After watching fuel prices rise to record heights in the summer of 2006, many motorists are showing renewed interest in fuel-efficient vehicles. So it should come as no surprise that thrifty diesel automobiles are more popular than ever. But their high efficiency of over 40 percent has a negative side: the engines produce so little excess heat – especially in city traffic and traffic jams – that heating systems that rely on engine heat are unable to warm the passenger compartment to a comfortable level in winter.
Since the early nineties therefore, carmakers have increasingly been fitting auxiliary heaters that begin warming the air as soon as the vehicle is started: warm air streams into the passenger compartment through the heating vents. This is not a simple hair-dryer principle. Glowing wires would be dangerous in a car because they can ignite plastics and would need a control system to limit the current as a function of the air temperature. For this reason, all manufacturers of auxiliary heaters now employ PTC thermistors. These ceramic semiconductors with a positive temperature coefficient have a low electrical resistance at low temperatures that rises suddenly by several orders of magnitude as soon as a predetermined temperature is exceeded. The secret of this steep response is that the current-carrying electrons are unable to cross the boundaries of the tiny ceramic crystals at high temperatures.
Temperature control superfluous
This special property makes the PTC thermistor into an ideal automotive heater because it obviates a separate temperature control. The PTC is completely self-regulating: a high current flows at low temperatures and the element heats up in a matter of seconds. As soon as the desired temperature has been reached, the current drops and a stable equilibrium is set up between the electric power supplied and the heat that is dissipated. This is quite irrespective of how the driver sets the blower and whether the air drawn in from outside is an icy -10 °C or a mild +20 °C (Fig. 1).
| ||FIGURE 1: R/T CHARACTERISTIC|
Typical R/T curve of a PTC thermistor.
Rated PTC resistance
Minimum resistance (resistance value at TRmin)
Temperature at Rmin (becomes positive)
Reference resistance (resistance value at TRef)
Reference temperature (resistance rises sharply)
Typical PTC thermistors have an output of 100 W. That is too little for fast comfortable warmth in a vehicle parked outside in wintry conditions. So manufacturers of PTC heating systems usually combine ten to 15 PTCs with cooling fins in a larger element for fast heat exchange (Fig. 2). The heating elements reach 160 °C in the car, while temperatures of up to 300 °C are possible for other applications.
World market leader EPCOS
The success story of PTCs in automobiles is linked closely to the name of EPCOS. Ever since demand rose in the early nineties, the company has been supplying PTC thermistors to all major manufacturers of heating elements. EPCOS ist the world leader in PTC heaters. Some 80 million PTCs are produced annually for automotive customers. This high figure can be explained by the fact that heating elements are currently used not only to supplement the interior heating but also to heat windshield-washer nozzles, fuel filters or the air vents in the crankshaft housing.
| ||FIGURE 2: AUXILIARY HEATERS|
Catem produces PTC heating units in a variety of sizes and designs (left). These include the latest, extremely compact models, which are ideal for local sources of heat, for example as heating elements (right) for the Airscarf in the Mercedes SLK.
EPCOS’s good market position is a result of both the company’s close cooperation with manufacturers in the development of application-specific solutions, and of the technically superior construction of the PTCs. Because the heating elements have a flat structure, they emit their heat very efficiently to their surroundings. The electrodes are made of three layers that supply electric current to the ceramic: they consist of the metals chromium, nickel and silver and are applied by screen printing or sputtering, depending on the application. EPCOS PTC thermistors are characterized by their long operating life: the heaters manage about 12,000 operating hours with no significant power loss and can withstand a good 75,000 switching operations without failure.
Today, 65 percent of all diesel vehicles in Europe are equipped with an auxiliary heater, and this figure is expected to rise to 80 percent by 2010. It is already apparent that PTC thermistors will not be limited to diesel vehicles but will find their place in gasoline-powered ones as well.
A scarf made of warm air
The dream of every convertible driver is a heater that warms the head and shoulder region even when the top is down. The German automotive supplier Catem, has made this wish reality by developing the Airscarf with PTCs for the Mercedes SLK convertible. Warm air wraps itself around the head and neck of the driver and front passenger like a scarf, so that cold drafts do not cause a chill or a stiff neck even at low external temperatures. As a result, SLK drivers are able to enjoy an extended convertible season in both the spring and fall.
The pleasant warmth comes from a PTC heating element only a few centimeters in size with an output of 215 W. It is built into the backrest and heats air drawn from the passenger compartment via aluminum vents before conveying it to the head rests via a plastic conduit (Fig. 3). Airscarf regulates its blower speed automatically in three stages depending on the car’s speed and temperature.
| ||FIGURE 3: AIRSCARF IN THE DRIVER’S SEAT|
|Headroom heating is accommodated in the seat structure of the head restraint between the foam cushion and backrest cover in the Mercedes SLK.|
Auxiliary heaters such as Airscarf benefit from the miniaturization of PTC elements. In the last ten years, automotive suppliers have worked with EPCOS to reduce the size of the modules by almost 40 percent. This paves the way for completely new options for individualized climate control: Instead of incorporating a single high-power module into the air-conditioning system, several small modules are installed so that passengers can set their own ambient temperature themselves.
In addition to comfort functions such as auxiliary heaters, PTC thermistors are also used to reduce the emissions produced by motor vehicles. Thus manufacturers of commercial vehicles use what is known as AdBlue technology to reduce the emission of nitrogen oxides. In this type of exhaust after-treatment, urea is injected into a catalytic converter so that the nitrogen oxides are converted to nitrogen and water vapor. However, the urea has the drawback of becoming viscous at low temperatures. Here too, PTC heaters are the answer: they are incorporated in the urea tank and ensure that this solution flows easily.
| ||TYPICAL PTC HEATING ELEMENTS|
| ||Ceramic heating elements can be manufactured in almost any geometrical shape desired. As such, they can also be used for heating washer nozzles or fuel filters. A particular advantage of the elements is self-regulation.|