Question for the Motor Heads

[ATAG_Bliss]

Originally Posted by Babi

I think we are expressing the same thing with different approaches. To me it's more intuitive to think that the intake pressure is dependant on "throttle" (throttle butterfly valve position) which causes a pressure drop, and engine RPM because it mechanically drives the supercharger (the relation between supercharger's RPM and its compression ratio may depend on the type of sc, not sure though).

A loaded engine at 2000 rpm achieves more boost (and consumes more fuel) than an unloaded one at 2000 rpm just because in the unloaded one the butterfly valve (throttle) is "more closed" (otherwise RPMs would go up) which causes a bigger pressure drop.

the point is that, ignoring the external pressure dependance, the intake pressure should depend on 2 variables, not three.

Do we agree on this?

Actually the actual intake pressure of a particular engine will depend on so many variables that I could write a 10 page essay about it. You can use RPM and throttle position as the variables, but without knowing the particular case of which you using the engine, what engine, what percentage of load you are using, it's hard to make any sort of analogy to what's actually going on in a given instance of operation.

Regarding the game, at constant altitude and throttle, if the prop pitch is coarsed (load increases, RPM decrease) the intake pressure goes up, which is not what one would think since the supercharger spins slower.

1st you have to understand the principles between the 2 (engine and supercharger) to realize that you will always have positive manifold pressure in that situation.

Think about it this way. Lets say we have a supercharger that creates a nominal output of boost of 8lbs. That means @ full load and full throttle situations you are going to pump out 8lbs of boost / positive manifold pressure of 8lbs. Lets say the operating RPM range of this engine is 600 RPMs to 6000 RPMs. 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. This creates a boost situation. This is exactly why a supercharger is used. Because now, when you jab the throttle, you not only have the fuel to feed it and increase RPMs, you also are exponentially increasing the amount of air the supercharger is positively charging the intake to feed the monster / increase RPMs / go faster / etc.

So the supercharger as you know, is always supplying air, but the engine (say a V12/V16) decreasing 500RPMs with the same throttle position, will significantly decrease the air it draws in (16 pistons/connecting rods all with w/e stroke - basically one big huge vacuum pump (the engine) being slowed down) while that 500 RPMs on little Mister super charger only slows down it's output marginally. The supercharger outlet is probably only slightly bigger in diameter than a single piston of said 16 piston engine. On most supercharged cars it's usually smaller in diameter than a single piston. And with a max output of 8lbs of boost (for instance) slowing down 500 RPMs of that monster vacuum pump is not going to effectively create a 1:1 vacuum pressure with relevance to each other (engine air sucking to supercharger air blowing). It's going to exponentially increase or decrease depending on the situation. That is why at full load and full throttle (say jabbing the throttle at 2000RPMs) you are going to maintain maximum boost and virtually identical boost completely through out the RPM range that you jabbed the throttle at (2000-6000 rpms)

That make sense? I've tried to type it out as best as I can explain it through text lol. I'm running out of analogies!

[louisv]

Boy, I learned in this thread today !!! thanxs everyone, especially for the three links...lots of detail here

[Blackdog_kt]

I think i'm starting to get a grasp on what SYN_Bliss is saying, i will still have to re-read it with a clearer head tomorrow (major lack of sleep today) but there's some good analogies in there that make it simpler to understand.

On another note, what surprised me is that the thread dived right into boost and superchargers. Maybe it would be much easier to explore and/or explain the basic effect of "RPM reduction leads to manifold pressure increase" if we had started with something simpler: a non-supercharged engine (say, a Lycoming on a Cessna) and absolute units of measure for manifold pressure.

In any case, very interesting and informative thread, cheers to all for their input

[MadBlaster]

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. This creates a boost situation.

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?

[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.

[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.

[MadBlaster]

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.

I understand what your saying and agree, but I think about it differently. Consider that this is a multi-piston situation, air being a fluid and the rpms being so high. So at any moment in a cycle, one piston has intake valve opening and sucking air (creating negative force flow on the downstroke), another piston has exhaust valve opening pushing spent gas (creating positive force flow on the upstroke) and the other cylinders have their valves closed because they are either compressing fuel/air or driving the shaft from ignition. So if the load increases and rpm drops, the engine cylinders are moving less air through the "system" (negative and positive flows from the system are reduced). In my mind, this creates the air "backstop" that allows the supercharger to build up pressure in the intake manifold, not just the longer time that the intake valves are closed because that's only half the system that is moving air through the engine.

[Blakduk]

Just had a reply from a pilot who often flies old warbirds (including Spitfires, P40's, Wirraways etc) as well as many other types of aircraft. He stated the boost gauge goes down when the revs drop on supercharged engines, regardless of whether it's due to increase load on the engine.
It seems CoD is wrong and A2A is correct.

[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.