When the injection is complete, the voltage cuts off, the piezo stack shrinks, and a spring closes the pintle. 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. The stack produces 0.004 inch of movement-enough to move the pintle far enough 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. 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. 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. One reason is that the expansion of the piezo crystals is minuscule. Making these devices work, however, isn’t easy. Bosch, Continental, 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. Take, for example, the precise metering necessary for modern-day fuel delivery. There are numerous places in a car where piezoelectric expansion can come in handy.

But the process is also reversible, in that these same materials expand slightly when electricity is applied to them. There’s even a word for it: “piezoelectric,” which describes electricity resulting from pressure. 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.

Gasoline piezo injector