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From: prins@cornell.UUCP (Jan Prins)
Newsgroups: net.auto
Subject: Re: SAAB IGNITION BREAKTHROUGH
Message-ID: <452@cornell.UUCP>
Date: Fri, 22-Mar-85 19:19:42 EST
Article-I.D.: cornell.452
Posted: Fri Mar 22 19:19:42 1985
Date-Received: Sun, 24-Mar-85 03:56:53 EST
References: <264@eneevax.UUCP> <189@osiris.UUCP>
Reply-To: prins@gvax.UUCP (Jan Prins)
Distribution: net
Organization: Cornell Univ. CS Dept.
Lines: 33
Summary: 

>> I'm just wondering ... is the sensor looking at the crankshaft
>> or is it looking at the camshaft ...

In article <189@osiris.UUCP> phil@osiris.UUCP (Philip Kos) responds:
>The sensor could look at either.  Just because the crank is spinning
>twice as fast as the cam doesn't mean it won't work - you just put
>two lobes on the distributor point cam instead of four (or however
>many cylinders your engine has).  .....  Sounds like Saab is probably 
>using the 1:1 crank drive with two lobes instead of four.
                                       
  You seem to be laboring under the assumption that a distributor shaft is
  present: the 'sensors' referred to actually replace the cam that actuates
  the ignition points and turns the distribution rotor.

  As for spark timing, you are right, this can be derived from crank or cam.

  But distribution of the spark to the right plug takes more care.  Don't forget
  that a complete engine cycle requires two crank revolutions.  Crank position 
  doesn't tell you what 'phase' of the cyle you are in.  The ambiguity requires
  you to fire 2 plugs at the same time, one of which will be wasted at best, 
  but at worst could cause intake fires with long duration cams.  In any case,
  spark plug life is cut in half and extra energy is required from the coils.

  Nevertheless, the fully electronic ignition systems I know use crank position
  and accept this loss because it reduces by half the number of coils required.

  In Saab's case, they have sprung for the full complement of coils, so there
  is no excuse for not 'doing it right'.  Thus they should take engine position
  from the cam, or disambiguate crank position by some cam reference.
  (It may be that the cam position is too "sloppy" because of the chain drive
  to use for Saab's high-accuracy spark timing program, but I really doubt it).

jan prins    {vax135, decvax}!cornell!prins