Question for the Motor Heads

[Babi]

Originally Posted by SYN_Bliss

So when you are maintaining 5000 RPMs with 50% throttle and jump the load up significantly to bog the engine down to 4500RPM's (500 revolutions per minute) all while maintaining the same throttle position, you have effectively increased boost because the engine has slowed down it's air intake significantly while the supercharger has only done this marginally.

Ok, i'm starting to get what you mean!

Found my notes on propulsion class, and i feel extremely embarassed for the fact that i completely forgot that in an aspirated engine, intake pressure drops with RPM because of the increased friction losses in the intake (higher RPM, higher air speed in the intake, more friction which cause a dissipation of energy and ultimately a decrease in static pressure when the air is brought to rest). Or if you want to think it more simply, more suction, less pressure.

Now, in a supercharged engine you have to take another effect into consideration, which is the fact that the compression ratio (p after sc/ p before sc) the sc is able to deliver is dependant on its RPM, which in turn is directly linked to engine RPM.

So generally the boost the sc is able to deliver increases with RPM, but also the friction losses increases with RPM, so it's all a matter of what is the most important effect. pressure loss in the intake because of friction should be proportional to RPM^2 if i'm not mistaken, hence very important at high RPM. The boost the supercharger is able to deliver as a function of its RPM will depend on the specific type i think, i will try to find some data.

In the end at higher RPM, if you apply more load and the RPM drop, probably the reduction of losses caused by slower air flow, can compensate for the modest reduction of boost by the supercharger; and the pressure slightly rises.

I think however that at slower RPM it should be the other way around (RPM goes up, pressure rises, because the losses are smaller and the predominant effect is the supercharger), can anyone test it in game?

[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 MadBlaster

And it has also significantly reduced outake of spent gasses too. In other words, the exhaust valves are cycling less now from the engine rpm drop and effectively create a temporary "backstop" while the supercharger pushes the air in and so the intake manifold increases like a balloon blowing up with a small hole in it instead of a large hole where most of the air would go right through it. So, the more the load jump and rpm drop, the higher the boost effect. Correct?

But the exhaust side of the equation has nothing to do with anything in the intake manifold. The exhaust valves only open during the exhaust stroke and only push exhaust out the exhaust manifold/header.

The intake valves start to open up as the piston is at TDC (top dead center) and as the piston goes down it fills the cylinder with air and fuel. This step is the only suction coming from the intake manifold where the supercharger is supplying forced induction. Next that same cylinder compresses and at a certain point in the ignition cycle during this compression stroke, the spark plug fires and creates an explosion. This causes the piston to go down again (cylinder is now full of exhaust gas) and on it's trip up the exhaust valve is open to let it leave the cylinder head through the exhaust manifold/header.

There's ongoing exhaust back pressure in almost virtually every combustion engine. If you didn't have back pressure, you would easily burn valves amongst other problems. To give you an idea about the Merlin (from memory) the exhaust manifold design was changed at a backwards angle because of the sheer amount of thrust it created. The exhaust gases are flowing either 700 to 1400mph out of the exhaust manifold (don't remember), and it didn't take long before someone realized that if they angled the manifold a certain way, that it would actually increase top speed, help with propulsion.

So, the more the load jump and rpm drop, the higher the boost effect. Correct?

Not necessarily. It depends on your starting RPM, the output of the supercharger, the engine etc etc. But generally if you are spinning 75% of max RPM, and significantly increase load to decrease RPM on a s/c'd engine boost will increase. There will be a point where this won't happen anymore. An engine creates vacuum. So at a certain point you will not have any positive pressure anymore because the blower will not keep up, unless****, while more load is added you are opening the throttle. Again, this all depends on so many variables and specifics of the engine/sc combo that I don't think anyone can give you a good answer. But if you remember that increased load and decreased RPMs will give you boost (in a boosted app) but only to a certain point.

[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)

[winny]

Originally Posted by Blakduk

Dutch- He makes me feel like i'm a 12yr old playing games while he's a grown-up doing things for real (which is all very accurate, except i'm well past 12).
I'll humiliate myself and ask him, i can handle his disdain.

Back a few years ago i met a guy called Alec Henshaw. He was a test pilot for Spitfires during WW2 and flew hundreds of them- now he would have been the one to ask!

Just for you.. Alex Henshaw in action over Castle Bromwich 1941
The 1st Min of the video is him.

http://www.youtube.com/watch?v=nCmzYccyBYM

[Blakduk]

Winny- I've been googling Alex Henshaw this evening. I didn't realise he was such a celebrity!
I basically just said 'G'day' to him as he was surrounded by other pilots & engineers- he was having afternoon tea in the hangar my brother was working in (i was helping by shifting boxes of gear in a ute, filling the esky with beer and getting food etc). They were working on Spitfire that was in bits- it was owned by a New Zealander and i think it ended up at Wanaka. They were pointing at bits and nodding knowingly at each other- about the only thing i contributed to was a discussion about the bolts being made of magnesium. I commented they would react with the aluminium and disintegrate over time- i got an approving nod and went to get a beer (it was after 4pm after all and i thought i better quite while i was ahead). I do recall him saying the Spitfire was 'only a 1000 hour airframe, so it's a wonder any are still flying today'.

[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.

[Sutts]

Originally Posted by SYN_Bliss

But the exhaust side of the equation has nothing to do with anything in the intake manifold. The exhaust valves only open during the exhaust stroke and only push exhaust out the exhaust manifold/header.

The intake valves start to open up as the piston is at TDC (top dead center) and as the piston goes down it fills the cylinder with air and fuel. This step is the only suction coming from the intake manifold where the supercharger is supplying forced induction. Next that same cylinder compresses and at a certain point in the ignition cycle during this compression stroke, the spark plug fires and creates an explosion. This causes the piston to go down again (cylinder is now full of exhaust gas) and on it's trip up the exhaust valve is open to let it leave the cylinder head through the exhaust manifold/header.

There's ongoing exhaust back pressure in almost virtually every combustion engine. If you didn't have back pressure, you would easily burn valves amongst other problems. To give you an idea about the Merlin (from memory) the exhaust manifold design was changed at a backwards angle because of the sheer amount of thrust it created. The exhaust gases are flowing either 700 to 1400mph out of the exhaust manifold (don't remember), and it didn't take long before someone realized that if they angled the manifold a certain way, that it would actually increase top speed, help with propulsion.



Not necessarily. It depends on your starting RPM, the output of the supercharger, the engine etc etc. But generally if you are spinning 75% of max RPM, and significantly increase load to decrease RPM on a s/c'd engine boost will increase. There will be a point where this won't happen anymore. An engine creates vacuum. So at a certain point you will not have any positive pressure anymore because the blower will not keep up, unless****, while more load is added you are opening the throttle. Again, this all depends on so many variables and specifics of the engine/sc combo that I don't think anyone can give you a good answer. But if you remember that increased load and decreased RPMs will give you boost (in a boosted app) but only to a certain point.

Agreed, boost can only rise as high as atmospheric pressure on a non-blower engine and as high as the blower output pressure on an engine fitted with a blower.

[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?