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'[EE]: Car Tachometer'
2001\03\22@195008 by

Well we just acquired a car and I am thinking of making the addition of
a tachometer with a PIC. The reason is that it is an 88 Olds Delta 88 and
those kinds of cars trigger the spark plugs more than just during the
compression cycle making standard digital tachs useless, plus it sounds like
an interesting project! Has anyone does this sort of thing? I was thinking
of using a freq to voltage converter but I'd rather do it all digitally. I
was thinking about measuring the time between pulses to determine RPM
(because counting the number of pulses per set amount of time would result
in WAY to slow display updates and lack of resolution at lower RPMs). 100us
resolution seems good enough for my uses. What does everyone think of that?
The only thing is I would have to use the inverse of this measurement, how
easy is this to do with a PIC? I was thinking of using a lookup table (with
maybe 256 entries and some linear interpolation) but I'd rather do it with
the math if possible. ANY comments and pointers would be greatly
appreciated. TTYL

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Herbert Graf wrote:
{Quote hidden}

Use this simple formula...

927C0h
------------------------  = RPM
100uS Loops X Cylinders

You might be able to simplify it even more

Eg. for 8 cylinders

124F8h
------------  = RPM
100uS Loops

Use a 24 bit counter to count the 100uS loops.

Start the count when the points open.
Increment the counter on every 100uS loop
The next time the points open...
Divide 124F8h by the count value.
The result is RPM.
Convert this to BCD and display it on 4 LEDs.
Debounce the points for 8 X 100uS loops.
Reset the counter

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Tony

mICro's
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salespicnpoke.com

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{Quote hidden}

Thanks alot, however I'd like to add that since the car uses an
electronic ignition system (no points) I'm planning to get my tach signal
from a fuel injector, anyone see problems with this idea? I realize that
injectors probably create a huge amount of transients on the line but I
figure I should be able to filter that out. Ideas? Thanks, TTYL

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On Thu, Mar 22, 2001 at 09:17:35PM -0500, Herbert Graf wrote:
>      Thanks alot, however I'd like to add that since the car uses an
> electronic ignition system (no points) I'm planning to get my tach signal
> from a fuel injector, anyone see problems with this idea? I realize that
> injectors probably create a huge amount of transients on the line but I
> figure I should be able to filter that out. Ideas? Thanks, TTYL

I have seen this done using one one of the signals available at the check
connector. I once saw a project that was done using the above method. I
can't seem to find the site again but I have a printout at home that I
will have a look at. The guy who did it used a signal that was
essentially logic levels except for a very nasty 400V spike.

--
Wesley Moore                      http://www.1moore.org/
RMIT - BEng (Comp Sys Eng)/BApp.Sc. (Comp Sci)  3rd Year

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2001\03\22@233918 by
In a message dated 3/22/01 11:32:06 PM Eastern Standard Time,
wmooreCS.RMIT.EDU.AU writes:

> On Thu, Mar 22, 2001 at 09:17:35PM -0500, Herbert Graf wrote:
> >      Thanks alot, however I'd like to add that since the car uses an
> > electronic ignition system (no points) I'm planning to get my tach signal
> > from a fuel injector, anyone see problems with this idea? I realize that
> > injectors probably create a huge amount of transients on the line but I
> > figure I should be able to filter that out. Ideas? Thanks, TTYL
>
>

I would use the signal present at the coil just like any other normal tach.
Any ignition system uses a coil and needs a trigger. Just identify the proper
wire and tap in. My 69 Chevy van is powered by a 91 Corvette 350 with TPI and
electronic ignition. It's tach works just fine.

Ray Russell
General Contractor

Pocahontas Division
Circa 1958
Visit The Pocahontas Website at:
OR
http://milliron.home.sprynet.com/Pocahontas/Pocahontas1.htm

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>      Well we just acquired a car and I am thinking of making the addition
of
> a tachometer with a PIC. The reason is that it is an 88 Olds Delta 88 and
> those kinds of cars trigger the spark plugs more than just during the
> compression cycle making standard digital tachs useless, plus it sounds
like
> an interesting project! Has anyone does this sort of thing? I was thinking
> of using a freq to voltage converter but I'd rather do it all digitally. I
> was thinking about measuring the time between pulses to determine RPM
> (because counting the number of pulses per set amount of time would result
> in WAY to slow display updates and lack of resolution at lower RPMs).

