[Diy_efi] ignition timing variation

Wed Jul 31 16:52:02 GMT 2002

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After speaking with the student writing the software, I now have more 
ammo to respond with....we're using a Motorola 6811 for both timing and 
fuel.  With 8 teeth on the crank, there is 45 degrees between teeth. 
 The time between teeth is inversely proportional to engine speed, 
however only engine period and throttle position are used in the 
ignition lookup table.  They have a bench version working with a hall 
effect sensor and wheel mounted on an electric motor, and I observed 
little or no fluctuation of timing here.  The engine is mounted on a 
water brake dyno with a large flywheel.  I looked at the timing with a 
standard timing light at 3000 rpm, and the timing signal jumps around, 
both advancing and retarding about 2 degrees.  I asked about programming 
errors, but the student ensures me that the timing is top priority on 
the interrupt list.  A quick calculation shows me that 100 microseconds 
at 3000 rpm is 15 rpm, thus if the engine changes 15 rpm in 45 degrees 
and the timing is based on the previous tooth information, a two degree 
error could easily occur.  I now have to find out if we're using speed 
averaging (as per Eric's email) as I think this may smooth out the 
pulsations.

Bruce:  I'll look into a better timing light, and no, it is not just a timing delay.  It really advances and retards

Joel:  More teeth mearn more accuracy, but when I was a student I designed a system based on one tooth and don't remember this being a problem.

Many thanks for all responses

STEWART PRINCE

PROFESSOR, MECHANICAL ENGINEERING

CALIFORNIA STATE UNIVERSITY, NORTHRIDGE

Bruce wrote:

>How are you detecting this jitter?.
>If you're using a conventional *home grade* timing light, that might be a
>characteristic of the timing light.  Some just don't trigger accurately.
>Might consider mounting a second sensor done differently, and then compare
>the two output signals, and see if the timing is really wandering around.
>  If the timing error is constant, then why even worry?.   Just include that
>in your timing table.    After all does it matter if the timing is a cal'c
>34 d or actual 34d as long as the engine is making max HP?.
>Bruce
>
>----- Original Message -----
From: "Stewart Prince" <sprince at csun.edu>
>To: <diy_efi at diy-efi.org>
>Sent: Tuesday, July 30, 2002 3:57 PM
>Subject: [Diy_efi] ignition timing variation
>
>
>>Does anyone have any information on "acceptable" variation of ignition
>>timing?  We're getting about +/- 2 degrees of jitter on our home-built
>>EFI system (Formula SAE).  By my calcs, 100 microseconds of error
>>corresponds to 1.8 degrees of rotation at 3000 rpm, and 7.2 at 12,000
>>rpm (assume constant angular velocity). I believe the students are using
>>the time between the last two teeth on an 8-tooth wheel (crank mounted)
>>to calculate engine speed.  Is it possible that some sort of averaging
>>scheme should be used to account for the speed variation due to
>>
>combustion?
>
>>--
>>STEWART PRINCE
>>
>
>
>
>_______________________________________________
>Diy_efi mailing list
>Diy_efi at diy-efi.org
>http://www.diy-efi.org/mailman/listinfo/diy_efi
>

-- 
STEWART PRINCE

PROFESSOR, MECHANICAL ENGINEERING

CALIFORNIA STATE UNIVERSITY, NORTHRIDGE

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After speaking with the student writing the software, I now have more ammo
to respond with....we're using a Motorola 6811 for both timing and fuel.
&nbsp;With 8 teeth on the crank, there is 45 degrees between teeth. &nbsp;The time
between teeth is inversely proportional to engine speed, however only engine
period and throttle position are used in the ignition lookup table. &nbsp;They
have a bench version working with a hall effect sensor and wheel mounted
on an electric motor, and I observed little or no fluctuation of timing here.
&nbsp;The engine is mounted on a water brake dyno with a large flywheel. &nbsp;I looked
at the timing with a standard timing light at 3000 rpm, and the timing signal
jumps around, both advancing and retarding about 2 degrees. &nbsp;I asked about
programming errors, but the student ensures me that the timing is top priority
on the interrupt list. &nbsp;A quick calculation shows me that 100 microseconds
at 3000 rpm is 15 rpm, thus if the engine changes 15 rpm in 45 degrees and
the timing is based on the previous tooth information, a two degree error
could easily occur. &nbsp;I now have to find out if we're using speed averaging
(as per Eric's email) as I think this may smooth out the pulsations. 
<pre class="moz-signature" cols="$mailwrapcol">Bruce: I'll look into a better timing light, and no, it is not just a timing delay. It really advances and retards Joel: More teeth mearn more accuracy, but when I was a student I designed a system based on one tooth and don't remember this being a problem. Many thanks for all responses STEWART PRINCE

PROFESSOR, MECHANICAL ENGINEERING

CALIFORNIA STATE UNIVERSITY, NORTHRIDGE

</pre>
<br>
Bruce wrote:<br>
<blockquote type="cite" cite="mid:02a201ba5eaa$bc3bd900$13258fd1 at nacelp">
  <pre wrap="">How are you detecting this jitter?.<br>If you're using a conventional *home grade* timing light, that might be a<br>characteristic of the timing light.  Some just don't trigger accurately.<br>Might consider mounting a second sensor done differently, and then compare<br>the two output signals, and see if the timing is really wandering around.<br>  If the timing error is constant, then why even worry?.   Just include that<br>in your timing table.    After all does it matter if the timing is a cal'c<br>34 d or actual 34d as long as the engine is making max HP?.<br>Bruce<br><br>----- Original Message -----<br>From: "Stewart Prince" <a class="moz-txt-link-rfc2396E" href="mailto:sprince at csun.edu">&lt;sprince at csun.edu&gt;</a><br>To: <a class="moz-txt-link-rfc2396E" href="mailto:diy_efi at diy-efi.org">&lt;diy_efi at diy-efi.org&gt;</a><br>Sent: Tuesday, July 30, 2002 3:57 PM<br>Subject: [Diy_efi] ignition timing variation<br><br><br></pre>
  <blockquote type="cite">
    <pre wrap="">Does anyone have any information on "acceptable" variation of ignition<br>timing?  We're getting about +/- 2 degrees of jitter on our home-built<br>EFI system (Formula SAE).  By my calcs, 100 microseconds of error<br>corresponds to 1.8 degrees of rotation at 3000 rpm, and 7.2 at 12,000<br>rpm (assume constant angular velocity). I believe the students are using<br>the time between the last two teeth on an 8-tooth wheel (crank mounted)<br>to calculate engine speed.  Is it possible that some sort of averaging<br>scheme should be used to account for the speed variation due to<br></pre>
    </blockquote>
    <pre wrap=""><!---->combustion?<br></pre>
    <blockquote type="cite">
      <pre wrap="">--<br>STEWART PRINCE<br></pre>
      </blockquote>
      <pre wrap=""><!----><br><br><br>_______________________________________________<br>Diy_efi mailing list<br><a class="moz-txt-link-abbreviated" href="mailto:Diy_efi at diy-efi.org">Diy_efi at diy-efi.org</a><br><a class="moz-txt-link-freetext" href="http://www.diy-efi.org/mailman/listinfo/diy_efi">http://www.diy-efi.org/mailman/listinfo/diy_efi</a><br><br></pre>
      </blockquote>
      <br>
      <pre class="moz-signature" cols="$mailwrapcol">-- 
STEWART PRINCE

PROFESSOR, MECHANICAL ENGINEERING

CALIFORNIA STATE UNIVERSITY, NORTHRIDGE

</pre>
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