fuel injector pulse width monitor


Here is a picture of the faceplate that holds the LED
bargraphs for air/fuel and pulse width as well as the pot to
control the fuel mixture above 7psi boost. The switch allows
either the primary or secondary injector pulse to be monitored.
The faceplate is made of polished aluminum. The air/fuel (O2)
monitor, pulse width monitor, and fuel controller are all on one
printed circuit board.

Fuel injector pulse width monitor

Shown below is the schematic for the pulse width monitor. The
heart of the circuit is the transistor current source which
charges cap C10 to form an integrator. The theory is that the cap
is charged and the voltage across it increases linearly for the
amount of time that the injector is energized. As the injector
is switched off the voltage in C10 is stored in cap C9 and
then C10 is reset to 0 volts to get ready for the next cycle. The
output of the circuit is the voltage across cap C9, this voltage
is fed directly into U3 the bar graph chip to be displayed as a
value betw1een 1-10 on the LEDs. Potentiometer R31 sets the
charge rate on C10 and basically sets the max injector on time
that can be displayed.

The calibration on this circuit is pretty tricky. You`ll need
an oscilloscope and a waveform generator. Setup a 62Hz 0-12v 50%
duty cycle square wave at the input to the circuit. Use the scope
to monitor the voltage across C10 Adjust pot R31 such
that the ramping voltage across C10 just reaches the peak (starts
to flatten out). Now adjust the pot R22 in the display circuit
such that LED 10 just comes on.

With the calibration described above LED 1 corresponds to
about 1.5ms and LED 10 corresponds to about 8ms of injector pulse
width (on time). Injector duty cycle in dependent on engine
speed according to the following relationship:

% duty cycle = ( pulse width / period )*100

period = 1/[engine speed (rpm) / 60]

Fuel injector pulse width monitor


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