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January 02, 2004
Hi, Guy and David,
There are varying degrees of symmetry and our engines are not symmetrical
vertically. I also no mathematician, but I am an engineer!
The simplest way I can describe it is to say that:- when the crank is at 90
degrees from TDC, the pistons are not half way down the bore! At this point
in a 1275 motor the piston is at 57% of its stroke. Hence when the crank is
in this position all 4 pistons are at 57% stroke. Compare this when the
crank is at TDC or BDC, you will have 0% stroke on two and 100% on the other
two, making an average of 50% overall. Hence my 7% volume variation.
Conversely when No 1 piston is at 50% stroke, the crank is 82 degrees after
TDC. This phenomenon is accentuated by the length of the con rod, ie the
longer the con rod the nearer to 90 degrees half stroke is.
Concerning pressure build up in the sump, I agree that this is most likely
to be caused through 'blow by' rings and/or guides.
But be aware that on a 4 pot motor, the motion of the pistons moving will
cause a pressure ripple at twice engine crank speed - 2 positive (and 2
negative) pulses per rev. This is why on an otherwise air-tight engine you
can feel pressure pulses if you remove the oil filler cap or some engines
will throw oil out of the dip stick hole if the latter is removed with the
engine running. This is also why you should never seriously overfill your
sump with oil (or water if a head gasket has gone for example) as this could
cause serious aggravation/damage even turning the motor over slowly on the
starter.
Geri.
"G & J Swann-Price" < geri@... > 27980
Posted on January 2, 2004 03:37 PM
Posted to category(s) Cooling System | Cylinder Head | Engine (external) | Engine (internal) | Gauges | Ignition System | Intake System | Lubrication System | Transmission | Tuning
