PWM injector driver

[Bernd Felsche]

Brian Michalk tapped away at the keyboard with:

> When you guys talk about a PWM injector driver, are you referring
> to SEFI, or just EFI?  If you are referring to EFI, then what is
> so special about the peak and hold features?  I thought it was a
> digital on off thing, and not an analog thing.

There are two types of pulsed injectors: peak-and-hold (P&H) and
saturation (SAT). The former are much more common; resistance is an
indicator of the injector type; about 2 Ohms for P&H and 8 Ohms for
SAT.

The peak current in P&H arises when the injector has been "fully
opened". A subsequent, reduced holding current is required to keep
the injector open for the required time. Maintaining the peak
current is likely to burn out the injector with long and/or high
duty cycles.

A SAT injector OTOH will happily operate as a "switch" with full,
nominal battery voltage applied for the entire cycle as long as
there's a flow of fuel.

The high, peak current in a P&H when opening provides a greater
magnetic flux for faster-acting injectors. The hold current
maintains sufficient flux to hold the injector open against the
closing spring - not enough to open the injector quickly.
Rapid opening provides for more predictable injected quantities and
better control.

Which begs the question of whether anybody uses a reverse current to
assist in closing the injector. One would probably not want to give
it much more than a "nudge" due to the needle valve seat accelerated
wear under excessive closing force. Hydraulic damping just before
contact *may* cushion the impact but I've not read of that as a
feature in the literature.

I'll leave that one to the experts.

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