Car OBC

[Dave Plowman (News)]

I have an old Rover SD1 fitted with a Smith's OBC which gives MPG, etc.
The injection system is Lucas/Bosch and the OBC works by adding one
injector circuit open time against distance travelled. And is pretty
accurate.

But I've changed the EFI to a MegaSquirt. This drives the low impedance
injectors directly - ie no resistor pack - and limits the current using a
flyback circuit. They are also driven PWM rather than straight DC as on
the old system.

This confuses the OBC. But not always. It sometimes gives an instantaneous
reading which could be correct - sometimes one which is obviously not.

I can't find a schematic for the Smith's unit so can only guess at the
input. And dunno whether it's the flyback current limiting or the PWM
which is upsetting it.

Any suggestions for an interface which would sort things?

--
*Change is inevitable, except from a vending machine.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[Dave Plowman (News)]

Anyone recommend a better newsgroup to try? I realise it's more a design
than repair thing.

--
*Whatever kind of look you were going for, you missed.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[JeffM]

Dave Plowman (News) wrote:

I've changed the EFI to a MegaSquirt.[...]
They are also driven PWM
rather than straight DC as on the old system.
This confuses the OBC. But not always.

A gated pulse train to simulate what the original saw?

 

[JeffM]

JeffM wrote:

Dave Plowman (News) wrote:
[Fuel] injection system[...]
I've changed the EFI to a MegaSquirt.[...]
They are also driven PWM
rather than straight DC as on the old system.
This confuses the OBC. But not always.

A gated pulse train to simulate what the original saw?

Forgive my inverted thinking. What you need
to turn a pulse train into a something resembling DC
is an integrator. TV vertical sections use those gizmos.
You need to put some numbers to this before proceeding.

 

[Dave Plowman (News)]

In article
<9d12baa7-a19b-4379-9b6e-7fb7df66368e@g4g2000prj.googlegroups.com>,
JeffM <jeffm_@email.com> wrote:

JeffM wrote:
Dave Plowman (News) wrote:
[Fuel] injection system[...]
I've changed the EFI to a MegaSquirt.[...]
They are also driven PWM
rather than straight DC as on the old system.
This confuses the OBC. But not always.

A gated pulse train to simulate what the original saw?

Forgive my inverted thinking. What you need
to turn a pulse train into a something resembling DC
is an integrator. TV vertical sections use those gizmos.
You need to put some numbers to this before proceeding.

Thanks, Jeff. Can you clarify what's needed spec wise - I'm more of an
audio sort of person. ;-)

--
*Funny, I don't remember being absent minded.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[JeffM]

JeffM wrote:
What you need
to turn a pulse train into a something resembling DC
is an integrator. TV vertical sections use those gizmos.

Dave Plowman (News) wrote:
Thanks, Jeff. Can you clarify what's needed spec wise

As stated:
You need to put some numbers to this before proceeding.

Got an oscope?

You need an RC time constant that will integrate this a bit.
In a TV, the integrator must combine the closely-spaced stuff
during the vertical sync pulse interval to make one fat pulse.
Your task is similar.

If you make the time constant *too* long,
the intervals between cylinders will be joined up;
**that** is what you need to figure out how to avoid.

Surely you've seen pictures of how a power supply filter
slurs one (sinusoidal) peak into the next.
Similar deal there.

You need to characterize what you have
and/or do some trial-and-error runs.

- I'm more of an audio sort of person. ;-)

RC filters are common in audio as well.

I already mentioned power supply filters
and their similarity to this application.

 

[JeffM]

JeffM wrote:
What you need
to turn a pulse train into a something resembling DC
is an integrator. TV vertical sections use those gizmos.

Dave Plowman (News) wrote:
Thanks, Jeff. Can you clarify what's needed spec wise

As stated:
You need to put some numbers to this before proceeding.

Got an oscope?

You need an RC time constant that will integrate this a bit.
In a TV, the integrator must combine the closely-spaced stuff
during the vertical sync pulse interval to make one fat pulse.
Your task is similar.

If you make the time constant *too* long,
the intervals between cylinders will be joined up;
**that** is what you need to figure out how to avoid.

Surely you've seen pictures of how a power supply filter
slurs one (sinusoidal) peak into the next.
Similar deal there.

You need to characterize what you have
and/or do some trial-and-error runs.
http://google.com/search?q=filter+capacitor+%22+63-percent%22
http://google.com/search?q=filter+capacitor+%22+63.2-percent%22
http://google.com/images?q=filter+capacitor+%22+63.2-percent%22

- I'm more of an audio sort of person. ;-)

RC filters are common in audio as well.

I already mentioned power supply filters
and their similarity to this application.