Question for the Motor Heads

[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.

[ARM505]

Thanks very much for that info.

I've also done a bit of asking (could only find a Mustang pilot though, and it seems the P51 has some sort of automatic boost control, so it pretty much stays where you set it, within limits - it was a short convo, and I didn't want to press too hard?), and this confirms what I've suspected and the info I've seen.