Driving force - automotive applications for ceramics
The capabilities of ceramic materials have made them an increasingly attractive choice for specifiers in the automotive industry. Scott Bentley of Morgan Advanced Materials, UK, looks at some of the ways ceramics are applied in the sector.
The unique physical, chemical and performance properties of modern ceramics are creating an increasing number of applications for technical and thermal ceramic materials in the automotive industry, both in vehicles themselves and during manufacture. Ultrasonic parking and level sensors based on piezoceramic transducers have been widely used in the sector for 20 years, while ceramic brake discs are a popular option at the luxury end of the market. Meanwhile, the move towards hybrid vehicles as well as advances in manufacturing techniques are creating new challenges that the latest generation of ceramic materials are ideally placed to meet. In addition, almost every component on modern vehicles – whether metal, glass or plastic – comes into contact with some sort of thermal ceramic material during manufacture. Here are just some of the ways these diverse materials are used in the automotive industry.
Next-generation hybrid vehicles
Advances in battery technology are creating new demands on thermal management systems. Precision engineered ceramic shafts and bearings for water cooling pumps are used in hybrid vehicles to cool lithium-ion batteries and critical engine electronics. Ceramic products are resistant to chemical attack from the glycol coolant and so have a long life, while their reduced weight compared to alternative systems contributes to low noise and increased fuel efficiency.
The manufacture of modern windscreens requires flat glass panels to be heated up before being pressed or bent into shape. The high value of the windscreens and tight production deadlines mean the risk of breakage during the forming process must be eliminated. Papers made from low-biopersistence high-temperature fibres placed on both sides of the glass protect the windscreen from physical damage caused by the extremes of heat used by specialist glass bending companies, while allowing sufficient heat transfer for the bending process to take place.
A key challenge for design engineers is protection of the vehicle’s electronic engine management system from high temperatures around the engine. Excessive heat from the exhaust manifold can be managed with a number of heat shield systems. The range of metal encased insulation options includes heat resistant blankets, felts, and thin papers. Not only do these systems provide protection from heat, they also help to reduce engine noise.
Diesel particulate filters
The ceramic paste used to bond together the segments of a diesel particulate filter (DPF) is loaded with specially engineered low-biopersistent fibre. The presence of the fibre controls the application properties of the paste during production and provides the thermal shock performance required during the high-temperature regeneration cycle that is used to clean the filter while the car is running.
Ultrasonic parking sensors
A piezoelectric ceramic transducer transmits high-frequency ultrasonic waves to detect the presence of obstacles. Ultrasonic systems use a number of sensors, much like speakers, which transmit a cone of sound waves. When these sound waves bounce off other cars and parking hazards, they are reflected back to the sensors, which can then calculate the time it took for the sound wave to travel out and back again. By comparing this against the speed of sound in air, they provide a reading of the distance travelled from the vehicle to the hazard, enabling the vehicle to be safely manoeuvred for parking.
Located in the engine block, cylinder head or intake manifold, the knock sensor detects harmful engine vibrations by tuning into the knock frequency, typically around 15kHz. The sensor’s detecting element is a piezoelectric ceramic that, when at rest, has no voltage across it. However, when subjected to mechanical pressure at the knock frequency, the element’s electrical structure becomes distorted, producing an output voltage. The generated voltage is directly proportional to the severity of the pressure or knock. This signal is interpreted by the vehicle’s engine control module, which alters ignition timing and, sometimes, fuel delivery, until the knock disappears.
Piezoceramic-based level sensors have been widely used since the mid 1990s to gauge the levels of fuel and other liquids such as coolants, oils, and screenwash liquids. In fuel tanks, the traditional float gauges have been replaced with ultrasonic ceramic level sensors for greater accuracy. A lead zirconate titanate (PZT) ceramic sensor is installed at the bottom of the tank’s interior, and an electric current is sent to the sensor, which responds by oscillating. The resulting ultrasonic soundwave passes through the liquid, rebounds off the surface and returns to the transducer, registering the fuel level based on a ‘time of flight’ measurement. By taking regular readings, an average figure can be communicated to the dashboard display.
These small piezoelectric ceramic components offer minimal wear with no moving parts, and the proven corrosion resistance and temperature stability of the PZT material ensures consistent, reliable operation. Automotive companies also apply this sensor technology to the additive reservoirs used in diesel engines. The additives improve emissions alongside diesel particle filters.
One innovative design incorporating PZT is the so-called ship-in-a-bottle fuel tank technology, comprising a blow-moulded plastic tank enclosing the fuel pumps, level sensors and other components. Originally introduced for the 2005 Ford GT, this solution reduces the number of openings required in the surface of the tank, minimising evaporative emissions, and also provides additional fuel space.
Wheel balancing transducers
Ceramic wheel balancing transducers provide dynamic force measurement in automotive applications. They are typically designed for use in wheel balancing equipment, but applications can vary and may include measurement of force exerted on the brake pedal during brake tests, or operation as a rotor-balancing sensor.