Using a CCP module makes this easy.

> 100us
> resolution seems good enough for my uses. What does everyone think of
that?

If you use a CCP module to capture the pulses, you will get much better
resolution than that.  I would still low pass filter the final result
(probably better to filter the engine speed than the period) a bit because
there may be jitter in the firing system, or in the circuit to detect a
pulse.

> The only thing is I would have to use the inverse of this measurement, how
> easy is this to do with a PIC?

No big deal.  It's just a divide.  If you use 32 bit math, you can probably
divide the period that would result from 1 RPM by the actual period, which
would directly yield integer RPM.  Again, I would keep 2 to 4 extra bits and
do a little low pass filtering on the value, then just update the display
with the official filtered RPM value every 250mS or so.

> I was thinking of using a lookup table (with
> maybe 256 entries and some linear interpolation) but I'd rather do it with
> the math if possible. ANY comments and pointers would be greatly
> appreciated.

Take a look at the 16C923.  It has a CCP module and can also drive a 4 digit
LCD display directly.  I've done that several times.  You end up with a very
low parts count system.

The tricky part will be the pulse detection circuit.  The spark pulse is not
just a single clean pulse, and there are other noise spikes in the system.

This is rather annoying because you have to manually edit it to reply to the
list (as I did).  It is easy not to notice you did this, then people reply
and their message doesn't show up on the list and they think the server is
broken or whatever.  Unfortunately James feels you should have the right to
send a message publicly to the list but have replies default to private, so
the list server won't fix this automatically.

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

--

>      Thanks alot, however I'd like to add that since the car uses an
> electronic ignition system (no points) I'm planning to get my tach signal
> from a fuel injector, anyone see problems with this idea? I realize that
> injectors probably create a huge amount of transients on the line but I
> figure I should be able to filter that out. Ideas?

Strobe tachs usually clip around a spark plug wire.  The clip completes a
magnetic ferrite loop when closed, and a coil is wound around the loop.
Again, you don't just get a single clean spike when the plug fires.  You
will need to do come filtering and conditioning, but the information is
there.

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

--

>      Thanks alot, however I'd like to add that since the car uses an
> electronic ignition system (no points) I'm planning to get my tach signal
> from a fuel injector, anyone see problems with this idea? I realize that

I don't know specifics about GM fuel injection, but I imagine that it is
similar to other systems.  With many Bosch fuel injection systems (as used
on many European cars) the fuel injectors are *not* pulsed once per
revolution.  Instead they are pulse width modulated; IIRC there is not a
correlation between engine speed and PWM period.

As others have suggested, you should look at the coil (or ignition control
module) to determine when you get spark.

-Steve

--

{Quote hidden}

Unfortunately that particular line of cars sparks MORE than once during
a full four cycles, the second spark being weaker than the primary. This
makes it very difficult to reliably get a reading from the spark plug line
which is why I will use a direct tie in to an injector line. Thanks anyways,
TTYL

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> -----Original Message-----
> From: pic microcontroller discussion list
> [PICLISTMITVMA.MIT.EDU]On Behalf Of Stephen B Webb
> Sent: Friday, March 23, 2001 09:50
> To: PICLISTMITVMA.MIT.EDU
> Subject: Re: [EE]: Car Tachometer
>
>
> >      Thanks alot, however I'd like to add that since the car uses an
> > electronic ignition system (no points) I'm planning to get my
> tach signal
> > from a fuel injector, anyone see problems with this idea? I realize that
>
> I don't know specifics about GM fuel injection, but I imagine that it is
> similar to other systems.  With many Bosch fuel injection systems (as used
> on many European cars) the fuel injectors are *not* pulsed once per
> revolution.  Instead they are pulse width modulated; IIRC there is not a
> correlation between engine speed and PWM period.

Doh, that's not good, I'm planning to have a look at it in a day or two.

> As others have suggested, you should look at the coil (or ignition control
> module) to determine when you get spark.

