Miata Mailing List: February 1997, Message #329
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   From: (none)

Subject: Re: Normally aspirated power

   Date: (none)



> This brings up an interesting question -- without *charging, what's the limit

> as far as hp modifications go?  ie, port and polish, headers, CAI, Exhaust,

> throttle body etc.  Could any combination of the aforementiond (plus anything

> else) add up to one supercharger?

> 

> Just curious

> 

> James Leasure

> 96 M

> Fairfax, VA





OK, Jim you asked for it.



First let me define the term "volumetric efficiency" (VE) as the ratio

of the actual volume of air which enters a cylinder divided by the

cilinder's displacement.  This term applies to engines, compressors,

turbines, pumps and similar machines.  In a perfect engine with a

displacement of, say, 1.6 liter, for each two revolutions of the

crankshaft 1.6 liters of air at 14.7 psia would be drawn into the

engine's cylinders, and it would have a VE of 100%.



In the real world there are lots of factors which reduce the VE of our

engines to (if memory serves me) something like 75 - 85%.  And it varies

with the speed of the engine.  The greater the pressure drop from intake

to cylinder, the faster the air will flow, but that means that the

pressure in the cylinder is below atmospheric and the density of the air

in there will be lower.  This means less fuel can be burned.  (And you

know what that means.)  Notice, if you will, that the emphasis is on

getting air into the cylinders, because this is what determines how much

fuel can be burned and hence how much power is produced.  (Putting more

fuel in the cylinders is easy; putting more air in is not.)



Now, volumetric efficiency can be improved by all of the means you

mentioned.  But it is impossible to get to 100%; I don't know what the

practical limit is, but each incremental increase comes at greater and

greater cost.  (Law of diminishing returns.)



This is what makes forced induction so attractive.  You can essentially

ignore the VE of the engine by pumping more air through the system. 

Let's say our 1.6 liter engine is turning 7,200 rpm with a VE of 80%. 

Then it would be sucking in about 162 CFM of air.  And let's say that

this air is about 100°F.  This means it would be inhaling 11.5 lbs. of

air per minute.  Now lets say we improved the VE to 85%.  Then the 11.5

lbs/min becomes 12.2 lbs./min.  But if we added a

turbocharger/intercooler which gives us a 10 psi boost and taking into

account a temperature increase of, say, 75°F., that 11.5 lbs/min.

becomes 17.1 (and the 12.2 would become 18.1)  This is actually a little

high because the increased backpressure caused by the turbo reduces the

effective VE somewhat.)



Another point,  Flow of gases (or liquids) in a system is caused by a

pressure difference.  Since the intake system in a naturally aspirated

engine has only atmospheric pressure (14.7 psia) as its supply pressure

and the exhaust system operates with much higher exhausting cylinder

pressure as its supply pressure, improvements which reduce the pressure

drop in the intake tract have a greater impact on performance than

improvements in the exhaust system.  I hesitate to say this, but you

can't suck as much as you can blow.  (Boy, that ought to draw some

fire.)



OK, now everybody wake up.  Class is dismissed.



If anyone out there knows for sure what the typical range of VEs for an

automobile engine is let me know; it will save me a trip to the library

to satisfy my curiosity.





Regards,



Bill Bowser

Black & Red '93 LE

Cincinnati, Ohio



If your car gets struck by lightning its and accident;

all other mishaps are the result of driver error.  Think about it.