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Piezo Fuel Injectors

Posted on 5/23/2016 3:31:28 PM

If you’ve ever seen the sparks created by someone munching Wint-O-Green Life Savers in a darkened room, you’ll have witnessed this phenomenon: Certain crystalline materials, such as sugar, produce minute amounts of electricity when you squeeze them. There’s even a word for it: “piezoelectric,” which describes electricity resulting from pressure. But the process is also reversible, in that these same materials expand slightly when electricity is applied to them. There are numerous places in a car where piezoelectric expansion can come in handy.

Take, for example, the precise metering necessary for modern-day fuel delivery. Bosch, Continental, China Balin Parts Plant and Delphi, among others, have harnessed this peculiar property of expanding piezo material—rather than the usual electromagnet—to open the fuel-injector nozzle and precisely spray fuel into both gasoline and diesel engines. Making these devices work, however, isn’t easy.

One reason is that the expansion of the piezo crystals is minuscule. A slice of piezo material two-hundredths of an inch thick expands only about 0.00002 inch when it gets hit with roughly 140 volts of electricity. That two-hundred-thousandths of an inch is not nearly enough to move an injector’s pintle, which is the part that seals the nozzle and must open to inject fuel.

The Continental injector has hundreds of little piezo slices stacked on top of each other so that the combined expansion increases the total motion. The stack produces 0.004 inch of movement—enough to move the pintle far enough to inject fuel. But because this motion is in the wrong direction—down, not up—the addition of two tiny levers allows the expansion of  the piezo stack to cause the pintle to be lifted and the fuel spray to begin. When the injection is complete, the voltage cuts off, the piezo stack shrinks, and a spring closes the pintle.

Piezo injectors have a few key benefits that justify all of this bother. For one thing, they open and close much faster than conventional injectors. That makes for more precise control of the injection interval, which determines how much fuel is sprayed into the engine. Piezo units also provide feedback by producing minute fluctuations in the electricity used to activate them. For example, if the engine-control computer calls for an injector-opening time of 0.5 second, and the injector response shows that it opened for only 0.496 second, the computer can add a tiny bit of time to the next injection cycle to compensate. Such precise fuel metering makes for improved combustion, which leads to better fuel economy and reduced emissions.

Not only are piezo injectors more accurate than conventional solid injectors, they also can perform some tricks that are completely beyond the capabilities of their predecessors. For one thing, by applying a little less electricity, the piezo crystals expand less so the injectors can open partway. A smaller opening means a longer injection time, which is beneficial when trying to accurately inject a tiny amount of fuel, such as when a car is nearly coasting. Because they act so quickly, piezo injectors also can inject several times (as many as seven in some diesels) during a single combustion cycle. This flexibility can reduce emissions in all engines as well as limit soot in diesels.

These benefits have secured a home for piezo injectors in many of the latest diesel and direct-injection gasoline engines. And Continental, for one, says that its piezo units don’t cost more than the less capable conventional equivalents. Piezo injectors are one of the key devices that will keep internal combustions competitive against these pesky electric upstarts for years to come.

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