Diesel Engine Piezo-Actuated Injectors
Piezo injectors evolved from the flow-control technology developed by Siemens VDO for ink jet printers. Piezo crystals expand when excited by voltage and, conversely, generate voltage when compressed. The latter feature is used in cigarette and barbecue-pit lighters.
The growth of piezo crystals under voltage is miniscule: a single wafer 80 μm thick elongates about 0.1 μm at 160V. Siemens stacks the wafers to produce a stroke of about 40 μm that is hydraulically amplified to unseat the spill valve. The wafer pack responds within 80 milliseconds of excitation, or three times faster than the best solenoids, and with an actuation delay an order of magnitude faster. As a result, injection can be divided into as many as seven precisely metered shots, each accurate to within less than a milligram of fuel. This translates into 3% more power and 15% better fuel economy than solenoids offer, and the ability to meet Euro 4 standards without particulate traps.
These injectors run on the same high-pressure regime as solenoid types: piezo actuated common-rail injectors, like the one shown in Fig. 6-3, see peak pressures of around 1600 bar, a figure that will be increased to 2000 bar in the next generation. Piezo unit injectors develop pressures in excess of 2000 bar. Depending upon the application, injector tips have as many as 10 orifices with diameters of less than 0.1 mm.
To date Siemens has built more than 15 million c-r injectors, used on upscale vehicles such as the Audi, Mercedes, Land Rover, Jaguar, and the 2007 Mercedes E320, which will introduce the technology to the North American market. Siemens also manufactures piezo unit injectors for VW and International. The latter operation is based in Blythewood, South Carolina, with all production going to the American diesel maker.
Solenoid injectors from Bosch, Denso, and Delphi continue to find buyers since they cost half as much as the Siemens version and meet current emission standards. Most observers believe that solenoids will hold their market share among low-end car makers until 2010 when Euro 5 standards come into effect.
Figure 6-3 shows a piezo injector in cutaway view. Wafer expansion opens a servo valve that stands between the high-pressure fuel delivery circuit and the low pressure return. With the valve open, pressure above the needle diminishes, while pressure in the nozzle cavity remains nearly constant. The differential unseats the needle to initiate injection.
Because wafer expansion is so miniscule, a hydraulic coupler is used to extend the stroke. The coupler also functions like a hydraulic valve lifter to compensate for lash caused by thermal expansion.