Unfortunately that is not an option since there is more than one spark per
four strokes. TTYL

--

>     Unfortunately that particular line of cars sparks MORE than once
> during a full four cycles, the second spark being weaker than the
> primary. This makes it very difficult to reliably get a reading from
> the spark plug line which is why I will use a direct tie in to an
> injector line. Thanks anyways, TTYL

Try the Hall sensor trick. Glue or attach a linear Hall sensor to the
bottom of the ignition coil. The sensor needs electrostatic shielding (Al
or Cu box or foil). This even works with magnetos except you need to get
inside the body first.

Peter

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> > As others have suggested, you should look at the coil (or
> ignition control
> > module) to determine when you get spark.
>
>         Unfortunately that is not an option since there is more
> than one spark per
> four strokes. TTYL

Just divide by 2. My motorbike, and many others i guess, does the same. My
bike has 3 coils. Coil1 fires when piston 1 or 6 is on combustion, coil2 is
for 2 or 5 and coil 3 is for 3 or 4.  When the one piston is on combustion,
its mate is on exhaust. Its for emmision controls i think, or just to
simplify things a bit.

John

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>      Unfortunately that particular line of cars sparks MORE than once
during
> a full four cycles, the second spark being weaker than the primary. This
> makes it very difficult to reliably get a reading from the spark plug line
> which is why I will use a direct tie in to an injector line. Thanks
anyways,

As long as it is consistant, there shouldn't be a problem.

What is the nature of these multiple spark pulses?  Do they happen in short
succession in an attempt to simulate one long spark?  Do they happen at
other parts of the cycle?  If so, why?  Does a normal strobe tach work on
this car?  I've never heard of this before, please explain.

The advantage of a magnetic pickup on a spark plug wire is that you will be
detecting the current, not the voltage.  There will only be current when the
plug actually fires.  I would at least hook one up and look at the signal on
a scope before deciding it can't be done.  If you have the ability to
capture the waveform, I'd be interested in seeing it.

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

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> >      Unfortunately that particular line of cars sparks MORE than once
> during
> > a full four cycles, the second spark being weaker than the primary. This
> > makes it very difficult to reliably get a reading from the
> spark plug line
> > which is why I will use a direct tie in to an injector line. Thanks
> anyways,
>
> As long as it is consistant, there shouldn't be a problem.
>
> What is the nature of these multiple spark pulses?  Do they
> happen in short
> succession in an attempt to simulate one long spark?  Do they happen at

I'm not quite sure, my analog scope is all I have at home and it's almost
impossible to see anything with it.

> other parts of the cycle?  If so, why?  Does a normal strobe tach work on
> this car?  I've never heard of this before, please explain.

That's the problem, a normal strobe tach does NOT work normally, it works
OK at some RPMs but not at others. BTW I have found what seems to be a tach
signal coming from the ignition module, I have to bring my scope to the car
to see if it's what I want or not.

> The advantage of a magnetic pickup on a spark plug wire is that
> you will be
> detecting the current, not the voltage.  There will only be
> current when the
> plug actually fires.  I would at least hook one up and look at
> the signal on
> a scope before deciding it can't be done.  If you have the ability to
> capture the waveform, I'd be interested in seeing it.

I have tried to do that but without a capturing scope it very difficult to
see anything, I do recall seeing the secondary pulse, it was there sometimes
and not others but that just might have been the sensitivity of my scope.
Thanks for the help. TTYL

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----------
> From: Olin Lathrop <olin_piclistEMBEDINC.COM>
> To: PICLISTMITVMA.MIT.EDU
> Subject: Re: [EE]: Car Tachometer
> Date: Saturday, March 24, 2001 5:44 AM
>
> >      Unfortunately that particular line of cars sparks MORE than once
> during
> > a full four cycles, the second spark being weaker than the primary.
This
> > makes it very difficult to reliably get a reading from the spark plug
line
> > which is why I will use a direct tie in to an injector line. Thanks
> anyways,
>
> As long as it is consistant, there shouldn't be a problem.
>
> What is the nature of these multiple spark pulses?  Do they happen in
short
> succession in an attempt to simulate one long spark?  Do they happen at
> other parts of the cycle?  If so, why?  Does a normal strobe tach work on
> this car?  I've never heard of this before, please explain.

