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