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?

what about the valve seats?

Cheers

Greg Locock


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about 2% of the mass flow into the cylinder exits past the piston rings.

There is a test called a leak down test that measures exactly how much leaks past under test condition. In a running engine the amount varies greatly with test conditions and the condition of the engine. Oil and temperature, rpm and especially wear all play a part.

Regards
Pat
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markhutch,

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

Thank you to all the responders. Great info. Now you've really got me going I'll probably be back with more questions! I can hear you all groan!! ;)

I got a JDM engine, and did a leak test on it. 2 cylinders were 0% leakage, and made no sound what so ever. 1st time for that one ever. Other 2 were 5% and 17%.
What is an "auto combustion chamber"?

I would have thought an auto combustion chamber is one in an engine for an automobile.

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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Dicer: sorry should have been more clear - by "auto combustion chamber" I meant a combustion chamber in a regular vehicle's engine.

Quote (patprimmer)

Zero leak down means the rings are butted out unquote



I haven't used "total seal 2 piece rings" but would assume that Dicer is running them in his j.d.m.

A JDM engine is Japanese Domestic Market. It is a term used in the second hand engine industry to identify engines sold OEM in Japan as JDM spect are often higher power output than engines for other markets. Once it is rebuilt to non OEM spec it is no longer truly JDM.

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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Even so-called (1 or 2 piece) "gapless" ring designs still have leak paths. It's just that the cross section area is reduced and the length of the leak path is greatly increased and more convoluted. This combination reduces the leakage flow rate by increasing the flow resistance.

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.

I'm not worried about it. 17 isn't as bad as 40 or 60, I've seen worse. Not valve leakage. I'll keep ya all posted on how it runs.

FYI- The "percentage" read by a leakdown tester is not conveniently translated into more precise terms (such as a flow rate of the "leak")- except at 0% and 50% and 100%. 0% and 100% are obvious. At 50% the "leak" flows indentically to the tester's internal orifice. The industry standard orifice is .040" diameter. So... although 50% leakdown is generally thought of as a trashed cylinder, the "leak" is equivalent to a .040" hole; as you ponder this, keep in mind the typical dimensions of ring end gaps.

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).

"Of course the cumulative result of the blowby was a noticeable volume of crankcase fumes out the vent (no PCV)"

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.

I had a Plymouth 170 cid slant six- if I didn't clean number five spark plug often enough it would pack up solid. (catch it soon enough, could just scrape the crud out and keep running it.) As long as it had a serviceable plug in that cyl, it ran fine.
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

The JDM is up and running, runs good and strong.