Variable Compression Explaination - MATH ALERT

[Frederic Breitwieser]

I volunteer to take a stab at this, even though I'm not a mechanical
engineer, but learning this stuff just as you are.

Lets begin.  Here are some mechanical constants:

Displacement = the amount of "stuff" your engine can hold (volume), if all
pistons were down simultaniously, measured at sea level.
Bore = The diameter of each individual cylinder
Stroke = the distance the top of the piston travels from Top Dead Center
(TDC) to Bottom Dead Center (BDC)
Compression Ratio (CR) = the ratio of volume from TDC to BDC.

Here is an example: 4.1L Buick Engine with OEM heads
Bore: 3.965 inches diameter
Stroke: 3.4 inches length
	max displacement = Bore Radius^2 * Stroke * PI
	cid = (3.965/2)^2 * 3.4 * 3.141592654
	cid = 251.89 cubic inches (or 4127.68 cubic centimeters, if you like metric)
The engine has a 9:1 compression ratio, which means:
	cidmin= cidmax / CR
	cidmin= 251.89 / 9
	cidmin = 27.99 cubic inch (or 458.63 cubic centimeters).

The 4.1L engine (for example) is a 6-cyl engine, therefore each cylinder
can hold 42 cubic inches and compresses it to 4.66 cubic inches (all I did
was divide the cidmax and cidmin values by the number of cylinders - 6, and
rounded quite rudely)

The above are constants, because cranks, rods, piston tops, head chambers
and cylinder walls, for all practical purposes, don't change (unless you
blow a gasket or bend something, but that's another issue altogether).
Since the chambers are solid, they are declared absolute, and you can
adjust what you can change.

This is where the confusion might be coming in - this is at sea level,
where the air pressure is at a certain level.  When you drive your car up a
mountain, there is less pressure, therefore less air takes up the same
amount of physical space.  When you drive below sea level, more air takes
up the same amount of physical space.  This is all due to gravity.  The
closer you are to the center of the earth, the more everything weighs, and
the more dense everything becomes.  Air being a very expansive "thing", it
compresses quite nicely, and expands equally nice based on height.  This is
why the atmosphere is so light at 20,000 feet, pretty much unbreathable.

This, in essense, is how a turbo/supercharger works.  By increasing the air
pressure, through a turbine, you are basically making your engine
"experience" a below sea level pressure situation, which results in more
air molecules within the same amount of space in your engine.  This is why
the term "boost" comes to play. 10 PSI of boost, is that amount over
ambient atmosphere.

I hope that explaination helped out, I admit its rather crude.

Now, about Volumetric Effeciency...
Most OEM engines sit at about 85% volumetric effiecency, this is determined
by the amount of "stuff" the engine "can" displace (in my example it would
be 252 cubic inches), versus how much it really does displace.  Engines
with high-overlapping cams, tend to have both valves open at certain
critical points, therefore are less efficient, mathematically wise.  The
exploded, highly compressed air, bleeds back into the intake manifold, thus
slowing down the air going into the engine.  Nature of the beast.  The
higher the VE, the more effecient the engine will be, and the more power
you will have, though the trade off is more hydrocarbons and of course,
more heat.

Turbo's and superchargers increase volumetric efficency by changing the
atmospheric pressure reference, from sea level (or wherever you are) and
moving it lower, below sea level at its rated amount.  Since everything is
measured from sea level (or where you are in relationship to sea level),
adding 10PSI will increase your engine's volumetric effeciency because
"more stuff goes in" on each stroke, into the same physical volume, thus
increasing your volumetric effeciency past 100%.

Mathematically, there really is no limit to the amount of boost, power, and
VE you can achieve with an engine, however their are obvious mechanical
limits.  Mathematically, you could simulate a 1 liter engine with 2000PSI
of boost, which would make a significant amount of horspower.   Assuming we
had structures that could hold and operate under that pressure (not this
year <G>).

Piston tops, head gaskets, cylinder walls, valve seals all have maximum PSI
ratings, the more pressure that develops within and engine, whether by
increasing the "stuff", or increasing the compression ratio, or increasing
the explosion power, all can reach the mechanical limitations of these
parts, and the result is a nice, big ka-boom.

I did all these concept drafts without taking into consideration intake air
temperature, as well as fuel quality.  Poor fuel obviously will detonate
without a spark plug, and we all know that warmer air takes up more space
than cooler air.  This is why intercooling, either by an air-air exchange
or an air-water exchange is almost a requirement for any serious turbo
vehicle - it keeps the temperature down, while still allowing the increase
in volumetric effeciency.

Personally, I'm in the same boat you are in, trying to understand and
comprehend all of these parameters.  I've been reading and studying two
books in particular, Corky Bell's Turbo book, as well as Jeff Hartmann's
Fuel Injection book.  Between the two of them (total cost of about $35),
you have a wealth of information, math formulas, etc, to take the above
much further than I have in this forum.  

And from practical experience, I can tell you overdoing the VE (or boost)
of an engine can be a dangerous thing.  I've been building a FWD 3.8L GM
block, attempting to breach a very high HP goal, and discovered on that
particular block, the bottom end can handle exactly 719 HP and 629 ft/lbs
of torque before the crank, rods, and pistons become part of the cement
floor.  The beauty of a dyno with safety gear.  Oh, I ran it without head
gaskets which is why they didn't blow :)

Happy motoring.