Optimum ignition dwell?

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Optimum ignition dwell?

Postby adam7 » Wed Nov 22, 2006 1:39 am

Can the picoscope (212/3) be used to find the optimum dwell timing?

Id be interest in any feedback, and in particular in any technical data anyone could provide on the denso plug top coils (NEC 100730 MB029700-8230 12V GCL209 as used on Rover 1.8 K-series EU3 engines).

I image that the optimum dwell time could be found using the colis primary current and voltage traces, and also knowing what peak engine rpm will need to be supported/reached.

I *think* that the two factors to consider for optimum dwell timing are:
1) long enough to fully "charge" the coil, or at least get a "good" spark.
2) short enough (particularly at peak rpm, eg high duty) that the ECU and coils dont over heat.

Some ECUs can use either a fixed dwell timing value, or indexed value based on RPM. Why would the optimum dwell value change with RPM? Is it related to coil/ecu heat at the higher duty cycle that comes with fixed dwell time and a short period (higher RPM), or maybe the energy required for a spark changes with RPM?

Cheers
Adam
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Optimum ignition dwell?

Postby davidj » Sat Dec 02, 2006 6:08 am

Hi Adam
Thank you for your post.
You could use your Picoscope to measure dwell (coil-on time) in ms if this interests you. In terms of 'optimum' dwell this may be harder to determine.
With conventional 'points' ignition, note that as engine speed rises the dwell time remains the same (set by the points gap) but the time is reduced. As a result at high engine speeds, with less time to charge the coil a lower kV spark was produced.
As things have moved on, many vehicles today employ constant energy ignition systems, using fast charge, low resistance and inductance coils that are under tight control of the PCM. By monitoring the voltage drop across a high power precision resistor in series with the primary coil, current is controlled and within limits, secondary kV is kept constant across the rev range.
Engine developers spend many hours getting the ignition mapping optimised for stock engines and so I would have thought there was little to be gained here in terms of tuning potential.

There are many text books available with more on this subject. We stock a particularly good one here:
http://www.picotech.com/auto/automotive-library.html

Regards,
David
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Postby adam7 » Sun Dec 03, 2006 11:09 pm

Thanks for your comments David. Ive had a look around your "library" section, and there is some very useful information there - particularly for fault finding etc.

In this instance, my interest is more in the tuning/development stages, eg taking a "stock" engine, modifying it and then programming an ECU (ECM) to control the ignition and fueling systems.

Thanks again for your response.
Adam
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