Compression Testers and Schrader Valve Location 6

[tc7]

This question regards engine compression testers and the effect Schrader valve location has on accuracy of compression readings.

Background: typical automotive shop style compression testers consist of a pressure gauge attached to a flexible tube (typical ~16-inches long) with an adapter at its opposite end which threads into the spark plug hole. Higher quality gauge brands such as Snap-on or MAC have a Schrader valve located in the adapter end of the tube closest to the spark plug hole while other lower cost models have the valve located at the opposite end of the tube, adjacent to the gauge.
Does this difference of valve position make any difference in the accuracy of compression readings?

Some professional mechanics would argue that if the Schrader valve is located at the gauge, the entire volume of the 16-inch long adapter tube adds to the combustion chamber volume and would reduce the resulting compression readings (for the sake of this question let's say that simple compression ratio of an engine is based on the relationship, CR = (Vd + Vc)/Vc where Vd = displacement volume, the volume inside the cylinder displaced by the piston from the beginning of the compression stroke to the end of the stroke and Vc = clearance volume, the volume of the space in the cylinder left at the end of the compression stroke.) So any volume added by the flexible tube connected at the spark pug hole would accrue to the Vc term and somewhat diminish the overall CR and reduce gauge readings accordingly; this would be much more pronounced on small 2-stroke engines.

Consequently they (i.e., mechanics and techs) declare that when the Schrader valve is located at the spark plug end of the tube, no additional volume is added to the clearance volume and compression readings will be most accurate. This makes no sense to me. My contention is that when the engine is cranked over and pressure builds sufficiently to lift the Schrader valve, (at ~3-4psi) then the pressure pathway is open to the flexible tube and the tube volume then necessarily becomes part of the clearance volume just the same as mentioned above, so there should be NO difference in gauge readings regardless of where the Schrader valve is located.

Hoping for truth-tellers to offer expert opinions, technical dissertations and/or criticisms. All input gladly invited and thank you very much.

 

[tc7]

Very nice explanation SwinneyGG, thank you for posting.

 

[gruntguru]

Positive displacement compressors are designed for clearance volume as close as possible to zero. Any compressed gas in the clearance space at TDC will expand on the intake stroke effectively delaying start of intake until expanded to intake pressure.

je suis charlie

 

[3DDave]

Each cylinder of a 2 liter 4 cylinder is about 500 ccs. A hose ID of ID 3 mm X 500mm is 3.5ccs; added to the original 500 ccs the resulting ratio of running compression to measured pressure is 503/500, less than a 1% error due to the tubing when the valve is at the far end of the tubing from the plug port. A one meter hose takes that to slightly more than 1%.

This only applies to conventional valve timing engines, not those with delayed intake valve closing.

 

[GregLocock]

Bear in mind that the hose is not rigid.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[3DDave]

Yeah, you can practically blow them up like a balloon animal. /s

 

[SwinnyGG]

Each cylinder of a 2 liter 4 cylinder is about 500 ccs. A hose ID of ID 3 mm X 500mm is 3.5ccs; added to the original 500 ccs the resulting ratio of running compression to measured pressure is 503/500

It's more complex than that.

Typical combustion chamber volume for a 2.0 liter 4 cylinder is 50cc or so.

Assume a perfect flat top piston to make the math easy. For a 500c displacement cylinder:

CR = (500+50)/50

CR = 11:1

Now add the hose:

CR = (500+53.5)/53.5

CR = 10.3:1

7 tenths of a point in compression ratio is a huge change.

11*14.7 = 161.7 psi

10.3*14.7 = 151.4 psi

10 psi is significant.

 

[Compositepro]

The 14.7 in the above calculation is in units of absolute pressure, psia. So the answer is in psia.

The pressure gauge reads in psig so the gauge readings will actually be 147 psig and 136.7, respectively. Details are important in engineering. Like, upon which end of the hose should the valve be placed?

 

[SwinnyGG]

Details are important... like how I never specified that those calculated pressures were gauge readings.

Anyway I think we all get the point by now.

 

[3DDave]

Sure - do what I originally thought but then f'd it up. /s

 

[gruntguru]

Wow. All that maths.

Clearance volume = 50cc.
Hose adds 3.5cc. (7%)
Reading changes about 7%.

je suis charlie

 

[3DDave]

The nice thing about the math is that it can be applied in other cases than just the one and the full explanation shows why it makes a difference.