The Function and Malfunction of the Diesel Thermostat

[wuzerk]

Originally Posted by Mike Noc

Mike two questions;

When cold is there any leakthrough at all?



Mike

Not in my experience. When experimenting I stood a new stat up in a saucepan with water sitting above the valve and it did not drain through until the stat opened at the correct temperature.

[Mike Noc]

My old one was the same Wuzerk, so interesting to see if the failed stat Mike has got is completely watertight when cold.

Mike

[NigelOBB]

Mines hit and miss if it gets to temperature say if I've been sitting in traffic then seems to work then as soon as it cools back to open ??

[FrenchMike]

Originally Posted by Mike Noc

Mike two questions;

When cold is there any leakthrough at all?

At what temperature does the stat start to open? Think it was Harry who suggested putting some cotton in the stat when hot and let it cool down, and then you can suspend the stat in water, gradually heat it, and as it starts to open it will fall into the pan.

Mike

Mike ,i just added the temp value for each pict .

At 50 deg it is perfectly closed and begin to open as you can see at 60 deg

[FrenchMike]

Originally Posted by wuzerk

Not in my experience. When experimenting I stood a new stat up in a saucepan with water sitting above the valve and it did not drain through until the stat opened at the correct temperature.

Hi ,

Exactly what we need to know !
so ,if i understand well ,a new one keep closed untill the 88 degrees are reached?

In that case ,it could be something like that:



Mike

[FrenchMike]

Originally Posted by wuzerk

So far I have never seen a report on here of a 'stat which failed open or closed on the diesels. I believe that the reason it fails to maintain the designed temperature is because the spring loses its strength allowing the valve to open too early. I know exactly when mine failed and that was after the first time I ever really thrashed the engine up a long hill thus reaching the highest temperature it had reached under my ownership (and compressing the spring the most)
The reading dropped below normal (9 oclock) straight afterwards.

I agree ,an overheating could damage seriously the behaviour of the bulb ?

Mike

[T-Cut]

The spring temper will not alter the way the valve opens. The wax melting/expansion characteristics control that. The spring is there to ensure proper closure as the wax cools down. It minimises hysteresis. See Wiki for more on this: http://en.wikipedia.org/wiki/Wax_the...perature_range

I believe tests were done on the spring to demonstrate that an 'overheated' spring has no effect. The spring was heated in a flame to de-temper it. In the absence of a mechanical explanation (valve or seat warping) then the extended opening range seems to be a good reason for cool running. This points to a change in the wax used for these stats. The wax used is typically a narrow range paraffin, which expands considerably when it melts. The expansion at melting point is very significant. It's not related the thermal expansion of the melt. So, if the melting range is broadening, I'd wonder whether it's not the usual paraffin wax at all, but something prone to chemical change through heat cycling. The wax would require a lab analysis to get further.

EDIT: I found this patent on improving engine thermostats, using a different method to seal the piston that actuates the valve. If the problem described in 'Background of the Invention' is commonplace, then we may have the answer to it.

Patent: http://www.freepatentsonline.com/4948043.pdf

TC

[FrenchMike]

Originally Posted by T-Cut

The spring temper will not alter the way the valve opens. The wax melting/expansion characteristics control that. The spring is there to ensure proper closure as the wax cools down. It minimises hysteresis. See Wiki for more on this: http://en.wikipedia.org/wiki/Wax_the...perature_range

I believe tests were done on the spring to demonstrate that an 'overheated' spring has no effect. The spring was heated in a flame to de-temper it. In the absence of a mechanical explanation (valve or seat warping) then the extended opening range seems to be a good reason for cool running. This points to a change in the wax used for these stats. The wax used is typically a narrow range paraffin, which expands considerably when it melts. The expansion at melting point is very significant. It's not related the thermal expansion of the melt. So, if the melting range is broadening, I'd wonder whether it's not the usual paraffin wax at all, but something prone to chemical change through heat cycling. The wax would require a lab analysis to get further.

EDIT: I found this patent on improving engine thermostats, using a different method to seal the piston that actuates the valve. If the problem described in 'Background of the Invention' is commonplace, then we may have the answer to it.

Patent: http://www.freepatentsonline.com/4948043.pdf

TC

Thanks TC for the wikipedia link;All is said:

Engines which require a tighter control of temperature, as they are sensitive to "Thermal shock" caused by surges of coolant, may use a "constant inlet temperature" system. In this arrangement the inlet cooling to the engine is controlled by double-valve thermostat which mixes a re-circulating sensing flow with the radiator cooling flow. These employ a single capsule, but have two valve discs. Thus a very compact, and simple but effective, control function is achieved.

The wax used within the thermostat is specially manufactured for the purpose. Unlike a standard paraffin wax, which has a relatively wide range of carbon chain lengths, a wax used in the thermostat application has a very narrow range of carbon molecule chains. The extent of the chains is usually determined by the melting characteristics demanded by the specific end application. To manufacture a product in this manner requires very precise levels of distillation.


Would be interesting to know if all the people who have had a faulty thermostat , had encountered an overheating some time before caused by a fan issue for instance....

Mike

[T-Cut]

Originally Posted by FrenchMike

Would be interesting to know if all the people who have had a faulty thermostat , had encountered an overheating some time before caused by a fan issue for instance....

Yes all data is good. In this case I somehow doubt that all the faulty stats reported on the forums are the consequence of overheating. Overheating in the diesel is vary rarely reported although obviously it can happen under circumstances of fan failure. What would be interesting, is to know what proportion of diesels have had this problem. My suspicion is a large proportion. I might even say most do. Clearly not all do since there are occasional comments on treads like this from a diesel owner whos engine always runs at 'normal'. The question that raises is what is meant by 'normal'. The gauge will be 'normal' at 75C, but that's not anything like optimum.

EDIT: The 'constant inlet temperature' note is very interesting and explains the design of the stat. I believe the V6 stat uses a similar design and while the 1.8 is a conventional single valve type, the later PRT introduced the pressure/temperature control principle.

TC

[FrenchMike]

Originally Posted by T-Cut

Yes all data is good. In this case I somehow doubt that all the faulty stats reported on the forums are a the consequence of overheating. Overheating in the diesel is vary rarely reported although obviously it can happen under circumstances of fan failure. What would be interesting, is to know what proportion of diesels have had this problem. My suspicion is a large proportion. I might even say most do. Clearly not all do since there are occasional comments on treads like this from a diesel owner whos engine always runs at 'normal'. The question that raises is what is meant by 'normal'. The gauge will be 'normal' at 75C, but that's not anything like optimum.

TC

Mine still is working fine at 88 degrees and i 'm pretty sure it has never overheated.

I have repaired my fan (do you remember ) before it fails .


Mike