Accuracy of timing?

[Bernd Felsche]

Mike Blakey tapped away at the keyboard with:

> I did some checks yesterday, and it turns out that the accuracy is
> 0.25 deg at worst or spot on at best. I am using a 60-2 tooth
> disk, but I could get greater accuracy if I use both the rising
> AND falling edge of the tooth and get 0.125 deg resolution. But is
> this necessary ?

0.25 deg resolution is more significant at low crank speeds and less
so at higher speeds. The variability of the combustion process is
greater w.r.t. crank angle at high speeds.

So although it's nice to know the crank angle to that resolution,
especially if you're using crank-speed variation as a metric of load
torque, it's not essential to have that "accuracy" because the
process is more variable.

See SAE papers 199-01-0203 "Enhanced Engine Position Ascquisition &
Treatment" by Leteinturier and Benning (Siemens) and
1999-01-0543 "Indicated and Load Torque Estimation using Crankshaft
Angular Velocity Measurement" by Azzoni, Minelli & Moro (Univ.
Bologna), and Flora & Serra (Magneti Marelli).

Both those papers are in SAE's "Electronics and Engine Controls 1999"
SP-1418, (which I ordered myself for Christmas) along with over a
dozen other papers including an interesting one on "smart" NOx
sensors in what is called a "chip-in connector" presented by NGK and
Siemens (1999-01-0202).  The smart sensor features direct-drive by a
microcontroller using PWM to provide pumping currents.

> What accuracy do OEMs run at ?

Some no better than about 72 degrees; VW Digifant, Bosch
L(H)-Jetronic which pick up the timing from the ignition distributor
Hall sensor with vane edges at typically 78 and 6 degrees BTDC. It's
difficult to get a smooth idle (amongst other things) because of
that.

-- 
Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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