Compression testing using the WPS500 Pressure Transducer

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Compression testing using the WPS500 Pressure Transducer

Postby Steve Smith » Fri Jan 27, 2017 7:00 pm

I wanted to share this one with you thanks to an inquiry received this week via the Help Desk.

Basically, when carrying out compression testing using the WPS500x Pressure Transducer with PicoScope 6 Automotive software, we do not incorporate a compensation feature to allow for the volume of the pressure sensor or compression hose. (Unlike Pico Diagnostics)

Such a feature becomes essential when looking at diesel compression given we add approximately 8 ml of volume to the combustion chamber!

The WPS500x internal volume = 5 ml, Pico Compression hose internal volume = 2 ml, and our dummy glow plug (volume varies depending on dummy glow plug length)

I have added a New Feature Request for the software team to evaluate but for now we can use maths to correct our values if peak pressure is what we are looking for using PicoScope software and not Pico Diagnostics.

If Peak cylinder pressure is all you require from your pressure transducer, then carrying out an Absolute Compression Test using Pico Diagnostics. Here you are only required to remove 1 x spark plug or glow plug to measure the peak compressions from all cylinders.

PD WITHOUT COMPENSATION.jpg
PD WITHOUT COMPENSATION


Included with the Absolute Compression Test is the Compensation feature that will allow for the size of the engine and volume of the measurement tool.

PD WITH COMPENSATION.jpg
PD WITH COMPENSATION


When using PicoScope 6 Automotive software, to compensate for the pressure transducer and compression hose we need to calculate a multiplication factor based upon the cylinder volume, compression ratio and sensor / hose volume.

The captures included in this post were taken from a Vauxhall Astra Diesel, 1.7 Litre, 4 cylinder engine with a compression ratio of 18:1 using a sensor volume of 7 ml (Transducer and hose)

Compensation formula required when using the WPS500x Pressure Transducer to measure peak cylinder pressure with PicoScope 6 Automotive software:

Cylinder displacement / (Compression Ratio -1) = Combustion chamber volume
Combustion chamber volume + Sensor volume / Combustion chamber volume = Multiplication Factor
The Multiplication factor is then used to multiply the pressure results obtained by WPS500x

4 Cylinder engine displacement 1686 cc Compression ratio 18:1
1686 / 4 = 421.50 (Displacement per cylinder)
421.50 cc / (18 -1) = 24.79 cc (Combustion chamber volume)
24.79 cc + 7 cc / 24.79 cc = 1.28
1.28 is the Multiplication factor required to correct peak cylinder pressure.

Multiplication factor x Obtained pressure = Correct Cylinder pressure allowing for the Pressure Transducer and Hose

Math Channel for WPS cylinder pressure captured on channel B
Pressure x Multiplication factor = B X 1.28

MATH CHANNEL.jpg
MATH CHANNEL FORMULA


Temperature will also effect our compensation values and can be incorporated into alternative formulas if required

Here we now have the corrected cylinder pressure value.

PS WITH MATH CHANNEL.jpg
PS WITH AND WITHOUT COMPENSATION


Using the same engine and cylinder above only now with a compression gauge we can see just how challenging cylinder measurements can be as a conventional gauge reads approx. 32.5 bar pressure! Why?

IMG_7248.JPG
CONVENTIONAL GAUGE


Here cylinder pressure is repeatedly captured and “built upon” by numerous rotations of the engine.
A conventional compression gauge adapter will contain a Schrader valve where the pressure is not allowed to escape and so often, we can convince ourselves a compression is good by holding onto the starter motor for prolonged periods to “build” our low compression to a decent compression.

With the Pico Scope software we obtain a clean measurement sweep of the cylinder as the Pico compression hose does not contain a Schrader valve so allowing peak compression to decay during the Expansion Stroke.

Here we can measure the ability of a cylinder to generate sufficient compression from a single stroke only whilst comparing this ability to neighbouring cylinders. Over and above peak compression is the ability of PicoScope to evaluate the four stroke cycle for timing and breathing ability of the engine.

To conclude here, for peak compression of all cylinders (using one cylinder only) use the Pico Diagnostics software with the compensation feature (Absolute Compression Test) and use PicoScope to discover why peak compressions are low based on 4 stroke anomalies.

I hope this helps, take care…….Steve
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Re: Compression testing using the WPS500 Pressure Transducer

Postby STC » Sat Jan 28, 2017 6:22 am

Hi Steve

I have added a New Feature Request for the software team to evaluate but for now we can use maths to correct our values if peak pressure is what we are looking for using PicoScope


In the interim here is a MS Excel Spread sheet I have tapped up. Hope it can fill the gap.
Compression Variation.xlsx
(10.99 KiB) Downloaded 130 times

(it is not locked so improve / alter it as you wish)

Hope it helps !!!
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Re: Compression testing using the WPS500 Pressure Transducer

Postby Steve Smith » Sat Jan 28, 2017 11:34 am

Nice work STC thank you.

I will keep this to hand myself and no doubt this will help many others too.

Take care.......Steve
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Re: Compression testing using the WPS500 Pressure Transducer

Postby volrem » Tue Aug 15, 2017 1:36 pm

Steve Smith wrote:The WPS500x internal volume = 5 ml,



Steve,


Where does this value come from? So far I have used 1,5 ml in my calculations. I got this value using syringe method.

Can you confirm that 5ml is indeed correct and therefore it's not possible to measure it correctly with syringe?

I rechecked it and still got volume of 1,5 to 2 ml.
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Re: Compression testing using the WPS500 Pressure Transducer

Postby STC » Tue Aug 15, 2017 4:15 pm

Hi Volrem

I saw that too some time ago.

The WPS500x internal volume = 5 ml, Pico Compression hose internal volume = 2 ml


Steve may know something that we don't ... but I thought that was a typo.

Perhaps that should read The WPS500x internal volume = 2 ml, Pico Compression hose internal volume = 5 ml ? I'm not certain on that neither am I able to measure that right now. Perhaps you could confirm or correct me.

Thanks
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