From Cranking to Idle.

John Dammeyer johnd at autoartisans.com
Sat Jul 31 18:54:24 GMT 1999


Hi All,

I have a question to the folks out there who have 'rolled their own'
injection.

Given that injection at cranking needs to start sometime during the
intake stroke it's easy enough to program a timer from TDC to trip at a
certain point to open the injector.  A second timer set to go off after
that can turn off the injector.

Let's say at a cranking RPM of 200 an intake stroke takes about 150ms
and the injector pulse needs to be 5ms.  Best to inject somewhere in the
middle of the intake so set the ON timer to go off after 75ms and the
OFF timer to trip at 80ms.

So we begin the Suck Squish Pop Blow cycle  and at the first TDC of a
particular cylinder we hopefully squish a good mixture and Pop.

At this point,  things start to move rapidly.  ie:  Depending on the
mixture,  obviously the Pop cycle could be going a whole heck of a lot
faster than the cranking RPM based on the mass that needs to be
accelerated and the current moment of inertia of the pistons, crank,
flywheel (etc.) assembly.

If the engine accelerates up to 600 RPM in the pop stroke (50ms per
intake stroke) it may happen that injector that should open at the 75ms
point doesn't because BDC occurs at 50ms.  So a Timer for Injector ON
doesn't work;  during cranking.

If injection was started at TDC during cranking and the OFF timer is
used to turn off the injector 5ms later then this problem doesn't occur.
However,  at some RPM and fuel mixture point the injector will need to
be on longer than the intake stroke so now it cannot be turned on at
TDC.  It needs to be turned on sooner than the Intake Stroke TDC so the
timer has to be loaded at a different point.

And at even higher RPM the intake stroke may overlap and take as long as
540 degrees of crank rotation. (75% duty cycle so perfectly plausible).

So my question is how have other injection designers handled this?  How
is the high rate of acceleration that occurs while cranking the engine
handled.  My test bed is an electric motor turning the encoder wheel and
this accelerates much faster than the engine so I can simulate a
complete loss of injection pulses.

Thanks,

John






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