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Electrotechnology II -- The Titanic

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September 19, 2014

When the Titanic struck an iceberg at night and sank in 1912, the loss of life strongly motivated  the search for ways of using the new electrotechnologies to detect submerged objects.  Shortly thereafter, during WWI, French physicist Paul Langevin, a student of Pierre Curie, employed piezoelectric quartz to detect another kind of lurking menace, German submarines, then decimating British shipping.  Accompanied by a raid on the available supply of large natural quartz crystals from opticians,  Langevin demonstrated the power of sonar in 1917 by killing a lot of fish with it.  His sonar converted electrical input into mechanical sound in water.

Naturally, navies took a keen interest.  Sonar transducers increased in power as the dominant electrotechnology see-sawed between piezoelectric and magnetostrictive.  By WWII, magnetostrictive nickel progressed rapidly but, curiously, the war induced a shortage of mica for use as a dielectric for capacitors.  Enter barium titanate as a substitute; its piezoelectric properties were soon discovered and found to be better than quartz, followed in short order by the superior lead zirconate / lead titanate composition still in use.

In the quest for ever-greater sonar power, in the 1960s the U.S. Navy undertook research into magnetostrictive alloys based on rare earths. The result is commercially available today as "terfenol-d," an alloy of terbium, dysprosium, and iron (elements 65, 66, and 26). Terfenol-d expands in a magnetic field.

This magnetostrictive alloy and the older lead zirconate / lead titanate are close to each other in terms of maximum power density.  However, in a continuously-controllable diesel engine fuel injector application that requires both speed and power, terfenol-d has the advantage of durability because the terbium atom is not completely round due to quantum mechanical effects.  In other words, its property of magnetostriction is natural and cannot be degraded or destroyed whereas the life of piezoelectric material is limited and degrades faster with stress.  This means that the alloy can be “pushed” much harder to operate the injector much faster and will still survive on an engine.

Our Navy is due much credit for their role in making these powerful electromechanical materials available and understood.  Piezoelectric quartz has matured as a stable electronic timer of choice, always vibrating mechanically at its fixed resonant frequency.  Magnetostrictive nickel has matured as the transducer of choice that drives durable, industrial-strength ultrasonic cleaning tanks.  Piezoelectric lead zirconate / lead titanate has matured into many different applications, from traditional sonar to medical and consumer uses to the latest generation of diesel fuel injectors.

Diesel fuel injection has been and will continue to be a field rich in innovation.  Electromagnetic solenoids, which can trace their lineage back to the clicking telegraph, are at present the majority of “electronic” injectors.  A solenoid is relatively lacking in force and precision.  Its magnetic field generates a force in the air gap between two surfaces, a fundamental limit on how much force it can generate and therefore how quickly it can accelerate a valve element.  Unfortunately, it is unnatural and therefore awkward and difficult to force a clicking solenoid to do the proper rate shaping of diesel injection to conform to the demands of low-emission combustion.  A solenoid is more suited to ring a doorbell by (eventually) banging into it than to precisely control diesel fuel injection.

While the piezo-driven injectors show the potential of controlling combustion through rate shaping by fast valve actuation, they cannot reach and stay at the most advantageous rate shapes because they degrade with use.  A voltage applied across a piezoelectric ceramic will cause it to expand very quickly and with high force.  The small expansion is proportional to the voltage.  In contrast to the solenoid, the force is generated within the solid body of the ceramic, enabling that force to be much higher.  The proportional speed and force, controlled by voltage, allow a fuel injector valve to precisely control diesel fuel injection.

Unfortunately, piezoelectric ceramics degrade with use, failing more rapidly when pushed hard to make them faster.  The piezoelectric effect is artificial, not inherent, in the high power ceramics such as lead zirconate / lead titanate.  Degradation is accelerated by higher voltage, stress, strain, and temperature, all of which are present on an engine.

Ruling out the solenoid and piezoelectric technologies leaves magnetostriction.  A magnetic field applied through a magnetostrictive alloy will cause it to expand very quickly and with high force.  The small expansion is proportional to field strength, that strength being proportional to electric current.  Like the piezoelectric ceramic, the force is generated within the solid body of the alloy, enabling the force to be much higher than the solenoid.  The proportional speed and force, controlled by current, allow a fuel injector valve to precisely control diesel fuel injection.

The critical feature of magnetostrictive alloys that distinguishes them from piezoelectric ceramics is that the alloys do not degrade or lose their functionality when pushed very hard.  This means that the force demanded from terfenol-d can be higher than from the piezoelectric ceramic and still be durable, allowing it to survive engine heat and vibration while operating a fuel injector valve at the fastest speed possible.


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