Fuel Atomisation

[Greg Hermann]

At 10:21 AM 12/10/01, Bob - Uni wrote:
>Hello Greg,
>
>Is that why bigger injectors that flow higher but dont atomise as well
>tend to rob you of idle quality and low down power? Would it be better
>then to have two smaller injectors per cylinder than one bigger one?
>
>Monday, December 10, 2001, 7:22:30 AM, you wrote:
>
>GH> The carby (talking IR carbys, one throat per cylinder, such as Weber DCOE;s
>GH> or IDA's with the butterflies fairly close to the intake valves) puts

(snip)

That particular aspect of it had not occurred to me, Bob, but it sure could
be a factor.

I tend to think that the biggest single factor with loss of idle quality
with big injectors is getting the pulse width down too short for the
injectors to produce truly repeatable flow quantities at idle, though.

To put it in "rigorous" control engineering terms, injectors only have so
much "dynamic range".

Absolutely, two, staged injectors per port will help idle and light load
performance and drivability a _LOT_ on an engine with a wide dynamic range
of output (such as a boosted engine).

I believe that the best approach to sizing staged injectors is 1/3 primary,
2/3 secondary. Play with the numbers a bit, and I think you will see why I
like this approach.

Have often wondered about ways to widen the dynamic range of available
injectors. The peak & hold approach is one pretty obvious approach to this,
but it hasn't been taken very far. The real problem is the finite time
necessary to build and then collapse the magnetic flux field in the
injector's solenoid. Dump circuits for collapsing it quickly are well
known, but I am not aware of such ever being used in EFI applications. Much
more radical approaches to peak and hold are also possible. A _REALLY_
competent, older EE guy once suggested to me that I take a peek at some of
the print hammer throw circuits that were used in the last and fastest
electromechanical teletypes and printers !! Apparently some of them were
using a (very) brief pulse of something on the order of 230 VDC to build
the flux in the hammer coil _quite_ quickly. Bob felt there was absolutely
no reason why such technology could not be applied to fuel injectors !!

I rather tend to agree with him, but don't really have the electronic
espertise to pursue it a lot further!

I really think that air shrouding, or "boosting" of injectors is the answer
for better atomization, though. Am plotting a way to do that with
conventional injectors, but the beginning emergence of "direct" injectors
may make it easier to accomplish this  before long.

One of the things that becomes possible with air shrouded injectors is
dynamic control of fuel rail pressure--since you don't need 3 bar of rail
pressure to get the current mess that passes for atomization, you could run
much lower rail pressure at light loads and idle, and higher rail pressure
at higher loads, thus widening the dynamic range of the injectors by a
factor of 1.5 or 2 without any electronic tricks.

Regards, Greg


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