Re: Ignition Redundancy

[Steve Stegmann <stegmann@domain.elided>]

On Fri, 14 Aug 1998, Tom Harais wrote:

> John:
> 
> One of the features that amazes me is, they put a "sniffer" in your
> tailpipe and a inductive clamp on your #1 cylinder wire. From this
> alone, these machines will give you engine RPM, real time ignition
> advance, and of course, HC PPM, CO PPM , NOx PPM, CO2% and O2%. How do
> they get real time advance from one spark signal? I know that RPM is

Tom,

They don't get realtime ignition advance from one spark signal.  That's 
not true.  You can't get advance without a reference point on the 
crankshaft.  Period.   Having said that I do believe they display 
somehting called advance.  I'm sure it is some kind of an estimate.

Given: 

They assume the base timing, since it's in the book and the factories and
any mechanics will set it that way.  Not worth the hassle to the factories
and mechanics to set it anywhere else.

OK, now we have the base timing.  From a careful measurement of the timing
of spark signals, they can watch the time between sparks change as the
engine changes speed.  From this change in interval between sparks you 
can fairly easily plot how the ignition advance is **changing**.  But 
rememeber all you have for a base is a scared mechanic & factory.  So 
maybe the display isn't so bad.

A 200 Mhz Pentium has a clock period of 1/200,000,000 or 0.5x10^-9 sec.  
The time between sparks at 1000 rpm for an 8 cyl engine is:
     rev     sparks       sparks
1000 --- x 4 ------ = 4000------
     min      rev          min


the time interval between sparks is 1/4000 or 0.25 x 10^-4 sec.

So that gives us 200,000,000 /4000 or 500,000 clock cycles between sparks. 
Given that they have a processor dedicated to this timing, it can be
something cheap, doesn't need to be too sophisticated.  Bet I can measure
that timing in 500,000 clock cycles.  Won't even have to be a really
amazing piece of code. 

But remember they don't really have the base timing.  They certainly can 
get the advance curve, but I don't see how they can get the base timing.  
That's why the factories still embed a magnet in they flywheel or 
something to give the computer a readout on the position of the 
crankshaft.  Especailly since there is no longer a distributor to do the 
timing.  It's all done with the computer and the crankshaft signal.  If 
there was a way to do without this signal, believe me Detroit wouldn't 
spend the money to generate a signal they don't need.

Have you seen this operate?  I would love to feed it an engine with the 
base timing off by 3 or 4 degrees and see what happens.

I can imagine the fun it would have with an MSD unit.  If somebody 
doesn't tell the programmer to expect multiple sparks he will have the 
same problem an inductive pickup tach has.  It'll read 4000 rpm at idle.

You'll love he OBD II fed requirements.  I can send a copy of an aritlce 
snail mail if you want.  But the feds are requiring that all auto engines 
keep a record of **EVERY** misfire.  The engine has to check each 
cylinder to be sure it fired and signal on the dashboard when the misfire 
rate gets above a certain percentage, & report the details when queried 
by the service coputer.

How do they do this you ask.  There are three methods.

1- cheapest and most likely.  Put a load cell in an engine mount and 
analyze cranshaft *vibration* from *each* cyl firing.  Since each cyl 
firing provides a distinct "push" to the crankshaft all you have to do 
is  look for the pushes and measure the time between them.  Any missing 
ones will easily show up.  Well there is a problem... The solid coupling 
between the engine, frame and driving wheels when you have a manual 
transmission on a somewhat rough road will fool the computer. Too much 
vibration is transmitted back up the driveline from the wheels.  The most 
likely outcome of this is the demise of manual transmissions in 
automobiles over the next 3 or 4 years.  Only wierdo's (like me) buy them 
anyway.

2- Next cheapest and least likely to work.  Since the engine produces a 
pulse for each cyl firing,  Just measure how the crankshaft moves.  It 
doesn't move smoothly.  It surges ahead (this is the driveshaft twisting 
to allow the surge) with each cyl firing, and then slows down till the next 
one fires.  This has proven to be unreliable.

3- Most expensive and very reliable.  This is being pursued by saab &
others with unlimited budgets. It is to used the spark plugs to analyze the
gasses in the cyl after the engine has fired.  You run a current through 
the plug after the cyl has fired (on the down stroke before the gasses 
get mixed with the incoming charge) and it measures the ionization 
potential of the gasses in the cyl from which you can decide if the plug 
has fired.  This relatively expensive and requires more wiring in the 
engine compartment, and special plugs.

The thing that surprised me is that the best of the most modern engine 
still misfire almost 5% of the time.  

The only good to come out of it is that the engines will be built to 
tolerances not believed possible, they really will be better.  Course I 
wonder if it will be possible to achieve this precision in a rebuild, 
probably not.  And this better engine will get you nothing.  Sure it will 
last longer, but the analysis system won't work after the engine loosens 
up a bit, so you'll still have to trash it at 200,000 miles.

Yes, all of this is going to be just as expensive as you think.  I really 
love it when Big Brother gets the bit in his teeth.

And you ask what will it accomplish.  Not a damn thing!!!!!!!!!  The 
eco-nazis are about to wet their pants in ecstasy.  But in the real world 
it won't help.  In most of the country, the auto is no longer the biggest 
problem.  

Just what are people who can't afford a new car every three years supposed
to do, walk.  I think this will be an interesting social experiment.  We
are really going to polarize the country with this one and nobody is
watching. BTW for this to work they will eventually have to make old cars 
illegal.  Flat out remove them.  Don't you see, this way everyone but the 
rich can be forced into mass transit where they should be.  This way 
those that can afford new cars all the time will have less traffic to 
contend with.  Isn't that as it should be.  You might try writing your 
congressman, but I doubt it will help.

I don't believe I wrote this. Sorry about the bandwidth

Steve

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Steven A. Stegmann
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