Fuel Injectors Work

The TBI system took the place of the carburetor as governmental automobile pollution standards increased , as well as a simultaneous increase in consumer requirements for greater fuel economy. In the TBI system, a throttle body sits atop the engine in the location where the carburetor use to reside. The throttle body in fact looks very much like a carburetor, however it performs very much differently. Located within the throttle body are one or two injectors (depending on the vehicle). When the engine is started, a continuous spray of pressurized fuel is discharged thru the injectors, into the intake manifold, and from there on to the cylinders for combustion. The injectors in the TBI system are always open, and spraying fuel when the engine is on. They do not pulse on and off. Furthermore, the one or two injectors are supplying fuel to every cylinder collectively, as did the carburetor, but with much greater precision. While the TBI system, and the advances in engine management that went along with it, were a great improvement over the carburetor, there were yet more advances to be made. In the TBI system, each cylinder pulled in on the intake stroke, the volume of air/fuel available at the back of the intake valve. If the cylinders were closer to the throttle body, they were going to get more air/fuel than than those cylinders farther away. The TBI had greater efficiency over the carburetor, but still needing a more balanced air/fuel delivery, which in turn would further reduce exhaust emission, increase fuel economy, and increase engine performance.

So, petrol prices continued to climb and regulations on air pollution continued to get tougher. And along comes the MPI injection system. This system replaced the TBI and is now the only system in production by auto manufacturers. In an MPI system, there is no throttle body. Each cylinder now has its very own fuel injector, located within the intake manifold and delivering fuel directly behind the intake valve. A major improvement, in that now each cylinder has its own individual fuel supply, and the balancing of air/fuel volume between all cylinders is much more precise.

The fuel injector is an electrically operated solenoid valve. It consists of a metal (or composite molded) body, coil windings, an electrical connector, a pintle, pintle spring, fuel filter, orifice, pintle cap, o-rings, and spacers. Current is delivered thru the injector coil, when the ignition key is turned on. On cranking, the injector driver circuit grounds the injector, the coil winding is energized and creates a magnetic field (a magnet). The magnetic field then pulls the injector pintle from its seat (the orifice) allowing pressurized fuel to flow through the injector, through the orifice, to the intake valve, and into the cylinder. When the injector ground is cut off, the magentic field collapses and the pintle is pushed back by a spring to its home, and fuel delivery stops. The actual driver circuitry is much more complex than stated. However, that is another topic.

The solenoid valve, in fact is a rather simple device, yet extraordinary at the same time. The history and principle of the solenoid, is attributed to the French physist, Andre-Marie Ampere, in the early 1800's. Note the name Ampere... (amps)... sound familiar? Tis doubtful that Ampere could have envisioned his discovery of magnetism induced from electrically energizing a coil winding, would literally, several hundred years later, been applied to so many hundreds of million components, that it literally "moves the world".

As brilliant, or a simple, as the injector is, depending on your perspective, it has but one and only one purpose. That is to allow fuel to flow into the cylinder. BUT, it must do so within the following parameters. It must filter the fuel, it must deliver the proper volume of fuel, it must deliver the proper fuel spray pattern, and it must close and seal properly. The injector has no "mind of its own". It has no self-correcting capabilities. Either it works within its manufactured specifications, or it doesn't.

Consider this. On the V12 HE engine, each injector fires once every other crankshaft revolution, two revolutions being one complete engine cycle. At 3,000rpm, when this particular engine is just beginning to reach its powerband, and also where we would most frequently be on the tach, each injector is firing at a rate of 25 times/second. At 6,000rpm...50 times/second. In one hour of operation at 3,000rpm, each injector has cycled 90,000 times. If you drive 12,000miles/year...a pretty astonishing 18,000,000 times/per injector!! And remember, this is a mechanical device with moving parts, coils windings energizing and collapsing, pintle valves up and down, and in an extremely hot environment. And it's doing this in the milliseconds....thousands of a second!! And if you want to think about its fuel flow...figure 18mpg at 12,000miles/year and you get 667 gallons/year or 55 gallons of petrol per injector. That's a 55 gallon oil drum full. For those of you with older vehicles...well...you can do the math. But... I will add one more. A vehicle with 100,000 miles on the clock...150 million cycles/per injector. That's a pretty hefty demand on such a small part, which is why they are so damned expensive to buy!

With such severe duty cycle, the injector has to be subject to failure....mechanically, electrically, or fouling from the volumes of fuel it flows. There are several failure modes that can occur...which I explain on another page. The only way to definitively know if injectors are operating within specifications, is to pull the injectors and have them "bench tested" off the vehicle, where they can be tested electrically, visually inspected for proper fuel spray pattern, flow tested for proper volume delivery, and leak tested for pintle/seat or mechanical leaks . They can then be cleaned, serviced, and in most cases restored to service, eliminating the high cost for new injectors. Remember...you can't fix what you can't see!