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Supercharger question (automotive application)
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Posted by: DeWayne

08/23/2004, 17:58:30

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This is a bit lengthy, but necessary to frame the question. Basically, a reputable supercharger manufacturer has stated that "cooling the air after compression doesn't (by itself) increase power." Specifically I am referring to an automotive engine with a PDM (roots style) supercharger:

 

 "...intercoolers do not "make" more horsepower. They cannot. Supercharger output is FIXED and doesn't increase merely because downstream air charge temperature is lowered. It's the same air by mass. The mass, the weight, the oxygen content of that blast of air discharged by the supercharger cannot be increased. You don't believe it? Try this. Capture the discharged air in a bottle, seal it and weigh it at 300, 200, -50. The weight (mass) won't change, the engine can't ingest more air so it won't make more power. Yes, the cooler denser air, at -50 for example, will "allow" more boost or advanced engine timing but that air, by itself, won't make more power. Now if the air entering the supercharger (ambient) is reduced, it is denser and cooler and then will make more power (1% increase for every 10 temperature drop according to our data)."

 

The question is this... Since the volume below the supercharger is constant prior to the intake valve opening and greater than the volume of a given cylinder (hence why boost stays constant and doesn't drop off and build up everytime a cylinder intake valve opens), doesn't it stand to reason that cooling that compressed volume air will

1) slightly reduce the boost (all else equal) because PV/T = constant? If V is constant and T goes down, then P must go down as well - obviously this assumes the supercharger "stops", the volume of compressed air is trapped and then cooled.

 and

 2) allow for more of the "available" oxygen (by mass) in the fixed volume (between the outlet of the supercharger and the bottom of the intake) to enter the cylinder (also a fixed volume chamber) for combustion?

 

Comments are certainly appreciated, but what I'd really like to see as an answer here is a Thermodynamics control volume solution to the problem. Though I'm a mechanical engineer, it has been well over ten years since I dug out my Advance Fluid Mechanics and Thermodynamics books to reduce this kind of question to a "text book solution." Any insight would be appreciated.







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Re: Supercharger question (automotive application)
Re: Supercharger question (automotive application) -- DeWayne Post Reply Top of thread Forum
Posted by: angelos

08/24/2004, 10:45:22

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The thing you said about PV/T= constant seems  wrong in this occassion.

The only thing that is (can be considered)  constant is the Pressure at the outlet of the Supercharger. It depends  on the superchargers rpm ,on the density of  air at the intake (not at the outlet) and finally on the air flow.

It is like this--- Say you have a bottle ,you continiously blow air in, and periodically open a valve that lets the out. The air that goes out of the valve is the one that fills the cylinder. The mass of air that goes into the cylinder will shlightly increase if the temperature of the bottle is low.

On the other hand  if the air is more hot  the mass  is less. We use intercoolers to avoid these losses, and by this way gain power. I think thats the reason the manufacter said that intercooler doesn't icrease power by itself .  If somehow we could lower the temperature of the air at the intake of the superpower (with no losses of course) then we would gain more power.







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Re: #2689 -- randykimball Post Reply Top of thread Forum
Posted by: Vengeance

08/24/2004, 04:32:43

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As we know, when you cool a charge of air, you increase it's density. This may drop boost pressure, but it will decrease pumping losses. So yes, cooling the air charge after it has been compressed DOES in and of itself increase power, no matter how little it may be; then add in all the other advantages of a cooler air charge...

Also, you don't want to overdo it with the water concept. Water injection is only a band-aid for a (I don't want to say poorly designed, but...) less than optimal system. Using water for an extended period of time, or using too much will eventually start to "clean" the base carbon crystals off of the piston dome, which are necessary for optimal engine performance.(unless ofcourse your pistons are ceramic coated)




"Give me a fulcrum and a place to stand, and I shall move the world." Archimedes


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