Date: Wed, 1 Oct 2003 12:25:48 -0700
Reply-To: Robert Keezer <warmerwagen@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Robert Keezer <warmerwagen@HOTMAIL.COM>
Subject: Re: The details of Motronic (long)
Content-Type: text/plain; format=flowed
Thanks for your questions on how Motronic differs from Digifant..
The answers here are given by our team of qualified technicians who are
standing by to take your call.
Joking!
I have read the Bosch FI book and included some quotes from it.
The rest of the narrative is mine.
Let's compare three models to better understand Motronic. (pretend I'm
standing in front of a blackboard and your'e seated in the classroom)
Look at conventional points ignition with mechanical/vacuum advance.. The
coil is energized in the primary windings to 12+ volts. When the points
close, the capacitor discharges, causing the primary voltage to collapse
which induces high voltage in the secondary winding.
This high voltage is sent to the distributor to fire a spark plug.
The points are a mechanical "input" to the coil, like an automatic switch.
The points are synchronized to open and close at a certain time- earlier as
the engine spins faster by the advance mechanism.
It helps to have an understanding of the old mechanical system before trying
to understand the electronic.
The electronic pointless ignition found in the Vanagon has a sensor in the
distributor called the Hall sensor. There are four apertures on the trigger
wheel that allow a signal to pass every time a window is opened to the
pick-up.
The pickup sends this signal to the ECU, which then triggers the coil to
send a spark to the plugs.
The computer is now doing the job the capacitor and points do in first
model.
It's the computer that bosses the coil around, not mechanical points.But the
advance is still controlled by mechanical or vacuum means .
Now that we understand that a computer can take the place of a mechanical
system and imitate it,
we are ready to understand Motronic.
It works the same way as Digifant in that the computer controls the coil but
with these changes:
The advance /retard is purely electronic. How does this work?
First, a few of the players have changed positions.
The Digifant distributor has four apertures- the impulse transmitted thru
these apertures four times per every 2 revolutions of the crank. These
impulses are used for spark timing but also for TDC input . The computer
uses this input to reference crankshaft position.
The Motronic has only one aperture on the distributor trigger wheel and this
is the camshaft position sensor. It no longer is used for spark timing, but
tells the computer the camshaft position..
TDC and RPM are inputs sent to the computer from the crankshaft position
sensor, (CPS)which consists of a toothed ring on the crankshaft with cutouts
to mark TDC and RPM. A pick-up on the block opposite this ring sends the
impulses to the computer.
The Motronic engine management program uses these electro-mechanical inputs
to control the spark timing, injector timing.
The temperature sender does the same as in Digifant- the computer looks at
engine temperature to enable cold-start longer injection times or warm
start.
The basic inputs are crankshaft and camshaft position, and temperature
sensor.
All the computer needs to command the system is one revolution of the
engine. If it sees TDC and 30 RPM crankshaft speed or greater, and camshaft
position-that is, valves closed on compression stroke number one cylinder,
and temperature- the computer triggers the coil to fire a spark off to the
plug and the engine starts.
The two inputs- the crankshaft telling the computer it's at TDC number one
cylinder, and the CPS indicating the valves are timed properly, is the
thumbs up to the computer to trigger the coil.
If either of these two are off, the computer won't act. The third, the
temperature, if it's off will not have the proper cold start air-fuel ratio
and be hard to start or not at all.
So now we see that since the Motronic computer doesn't receive it's TDC
input from the the distributor, which also is a retard/advance input for the
Digifant , the distributor has been assigned a new purpose and the spark
timing function has been dropped .
Motronic runs the everything now, even the vacuum evap valve has been
replace by a solenoid.
Now, to understand how the Motronic ECU calculates timing I'll quote this
reference from the Bosch fuel injection book:
I quote:
"I'm talking here about more than electronic ignition-the systems that have
replaced the points and condensors of yesterday.I'm talking about
microcomputer control of ignition advance angle for every plug firing, or at
most, every two plug firings.The millisecond response time of electronic
ignition advance controls far faster than the traditional mechanical
flyweight/vacuum advance systems."
"In Motronic systems.. The control unit processes a number of inputs as
shown in fig. 4-3, and then adjusts timing for all conditions based on it's
internal data map.
(fig 4-3 shows inputs/outputs: Control unit; engine speed: RPM sensor;
engine timing: TDC sensor;(Combined in VW Motronic),"engine load: Air-flow
sensor(MAF) ,coil, distributor, spark plug (outputs)."
" For any combination of of engine load and rpm, the control unit can supply
the best ignition timing.
