An engine breathes air, but it needs fuel for power - 3 of the 4 strokes each piston makes consume energy - the only stroke that provides power is... you guessed it... the power stroke.

Fuel injectors are little solenoids - wire wound electromagnets that move a core with a needle-like pintle on the end of it.  When the end of the pintle has the injector's spray port stopped up, no fuel can flow.  The injector in the photo is a faulty one - it flows fuel even with no current flowing through its coil.

On a properly operating injector, when the pintle moves  away from the spray port hole it's because the wire wound electromagnet receives a ground pulse - and the engine controller is the brain that operates the injectors.  So how does the PCM know what to do with the injectors?

Well, the PCM is kind of like you and me - it takes inputs and uses them to decide what to do with its outputs.  When you interact with the world, you use your eyes, ears, nose, limbs, and your taster. You smell food and your mouth automatically waters.  You use your hands and feet to search for the food you smelled so you can appropriate some of it.  Your ears provide stereo input so you can listen to things and enjoy the sound or figure out where a sound is coming from.  Your eyes provide 3D vision.

One way or another, you act on all those things - reaching for a pipe, you touch it.  That's an output.  Find that the pipe is hot, that's an input - a feedback mechanism, if you will.  Your brain then sends the command for your hand to move.  You just applied an output, gathered data, processed that data, and acted on it.  Sometimes your brain will move your hand before you even feel that the pipe is too hot.

The computer I'm typing with here is receiving input from my keyboard and mouse.  Its outputs are the monitor, speakers, and storage drives.

A computer on a vehicle takes input from the ignition switch and its related electrical relays (electromagnetic switches) and begins to check other inputs to decide what it needs to do next. 

It checks Engine Coolant Temperature through  a sensor.  It checks Air Temperature and Transmision Oil Temperature through other sensors.  It checks to see what the barometric pressure is and it looks in on the Throtlle Position sensor to determine what voltage reading it can use to determine that you don't have the accelerator depressed.

When you turn the ignition to the crank position and the starter begins spinning the engine, the speed of the spinning engine is detected by the computer - incidentally, we call the computer a Powertrain Control Module, or PCM, because that's what the boys in Washington tell us to call it.  If it only controls the engine, it's called an ECM - you can figure out what that stands for.

As the PCM determines how fast the engine is spinning and where number one cylinder is, it can figure out where the rest of the pistons are, but it doesn't know which stroke they're on until it checks in with the cam sensor - the camshaft, if you remember from "How Does an Engine Work," only turns half as fast as the crankshaft, thus the crankshaft to which the pistons are connected has to turn two full rounds (720 degrees) before a cycle is complete.

Okay, so the computer knows how fast the engine is spinning and it knows where the camshaft is, so now it knows when each injector should spray.  The computer's doesn't only operate the injectors; it also fires the ignition coils, and today's vehicles usually have several of those, either one for each pair of 'companion' cylinders or one for each cylinder. 

The computer also tells the fuel pump when to run and in some cases it even tells the fuel pump how much pressure to produce.  Fuel injected engines can have as little as 12 pounds of pressure (like the old GM and Ford throttle body systems or as much as 80 or more pounds, depending on the vehicle and engine platform.

Okay, so now the injectors are squirting fuel, clicking at a rate based on the speed of the crankshaft signal and timed based on the information that comes from camshaft position, which, by extension, tells the computer when the fire has gone out from the previous burn and when the intake valves will open - the injector sprays just before the intake valves open on a gasoline engine, but it doesn't need to spray if there's still any fire in the cylinder. That fire goes out at about 24 degrees of crankshaft rotation After Top Dead Center on the power stroke.  

So now, how does the PCM determine much fuel is to be delivered?

The amount of fuel injected is determined by the pulse width of the injector, i.e., how long the injector remains open when it opens. 

 The oscilloscope pattern of a normal injector pulse looks like the one you see on the black screen below.  The little bucket shaped area represents the amount of time the injector is open... after the injector closes, the collapse of the magnetic field creates the spike you see right after the bucket. When the computer calls for more fuel, the little bucket gets wider. The table right before the bucket is 12 volts as fed through the coil, and when the PCM energizes the injector, that 12 volts is pulled to ground and the pintle moves.  The 12 volts is still coming in on the other side of the injector coil, but the only side of the injector we're looking at with the scope is the side triggered by the PCM. Notice the voltage spike right after the injector is de-energized. That spike is induced voltage that comes from the collapse of the magnetic field sweeping across the windings in the injector.  An ignition coil uses this principle with two windings (one with a few turns and another with thousands of times more turns) to create its spark.

That pulse width is basically modified based on Engine Coolant Temperature, engine load (from the Manifold Absolute Pressure Sensor or the Mass Airflow Sensor), Intake or Manifold Air Temperature, Throttle angle provided by the Throttle Position Sensor, and the rate of throttle movement.

At higher altitudes (detected by the barometeric pressure input), the air fuel mixture is modified along with the ignition timing because the air is thinner up there and not as much fuel is needed.

  Send me an email through the "Contact Us" section if you have questions or comments on this article.                                                                                    Richard