compression ratio

["Walt Philipson" <wphilipson@xxxxxxxxxxx>]

I don't know the specifics of bmw cam timing, but in most motors the intake
valve is open well after bdc on the compression stroke to allow the momentum
of the intake stream to fully charge the combustion chamber. There are also
significant pressure losses in the intake system which lowers the initial
pressure. (That's why spark ignition engines have such terrible thermal
efficiencies at idle). Because of this, it is very difficult to predict what
your cylinder pressures should be.

Walt Philipson

looking for a good E28





Jim,
  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

  I haven't thought in too much detail on how the closing of the intake
valve affects this, but the exhaust valve opening doesn't affect this
because the exhaust valve is closed at TDC when the highest pressure
occurs and doesn't open until afterwards.


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.

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