Question for the Motor Heads

[Seeker]

Originally Posted by ARM505

Funny that A2A is mentioned, because I seem to remember that LOWERING RPM in the A2A Spit LOWERS boost (ie, lowering prop RPM's via the prop control lever at a constant throttle setting results in less boost indicated). This is explained in the A2A docs - the supercharger, being directly linked to the engine, is now being turned at a lower RPM, hence a lower manifold pressure - the opposite to unforced induction, where lowering RPM at a constant throttle setting will result in higher manifold pressure, the 'car going up a hill in a high gear' analogy.

Yup, just checked the A2A Spit, and that's what happens. COD seems to behave in the opposite way - lowering RPM's lowers manifold pressure. One is right, the other wrong I suppose. Funnily enough, once again we seem to have a multitude of inputs explaining why COD is correct. Oh well. I'll just go and fly my Spit IIa IRL to check it out quick

To be honest, my vote goes to the A2A Spit, which is frankly awesome, even though the engines tend to wear too fast!

There's a thread about the Spít I and II in the FM forum where people are beating each other over the head with docs and charts as usual.

In the thread there's a comparative test between 109 and Spit, and the RAF docs clearly state that the Spit pilot reduced revs to 2600 which raised the boost...

[ATAG_Dutch]

Originally Posted by Seeker

There's a thread about the Spít I and II in the FM forum where people are beating each other over the head with docs and charts as usual.

In the thread there's a comparative test between 109 and Spit, and the RAF docs clearly state that the Spit pilot reduced revs to 2600 which raised the boost...

If you watch the A2A vid referenced by louisV, the A2A spit is provided with three different props, so it would depend on which one was being used. From what I can make out, the spit in the comparison would have the constant speed in order to set revs at 2600. I think!

[Blackdog_kt]

The efficiency of direct-drive supercharges is tied to the RPM. This is true.

The above effect is not always linear and probably each supercharger has its own "powerband" so to speak.

There's also another thing to consider. RPMs=amount of combustion cycles per minute. The engine is essentially a vacuum pump, it sucks air, mixes it with fuel and burns it to produce power.

If you lower the RPM you essentially lower the amount of combustion cycles during a given time frame. This means that for the same throttle position, less air is being "used up" by the engine.

Where does this air go then? I guess it stays in the manifold for a while longer because at one end (the intake) air is still being forced in, while at the other end (the actual engine/carbs/pistons) less air is being drawn out of the system. This would easily result in an increase of pressure in the intake manifold system and since this is what the boost/ata/manifold pressure gauges measure (just with different units), it shows up in the instruments.

As another interesting bit of information, the less amount of stress on the manifold is not with the throttle closed. At low throttle settings the intake "tube" is trying to implode, because the inside pressure is less than the outside pressure. In fact, the lower amount of stress on the intakes occurs when running throttle that gives a manifold pressure equal to the outside (ambient) air pressure: at sea level this would mean running full throttle on a non-supercharged engine.

Just goes to show how things are not that much set in stone but there's a lot of inter-dependency between different conditions.

If any of you want some pretty long winded explanations that cover everything, check out the following links. Someone else posted them here and i didn't miss a chance to bookmark them after reading, very useful stuff.


Manifold pressure: http://www.avweb.com/news/pelican/182081-1.html

Propellers: http://www.avweb.com/news/pelican/182082-1.html

Mixture: http://www.avweb.com/news/pelican/182084-1.html

[Skoshi Tiger]

Originally Posted by Blackdog_kt

If any of you want some pretty long winded explanations that cover everything, check out the following links. Someone else posted them here and i didn't miss a chance to bookmark them after reading, very useful stuff.


Manifold pressure: http://www.avweb.com/news/pelican/182081-1.html

Propellers: http://www.avweb.com/news/pelican/182082-1.html

Mixture: http://www.avweb.com/news/pelican/182084-1.html

Blackdog,

Thanks for re-posting those links. I've only managed to get through the first one but it was really informative.

Cheers!

[ATAG_Bliss]

Originally Posted by MadBlaster

Manifold pressure, I'll take a guess as I'm not a pilot either. I think it's just like a car. Natural aspirated, the engine creates vacume on the piston downstroke and sucks in the fuel/air mix. The higher the rmps, the more sucking power and the higher the manifold pressure. In supercharger setup, pump is belted somehow to the drive shaft and forces air/fuel into the manifold via a pump. I guess what the guys are saying up above to explain your observation about manifold pressure going up when prop pitch is changed to 'course' is that at high rpms, output of the supercharger pump somehow hinders the engine vacume (ala "law of diminishing returns"), causing it to be lower than it would be if the engine was naturally aspirated. So, it's a trade off when you use a supercharger. What you lose in power at the high end of the rpm scale you gain on the low end of the scale (i.e., higher manifold pressure at course pitch setting where the load on the prop in static state (no acceleration or deceleration) is highest).

