Gee- I had overlooked the current draw for the injectors! It would seem that a 40 amp
 alternator is not sufficient for my application, and I have pretty minimal draw after
 accounting for fuel pump, injectors, and minimal avionics. This would explain why I'm
 getting good voltage on the ground at 1800 rpm, but not in the  air at 6400rpm.
 Anyone know what the injectors draw?

   There is a 45 amp alternator that fits my mounting application, but only bumping it
 up 5 amps is not enough. I might have to see about mounting a second alternator. Sort
 of a PITA. That Ammeter from Digikey looks good though.

 Brian Trubee


 Unplug one and use an ohmmeter to measure the coil resistance.

 The max current can be found by dividing the number of ohms found into the max
 voltage or 15V.

 Which injector are you using. Some of the older ones used peak and hold and the above
 scheme won't work. If it is around 10 or 12 ohms it is not peak and hold.

 If it is 12 ohms it is drawing 1.25 amps wide open max. However they never run wide
 open all the time so 80% of that is about right. Say 1 amp. A larger alternator
 won't hurt.

 Paul Lamar


Fortunately, injectors have have a property called inductive reactance ,
which dramatically reduces the actual current drain of normal operation.
My injectors measure 14 ohms, but the measured current flow at 80% duty
cycle using a maximum pulse width of 17 milliseconds (4-stroke engine at
5400RPM) is about 80 milliamperes, or .08 amps, only 10% of the Ohm's
law calculation.

The instaneous initial current is what is calculated by Ohm's law, but
the Inductive reactance reduces the initial inrush current as the energy
from the current creates a magnetic field around the coil. This field
expands and generates a voltage in the coil opposing the voltage applied
by the electrical system reducing current flow while the field builds.

As a practical matter the duration of the injector pulse has relatively
little effect on the total current consumed, as most of the energy is
used in the first half-millisecond to perform the work of opening, and
the reactance reduces current flow for the duration of the pulse.
Because the density of the magnetic field is the product of current
(amperes) multiplied by the number of turns of wire in the coil, it is
possible for a lower resistance injector to draw less power that one of
higher resistance, for resistance represents power lost to heat. The
ideal injector would have no resistance. Therefore do not assume that a
higher resistance injector will need less current.

Once the pulse is on long enough for the field to become saturated, the
current rises to the initial level, and the injector begins to heat
internally, so don't test them by connecting to a battery - they can
burn out before you are aware  of the temperature rise.

In a rotary the pulse width should never exceed 8ms and  for injectors
of 10-14 ohms it is reasonable that maximum average injector current
would be less than 150 milliamperes per injector at 7500 rpm at max
power, decreasing linearly with rpm, and slightly lower with reduced
pulse width.

Victor Roberts

A rotary injector fires every rev. A piston engine injector fires every other rev.
7500 RPM is 125 revs per second. That is one rev in 8 millisecond. (0.008)  Therefor
the rotary  injector is on (7 ms) most of the time at full power 7500 RPM in a
rotary. The fuel flowing through the injector cools it.

Paul Lamar
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