Thermostats: in control

In our new series introducing products, we want to give you the essential information from the experts about different vehicle technologies in a compact, simple form. Today, we’re starting by examining the thermostat in detail.

This is an area where MAHLE Aftermarket offers not only a broad product portfolio but also extensive experience in all the leading thermal management technologies. Whether you’re looking for passenger car or commercial vehicle options, MAHLE supplies thermostats, thermal switches, and thermocouples for all kinds of applications.

The thermostat—an “expandable” term

Components of the expanding element

How do engines and auxiliary components reach operating temperature quickly, yet stay cool when they have to work hard? What temperature range is the most energy-efficient and low-wear for the engine? Thermostats are responsible for regulating heat. Smart control of the coolant flows ensures that the optimal temperature is reached more quickly and held constant. The heart of the coolant thermostat is the thermal expansion element.

As the temperature changes, this wax element changes volume and acts as a regulator through the stroke movement of the piston. When the engine is cold, the coolant circuit remains closed, and the engine components warm up more quickly. Instead of flowing through the radiator, the coolant passes through a smaller circuit known as the by-pass loop.

Heat from the engine warms the coolant up to a defined temperature. Just before the optimum temperature level is reached, the thermostat opens the flow inlet to the radiator, and the coolant is fed through the radiator to prevent the engine overheating. If the temperature rises too sharply, all the coolant circulates through the radiator while the by-pass loop closes, again avoiding overheating of the engine. The cooling circuit of the auxiliary components can also be controlled separately following the same principle used by the engine’s primary cooling circuit.

All details you get with this brochure: >> Download [PDF; 1431 KB]

Thermostat insert with thermal expansion element

Take a look at this animation showing how a thermostat works:


Map-controlled thermostats:

thermal management 2.0

But that’s not all: thanks to modern electronics, thermostats are also able to improve efficiency and consumption values. But how does map-controlled thermostat technology actually work? How do we safely raise the operating temperature of passenger car engines to a constant higher level, thus optimizing combustion and all accompanying parameters?

The vehicle’s control unit electronically activates an integrated heating element on demand, shortening control times compared with conventional wax elements. As a result, the map-controlled thermostat controls the engine temperature more quickly and more precisely in different load and operating conditions, always keeping it in the optimal range.

The operating map is a dataset in the engine control unit. Depending on the operating conditions, it provides optimal values for the temperature and mode of the cooling circuits. In addition, the electronic control unit analyzes the current output profile to determine, for instance, whether the driver is calling on the engine to perform at a high or low level. Load-dependent regulation of the temperature level allows for optimal engine temperatures, enhanced efficiency, and fuel savings of up to 2 percent.

Map-controlled thermostat design

Thermostats—in charge of the cooling circuit

Depending on the application and size of the unit to be cooled, other thermostat variants include integral thermostats, housing thermostats, and sleeve valve thermostats. EGR thermostats and oil thermostats are also used to regulate exhaust gases and oil temperature, respectively.

In brief: key thermostat tasks

  • Optimized engine efficiency
  • Minimized wear on engine components
  • Taking the engine up to operating temperature quickly
  • Reliable control in all operating conditions (preventing overheating/cooling)
  • Rapid heating of the cabin
  • Reduction in fuel consumption
  • Minimizing emissions

Defect detection

Small errors when installing a temperature regulator can have serious repercussions for the cooling circuit and the engine.

A thermostat malfunction may be caused by:

  • The working piston jamming
  • Component corrosion
  • Defective thermostat housing or damaged gasket
  • Foreign matter in the cooling system
  • Faulty electric actuation of map-controlled thermostats

The vehicle is then at risk of the following consequential damage:

  • Engine overheating to the point of engine damage
  • Greater wear and inefficient operation due to improper operating temperature
  • Insufficient cabin heating
  • Malfunctioning of the climate control system
  • Noncompliance with emissions standards
  • Increased fuel consumption
  • Loss of coolant

To prevent mistakes and make sure you’re fitting thermostats correctly, why not top up your knowledge using our installation poster [PDF; 569 KB] or Technical Messenger, our regular information leaflet?

Even more information, tips, and guides

As a workshop professional, you can benefit from a comprehensive range of information, advice, and services relating to thermal management control units. Our technical posters and newsletters, catalogs in print and in digital form, plus animations give you practical, expert support that’s always tailored to your needs.

On top of that, we also offer regular theoretical and practical training. Find all the content at www.mahle-aftermarket. com under >> 'Services' and in the >> 'Download Center'.

More practical support and invaluable tips are available right here in MPULSE as well:

>> Article 'Ventilation lightens the load'

>> Article 'Completely sealed?'


We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.


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