Pulsed injectors on/off times

[Jorgen Karlsson]

Ammeer Antar wrote:

> This sort of general auto tech question, but I'm considering
> using it on an
> Audi.


> For injectors, how do you find the values such as max t-on and min
> t-off. In other words, is there some standard percentage of rated
> flow that
> designers use for an EFI setup?

There is a simple way to measure the time it takes for the injector to open.
A signal generator an oscilloscope, a .1ohm power resistor and a drive
circuit is needed to do this. Using a car running at idle works.

Connect the resistor in series with the injector, measure the voltage over
the resistor with the oscilloscope, when you have got it to trigger
correctly you will have the shape of the current on the display. It will
increase rapidly at the beginning and then start to stabilize. Then at a
well defined point the current will start to increase quicker again, this is
the point where the pintle bottoms out, the injector is fully open.

If my memory serves me right the time before the injector is fully open it
can be considered as half open.

General times for a Bosch injector are 1-1.5mS, A Lucas disc type injector
opens in 0.5mS. These are the only brands that I can recommend. Bosch has
the better fuel atomization of the two, this to some degree compensates for
the slow opening time.

The max opening time can also be found in a similar way, but it is a lot
harder to find the point. Different fuel pressures change the max open time.

IMPORTANT:
When the injector is used above it's maximum time it will start floating.
This mean that the injector will deliver half as much fuel as it is supposed
to!

Stay with the 80-85% design rule, most injectors tend to float at around
90-95%. some Rochester injectors float at 80% when the fuel pressure is
high, raise the fuel pressure a bit more and they will not open at all!!!

The general rule is to design with half a pound per hp per hour. That mean
that a 40lbs injector will support 80hp when fully open. By following the
above rule it also supplies fuel for 40hp when it is 95% open!!! Remember
this. 100% is OK! 85-99% is begging for trouble!


Then we have different types of fire sequences, many cars use batch fire.
They Fire all injectors at the same time.

Bank to bank fire is an other scheme, the injectors are used in two
alternating groups.

Then we have the best way of doing it, Full sequential fire. Each injector
is fired just in time for its intake valve to open. This ensures that the
mixture will be perfect for all cylinders. Since the injector only fires
once every other engine rev it will also have more time to inject and to
close. This mean that the 80% duty cycle will be equal of a conventional
scheme running at 90% duty cycle. The same goes for small opening times, the
injector only fires at the open valve. The resulotion is far better when
using full sequential.

Audi uses sequential fire on the 20v engines.

Good sequential EFI systems can also vary the balance between the injectors,
really good injection systems can change the balance between the injectors
depending on rpm and load! Good systems can also vary the phase when the
injector fires and you guessed it, really good systems can vary the phase
with rpm and load.

Systems of the last type compared to a batch fire system has far better idle
with 80LBS injectors then the batch fire system has with 40lbs injectors. If
we compare with CIS we could probably fit 160lbs injectors and still have
far better idle quality.

I hope that this answers a lot of questions.

Jorgen Karlsson
Gothenburg, Sweden.