Thermostats and thermoregulation
How a wax thermostat works, why the coolant temperature should change with load, how the control unit heats the thermostat using maps, and how a rotary slide valve module controls the coolant flows — with examples from different engines.
The thermostat decides where the coolant goes — past the radiator or through it. Based on the self-study programs, we look at how a conventional thermostat works, why different temperatures suit the engine at different loads, how the control unit heats the thermostat and how a rotary slide valve module controls the flows.
Conventional thermostat
The heart of the thermostat is a capsule with a solid wax filling: hot coolant melts the wax, it expands and pushes out the pin. (solid filler: heating → liquefies, expands → pin)
The pin opens the large disc, and the coolant goes to the radiator — through the large circuit. (large disc open → large circuit, to the radiator)
Through a mechanical link, the small disc meanwhile closes the short path to the pump — the small circuit. (small disc closes the short path to the pump — small circuit)
When the wax cools down, the spring returns the discs: the large circuit closes and the small one opens. (cooled down — spring closes the large circuit, opens the small one)
Temperature by load
Different temperatures suit the engine: at part load from ninety-five to one hundred and ten degrees, at full load from eighty-five to ninety-five. (partial load 95–110 °C · full load 85–95 °C)
Hot coolant at part load reduces consumption and emissions, while cooler coolant at full load slightly cools the intake air — and power increases. (hot – less consumption and toxicity · cool – air cooler, higher power)
That is why a heating resistor was built into the wax filling, and the engine control unit supplies it with a pulse-width modulated signal. (heating resistor in the wax filling · PWM from the control unit)
The stronger the heating, the further the pin extends: with current the coolant temperature is lower, without current — higher. (current on — coolant cooler · no current — hotter)
Without current — when starting and with the engine off — the thermostat works like a conventional one and opens at one hundred and ten degrees. (without current (start, engine off): opens at 110 °C)
The control unit takes the target temperature from two maps — engine speed and load, and vehicle speed and intake air temperature — and uses the lower value. (2 maps: engine speed and load · vehicle speed and air t → the lower is used)
The actual temperature is measured by two sensors: one at the engine outlet and the other at the radiator outlet. (NTC sensors: engine outlet · radiator outlet)
Based on the difference between these temperatures, the control unit switches on fan stage one or two, and above one hundred kilometers per hour the fans are switched off. (Stage 1 or 2 – temperature difference · > 100 km/h – off)
Rotary slide valve module
In newer engines, the flows are controlled by a rotary slide valve module: an electric motor turns the slide valve through a worm gear, and a Hall sensor reports its angle. (rotary slide valve module: electric motor → worm → slide valve · Hall sensor)
The second slide valve, for the cylinder block, is linked to the first through an intermediate gear and turns only over part of the travel. (slide valve 2 (block) — via intermediate gear, only 145…85°)
During warm-up, the first slide valve is at one hundred and sixty degrees: the radiators are closed, the coolant in the block does not circulate, and the block warms up faster. (warm-up, 160°: radiators closed, coolant in the block not circulating)
At about one hundred and forty-five degrees, the first slide valve picks up the second: a small flow passes through the block, the cylinder head and the turbocharger so that the exhaust manifold integrated in the head does not overheat. (≈145°: small flow — block, head, turbocharger)
From one hundred and twenty degrees, the engine oil cooler opens, and the second slide valve opens the block further. (120°: + engine oil cooler · block wider)
From eighty-five degrees down to zero, the main radiator opens: at part load one hundred and seven degrees are maintained, at high load eighty-five. (85…0°: + main radiator · 107 °C partial · 85 °C high load)
After the engine stops, the slide valve moves into the range from one hundred and sixty to two hundred and fifty-five degrees, and the electric pump circulates coolant through the cylinder head and turbocharger for up to fifteen minutes. (after stopping: 160…255° · electric pump up to 15 min)
For the event of a failure, the module has a fail-safe thermostat with a wax filling: at one hundred and thirteen degrees it opens a bypass to the radiator. (emergency thermostat: 113 °C → bypass to radiator)
Engine specifics
- System with electronic control (self-study program 222): the control unit supplies the thermostat heating resistor with a pulse-width modulated signal; if the engine outlet sensor fails, control runs at a substitute temperature of 95 °C with fan stage one permanently on.
- 2.0 l TSI engines (program 522) and 1.8 and 2.0 l TFSI EA888 engines (program 606): the rotary slide valve module sits on the cylinder block on the intake side together with the coolant pump, which is driven by a separate toothed belt from the balance shaft; the module's fail-safe thermostat opens at 113 °C.
- If the module fails (program 522), engine speed is limited to 4000 rpm, the EPC warning lamp lights up, the coolant temperature is shown in degrees on the instrument cluster, and the after-run pump switches on after the engine stops to cool the cylinder head.
- The gearbox oil cooler (program 606) is switched in at a coolant temperature of 80 °C without cabin heating or 97 °C with heating — surplus engine heat warms the gearbox oil.
Summary
- The heart of the thermostat is a capsule with a solid wax filling: hot coolant melts the wax, it expands and pushes out the pin.
- Through a mechanical link, the small disc meanwhile closes the short path to the pump — the small circuit.
- Different temperatures suit the engine: at part load from ninety-five to one hundred and ten degrees, at full load from eighty-five to ninety-five.
- That is why a heating resistor was built into the wax filling, and the engine control unit supplies it with a pulse-width modulated signal.
- The actual temperature is measured by two sensors: one at the engine outlet and the other at the radiator outlet.
- In newer engines, the flows are controlled by a rotary slide valve module: an electric motor turns the slide valve through a worm gear, and a Hall sensor reports its angle.
Next episode: coolant pump and electric pumps.