On the MSD (Multi Spark Discharge) boxes, there is a short burst of voltage
spikes to fitre the plug several times. It simulates the type of plug
firing you get with points. The multi sparks stop at about 3k rpm or so
because of time constraints.
>
> The advantage of a magnetic pickup on a spark plug wire is that you will
be
> detecting the current, not the voltage.  There will only be current when
the
> plug actually fires.  I would at least hook one up and look at the signal
on
{Quote hidden}

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Is there a manual transmission version of the car in question (or another
car with the same engine?).

If so, you could probably find out how the factory supplied tach for the
manual transmission model interfaces from an apppropriate service manaul.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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> Is there a manual transmission version of the car in question (or another
> car with the same engine?).
>
> If so, you could probably find out how the factory supplied tach for the
> manual transmission model interfaces from an apppropriate service manaul.
>
Just a thought but diesel engined vehicles do not have an ignition system
perhaps some petrol engined vehicles use the same technique as a diesel
engined vehicle for a tachometer drive.

Clue the alternator has an extra terminal which is connected to one of the
coils within the alternator.

Regards

Chris

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>         That's the problem, a normal strobe tach does NOT work normally,
it works
> OK at some RPMs but not at others.

So what does it do when not working "normally"?  Do you get the extra pulse
half way between the real ones, right after the real one, or what?

> BTW I have found what seems to be a tach
> signal coming from the ignition module, I have to bring my scope to the
car
> to see if it's what I want or not.

That would be great if it is really a tach signal.  Maybe you can get hold
of the shop manuals for your car and see how the optional tach is hooked up.
Most cars have a factory option for a dashboard that includes a tach.  I
wouldn't be surprised if the ignition module is the same in either case,
with the tach output not connected when the normal dashboard is installed.
Maybe you found just that signal.

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Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

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> On the MSD (Multi Spark Discharge) boxes, there is a short burst of
voltage
> spikes to fitre the plug several times. It simulates the type of plug
> firing you get with points. The multi sparks stop at about 3k rpm or so
> because of time constraints.

What a concept!  So we now have sophisticated electronics going out of its
way to simulate bouncing of old fashion mechanical contacts.  What's next,
tuners with DSP filters to simulate the old fashion AM radio sound?

Anyway, if this is the case then it should be easy to get rid of the
multiple pulses by something akin to a debouncing circuit.  You should be
able to push most of the logic into the firmware and keep the electronics
rather simple.  You know that engine speed can't change drastically from one
revolution to the next, so it should be rather easy to ignore all pulses for
the first half cycle (for example) after detecting one.

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

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> > spikes to fitre the plug several times. It simulates the type of plug
> > firing you get with points. The multi sparks stop at about 3k rpm or so
> > because of time constraints.
> What a concept!  So we now have sophisticated electronics going out of its
> way to simulate bouncing of old fashion mechanical contacts.  What's next,
> tuners with DSP filters to simulate the old fashion AM radio sound?

Well, not quite.

They wind up generating even pulses per "fire" to cause the plug to fire 3
times for every initial firing. Points would hardly do that. But a PIC
could do the job quite easily.

-->Neil

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ICQ #29402898

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----------
> From: Olin Lathrop <olin_piclistEMBEDINC.COM>
> To: PICLISTMITVMA.MIT.EDU
> Subject: Re: [EE]: Car Tachometer
> Date: Saturday, March 24, 2001 2:55 PM
>
> > On the MSD (Multi Spark Discharge) boxes, there is a short burst of
> voltage
> > spikes to fitre the plug several times. It simulates the type of plug
> > firing you get with points. The multi sparks stop at about 3k rpm or so
> > because of time constraints.
>
> What a concept!  So we now have sophisticated electronics going out of
its
> way to simulate bouncing of old fashion mechanical contacts.  What's
next,
> tuners with DSP filters to simulate the old fashion AM radio sound?

How about transistors that simulate tubes in guitar amplifiers?
>
> Anyway, if this is the case then it should be easy to get rid of the
> multiple pulses by something akin to a debouncing circuit.  You should be
> able to push most of the logic into the firmware and keep the electronics
> rather simple.  You know that engine speed can't change drastically from
one
> revolution to the next, so it should be rather easy to ignore all pulses
for
{Quote hidden}

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>My motorbike, and many others i guess, does the same. My
>bike has 3 coils. Coil1 fires when piston 1 or 6 is on combustion, coil2 is
>for 2 or 5 and coil 3 is for 3 or 4.  When the one piston is on combustion,
>its mate is on exhaust. Its for emmision controls i think, or just to
>simplify things a bit.

