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Ring leakage is a recycle leakage between the cylinder ends. The leakage can be over the rings, through the rings, under the rings or around the rings (through the piston).
My comments here are limited to a double acting cylinder with the rings leaking relatively the same in both directions (broken and/or missing rings). Yes, it is possible for rings to leak in one direction and not the other.
As with any internal recycle leakage, at a constant ratio of compression, the cylinder discharge temperature goes up as it is directly related to the gas temperature existing at the suction toe and the compression ratio. The actual discharge temperature will be greater than the theoretical discharge temperature.
I will attempt to explain the leakage characteristics using the attached PV illustration as an example.
When the piston is at the BDC position, there is maximum cylinder volume and low pressure in the head-end and minimum cylinder volume and high pressure in the crank-end. At this condition, gas is leaking from the crank-end into the head-end. This is like a suction leak to the crank-end and a discharge leak to the head-end. This condition continues at a diminishing rate until the differential pressure across the rings equalizes. In our example, this occurred around 10%-cylinder displaced volume. For the rest of the piston travel toward TDC, the pressure is higher in the head-end than the crank-end. This becomes suction leakage to the head-end and discharge leakage to the crank-end. This transition near the pressure reversal point, from discharge to suction leakage, creates a “hump” in the pressure trace. From this point until the second pressure reversal occurs, gas continues to leak from the head end to the crank-end.
As the piston approaches TDC, and if the leakage rate is great enough, there may be a premature pressure drop in the head-end cylinder and a corresponding pressure rise in the crank-end cylinder, creating “rounded suction and discharge toes”. As the piston now moves toward BDC, the head end continues experiencing suction leakage and the crank-end, discharge leakage until the pressure reversal again occurs, in this example, approximately 10% into the stroke (there is that “hump” again). The conditions again reverse and now the head end is experiencing discharge leakage, and the crank-end, suction leakage. This continues back to BDC and again, we see the “rounded toes”.
It's all rather complicated but wait till you have a cylinder with multiple defects. We will save that conversation for a later article.
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