fuel injector pulse width monitor

Posted by Author makecircuits

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.

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]