This system has been used on motorbikes for many years that I am aware of. It is
a minimisation of ignition components by having the high voltage winding on the
coil totally floating. each end of the winding is connected to separate spark
plugs, one on the cylinder under compression, the other on a cylinder in exhaust
phase. By doing this only one coil is required for a two cylinder engine.

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>The advantage of a magnetic pickup on a spark plug wire is that you will be
>detecting the current, not the voltage.  There will only be current when the
>plug actually fires.  I would at least hook one up and look at the signal on
>a scope before deciding it can't be done.  If you have the ability to
>capture the waveform, I'd be interested in seeing it.

always assuming no capacitive coupling into the pickup coil. Even though this
would appear as a common mode component, I would bet it would tend to foul up
the signal.

If you are going to try magnetic pickup, why not use a sensor on the ring gear
teeth where the starter motor engages. Will even work on diesel engines with
mechanical injectors:) You just get many pulses per single revolution.

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Alan B. Pearce wrote:
>
> >My motorbike, and many others i guess, does the same. My
> >bike has 3 coils. Coil1 fires when piston 1 or 6 is on combustion, coil2 is
> >for 2 or 5 and coil 3 is for 3 or 4.  When the one piston is on combustion,
> >its mate is on exhaust. Its for emmision controls i think, or just to
> >simplify things a bit.
>
> This system has been used on motorbikes for many years that I am aware of. It is
> a minimisation of ignition components by having the high voltage winding on the
> coil totally floating. each end of the winding is connected to separate spark
> plugs, one on the cylinder under compression, the other on a cylinder in exhaust
> phase. By doing this only one coil is required for a two cylinder engine.

Yes Harley Davidson have done this since the 1920's,
used one coil for 2 cylinders.
Modern motors are starting to do this more, it's common
on motorcycles, where you don't want a distributor.
Some cars are following suit now, with V6 car engines
having 3 separate coils and no distributor.

But regardless of the coil windings remember that both
spark plugs always fire together, the designers rely
on 4-stroke motor ignoring the extra spark after the
exhaust stroke.
-Roman

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Seems like one plug or the other would have a slightly
lower breakdown and fire first - Preventing the other
from ever firing.

In fact, assuming the cyls are 180 deg out of phase,
one cyl is ~ TDC power stroke, the other TDC exhaust -
Ionization conditions in the cyls would seem to favor
the wrong plug firing.

What am I missing?

regards, Jack

Roman Black wrote:
> But regardless of the coil windings remember that both
> spark plugs always fire together, the designers rely
> on 4-stroke motor ignoring the extra spark after the
> exhaust stroke.
> -Roman

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Oops, cyl phasing TDC pwr, BDC t'other. Makes
a bit more sense.

Jack

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>Seems like one plug or the other would have a slightly
>lower breakdown and fire first - Preventing the other
>from ever firing.

>would seem to favor the wrong plug firing.

The two spark plugs and the high voltage coil are in series.

todd

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>Seems like one plug or the other would have a slightly
>lower breakdown and fire first - Preventing the other
>from ever firing.
>
>In fact, assuming the cyls are 180 deg out of phase,
>one cyl is ~ TDC power stroke, the other TDC exhaust -
>Ionization conditions in the cyls would seem to favor
>the wrong plug firing.

>What am I missing?

What you are missing is that the coil has no connection to ground for the
secondary winding. The two plugs are therefore in series, and so any lowered
breakdown voltage helps the situation. The worst case voltage from the coil to
engine is half the total coil voltage, so presumably the coil has extra turns
ratio to give a higher voltage than that for a coil which feeds a single plug at
a time. Alternatively because there is now no loss due to the spark gap in the
distributor then maybe no extra voltage is needed.

