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Why Did They Start Putting Thermostats On Lower End Of Engines?

Why Did They Start Putting Thermostats On Lower End Of Engines?

Since the beginning of time thermostats have always been at the cylinder head outlet.

Within the past 15-20 years they seem to be trending to the radiator hose inlet.

To me this seems a little strange. Why do we care to more carefully regulate the water temperature around the cylinders than in the heads?

The heads are where all the [heat] action seems to be so it would seem to be more critical to regulate them like it was always done in the Olden Days.


RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

The thermostat acts as a simple flow restrictor. The behavior of the total system will be similar no matter where the thermostat is in the system, as long as the thermostat is properly sized and temperature matched to its location in the circuit, as obviously temperatures will vary in different areas of the cooling loop.

I suspect the move due almost entirely to cylinder heads becoming more complicated with the addition of more valves and much more complicated works to control them (VVT systems of various types) making packing challenges more difficult.

RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

although you are correct that with a thermostat we want to regulate the temperature of the hardware (cylinder and head), we are doing so by regulating a intermediate (the cooling fluid). there is a relationship between fluid temperature and hardware temperature, but that is a very complicated one, depending on load, speed, coolant inlet and outlet temperature and maybe some other factors as well. and you still are not capable to directly regulate the temperatures you really want to know - the surface temperature of the head of the combustion chamber and the contact temperature between piston ring and cylinder. thus, the standard placement of the thermostat just outside the cylinder head has and had its drawbacks.

with the introduction of various emission regulations over the years getting the engine within its working temperature range as soon as possible after a cold start and keeping it within the optimal range as closely as possible a more refined regulating system is required. by controlling the inlet temperature keeping the amount of cooling within the optimal range becomes somewhat easier because you more or less stabilize the cylinder water inlet temperature depending on the load of the engine. in the classic system where you control the outlet temperature of the head you have less direct control over what happens because the inlet temperature will vary depending on speed, whether it rains or not etc with as a result the engine temperature will fluctuate more. you may also have noticed that regulating the cooling system has been increasingly sophisticated by the use of less fluid volume (leading to faster heating after a cold start) and more then one electrically driven multispeed fan effectively widening the range within regulating is possible. with "classic" system the range was managed by the thermostat only, in modern systems thermostat and fan(s) together do the job, making it possible for a faster reacting system capable over a wider range with less fluid.

RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

Quite a few powertrains are also sending that coolant into the transmission to speed it's warmup and then stabilize it's temperature so for that purpose a radiator out temp control would also be superior.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

Romke is right. Engine metal temperatures are more stable with a cold side T-stat - this is particularly important with the aluminium block engines that are very common presently. Cold side T-stat still sees head coolant temperature via the bypass route and it responds to that temperature, but when it starts to open, it sees both hot and cold (from the radiator) coolant and blends accordingly, and eventually closing off the bypass pathway when the engine is warm. With hot side T-stat, it takes a while for the cold water from the radiator to reach the T-stat so that it can start regulating appropriately and this transport delay leads to undesirable temperature cycling.


RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

VW has used a thermostat on the inlet of the water pump (bottom/front of engine), basically regulating the inlet temperature of the water pump, as long as I can remember.

The bypass loop through the heater core is connected all the time and joins in between the thermostat and the water pump inlet. The radiator loop comes in through the thermostat into that housing. One other advantage of this layout is that the thermostat doesn't need a bleed hole to allow a small amount of circulation for it to "see" the temperature. It means the flow through the radiator loop can be completely shut when the engine is cold. For a diesel engine in winter where you want every bit of heat you can possibly get for the interior heater, this is a good thing!

RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

With an outlet thermostat your get an in rush of cold coolant when the thermostat opens. This can make an aluminum engine contract quite a bit and work the seals and gaskets. If you ever get a chance to hear an engine run on a thermal shock test you'll know that this affects performance as well as the sealing surfaces. Fast warm up, as s previously stated, and more stable operating conditions are the plus side.

The somewhat more complicated re-circulation system and the dual function thermostat, which modulates re-circulation and the inflow of lower temperature coolant, is the down side. We also get to learn a new term: 'de-gassing bottle', with new cooling technology.

It is worth your while to get a hold of a new thermostat which has a disc on the 'back' to regulate the re-circulation flow. You will understand instantly what it is doing - opening one path while closing off another.

RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

Mercedes may have a heating element in the thermostat. Effectively adds some software-based control.

RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

. . . and probably about $500 to replace.

RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

Good guess. It was replaced on mine about two years ago (when I learned about the self heater complication). From memory: part $280, and about an hour at $130. Add another $25 for Shop Supplies, and 15% for 'Free Health Care', $500 Canadian dollars, about $380 US.

RE: Why Did They Start Putting Thermostats On Lower End Of Engines?

I saw a Merc the other day with a sheet of aluminium foil over half its "radiator". It must have been 15-20 years old, so a new thermostat may have cost more than the car was worth.


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