- Common rail injection can best be thought of as the child of direct injection and electronic multipoint fuel injection. Multipoint fuel injection systems, which are used on almost all modern cars, use a single fuel injector mounted in the intake port of the engine to spray fuel into the engine's cylinder via its intake valve. Direct injection moves the fuel injectors into the cylinder, where the spark plug would be on a gasoline engine, to inject fuel directly where its needed.
- Direct injection is by far the best injection method, especially for diesels that depend on accurate and well-timed fueling. However, direction injection requires high fuel pressures to overcome the air pressure caused by the rising piston. This has traditionally necessitated a positive-displacement fuel pump, which resembles a small four-stroke engine. These pumps are timed to engine movement. What manufacturers needed for their modern diesel engines was the accurate injection of electronic MFI with the raw power of a traditional DI system.
- A common rail direct-injection system uses a single fuel reservoir to connect the fuel injectors, but feeds that rail from a powerful pump like those used with direct injection. The fuel pump pressurizes the rail to pressures exceeding 23,000 pounds per square inch. A computer opens and closes the fuel injectors to send a jet of pressurized fuel from the rail to the cylinder.
- The switch from DI to CDI is similar to and every bit as revolutionary as last century's transition from carburetors to electronic fuel injection. Mechanical DI systems are known as "open-loop," which means they perform the same way no matter what the outside conditions or engine condition. CDI uses an array of sensors to determine how much fuel to inject and when to inject it for best performance. CDI-equipped engines are far cleaner, quieter, more powerful, more fuel efficient and more versatile than any mechanical system could ever hope to be.
- Even though CDI technology is still in its infancy, Caterpillar Diesel has already taken the next step in CDI evolution by eliminating the fuel pump and its dangerously pressurized fuel rail altogether. CAT's new HEUI (Hydraulically Actuated, Electronically Controlled, Unit Injection) injectors use engine oil pressure to actuate a quivering diaphragm inside the injector. This diaphragm pressurizes the fuel to the requisite psi right inside the injector, allowing the computer to adjust not only injection timing but fuel pressure to meet real-time engine demands.
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