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I do not know this type of ignition but why insist on the ignition ? Does
it have a belt driven fan ? You can read that using an opto. You can put a
Hall sensor on one of the coils, you can use a proximity sensor on the
clutch/starter gear disc etc. Once it is clear that the ignition is not
tinker-friendly, why try harder to interfere with it ? Other
electromechanical pickoffs for tachometers are vibration of the engine
block and camshaft(s) and sound of same. A microphone in the intake
manifold will also work. It requires some filtering but the suction is
unmistakably the highest signal generated (several tens of millivolts
using a normal dynamic microphone capsule). This is the only way with
small diesel engines that need to be modified.

Peter

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> how are you doing a low pass filter in software ?
>
> maybe just point me in the right direction with the "extra 2 or 4 bits"
part

A single pole low pass filter is easy and fast in software.  If NEW is a new
reading and FILT is the filter value to update with the new reading then the
algorithm is:

FILT <-- FILT * (1 - W) + NEW * W

where W is a 0 to 1 weighting fraction for the new value.  This type of
filter has the same response as an RC low pass filter with NEW as the input
and FILT as the output.  W determines the time constant of the filter if new
values come in at regular intervals.

Another way of looking at this filter is that the output is a weighted sum
of all input values.  The most recent input has a weight of W, the nextmost
recent W**2, the next more recent W**3, etc.  Or you could say that each new
reading starts out with a weight of W in the filtered value, and decreases
by W for each additional new reading.  In other words, the impulse response
is a step followed by an exponential decay.

Computing such a filter is particularly easy if W = 2 ** -N where N is a
positive integer.  In this case, NEW * W is just NEW shifted right N bits.
FILT * (1-W) is FILT shifted right N bits subtracted from FILT.  Using
C-like syntax, the algorithm is:

FILT = FILT - (FILT >> N) + (NEW >> N)

It is often useful to maintain FILT and N as fixed point values with N
fraction bits to eliminate roundoff error.  That's what I was alluding two
with "extra 2 or 4 bits".

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

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On Sun, 25 Mar 2001, Olin Lathrop wrote:

> > how are you doing a low pass filter in software ?

<snip>
{Quote hidden}

Here's an efficient implementation of that algorithm

http://www.dattalo.com/technical/software/pic/twist.asm

Here's a small snippet of the comments.

;;; --------------------------------------------------
;;; twist
;;;
;;; The purpose of this routine is to evaluate:
;;;
;;;    A*x + B*y
;;;  ------------
;;;     A  +  B
;;;
;;; Such that A+B = 2^N

'x' and 'y' are the "FILT" and "NEW" parameters. The weighting factor is

W = B/(A-B)

(note:

1-W = A/(A-B)

Which is the coefficient on 'x' and "FILT".)

My algorithm limits 'x' and 'y' to 8 bits and 'A' and 'B' to 4 bits. The
algorithm is computed with 8-bit arithmetic, so the "2-4" extra bits alluded to
above will not be obtainable. Now, Tony K. has written a similar routine for
24-bit numbers. (See http://www.dattalo.com/technical/software/software.html ).

Scott.

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> But regardless of the coil windings remember that both
> spark plugs always fire together, the designers rely
> on 4-stroke motor ignoring the extra spark after the

My understanding is that only the spark plug that is currently under
compression (air fuel mixture ready to be ignited) actually fires.  It
seems unlikely to me that two plugs would fire simultaneously.  The spark
gap tolerances (or real world values, anyway) could vary by 30% or more
pretty easily (additionally partial plug fouling would contribute to
different voltage requirements).  So once the first plug fires the voltage
at the coil would drop, never reaching a high enough voltage to fire the
other plug.

These setups are often called "wasted spark", and I believe the name is
misleading to what actually goes on in the engine.

-Steve

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All,

Actually, both plugs do fire.   But the one under compression gets
the bulk of the voltage due to the highly compressed fuel/air
mixture having a very low resistance.  Unlike the cylinder on the
exhaust stroke having a very high resistance.   Due to the laws
of electrical physics, seeing that both of the plugs and the coil
are in series, both plugs have to fire to complete the circuit.
Otherwise, neither would fire due to an incomplete circuit.  You
are absolutely correct about the term 'wasted spark' though.  The
cylinder on the exhaust stroke gets the wasted spark.  It's called
wasted, because it's firing in a cylinder full of spent exahust
gasses, and therefore, does not contribute to power generation.

