how air-tight are auto combustion chambers
how air-tight are auto combustion chambers
(OP)
I'm making some calculations regarding compression ratio and wondered if auto-engine combustion chambers are 100% air-tight?
It seems to me that piston rings do a great job but can they really make 100% seals?
It seems to me that piston rings do a great job but can they really make 100% seals?





RE: how air-tight are auto combustion chambers
Cheers
Greg Locock
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RE: how air-tight are auto combustion chambers
RE: how air-tight are auto combustion chambers
Regards
Pat
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RE: how air-tight are auto combustion chambers
There is no such thing as a 100% air-tight recip piston engine cylinder. As others noted there are two areas where gas leakage occurs in the cylinder working space- past the valve seats and past the rings. Leakage past the rings accounts for the largest percentage, while leakage past the valve seats is usually fairly small under normal conditions. Of the leakage past the rings, some leakage is between the ring face and bore surface, some goes past the piston ring land and lower ring face, and some goes through the ring gap space.
The typical static leakdown test and motoring compression pressure test will give a qualitative indication of the cylinder's sealing ability versus an established benchmark. But what really matters is the cylinder sealing ability under dynamic firing conditions. The high temperatures, high pressures, and significant dynamic forces that exist in a firing engine at high rpm can all affect the sealing function of rings and valves.
Here's a good technical reference on piston ring conformability:
http://www.federalmogul.com/korihandbook/en/sectio...
Hope that helps.
Terry
RE: how air-tight are auto combustion chambers
RE: how air-tight are auto combustion chambers
What is an "auto combustion chamber"?
RE: how air-tight are auto combustion chambers
Zero leak down means the rings are butted out. 17% means rings or valve seats or gasket is real bad.
Regards
Pat
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RE: how air-tight are auto combustion chambers
RE: how air-tight are auto combustion chambers
I haven't used "total seal 2 piece rings" but would assume that Dicer is running them in his j.d.m.
RE: how air-tight are auto combustion chambers
The odds of one being retrofitted with gapless rings is about ten zillion to one, but I suppose it is possible but it's no longer JDM.
Regards
Pat
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RE: how air-tight are auto combustion chambers
Due to the high peak combustion pressures and small compressed gas volume at TDC conditions, even an extremely small piston ring leak path can impact engine efficiency.
RE: how air-tight are auto combustion chambers
RE: how air-tight are auto combustion chambers
Also consider how slowly a gas can pass through a "leak" in relation to a piston's cycle time of a running engine. An illustration is the old Falcon (170 c.i., about 8:1 C.R.) that I drove (on all six cylinders) for many tens of thousands of miles with a "dead" cylinder (tester showed over 99% leakdown). Yet the only way to make that cylinder misfire was to pull the plug wire! Of course the cumulative result of the blowby was a noticeable volume of crankcase fumes out the vent (no PCV).
RE: how air-tight are auto combustion chambers
I had a similar run out engine (in an econoline van) that I simply vented the crankcase vent (disconnecting the road draft tube) to a 1 gallon clorox jug, loosely filled with teflon plumbers tape. The tape attracted the oil mist, which eventually formed droplets settled to the bottom of the jug, to be dispensed back into the sump at next oil check interval. In this way, I was able to extend my oil usage from 1 qt/ 50 miles, to 1 qt/ 150 miles.
RE: how air-tight are auto combustion chambers
After I replaced it with a fresh 225, I tore it apart for inspection. The 3-piece oil ring in that bore was worn to where the central expander was riding on the cylinder bore.
Jay Maechtlen
http://www.laserpubs.com/techcomm
RE: how air-tight are auto combustion chambers