How to calculate cranking rpm from a relative compression waveform?

Joe Diagnostician New Jersey Posted   Latest  

I know it is possible to calculate rpm from a cranking waveform. I just haven't been able to figure it out. Is there someone who will shed some light ?

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Stephen Technician
Tennessee
Stephen Default
 

Use cursors or draw lines that start at a point, like the peak of a cylinder draw and end at the same point on the next time that cylinder peaks. That is 2 revolutions(720*). Measure the time between cursors in seconds, from the scope’s horizontal scale. Multiply that by 1800 to get RPM. Pico has rotation rulers that will readout the RPM in the ruler data “box”.

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Dmitriy Analyst
Ontario
Dmitriy Default
 

Stephen, something is not quite right with this calculation. You have that the longer the time between the cursors, the higher the RPM, but it should be the other way around...

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Stephen Technician
Tennessee
Stephen Default
 

Thanks for catching that. You are correct. I should have said divide 1800 by the seconds for 2 revolutions to get RPM.

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Geoff Diagnostician
Hawaii
Geoff Default
 

Multiply....divide.....you had a 50/50 shot Steve! ;-) Now I will go get a pencil and paper and derive the "magic 1800" for fun....LOL

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Stephen Technician
Tennessee
Stephen Default
 

I’ve got a mind like a steel trap. Unfortunately the auto-correct feature is slow to free incorrect information from the trap. About 1:AM, I woke to the realization that 1800 would give rev per HOUR. Dividing 30 by the seconds for 2 revolutions would give RPM. I said to mark out 720*(2 rev) because that is the most common way that cursors are applied to in-cylinder waveforms. Then tripped over…

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Dmitriy Analyst
Ontario
Dmitriy Default
 

Still no go... For example, take 1 revolution per second. That's 2 revolutions per 2 seconds, so the formula now gives 1800 / 2 = 900 RPM. However, 1 revolution per second is 60 revolutions per minute, or 60 RPM. A very quick way to check if a formula is misremembered! Let's see what Geoff comes up with, he still has an hour till the sunset.

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Stephen Technician
Tennessee
Stephen Default
 

Thanks for your support. I finally got mathematical assistance(calculator) to figure out where I was wrong. Turned out it was multiple places. But I now know that dividing 120 by the time of 2 revolutions gives RPM. I also know not to do formulas in my head.

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Dmitriy Analyst
Ontario
Dmitriy Default
 

Hurray, Stephen, 120 divided by Time2 checks out! I am interested in methods or tricks for "calculations under pressure". What works in the office chair may go sideways in less comfortable conditions... I typically split my calculations into a series of steps, with a "guard rail" check for each of them. Going through a real-world example like we did is invaluable for getting a sense for what…

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Dmitriy Analyst
Ontario
Dmitriy Default
   

Joe, these methods should be easy enough to adapt to cranking waveforms, as long as the same cylinder can be reliably identfied: diag​.​net/msg/m3t0a79qvr…

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Justin Mobile Technician
Utah
Justin Default
 

What scope are you using? If you have a pico this is as simple as placing two rulers.

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Stephen Technician
Tennessee
Stephen Default
 

I apologize for giving you incorrect information. I appreciate Dmitriy catching it and letting you & I know.

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Rick Technical Support Specialist
Missouri
Rick Default
   

I'm lost? Can you put the cursers at the center of two peaks? Two Compression points of the same cylinder. Take that time divide by 60,000 then divide that by two? I'm on the injured reserve list so I cannot test this thought? Thinking of the old fuel pump rpm amp reading formula. I have to go look it up.

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Paul Mechanic
Illinois
Paul Default
 

I believe you would multiply your solution by 2 because the crank goes around twice for every four cycles. Curious as to why you need to drive cranking speed from your scope shots?

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Paul Mechanic
Illinois
Paul Default
 

I have had the pico so long, I haven't needed to figure rpm out from Delta time in years, the scope always calculates it and frequency when ever the cursors are used.

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