Perfect piezo

Traditional fuel injection systems are solenoid based. Now a growing number of automotive companies are backing piezoelectric systems to deliver fuel more accurately. Jon Excell reports.

Most experts agree that it will be some time before we’re all driving around in hydrogen-powered vehicles.

But while car companies appear to be waiting for politicians to take their risks for them, they can happily continue with the relatively risk-free development of the internal combustion engine. And one of the most fertile areas of improvement in the IC engine is the fuel injection system, where higher fuel pressures and faster, more accurate dosing of fuel both improve engine efficiency and help manufacturers meet tough new emissions standards.

While fuel injection systems are traditionally based on the electromagnetic switching effect of a solenoid, a growing number of automotive companies are throwing their weight behind systems that exploit the piezoelectric phenomenon.

In such systems, the application of an electric charge to a piezoelectric element causes slight, controllable and rapid changes in its geometry that can be used to mechanically open the injector needle. In theory piezo injectors are smaller and faster than their solenoid equivalents. This makes it possible to deliver even smaller amounts of fuel accurately. As a result, the number of injection strokes on each working cycle of the engine can be varied almost without restriction.

At the recent Frankfurt motor show, Siemens, Bosch and Delphi all launched piezoelectric fuel injection systems.

Developed for diesel-powered vehicles, the third generation of Bosch’s common-rail system uses piezoelectric in-line injectors and is said to reduce pollutant emissions by up to 20 per cent. Dr. Ulrich Dohle, president of the diesel systems division at Bosch, explained how the system works.

Bosch has replaced the magnetic coil usually used to control the injection valve with an actuator made of piezo crystals, which expand within an electrical field. The piezo element is built into the body of the injector – hence the name ’inline injector’ – and its movement is transmitted non-mechanically to the rapidly switching nozzle needles. This, said Dohle, allows a more precise measurement of the amount of fuel injected and thus leads to a reduction in harmful combustion products.

Piezo actuators will appear on Bosch’s third-generation common rail system, which debuts on Audi AG’s new 3L V-6 TDI that is optional on the A8 flagship.

Delphi also plans to join the Piezo-party, and announced that it has developed a piezo-injector designed to help vehicle manufacturers meet Euro V emissions regulations (a low-emission regulation for diesel vehicles due to come in to force in 2005.)

The system, known as piezotec, will deliver up to seven or more injection events per cycle, Delphi said. The company added that the technology allows reduced spacing between injections, giving engine designers freedom to shape the flow of fuel through the combustion stroke.

Piezotec is expected to generate a 25-30 per cent reduction in emissions, according to Delphi, outperforming Bosch’s third generation common rail system

’Piezotec will take piezo-based injection system performance to a new level,’ said Guy Hachey, Delphi vice-president and president, Delphi Energy & Chassis Systems. ’Vehicle makers will be able to improve performance and have greater designflexibility as they develop diesel engine systems compliant with more stringent environmental and customer acceptance standards.’

Piezo injection for diesel engines has been around for a couple of years. But petrol engines are pretty much untouched by Piezo’s wand. With diesel engines at the heart of almost 40 per cent of the new cars sold in western Europe, it’s surely time for a petrol-engine backlash?

Enter Siemens VDO, which is launching the world’s first piezo injection system for a petrol engine. Siemens’ piezo direct-injection (PDI) enables the use of a jet-directed combustion process that transports the fuel directly to the spark plug.

Until now, petrol direct-injection engines have relied on wall-directed or air-wall-directed combustion processes, which redirect fuel off the cylinder wall and piston base toward the direct vicinity of the firing spark plug, where the mixture immediately ignites. This process only works if the amount of fuel and injection timing are precisely matched.

The Piezo element – made from titanium based ceramics – switches four to six times faster than conventional solenoids, allowing the fuel to be far more accurately metered. The piezo actuator is integrated into the fuel injector and directly connected to the nozzle needle. As soon as the piezo element receives electric current, the needle completely unseats at the tip, moving into the cylinder. A precise amount of fuel is delivered toward the spark plug in 0.0002 seconds.

While the Frankfurt show also saw the launch of plenty of advancements to solenoid-based injection systems, piezo has so much to offer that it is bound to take over completely one day.

Johannes Winterhagen of Siemens VDO Automotive AG summed it up when he said: ’We suppose that by the end of the decade nearly all new diesel passenger cars will be equipped with piezo common rail or piezo unit injector systems.’