Tech puts squeeze on motorists

Piezoelectric fuel injectors, which are becoming more and more popular on diesel engines, illustrate an important trend in automotive engineering.

First, some historical background: in 1880 brothers Pierre and Paul-Jacques Curie discovered that a voltage appeared across the faces of objects made of certain crystalline substances when a force was applied to compress the object.

This phenomenon was named the piezoelectric effect, after a Greek word meaning to squeeze or press.

Mathematical calculations predicted that the converse should also happen; if a voltage is applied across the faces of a piece of piezoelectric material, the material should expand.

The prediction was promptly confirmed experimentally by the Curies.

The piezoelectric effect remained a laboratory curiosity, stimulating interest mainly in the field of theoretical crystallography, until 1917 (during World War I) when the French developed an ultrasonic submarine detector using a piezoelectric pulse generator.

This triggered a wave of interest in the practical applications of piezoelectricity, with the Japanese being in the forefront. From an automotive perspective further impetus was given to the development of piezoelectric devices when Bosch took an active interest.

Today Bosch, Delphi, and Continental AG (through its acquisition of Siemens VDO) are all producing piezoelectric fuel injectors for common rail diesel engines and direct injection petrol engines.

Because the expansion of a sliver of piezo material when a voltage is applied across its faces is so minuscule, stacks of thin discs are used in the injectors, all contributing to produce a useful deflection which can actuate an injection pulse, a task for which a solenoid valve was used in the past. The advantages of piezo-actuators are considerable: they are lighter, more compact and operate faster, with more finely controllable and adaptable injection intervals. Thus they allow several injections per injection cycle, for instance two pilot injections, one main injection and two post-injections. They also allow smaller highpressure pumps to be used.

The disadvantages are sometimes glossed over, as often happens in the headlong rush towards technological nirvana.

For one thing, piezo-injectors are expensive — on average about R18,000 per injector, depending on the application — and they cannot normally be dismantled and reconditioned, at least not yet.

For a set of four the cost amounts to R72,000, before labour is added. For another thing, they are not bulletproof. Water in the fuel getting past the filter(s), or petrol accidentally ending up in a diesel injection system, will destroy them in no time. I believe Delphi is aiming to develop the “everlasting” injector, good for more than a million kilometres. I wish them good luck.

For the average owner of a car with piezo-injectors, the lessons in this story are to renew the fuel filter(s) at least as frequently as the manufacturer stipulates, to watch like a hawk what goes into your fuel tank, and to budget for new injectors (as well as a new turbocharger, where applicable) at any time from 160,000km onwards. These have to be regarded as wear items. Such is the price of technological refinement.

For all your motoring queries, contact Gerrit Burger: geb@mweb.co.za