Regards,

Jim

On Mon, 26 March 2001, Stephen B Webb wrote:

{Quote hidden}

jimjpes.com

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>  Actually, both plugs do fire.   But the one under compression gets
>  the bulk of the voltage due to the highly compressed fuel/air
>  mixture having a very low resistance.

Having lower resistance would cause it to get less of the series voltage.
However, none of that matters anyway since they are in series and therefore
the current thru both plugs is the same.  Once the spark ionizes the gasses
in the gap, their resistance goes way down, greatly decreasing the voltage.
But a spark is sustained by current, not voltage.

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

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> What am I missing?

They will fire together because they are in series. The one that 'goes'
harder will dictate when the spark really fires. This is the one in the
cylinder with more pressure at the time (the 'right' one).

Peter

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I wasn't going to get into this discussion as the original poster seems to
have figured out on his vehicle where to get a tach reading, but there seems
to be a lot of missunderstanding on non distributor ignition systems. So
here is some information I retyped from a manual SPX supplies with its
PACE400 engine analyzer. (you know the machine a auto garage hooks up to
your car when you get a scope check) I may have added some spelling mistakes
ect,.

Appendix D - Lab Scope Use.

Section Four - Distributorless Ignition System (DIS)

Instructional Objectives
Review the basic operation of the Distributorless Ignition System
Understand the DIS Exhaust Average KV reading
Review variables that affect DIS Exhaust Average KV

Description
The terms Distributorless Ignition System and DIS have become generic to
mean any engine without a distributor. The purpose of the following
information is not to explain or categorize all the various types of DIS,
but to offer some fundamentals.

Currently, there are two basic designs of ignition systems without a
distributor. Direct Ignition, as used on many import products, has one coil
per spark plug. The coils are fired in Firing Order, one at a time, much the
same as fuel injectors in a sequentially fuel injected system.

ignition wire---ignition coil secondary---ignition wire
+                                                                         +
spark plug-------metal of engine---------spark plug
Figure 4-1

Figure 4-1 shows a diagram of the more common "Waste Spark" method of DIS.
In this system a spark plug is attached to each end of the ignition coil
secondary. One plug fires with a negative polarity, as in distributor
ignition, the other fires with a positive polarity. Each coil fires the
plugs in two companion cylinders - that is, cylinders that reach TDC (top
dead center) at the same time. The cylinder that is at TDC on the
compression stroke is referred to as the Event Cylinder while the cylinder
at TDC on the exhaust stroke is referred to as the Waste Cylinder. When the
coil discharges, both plugs fire at the same time to complete the series
circuit.

As with distributor ignitions, there are internal cylinder influences
present in the compression stroke that are not present in the exhaust
stroke:
+Higher pressure during the compression stroke increases the voltage
demand.
+The air/fuel mixture can alter the required voltage. A rich mixture
lowers the voltage demand, while a lean mixture increases voltage demand.
+Timing also can change the voltage required. Advanced timing lowers
voltage demand and retarded timing increases voltage demand.

Analyze DIS Compression KV's much the same as convention ignition, keeping
the previous information in mind. The DIS Exhaust KV's can be used as an
indication of the ignition secondary condition as well as a base comparision
for the DIS Compression KV's.

DIS Exhaust Average KV
Unit of Measure  Kilovolts (KV)
Normal Range  1 to 5 KV
Measurement  The average voltage required to "ionize" or initially develope
the arc across th plug gap (Firing KV) over 10 plug firings in the exhaust
stroke.

Theoretically, in the exhaust there are no internal influences such as, an
air/fuel mixture or compression. The exhaust values normally indicate the
quality of the voltage supplied for the combustion event. Abnormal exhaust
readings usually indicate a fault in the plug, plug wire, or coil. Observe
the companion cylinder for a diagnostic comparision. Abnormal readings in
both companion cylinders can indicate a defective coil or module. However,
it would not be unusual to have a problem in one cylinder  show in the
other's firing pattern or digital readings. Normally, performing a cylinder
balance (Cylinder Performance Test) confirms the problem cylinder or
Cylinders.

There is more but that should be enough to explain. So compression KV's
typically 10-20 KV Exhaust KV's Typically 1-5 KV.  BOTH PLUGS FIRE AT THE
SAME TIME!!!! ;-).

Thanks, Jamie

And it continues on

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