Anyone remember how long a cylinder is pressurized during a leak down test?
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Re: Leak down test
Just long enough to get a steady reading on the pressure gages so you know what the pressure loss is.
duke -
Re: Leak down test
I apologize if I appear dense but that's just it, the pressure won't be steady on a leak down test, will it? Won't the pressure continuously decay as the cylinders leak (and they will leak some)? I know I appear anal on this topic but I work in an industry where everything is done under tight tolerances and every test has a time limit. I'm afraid this has carried over to my personal life as well, with some positive effects and some negative!!!Comment
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Re: Leak down test
No. using a leakdown tester, makeup air is suplied and what actually is being measured is the % of makeup needed to maintain pressure.Comment
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Que?
Now I'm confused. Duke, Bill, thanks for replying.
I don't have a leak down test rig in front of me for reference.
Maybe this is the key, is the gauge on the rig a flow gauge and not a pressure gauge?Comment
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Fluid Mechanics 101 Lesson 1 - "critical flow"
There are a couple of different ways to do a leak down test. One would be to pull a vacuum or pressurize a cavity and then count the time required to bleed down to some other value. Maybe we can call this a "bleed down test". Commercial engine leakdown testers express the flow rate (leakage) as a "percent', which is really just a pressure difference across a metering orifice. This way we don't have to use a stopwatch.
Usually 100 psi is supplied on the inlet side. As long as the cylinder absolute pressure remains high enough to keep the pressure RATIO (downstream absolute press./upstream absolute press.) BELOW 0.528, the flow through all the tiny leak paths will be sonic and will not increase no matter how high the pressure is in the cylinder. This is a useful property of ideal gases. (Air behaves as an ideal gas at "normal" pressures and temperatures.) This "critical pressure ratio" corresponds to a gage pressure reading in the cylinder of about 13.2 psi. Remember, were taking ABSOLUTE pressure RATIO here (downstream divided by upstream), not gage pressure DIFFERENCE: 14.7/ (14.7+13.2) = .528. The pressure we read on a gage is "gage pressure", which is relative to atmospheric, so absolute pressure is gage pressue plus 14.7, and absolute atmopheric pressure is (sea level "standard")14.7 psi.
Commercial leakdown testers are usually calibrated in percent, and if the input is 100 psi one percent leakdown equals one psi, so the downstream pressure beyond the metering orifice in the gage set will be 95 for five percent leakdown. I prefer just quoting leakdown rates in psi, and you don't need 100 psi to do a leakdown test. As long as cylinder gage pressure reads above 13.2 the pressure difference across the metering orifice will remain the same, so if you lose five psi at 100 psi input the output gage will be 95. Likewise, a 50 psi input will read 45 psi at the output. The wild card here is that the higher the cylinder pressure the more force is behind the rings pressing them against the wall, so higher pressure will usually result in less leakage, and 100 psi input is considered "industry standard" input pressure for cylinder leakdown testing.
The pressure drop across the metering orifice is proportional to the square root of the flow rate, so if you have one cylinder with five percent (or five psi) leakdown and one that is 10 the latter has about 40 percent more leakage.
A bleed down test is what can be done on a cylinder head off the car. You make a plate with a vacuum tube nipple to cover the port and seal it with a gasket or grease. Then use a mightly vac type vacuum pump to pull a vacuum and count the time for the vacuum to bleed down from say 10 to 5 inches Hg. If it takes more than five seconds your valve seats and guides are probably in decent shape. So in this type of "bleed down" test where you start with a fixed vacuum or pressure, the important parameter is how long it takes to bleed down from one value to another. A commercial leakdown tester just uses a pressure drop across a metering orifice as a surrogate for flow or leak rate.
Both tests are valid, but the commercial leak down tester is a snap to use and the time it takes is just a matter of reading a gage, so it's more convenient than doing a bleed down test with a stop watch. One hundred psi input, read the "percent" leakage on the gage and you're done.
The critical pressure ratio is also useful in working with carburetors. Say your engine idles at 16" vacuum. The absolute manifold pressure is 14" and atmospheric is 30", so the absolute pressure ratio is 14/30 = .466. Flow is critical and velocity past the throttle plate is sonic. Flow is controlled only by the area. That's why a 300 HP engine has a nice stable idle speed.
Most SHP engines don't generate enough idle vacuum to achieve critical flow past the throttle blade, so random vacuum variations will vary the air flow, and this is the primary reason for their "lope" at idle. If the idle vacuum is 12", absolute manifold pressure is 18" and the pressure ratio is 0.6, which is above critical, so velocity past the throttle plate is less than sonic and flow will respond to the square of the pressure difference.
DukeComment
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Thanks! *NM*
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