With the introduction of the WPS500x, we’ve elevated cylinder pressure analysis, gaining insight into crucial metrics like positive/negative cylinder pressures, valve open/close events, and valve duration. We’ve discovered that to achieve accurate peak cylinder compression values using a pressure transducer with PicoScope, the internal volume of the sensor must be compensated for. This guide revisits the core mathematics involved in this essential compensation process.
The following forum post has this covered: topic16131.html
Understanding Sensor Volume and Compensation #
The internal volume of our pressure sensor (sensor volume) must be considered and compensated for if we require accurate peak cylinder compression values using the pressure transducer.
Therefore, I wanted to revisit the maths involved regarding Compensation when measuring peak cylinder pressure with a pressure transducer using PicoScope (Not Pico Diagnostics).
Here is another occasion where our users are right and my theory was wrong. (Not for the first time either)
I have (in the past) incorrectly assumed our pressure transducer to have an internal volume of 5 ml, but thanks to PicoKev, Volrem and others, the internal volume of the pressure transducer alone is approx. 1.22 ml.
If we then add our compression hose to the pressure transducer (with an approximate internal volume of 2 ml), we have a total of approx. 3.22 ml.
Add to this the internal volume of a compression hose adapter (for spark plug replication) and we have an average of 5 ml to cover the internal volume of the transducer, compression hose and spark plug adapter.
This is a “one size fits all” sensor volume measurement for petrol engine compression testing.
Remember this will increase dramatically when we add a dummy glow plug (diesel compression testing), which can vary in size and so volume.
Basically, what I am trying to say here is:
For petrol engine peak compression measurements via the pressure transducer, use 5 ml as your sensor volume value when applying the relevant math channel.
For diesel engine peak compression measurements via the pressure transducer, use 5 ml + the volume of your dummy glow plug as your sensor volume value when applying the relevant math channel.
To quote now from forum post topic16131.html
Calculating the Multiplication Factor #
When using PicoScope 7 Automotive software to measure peak cylinder pressure with a pressure transducer coupled to a compression hose (internal volume 5 ml), we need to calculate a multiplication factor based upon the cylinder volume, compression ratio and sensor volume value.
Here we have a Vauxhall Astra Diesel, 1.7 Litre, 4 cylinder engine with a compression ratio of 18:1, using a sensor volume value of 7 ml (Transducer and compression hose 5 ml + 2 ml for our dummy glow plug).
Below is the compensation formula required when using the WPS500x Pressure Transducer to measure peak cylinder pressure with PicoScope 7 Automotive software:
Cylinder displacement / (Compression Ratio -1) = Combustion chamber volume
Combustion chamber volume + Sensor volume value / 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 cc Displacement per cylinder
- 421.50 cc / (18 -1) = 24.79 cc Combustion chamber volume
- 24.79 cc + 7 ml / 24.79 cc = 1.28
- 1.28 is the Multiplication factor required to correct peak cylinder pressure.
Multiplication factor x Obtained pressure = Correct peak cylinder pressure allowing for the internal volume of the Pressure Transducer, compression hose and dummy glow plug.
Math Channel for WPS cylinder pressure captured on channel B
Pressure x Multiplication factor = B X 1.28.
Below, we now have the corrected peak cylinder pressure value.
Be aware that temperature will also affect our compensation values and so temperature can be incorporated into alternative formulas if required.
Given that the maximum operating pressure of the WPS500x is 34.5 bar (500 psi), there may be times when diesel engine cylinder pressure will exceed this value (Especially if the injector remains connected during the compression test!).
Whilst the following does not apply to all engines, we could take advantage of pressure transducers that are integrated into glow plugs.
Now we can measure peak cylinder pressure with combustion across all engine speeds and loads without intrusion and without increasing the volume of the combustion chamber.
Creating the Math channel for pressure transducer analysis #
The following formula applies to the pressure transducer incorporated into the glow plug of a current VAG TDI engine.
With thanks to VAG regarding an approximation of the specifications for this sensor, we can create a math channel that converts the measured voltage from the glow plug (pressure transducer) into a pressure value (bar).
The formula required:
Pressure = (Sensor Measured Voltage – Nominal Sensor Voltage) / Sensor Incline or slope.
Sensor measured voltage = Voltage output from sensor in proportion to cylinder pressure.
Nominal sensor voltage = Voltage output from sensor at ignition on engine off (0 bar), Approx. 0.575 V.
Sensor incline or slope refers to the characteristic output of the sensor across its entire operation range (0-210 bar) in response to cylinder pressure (1 V = Approx. 55.555 bar).
Think of this value as similar to a current clamp with a specified characteristic output of 1 mV/A. For every mV output of the current clamp, the scope display reads 1 A, therefore, a 1 V output from the clamp (1000 mA), the scope display reads 1000 A.
Be aware that the sensor incline (or slope) is a rounded approximation that depends on the pressure sensor supply voltage (Ratiometric). Here, we assume the sensor is supplied with 5 V, but in the real world, this could deviate from 4.8 V to 5.2 V, etc., and this most certainly impacts the behavior of the pressure sensor across its operating range. Consider the pressure values obtained from your math channel as relative cylinder pressure.
The math channel (assuming your glow plug pressure sensor is connected to channel A) is (A-0.575)*55.555.
The waveform below highlights the dramatic increase in cylinder pressure under the acceleration of the engine. This certainly brings home the events taking place inside the cylinder during combustion and reveals the stresses these engines are placed under during a typical driving cycle.
Please be aware of the limitations of the glow plug pressure sensor at low engine speeds/pressures, given that the sensor is designed to measure pressure up to 210 bar. The resolution is also questionable, but certainly invaluable for an insight into combustion events across the entire engine speed/load range.
The psdata file below contains the relevant math channel along with cylinders 2 and 4 pressure waveforms. These can be revealed by right-clicking on the screen and selecting Channels.