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Diesel Fuel Injection - How-It-Works

Inside Diesel Pumps And Injectors

Photography by Stephen Clark, Courtesy of General Motors Corp., Courtesy of Bosch

The key ingredient to maximizing the peak performance of a diesel engine is increasing the amount of diesel being burned. On old mechanical-injection engines, the only way to do this was to modify the injectors and/or the injection pump. The new electronic-injection systems have several ways to increase the fuel going into the cylinders, but ultimately, peak power production still comes down to the mechanical limitations of the injection components that build fuel pressure and inject the diesel into the combustion chambers.

The fuel system on most diesel engines is composed of three main parts: the injector, the injection pump, and in some cases, the engine control unit (ECU). In most diesel engines, fuel injectors are mounted in the engine's cylinder head(s), and the tip or nozzle of the injector sprays directly into the combustion chamber. In many cases, the injector is mounted just like a spark plug would be in a gas engine. But unlike fuel-injected gas engines that inject fuel at 10-60 psi, diesel fuel-injection systems run in the 10,000-30,000-psi range.

The VE pump is a distributor-style, mechanically controlled, axial-piston pump. Its input shaft is driven by the engine, and fuel is pressurized by axial pistons. Fuel is fed to the injectors by a port-controlled distributor; this mechanical device controls the timing and quantity of fuel going to each injector.










The CP3 is a radial-piston pump for high-pressure, common-rail injection systems. Manufacturers seem to be taking all diesels in the direction of the common-rail injection system. With Ford's new 6.4L Power Stroke switching to a Siemens common-rail system, all the domestic 3/4- and 1-ton diesel trucks will now be using common-rail technology. The common-rail system uses an accumulator rail(s) to maintain fuel at high pressure; this rail(s) feeds the fuel to the injectors. The CP3 pump functions similarly to the VP44, but the main difference is that the CP3 doesn't have a solenoid for fuel delivery to the injectors. The common-rail system uses either solenoid-valve or piezo-electric injectors to control fuel quantity and timing. The CP3s used on the Cummins and Duramax engines are very similar. The only difference is that the Duramax CP3 uses different fittings to feed two rails (one for each bank of cylinders), whereas the Cummins CP3 feeds only one rail for all six cylinders.

Modified CP3 pumps are available to increase fuel flow by 30 percent, and depending on other engine mods, this will add 60-100 hp. There are also kits to run dual CP3s on the Duramax or Cummins. A second CP3 is added with this kit, and it's driven by a belt pulley. With twice as much pumping capacity, good fuel pressure can be maintained when using aggressive injectors and electronics.

The P7100, or P-pump, is an inline-injection pump that uses a cam to actuate plungers to pressurize the fuel. According to some diesel fanatics, this is the mother of all injection pumps because of its extraordinary capabilities. Although it was replaced by the electronic VP44 pump on the 24-valve Cummins, some heavily modified trucks have taken a step back and switched the VP44 for a P-pump because of its ability to flow large amounts of fuel.

The aftermarket offers dozens of performance upgrades for the P-pump, making it the Holley 4-bbl of diesels. Industrial Injection alone has three levels of modified P7100s: The Dragon Fly has mild modifications and uses stock 12mm pumps capable of 550 cc of fuel delivery, the Dragon Flow uses 13mm pumps for 800 cc of fuel delivery, and the Super Dragon Flow uses 14mm pumps for 1,400 cc of fuel delivery. All these pumps can have the timing modified.

This part from Industrial Injection increases the fuel volume to the common-rail fuel system by adding an additional fuel-feed line between the pump and common rail. The downfall of the common-rail system is that after a full-throttle burst, the rail takes time to recover to maximum fuel pressure. Dual-feed lines are designed to cut rail recovery time in half. Less restrictive fittings are also used to increase fuel flow. Industrial Injection claims this simple modification can add up to 50-70 hp.

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