supercharger

Jacka$$withawrench

Jacka$$withawrench

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bob58o said:
Engine capacity 212cc
Air pumped 180cc
VE = 0.85

Engine capacity 212cc
Air pumped 300cc (forced induction)
150cc per revolution
VE = 1.41
Boost = 6 psi

I’m not sure I agree that volume of air doesn’t increase as well as air density. VE isnt even constant at different RPMs because of intake / exhaust tuning and scavenging.

Unless the VE was originally 1.00 when NA. I have to think about this. Air would flow faster with a greater pressure difference between runner and chamber and more volume could be moved in a certain amount of time. But the chamber being filled with dense air might offset the boost in the runner? I dunno. My fluid mechanical ignorance is showing.
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The engine would still take in the same volume of air, just a higher density of air, no? I looked into it online and that's what an old guy (Banks Power on facebook) is saying, that a supercharger increases density not volume and therefore doesn't increase Volumetric Efficiency.
 
bob58o

bob58o

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It’s a complex system. Piston moves down- creates low pressure in chamber. Piston moves up- increases pressure in chamber. Intake and exhaust valves open and close and have overlapping open periods. Air flows based on pressure difference. Intake and exhaust pressure waves pulling out spent change and pulling in fresh intake charge. People like to make simple models explaining what’s going on, but the reality is much more complicated.

Measuring the mass of the air and its density will give its volume.

We need Alex here to explain maths and to remind us jokes about German sausages are the wurst.
 
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bob58o

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The word I find most interesting in the following is “overfilling” of the cylinder


Forced induction has the ability to raise the VE of an engine to 100 percent and way beyond. It is a function of the airflow in cubic feet per minute (cfm) of the compressor and the boost pressure created. Boost is the positive pressure above atmospheric that is generated in the intake manifold.

…

Most of the power gain from forced induction is realized from the improved VE; the denser charge (the fuel and air molecules are closer together) put there is a side benefit—that is the pressure rise in the bore. In a normally aspirated engine when the piston is at bottom dead center, there is atmospheric pressure in the cylinder. Then as the piston sweeps toward top dead center, the area is made smaller and the pressure in the bore rises. This is a result of the compression ratio of the engine (the mathematical change in volume), the atmospheric pressure, and the volume taken by the charge.

When forced induction is applied, the beginning pressure in the cylinder is greater than atmospheric, and is a function of the boost level. Let's apply some numbers to make this clear. We'll assume that the atmospheric pressure is 15 psi and the boost pressure is 5 psi. Then the cylinder will be pressurized to 15 plus 5 or 20 psi before the piston starts to move upward. This higher pressure will then translate to a greater cylinder pressure when the flame expands across the bore and pushes against the piston.


There is no denying that the pressure rise from the forced induction is minimal compared to the gains from overfilling of the cylinder (VE) and the denser charge, but it does make a difference. For this reason, along with the improved VE, a forced-induction engine (especially one with a turbo) feels more powerful even when the boost gauge is reading zero. This is often not the case with a supercharger since it takes power to turn the compressor, and that is coming from the crankshaft.
 
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Thepartsguy

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The engine runs just fine it’s very smooth the slide does hang up I think due to the pressure so I’ll need another cap spring.
 
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