This guide, written by our expert Steve Smith, demonstrates how to use PicoScope’s powerful math channels to analyze complex engine data. We’ll walk you through a practical example of how to calculate engine speed (RPM) from a simple ignition trigger signal and even analyze water pump speed using a pressure transducer. This method provides a clear, visual way to monitor engine performance that goes beyond simple readings.
Your guide to calculating RPM #
Stop guessing and start analyzing. We’ll show you how to apply a simple math formula in PicoScope to instantly convert a captured ignition signal into a real-time RPM waveform. This makes it easy to diagnose issues related to engine speed and provides a clear picture of engine behavior under different loads.
There are times when I could kick myself for not obtaining the crank sensor signal during a typical engine capture!
Whilst we may not be looking for a crank sensor-related fault, it just helps to know engine speed (https://www.picoauto.com/support/topic11511.html) during waveform analysis to determine if the vehicle was accelerating/decelerating
We can, however, use other inputs and here is a typical example using an ignition trigger signal (IGT) from number 1 Coil on Plug unit
Below, we have the WPS500 pressure transducer connected to the coolant expansion bottle to detect a change in pressure attributed to water pump impeller activity. (Confirming the water pump impeller is rotating)
While the capture above appears cluttered, there are a number of software features in PicoScope 7 that can be used to improve the view of the data we require.
When you download the .psdata file below, we have Channel B turned off. You can turn on the Channel by clicking the Channel B button to open it up and turn it on.
Close the Notes and Channel Labels to increase the size of your scope grid.
Click on the input ranges (V, Bar, etc.) and slide each waveform to a chosen position.
Use the Channel Option button to apply a “Lowpass” filter to noisy channels. (Channel A below has an 8 kHz lowpass filter applied.)
Now for the maths:
To obtain engine rpm from the IGT signal (Channel A), we need to know the number of cylinder 1 IGT events per engine revolution. As the number 1 IGT signal is present once every 2 rotations of the crankshaft, we represent this as:
1 (IGT event) / 2 (Engine revolutions) = 0.5
We now need to convert “cycles per second” (Hz) to “cycles per minute” (RPM). One formula to remember here is: Frequency (Hz) x 60 = RPM, and likewise: RPM / 60 = Frequency (Hz).
So how does all this fit together in a math channel?
The formula required to obtain engine rpm from channel A is 60/0.5*freq(A) and can be easily installed in your PS7 application by downloading the psdata file and saving the Math channels.
After saving each math channel, you can then open them up and edit names, colors, check the equations, etc.
Here we have graphed engine speed from 678 rpm through to a maximum of 5808 rpm.
Now that we have engine speed, if we know the relationship between the crankshaft and water pump pulleys, we can obtain the water pump speed.
Example:
- Crankshaft pulley: 175 mm
- Water pump pulley: 145 mm
- Using the formula “Driven / Driver”: 145 mm/175 mm = 0.83:1
- 0.83 rotations of the crankshaft to 1 rotation of the water pump
- Engine at 1000 rpm / 0.83 = Water pump 1204 rpm
We can now create another Math channel to graph the water pump speed. The formula is the same as engine speed, but now we include the water pump speed ratio: 60/0.5/0.83*freq(A)
I have included the .psdata file below with the relevant Math channels. Remember to use a lowpass filter on Channel A set to 8 kHz.
Test 8 IGT IGN water pump.psdata
Note: the psdata will have Channel B turned off, you can simply turn it on to see the data
Food for thought here: an injector signal could also be used to graph rpm, but remember that during overrun, the injector signal will be cut and so there will be no data for the math channel calculation. Also, with direct injection vehicles, we have the added complexities of variable injection strategies too, either during the induction or compression events and often during the same waveform time span.