ra12r
Zone Head
Posts: 919
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posted March 11, 2004 07:59 AM
Compression Ratio vs Engine Temp
I would like to further understand how cylinder compression is affected by engine temp. I an thinking that cylinder compression is a constant regardless of engine temperature.
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BA
Pro
Posts: 1592
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posted March 11, 2004 09:33 AM
 Edited By: BA on 12 Mar 2004 15:58
Hey man, I think you're right that the compression is a constant, but I wonder if you are really wondering about 'pressure'. ie. the effects of engine heat upon the incoming air/fuel charge and it's "density", thus it's pressure on the compression stroke.
Seems that the cooler, denser charge would bring more 'pressure' and thus the slight increase in power we all see in cool weather.
I'd expect the reverse to be true in hot weather, with less oxygen density.
maybe I'm just over-reading, but otherwise, I'd say your statement is correct.
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ra12r
Zone Head
Posts: 919
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posted March 16, 2004 06:36 AM
Well my interest is concerning hot-starting. The references to compression ratio and hard starting should be consistent. But it is not, the motors can start cold but not hot....hmmm So i will just ask the simple question. Why is it hard to start a hot motor??? Because unless the characteristics of compression changes with the temp of the motor, the compression is not the reason...
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engineermike
Expert Class
Posts: 101
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posted March 24, 2004 07:17 PM
I believe there are a couple of things working against each other here.
1. The starter, as any electrical component, doesn't have as much torque output when it's hot.
Couple that with:
2. When the motor is hot, the pistons are physically larger and fill the bore better. Furthermore, the rings are circumferentially longer, so the end-gap is less. They combine to help piston sealing, so compression is higher.
Mike
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'04 Orange 10R
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