Designers of the new Peugeot 307 automobile have made a breakthrough in automotive diesel engines - thanks to their use of a piezo stack developed by Epcos. The outstanding features of the car's innovative propulsion system are economy, high torque and low emissions.
Until around 20 years ago, diesel-powered cars enjoyed the advantages of lower fuel prices and longer engine service life, but these were offset by the sluggish acceleration and higher price of diesel engines. Later, turbocharging and common rail technology, in which all injection nozzles are supplied with fuel at a constant pressure of 1600 bar and electrically controlled, combined the lively response of gasoline engines with the traditional economy of the diesel.
Developed and manufactured exclusively by Epcos, the piezo stack boosts the efficiency of common rail systems. Piezo stacks belong to the family of multilayer ceramic components. They take advantage of the inverse piezoelectric effect discovered by Curie and Lippmann in 1881: if a voltage is applied to a piezoelectric crystal, its dimensions change. But this effect cannot be exploited and no significant change in length obtained until several piezoelectric elements are superimposed. Made up of 350 thin ceramic layers, a piezo stack has a length of 30 mm and crosssection of 7 x 7 mm. When a voltage of about 160 V is applied to the stack, its length increases by 40 µm. A maximum field strength of 2 kV/mm is constant within the piezoceramic material. Depending on the steepness of the control pulses, currents in the region of 30 to 40 A occur momentarily, however, the mean value is only in the milliampere region.
Up till now, common rail systems have used solenoid valves to control the injection nozzles. Piezo stacks have the advantage of substantially lower inertia. If the power dissipation limit, which is also determined by external parameters such as ambient temperature, is observed, continuous pulse sequences of several hundred hertz can be mastered. That means substantially greater precision for timing and dosing fuel injection. In practice, five or six fuel injection operations can take place per cycle today, but injection is limited to two operations per cycle in the Peugeot 307. This yields the following advantages: improved fuel combustion; higher efficiency and 15% lower fuel consumption; substantially reduced exhaust emission; reduced noise emission, as uniform combustion prevents diesel knock.
Epcos is pushing piezo technology. The silver palladium electrodes originally used have been replaced by copper contacts to reduce costs. Minimum service life, currently at 109 operations, will increase by a factor of three in future generations. Assuming two injection operations per cycle, equivalent to a vehicle life of one million km. Piezo stacks are, of course, suitable for injection systems in petrol engines as well, a project on which Epcos is currently working.
But they are by no means restricted to fuel injection. Potential applications include precision adjustment of optical systems, atomisation of liquids and pulverisation of solids.
For further information contact Electrocomp, Value-Added Reseller, 011 458 9000.
| Tel: | +27 11 458 9000 |
| Email: | [email protected] |
| www: | www.electrocomp.co.za |
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