about 900 lbs per hour per engine with a 1500 hp cruise setting whilst the R3350 turbo-compound at the same setting would burn about 600 lb/hour/engine. A significant improvement IMHO, not to mention the lower oil consumption of the 3350.

~DC

 

[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.