Re: Big 6 compression

[Brian Mentiplay <bmentiplay@xxxxxxxxxxxxxxx>]

For the edification of whoever cares, can anybody out there describe the
relation between compression-test results and compression ratio more
precisely?  I also am not sure the valve timing has anything much to do
with it, unless the intake valves close significantly before the piston
reaches BDC -- do they?

The estimate

       P    = C.R. * 1 atm
        TDC

depends, among other things, on the assumption that the compression is
isothermal; but if a fluid is compressed quickly (?), the compression will
tend to be _adiabatic_, and obey the relation

            k
	P * V  = constant

where k is a different constant for different fluids; for air, it's 1.4.
But that doesn't provide the whole story; working that calculation for the
10:1-C.R. M42 yields the result

	P    = 25 atm = 369 psi
	 TDC

which is obviously ridiculous (The Bentley E36 manual specifies 142-156,
averaging 149 which happens to be pretty close to atmospheric pressure
multiplied by the 10:1 compression ratio).  Maybe the air is heated to
"engine temperature" by the time it gets to the intake port, then
compression heats it up more but that heat is lost (yes, lost) through
contact with the cylinder walls?

All this assumes also that the pressure at BDC is 1 atm; in general it will
be somewhat less, particularly if the intake valves close significantly
before BDC.


>  My thinking is based around Boyle's law (at least I think it was
>Boyle!) which says that:
>
>     P   *  V          P  *  V
>      1      1          2     2
>   ------------  ==  -----------
>        T                 T
>         1                 2
>
>  So assuming adiabatic compression, the temperature doesn't change and we
>get: P*V=P*V.  The volume ratios get plugged in and we're left with (for a
>10.5 to 1 engine):
>
>     P *1 = P *10.5
>      1      2

If the temperature doesn't change, it's isothermal; adiabatic means with no
loss or gain of heat.

Sorry to be long-winded, especially since I don't have the answer.  Maybe
someone else does?

Incidentally, the original question was, was a particular compression-test
result appropriate -- wasn't the answer in the service manual? :-)



Brian Mentiplay
1994 318is

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