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Diesel-injector nozzles and their care

11/05/2013 @ 9:05am

If you were to talk to diesel manufacturers, they would say the majority of the cost in the engine is found in the fuel system. On a mechanical-injection diesel, this consists of the injection pump and nozzles. These components are the heart of the diesel. They are not only critical to its operation but also extremely costly to replace if a failure occurs.

Many refer to the part that delivers fuel to the cylinder as an injector. To a diesel expert, however, the injector is the nozzle-holder assembly. Over time, it has been used to describe the actual nozzle. This misnomer has become complicated by the fact that there are different fuel-system designs, which include mechanical-unit injectors, electronic-unit injectors, and hydraulic-actuated electronic-unit injectors. There are also brand-specific variations in the nozzles by manufacturers, but the basic function of nozzles, their service procedures, and maintenance tips all apply.

To complicate matters within the mechanical category, there are many different designs, and they generally share operating characteristics. (I say, “generally” because these shared characteristics don’t always apply.) Compare this to hydraulic injectors, which are usually classified by the nozzle design including:

  • Poppet

  • Pintle

  • Multiorifice

  • Electrohydraulic

With each design category, there are often subsets of styles such as those used strictly with indirect-injection or direct-injection applications. Regardless of the design, a mechanical injector that contains no electronic parts can be serviced and needs to be serviced. Electronic-enhanced injectors in light-duty applications, on the other hand, are traditionally not serviceable and need to be replaced as a unit.

There are three terms that pertain to nozzle testing and service. They are nozzle-opening pressure (NOP), back leakage, and forward leakage.

An injector nozzle can be considered a hydraulic switch. One of its design elements is the pressure at which it opens. This is usually set with either a spring-tension adjustment or, on some models, with shims. The terms pop-open pressure and popping pressure are also used instead of nozzle-opening pressure.

All about pressure

Whichever term is used, it describes the amount of pressure that must be created by the injection pump before the nozzle will pass fuel into the cylinder.

Each model of engine and nozzle design has its own NOP value that typically varies from 1,000 psi to 5,880 psi.

Some nozzles employ an internal opening valve that returns unused fuel to the tank. The internal leakage is a result of the nozzle valve-to-nozzle body clearance. It is measured during bench testing for 10 seconds and recorded as back leakage.

Forward leakage is the nozzle’s ability to not drip or leak until the NOP is realized. It confirms the nozzle’s ability to seal. To test for forward leakage, a pressure of approximately 150 psi below the NOP is created on the bench test. No visible dripping is allowed.

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