Originally Posted by Blackdog_kt

The efficiency of direct-drive supercharges is tied to the RPM. This is true.

The above effect is not always linear and probably each supercharger has its own "powerband" so to speak.

There's also another thing to consider. RPMs=amount of combustion cycles per minute. The engine is essentially a vacuum pump, it sucks air, mixes it with fuel and burns it to produce power.

If you lower the RPM you essentially lower the amount of combustion cycles during a given time frame. This means that for the same throttle position, less air is being "used up" by the engine.

Where does this air go then? I guess it stays in the manifold for a while longer because at one end (the intake) air is still being forced in, while at the other end (the actual engine/carbs/pistons) less air is being drawn out of the system. This would easily result in an increase of pressure in the intake manifold system and since this is what the boost/ata/manifold pressure gauges measure (just with different units), it shows up in the instruments.

Superchargers are Load Sensitive. This means that if the engine is not under load the superchargers will not make boost REGARDLESS of engine RPM. This is something I think many people are missing.

Even though your 1st thoughts are to think that a supercharger is pumping air directly linear with how fast the the supercharger screw(s) are turning (engine RPM), you gotta realize that at even high RPM's unless you are under a load and needing to be heavy on the throttle (aka opening up throttle plate and allowing the carb jets more fuel flow) you are not creating much boost. The engine needs to be working to get ANY boost at any RPM. That same throttle plate limits the amount of air that can enter the manifold to build up positive pressure (boost). It's the same reason why increasing engine load (with the same throttle setting applied) will lower RPMs and increase boost.

It would be like cruising down the interstate at 70mph on a flat road then trying to go up a hill without giving it any more gas (throttle pedal position). You are going to slow down (because of load) unless you increase throttle which in turn, is going to increase fuel consumption, to be able to maintain 70mph. In one instance you are at, say 10% throttle while driving on a flat road. In the other instance you could be at 50% throttle while driving up a hill to maintain the same speed as the 1st instance.

[MadBlaster]

Don't disagree with anything said. Backpressure plays a role in the deal too (e.g., number exhaust valves, header/manifold/cat). Each engine is engineered out for a specific task. I guess I was thinking about the example more literal. The plane is on the ground with the chocks on no aero forces on prop, the throttle valve on carb is fixed at some position and the only thing being done by the pilot is toggling the prop pitch from fine to course to increase the thrust load and lower the rpms. If the rpms on the supercharger pump are directly proportional to turns on the driveshaft (i.e., the belt drive gearing is a fixed ratio), what would make the manifold pressure increase if the rpms of the engine and supercharger are both proportionally decreasing from the increased prop load and fuel being metered is unchanged? I think what blackdog is saying makes sense. Backpressure on the exhaust side holds the air in the intake manifold longer for that particular engine at that range of rpm/load change and fuel rate. Another thing to consider, the supercharger being belted to the driveshaft. I guess is a variable load on the engine. It takes energy to pump air. So while the prop is increasing load to lower engine rpms, the lower rpms on the supercharger pump are decreasing load (still assuming fixed fuel meter). That's intuitively what I was thinking about above where cranking that supercharger pump at certian rpm range with the increase in load that it brings to do that work doesn't pay off verses normal aspirated. Jeez, what a brain drainer this one is.

[ATAG_Bliss]

Originally Posted by MadBlaster

what would make the manifold pressure increase if the rpms of the engine and supercharger are both proportionally decreasing from the increased prop load and fuel being metered is unchanged?

Manifold pressure is going to increase when you compensate for the loss in RPM's through engine load and throttle increase. But that same metered fuel that required 50% throttle to maintain the RPMs you wanted to achieve, is now doing it at less RPM's. This creates positive pressure - boost (but only until the engine is normalized) then the boost guage (depending on the engine / charger combo) will remain fairly steady, but again, this is under a load. Superchargers do take HP to turn and cause a load on themselves, but they are generally designed for the application it's being used on.

A boat is the easiest way to see a supercharger in action with regards to boost, because the prop is always submerged in water at any RPM.

As far as back pressure is concerned, I think you meant to say exhaust manifold instead of intake, but your points are exactly right. That all factors into the equation of a boosted combustion engine.

[Babi]

Maybe i don't undestand because english is not my language, but i don't really grasp the fact that pressure output from the supercharger is dependant on "load"; probably because i dont understand the term.

The pressure should just depend on the engine RPM (wich determine the supercharger's RPM) and the throttle position (and of course external pressure). In the example of the car, the supercharger itself couldn't care less if the car is going uphill or downhill. Then if more "load" means that the engine RPM decrease, this will affect pressure output from the supercharger, but it's not DIRECTLY tied to load.

Then again maybe i undestand because of my bad english.

I think the discussion was about the fact that with constant throttle position and constant altitude manifold pressure drops with RPM