Engine running compression tester.

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wreeve
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Engine running compression tester.

Post by wreeve »

In BMW land we are used to a tool called the rough running test. Basically, what this does is watch the crank-shaft rotating and notice the acceleration caused by each piston's compression stroke. In a perfect engine, the acceleration from each piston will the same – you have a perfect “rough running” result!
Can you see the compression events on the crank sensor with a rotary engine or is it just too smooth? Or is it so sensitive you can see both spark plug events on the crank speed?
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Re: Engine running compression tester.

Post by 350matt »

I doubt this will work on the rotary as in terms of combustion events per crank rotation its the same as a normal V12
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Re: Engine running compression tester.

Post by warpc0il »

This is exactly the mechanism that the 8s PCM uses to identify "misfires".

In some cases it's sensitive enough to determine which rotor isn't pulling its weight.

The causes of such misfires include ignition, fuel or mechanical issues.

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Re: Engine running compression tester.

Post by wreeve »

Works well on the BMW V12. I am wondering if we can monitor the signal from the crank sensor (or the rotary equivalent. I assume it's x number of teeth with one missing). Hook up to the 1st spark plug as well for reference. And with some electronics see if we can determine pseudo compression, and as you say might even diagnose a weak spark (or its associated coil etc).
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Re: Engine running compression tester.

Post by 13Black »

350matt wrote:
Wed Jul 21, 2021 12:06 pm
in terms of combustion events per crank rotation its the same as a normal V12

Not so:

A 13B will have 4x combustion events in 2x crank rotations.

Same as a run-of-the-mill 4 pot.


edit as it's slightly off topic:
it does however mean the combustion events take a loooong time
Yup, 50% longer, one of the reasons why rotaries can make power at high revs compared to piston engines, air has so much more time to get in and out
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Re: Engine running compression tester.

Post by wreeve »

How many teeth/pulses per revolution are on the "crank" shaft/sensor, we can measure the time (very accurately) between those pulses/teeth.
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Re: Engine running compression tester.

Post by 350matt »

13Black wrote:
Wed Jul 21, 2021 4:55 pm
350matt wrote:
Wed Jul 21, 2021 12:06 pm
in terms of combustion events per crank rotation its the same as a normal V12

Not so:

A 13B will have 4x combustion events in 2x crank rotations.

Same as a run-of-the-mill 4 pot.
ah yes you're correct I 'd overlooked the eccentric shaft to rotor gear ratio is 1/3rd as it takes 3 revolutions of the E shaft to complete 1 one rotor revolution
it does however mean the combustion events take a loooong time
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