Diesel injectors have gotten increasingly complex over the last twenty years of diesel development, but their basic design is fairly simple. Diesel fuel from the injection pump enters the mechanical injector body and starts to build pressure. Once the pressure is high enough (about 4,000 psi), a check valve in the injector is lifted off its seat, and fuel sprays through. Any excess fuel that remains after the valve is opened is then returned back out through the injector body, then back to the injection pump.
Ford Takes A Different Route
In 1994, Ford changed the engines in its F-series from indirect-injection, to a direct-injection HEUI system. HEUI stands for hydraulic electronic unit injection and uses engine oil as an actuator for the injector. The engine oil is used to boost fuel pressure inside the injector, so if you have a 7.3L or 6.0L Power Stroke, make sure your engine is full of oil-and change it often-it's what's helping fuel your truck.
Common-Rail Systems
In 2001, General Motors introduced a new line of diesel pickups, which featured a Bosch common-rail injection system. While common-rail injection isn't a new idea, it has helped modern diesels become quieter, more efficient, and more emissions-friendly. In 2002, Dodge jumped on the common-rail bandwagon, as did Ford in 2007. Common-rail injectors are much more complex than their earlier counterparts because they use a solenoid and two pressure chambers to create an injection event. The solenoid is triggered by the vehicle's computer, which is used to vary injection timing and fire multiple injection events. Many of the newest diesels on the market use super-fast injectors controlled by piezoelectricity, called piezo injectors for short. They use crystals and electricity as an actuator and can fire up to five injection events per power stroke, which helps reduce emissions and quiet the engine.
Modifying Injectors
At the very tip of the injector is the injector nozzle, which is the part that is most commonly modified when you purchase aftermarket injectors. New nozzles are fitted to the old injector bodies, usually resulting in horsepower gains from larger or additional injector holes. As long as the injection pump and turbocharger can keep up, having larger injectors will flow more fuel into the engine, and produce more power. When a custom set of injectors is ordered, each injector is referred to by the number of holes multiplied by the hole size. Therefore, 5x13 injectors would have five holes at thirteen-thousands of an inch. In the case of very large injectors, internal modifications can be present as well, which is why price usually goes up for the higher-horsepower models.
 Perhaps the most important...  Perhaps the most important part of an injector is the nozzle. Spray angle and pattern are very important to combustion and will affect horsepower as well as fuel economy. |  Here, an injector is being...  Here, an injector is being tested by using a laser to measure its spray pattern. |  Here are the two latest injector...  Here are the two latest injector designs. On the left is a solenoid-activated common-rail injector. On the right is a piezo injector. |