At 16:47 +1000 12/10/97, Egil Kvaleberg wrote:
>
>Problem with your theory is that you seem to get a higher pressure from
>the compression reading than you get by multiplying the atmospheric
>pressure by the compression ratio. For instance, my 10:1 compression
>engine gives a reading of around 180 PSI.
>
There are at least two factors which would account for this discrepency.
The compression of a gas will increase its temperature and therefore its
pressure in a fixed volume. There will also be some heat transfer from the
engine block if it is warm (above ambient) temperature.
I do not know which is the more significant factor, but I would suspect the
gas compression is more important than heat transfer (which would need more
time). Compression testing a cold engine would eliminate the heat tranfer
(in fact, there may be some heat transfer from the gas to the block) and
should produce an result which is closer to that calculated from the
compression ratio and local atmospheric pressure.
Also keep in mind that a compression gauge reads pressure above atmospheric
and not absolute pressure. So in the example above, gas (fuel/air mixture)
at about 15psi (0+15) is drawn into the cylinder and compressed to about
195psi (180+15).
If you want to measure a compression ratio, it would be best to use a
liquid to measure the volume at BDC and TDC. (I am not suggesting that it
would be easy, as leaks around the rings, etc could cause misleading
measurements). Is there an easier method?
Trevor Jordan
74 TR6 CF29281U
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