Rev counter mystery

While still playing with my new 4425 kit (new to this so be gentle) I set up a relative contribution test on a diesel 2.5 VW Crafter. It didn't show a dripping injector but hey I'll keep going onwards and upwards! Can anyone explain simply how a scope gets accurate engine RPM off the alternator output. We also have an MoT emissions tester that does the same by attaching clips to the battery posts'. All it needs to know is how many cylinders the engine has. How does it know the gearing ratio of the drive belt crank to alternator as they are different across manufacturers and models?

I can only assume that the scope is looking for a very slight quickening up of the alternator as each piston is fired down on it's power stroke. Thats why it needs to know the number of cylinders so it can look at an event for each cylinder. Am I in the right area?


Steve

Indeed there are variables there with alternator ripple.

Capture the CKP, Injector 1 current, then add a Maths channel for RPM and it will become clear.

steve-amt wrote:While still playing with my new 4425 kit (new to this so be gentle) I set up .....................................................................................................................................

STC wrote:
Capture the CKP, Injector 1 current, then add a Maths channel for RPM and it will become clear.

Quick Google look up for "Pico RPM Maths Channel"

and this pops up. topic11511.html Good reading and very easy to follow.

I suppose you would have to do some trial and error testing to conclude your own results, but here is a theory;

Suppose you count the number of humps displayed on your scope from the alternator ripple, alter your time base to achieve a good waveform in one revolution, then as there are 60K ms in one minute, divide this by your time base setting that shows one revolution of the alternator, and hence the result is the alternator speed.

example;

60 000 / 63 = 952 RPM.

Just my tuppence worth

HI, Technician,

I was just wondering how the scope knows the crankshaft speed by looking at the alternator voltage across the battery as the ratio between alternator and crank pulley is not always the same. As I have to input the number of cylinders into the scope I am assuming it is looking at a slight speed up of engine speed as each piston is on it's power stroke.

steve-amt wrote:HI, Technician,

I was just wondering how the scope knows the crankshaft speed by looking at the alternator voltage across the battery as the ratio between alternator and crank pulley is not always the same. As I have to input the number of cylinders into the scope I am assuming it is looking at a slight speed up of engine speed as each piston is on it's power stroke.

Hi Steve,

if I try and explain it myself I might not be clear enough, it is a long time ago since my college days , however I found this link, please read it through and see what you think, the author is using a drill to explain basics and a scope to show the results.

http://paulorenato.com/index.php/113

Steve

Clearly I didn't read your post properly when I replied the first time, hence answered the wrong question. Well not entirely! You did mention a dripping injector -Perhaps I latched on to that, a real fault ? I would be surprised if CKP with a RPM Math channel would not highlight an issue in that cylinder.

Let's try again.

Can anyone explain simply how a scope gets accurate engine RPM off the alternator output. We also have an MoT emissions tester that does the same by attaching clips to the battery posts'. All it needs to know is how many cylinders the engine has.
I can only assume that the scope is looking for a very slight quickening up of the alternator as each piston is fired down on it's power stroke. That's why it needs to know the number of cylinders so it can look at an event for each cylinder. Am I in the right area?

I would say you are right on the money, I cannot think of another variable to that theory. Perhaps some one will come along and put us out of our misery.