exactly how rough is a rough idle?

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exactly how rough is a rough idle?

Postby hillp » Mon Mar 10, 2014 4:44 pm

What kind of measurement would one do to quantify how rough an idle is? Some kind of measure of rpm variability? Would a spectrum analysis (I've never done one) of some sort on the primary or secondary ignition do it?
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Re: exactly how rough is a rough idle?

Postby Steve Smith » Wed Mar 12, 2014 11:11 am

Hello and thank you for your enquiry and sorry for my late reply.

RPM variability is a possible quick non intrusive test with an allowance made for finite idle control and fluctuations. This test would depend differ depending on fuel types

Speaking from a personal point of view a good assessment of a smooth idle speed on a petrol engine would be a measure of its manifold pressure.
A good engine running at a smooth idle with no electrical or mechanical defects will return a stable 20"Hg of vacuum.

Mechanical defects, mixture errors, and engine management issues will all have a characteristic impact on the gauge, or in our case, the reading obtained using the WPS500X pressure transducer. Depending on the readings obtained, vehicle history, emission data and accompanying serial data, you could then introduce other relevant tests using the remaining channels on your scope to identify "rough idle"

Diesel engines are different given the absence of manifold vacuum. Once again Vehicle history, serial data evaluation, mechanical assessments, smoke tests all play an essential part in idle evaluation. Thereafter the scope will enable you to home in on areas identified from the above tests/checks

I hope this helps.....take care, Steve
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Re: exactly how rough is a rough idle?

Postby hillp » Wed Mar 12, 2014 12:36 pm

RE:
A good engine running at a smooth idle with no electrical or mechanical defects will return a stable 20"Hg of vacuum.
OK, I suppose this makes sense, at least intuitively, that the smoother the idle the higher the vacuum, though I wonder what the mechanism for this is. (So a 1960's muscle car with a lumpy idle due to its camshaft profile would show less vacuum.) I suppose we'd have to average the signal over some time frame, since any very fast pressure sensor in the intake will show a cyclic pattern as the valves open and close. In the example waveform for the WPS it varies over a span of about 1" Hg (converting from psi). You mentioned the WPS500X; would the FirstLook be any different?

Are there any math (or other) functions that will quantify variability in measured scope values, or would I have to export the data to Excel and calculate std deviation, max/min, or other such measures? This is easily done. How about a function to calculate RPM, instead of manually counting time between one ignition spike and the next, then the next, etc?
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Re: exactly how rough is a rough idle?

Postby Steve Smith » Thu Mar 13, 2014 3:48 pm

MIN AND MAX MATHS CHANNELS 2014-03-13_15-46-17.jpg
Hello and thanks for taking the time to feedback.
Trying to find a “one size fits all” accessory to determine rough idle was going to be ambitious to say the least, and the muscle car is just one example.
Both the pressure transducers you mention are excellent tools for looking at this specific test procedure, but my preference here for reading a specific measurement value “unit” (mbar etc.) would be the WPS as this transducer will read static pressure values as well as dynamic pulsations. Both these sensors perform excellent in the intake manifold given the pulsations present at idle speed and so “either/or” is fine
Saving the scope capture for use in Excel to calculate STD deviation is an excellent technique as you have mentioned, but for an on-screen graphical representation of an intake pressure waveform you could add a maths channel by clicking TOOLS-MATHS CHANNELS-CREATE and click on ADVANCED. Select “min” and place your cursor between “( )” and type “A” whereby you will have created a maths channel “min(A)”. Repeat this procedure using “max(A)” and once created, reopen the MATHS CHANNEL selection box “check” both the new maths channel boxes and click OK.
You will now have 2 x additional waveforms plotting the min and max values throughout the entire duration of your capture.
I hope this is some help to you…..take care……..Steve
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