Question for the Motor Heads

[Blakduk]

Just had a reply from an engineer who works on superchargers all the time- the boost gauge should not show higher boost when RPM drops regardless of whether the RPM decrease is due to higher loading of the engine. It relies entirely on what the supercharger is doing and it is directly linked to engine RPM.
It seems CoD is wrong.

[ATAG_Bliss]

Originally Posted by Blakduk

Just had a reply from an engineer who works on superchargers all the time- the boost gauge should not show higher boost when RPM drops regardless of whether the RPM decrease is due to higher loading of the engine. It relies entirely on what the supercharger is doing and it is directly linked to engine RPM.
It seems CoD is wrong.

Well no offense to your engineer buddy but he is wrong m8. I'm an engine builder (drag racer) and a mechanical engineer.

Take this analogy: Easiest way to test this would be with a supercharged car with a manual transmission. Say you are in 2nd gear winding up around 5000 RPMs - you are accelerating at the time, not crusing (with a 6500 redline). Now without changing the throttle, press on the brakes (create a load) that not only stops your acceleration but decreases your RPM's by say, 500. Your boost will increase. Now just for the fun of it, keep pressing the brakes even harder to really lug it down and floor the throttle. Boost is still not dropping, but RPM's are falling off. Now if you press the brakes hard enough while you are floored to essentially kill the engine, you will maintain boost up until a certain point (even more so than what you originally started with) until the engine dies. That scenario just shows you that boost (just like the definition of how a supercharger works) relies on both engine load and RPM.

If you don't believe me. Please go to a car dealership and test drive a supercharged car, borrow a supercharged car, steal a supercharged car, do w/e you want to get one. But do just what I said and you'll see the same thing. I don't claim to know anything about an aircraft engine, but if it's a V configuration, combustion engine, that has a gear driven supercharger going through a carb, it can't be that much of a different principle than a similar setup on a car.

[Blakduk]

Bliss- unfortunately i live in an area where supercharged hotrods are rare so my opportunities to steal one are very limited. Also i'm a wuss and extremely scared of getting caught. Basically i have to take other people's word for how the real thing works.
Your argument makes sense, it seems plausible but.... i haven't seen the proof.

Basically i've got two engineers telling me different things, and two sims displaying different things (this could turn into another 'plane on a conveyor belt' thread)

[Sutts]

Originally Posted by Blakduk

Bliss- unfortunately i live in an area where supercharged hotrods are rare so my opportunities to steal one are very limited. Also i'm a wuss and extremely scared of getting caught. Basically i have to take other people's word for how the real thing works.
Your argument makes sense, it seems plausible but.... i haven't seen the proof.

Basically i've got two engineers telling me different things, and two sims displaying different things (this could turn into another 'plane on a conveyor belt' thread)

If boost rises when rpm falls (at a fixed throttle setting) and boost falls as rpm rises then the sim is correct.

As mentioned earlier, an engine is just like a big suction pump, sucking air fuel mixture out of the inlet manifold and thereby reducing the pressure in that manifold (which shows on the boost gauge).

The higher the RPM (at a fixed throttle setting), the faster the air is pumped out of the manifold and the lower the manifold pressure becomes. On the other hand, when RPMs are reduced (at a fixed throttle setting), less air is being sucked out and the pressure rises.

When the engine isn't running, the suction pump stops and the pressure in the manifold returns to the atmospheric pressure at the current altitude being flown.

On a non-supercharged / non-turbo charged engine, the highest boost pressure available is the atmospheric pressure at the current altitude.
On a supercharged / turbo charged engine, the highest boost pressure available is the max pressure provided by the compressor at the current altitude.

That's my understanding anyway.

[ARM505]

Again, I must ask/clarify:

In the following situation - constant everything (ie altitude, throttle setting, mixture setting, airspeed etc), a change in RPM via the prop control lever will result in (and I have tried both the A2A model and CLOD in similar density alts/airspeeds, for what it's worth):

a) a slight decrease in indicated boost (ie the A2A model Spit)

b) an increase in indicated boost (ie the CLOD model)

One is correct, the other not. So....which is it?

[Sutts]

Originally Posted by ARM505

Again, I must ask/clarify:

In the following situation - constant everything (ie altitude, throttle setting, mixture setting, airspeed etc), a change in RPM via the prop control lever will result in (and I have tried both the A2A model and CLOD in similar density alts/airspeeds, for what it's worth):

a) a slight decrease in indicated boost (ie the A2A model Spit)

b) an increase in indicated boost (ie the CLOD model)

One is correct, the other not. So....which is it?

You haven't said whether you're increasing or decreasing RPM with the prop control.

Increased RPM.....lower boost
Reduced RPM....higher boost

[ATAG_Dutch]

Originally Posted by Sutts

You haven't said whether you're increasing or decreasing RPM with the prop control.

Increased RPM.....lower boost
Reduced RPM....higher boost

He must be reducing rpm as the CoD model would show an increase.

[Babi]

Originally Posted by Sutts

If boost rises when rpm falls (at a fixed throttle setting) and boost falls as rpm rises then the sim is correct.

As mentioned earlier, an engine is just like a big suction pump, sucking air fuel mixture out of the inlet manifold and thereby reducing the pressure in that manifold (which shows on the boost gauge).

The higher the RPM (at a fixed throttle setting), the faster the air is pumped out of the manifold and the lower the manifold pressure becomes. On the other hand, when RPMs are reduced (at a fixed throttle setting), less air is being sucked out and the pressure rises.

When the engine isn't running, the suction pump stops and the pressure in the manifold returns to the atmospheric pressure at the current altitude being flown.

On a non-supercharged / non-turbo charged engine, the highest boost pressure available is the atmospheric pressure at the current altitude.
On a supercharged / turbo charged engine, the highest boost pressure available is the max pressure provided by the compressor at the current altitude.

That's my understanding anyway.

It's true that pressure drops with increased RPM in a normal (aspirated) engine because of "suction". But you completely miss the other important factor: the supercharger is not some magical box that gives its nominal boost output regardless of what the engine is doing. Its mechanically driven by it, and the faster the engine RPMs, the faster the supercharger spins and the more boost is able to deliver. The pressure ratio the supercharger is able to achieve does not vary linearly with its RPM, and depends on the specific type of compressor installed. So you see? the pressure tends to go down because of increased RPMs, but the supercharger tends to deliever more boost because of this RPM increase. What is the predominant effect? difficult to say, depends on supercharger and intake design, and also if the engine is running at low or high RPM.

Read my post on top of page 5.

In the end we can't say anything about the accuracy of the model, until we have specific engine test data or supercharger characteristics.