For example, for an rpm-input signal of 2000 rpm,the computer would look up
the timing advance angle(in the ROM of stored timing data maps)" let's say
it should be 22 degrees BTDC."
"But the control is even more precise. Suppose the rpm was 2050, and the
memory contained only data points for 2000 and 2100;then the computer would
look up both 2000=22 degrees BTDC, and 2100=24 degrees BTDC, and
interpolate.It would calculate an advance for the 50 rpm difference between
2000 and 2050, and would output timing od 23 degrees BTDC."
"In the control unit, timing is computed so fast that Motronic can adjust
timing for the firing of each spark plug!"
Knock control: (quoting)
"The Motronic control unit has accurate information on crankshaft position
and firing order so it is possible which cylinder is knocking(one cylinder
usually knocks before the others).
Further,circuits operate with such speed that ignition timing can be
retarded only for the knocking cylinder,and advanced for the firing of the
next cylinder"
Now that you know how Motronic computes spark timing for each cylinder
separately, as opposed to the same value applied to all cylinders in
Digifant, it becomes easier to understand the knock control system.
One of the striking differences of Motronic vs Digifant is how the function
of the distributor has changed.
You can not adjust anything on Motronic, except the idle 100 rpm via
VAG-COM. Turning the distributor no longer advances or retards the timing,
because it no longer sends the TDC data..
I have talked with my local VW tech and he says that he has not yet seen a
Motronic engine knock or ping. It's a better system and now if you like
your WBX but hate Digifant, there should be a way to adapt Motronic.
Motonic will also squeeze more power out of your engine- switching from
Digifant 2 to Motronic for me has had the effect like going to a higher hp
engine.
If you want to better understand VW fuel injection systems I recommend the
Bosch Fuel Injection & Engine Management book by Charles O. Probst, SAE
published by Bentley. ISBN 0-8376-0300-5
The date of this printing is 1989 so the Motronic system here described
doesn't cover newer versions, but all are similiar in basics.
It's very well written for persons with a basic understanding of fule
injection theory who wish to expand their knowledge .
It also has troubleshooting and testing chapters that are better explained
than in the shop manual.
Robert K
1982 Westfalia
----Original Message Follows----
From: Alex Towner <altowner@COMCAST.NET>
Reply-To: Alex Towner <altowner@COMCAST.NET>
To: vanagon@GERRY.VANAGON.COM
Subject: The details of Motronic
Date: Wed, 1 Oct 2003 00:58:31 -0400
The following post from a couple of days ago leads me to a few questions:
Date: Mon, 29 Sep 2003 12:12:12 -0700
From: Robert Keezer <warmerwagen@HOTMAIL.COM>
Subject: Re: The "Other" System: Motronic (long)
Individual spark timing:
The ECU controls spark timing and calculates this for each plug individually
making for very precise ignition timing.
Minor variances such as plug gap and wear can be compensated for . This
translates to mean that one plug may get a slight ly earlier or later spark
to compensate for any variance.
Knock control:
The Motronic control unit has accurate information on crankshaft position
and firing order so it is possible to determine which cylinder is
knocking(one cylinder usually begins knocking before the others) Further,
circuits operate at such speed that ignition timing can be retarded only for
the knocking cylinder, and advanced for the firing of the next cylinder.
Each knock signal can modify timing in milliseconds.
1) how is it possible for a Motronic computer - on a car with a distributor
- to adjust the spark timing for each cylinder independently? The ignition
rotor passes a contact for a given cylinder on the distributor cap, which
in turn sends voltage to the spark plug for that cylinder - we all know
this. How could the computer adjust this mechanical process for an
individual cylinder? It can't adjust the relationship between the rotor and
the distributor cap, and there is no electronic regulating system
interfering with the ignition wires before they reach the spark plugs. The
computer can adjust the timing of the entire distributor, by changing when
electrical energy is released inside the distributor via the hall sensor,
but it doesn't seem like it could possibly do that once for each spark plug
with each revolution of the rotor. I can understand how an engine with
individual coils for each spark plug could regulate the timing for each
cylinder independently, but not one with a conventional distributor.
2) The same thing makes no sense to me with respects to knock control. If
knock is detected anywhere on a distributor equipped engine, I would think
the only thing the computer could do is adjust the timing for the whole
engine by altering its signal to the distributor. I don't see how it could
adjust cylinders independently. In this sense, it would seem it makes no
difference which wires from the computer are attached to which of the two
knock sensors. If knock is heard anywhere, the timing for the whole engine
will be adjusted.
Comments?
p;
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