The Function and Malfunction of the Diesel Thermostat

[Jules]

Interesting Jim !
Glad I kept all the old housings to have a fiddle with!

[T-Cut]

Thanks for your observations. Is your old stat like the one in the photo above (barrel-ended piston) or the simple rod (later) type?

Originally Posted by jim lee

- - - In the cold condition there is some clearance between the end of the central rod and the bottom of this hole.

How did you determine the clearance in the assembled unit?
Obviously any significant slop in the piston will increase 'opening' temperature.Was yours replaced because it opened too early?

Any alteration in the cold clearance between the end of the central rod and the bottom of the hole will alter the temperature at which the 'stat begins to open. I suspect the factory sets the opening temperatures of their various 'stats by adjustment to this clearance.

It's an interesting idea, but I suspect stat manufacturing isn't precise enough to use clearances to control their settings. The literature suggests that 'high' and 'low' type stats use waxes of 'high' and 'low' melting points. I'd agree that for a batch of stats manufactured with a particular wax there will be a small range (tolerance) in the opening point due to physical/dimensional differences.

My thought is that any in service change in this clearance on 'our' stats would result in a change in opening temperature. In our case an decrease in this clearance causing a reduction in opening temperature. Is it possible that plastic 'creep' of the 'stats casing due to heat, stress and age could cause such a reduction?

The plastic case seems to be a thermostable resin rather than one that might change shape through heating. I found it to be exceedingly tough, something like modern Bakelite or a GRP.

To confirm my theory I will increase the relevant clearance on my old unit and see what difference that makes to the opening temperature. If anybody is interested I will post the results.

I think you can rightly predict what effect it will have. I look forward to the results. The more experiments are conducted, the more the answer is likely to appear.

Are you able to extract and test the wax as shown earlier?
Can you examine the capsule interior?
If it's the earlier stat type, can you send me a sample?

TC

[Trevor1975]

Originally Posted by T-Cut

The plastic case seems to be a thermostable resin rather than one that might change shape through heating. I found it to be exceedingly tough, something like modern Bakelite or a GRP. TC

Just to clarify, the comment on plastic creep of the thermostat case is talking about the metal casing of the thermostat itself(not a plastic material).

Steel can deform in an 'elastic' or 'plastic' way, where elastic deformation as seen in springs etc, the material returns to its original size / shape, whereas plastic deformation results in a permanent change in size / shape. All Steel creep is 'plastic' in nature, so just imagine the metal body of the thermostat slightly changing in shape over many years of thermal and mechanical cycling. Does that make sense? This would in theory change the opening temperature.

Although I would say that reduction of spring rate through metal fatigue would be a more likely cause... I would imagine that the thermostats are designed to fail cooler or open, rather than hotter or closed, to minimise the risk of over heating.

[Frank Incensed]

Originally Posted by ice man

..................

Steel can deform in an 'elastic' or 'plastic' way, where elastic deformation as seen in springs etc, the material returns to its original size / shape, whereas plastic deformation results in a permanent change in size / shape. All Steel creep is 'plastic' in nature, so just imagine the metal body of the thermostat slightly changing in shape over many years of thermal and mechanical cycling. Does that make sense? ....................................

You've brought back to mind a visit to the ORTF building in Paris in 1968. IIRC, the huge glass windows were suspended from the top because of their weight. We were told that, over time, they would become thinner at the top, as the glass flowed downwards under gravity.
(Not suggesting gravity affects our stats)

[Trevor1975]

Originally Posted by Frank Incensed

You've brought back to mind a visit to the ORTF building in Paris in 1968. IIRC, the huge glass windows were suspended from the top because of their weight. We were told that, over time, they would become thinner at the top, as the glass flowed downwards under gravity.
(Not suggesting gravity affects our stats)

OMG, don't stand under the windows then!!!

[T-Cut]

Originally Posted by Frank Incensed

- - - the huge glass windows were suspended from the top because of their weight. We were told that, over time, they would become thinner at the top, as the glass flowed downwards under gravity.

The belief that glass is super-cooled liquid - a myth I afraid.
Wiki explains it:
The observation that old windows are sometimes found to be thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a timescale of centuries. The assumption being that the glass was once uniform, but has flowed to its new shape, which is a property of liquid. However, this assumption is incorrect; once solidified, glass does not flow anymore. The reason for the observation is that in the past, when panes of glass were commonly made by glassblowers the technique used was to spin molten glass so as to create a round, mostly flat and even plate (the crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk became a different thickness as the glass spun. When installed in a window frame, the glass would be placed with the thicker side down both for the sake of stability and to prevent water accumulating in the lead cames at the bottom of the window. Occasionally such glass has been found thinner side down or thicker on either side of the window's edge, the result of carelessness during installation.

TC

[Frank Incensed]

Originally Posted by T-Cut

The belief that glass is super-cooled liquid - a myth I afraid.

Destroyed - a belief I've held for 45 years. I'll never trust a Frenchman again.

Still at least I know that thunder is clouds banging their heads together. My parents told me that.

[Trevor1975]

Originally Posted by ice man

.... I would say that reduction of spring rate through metal fatigue would be a more likely cause... I would imagine that the thermostats are designed to fail cooler or open, rather than hotter or closed, to minimise the risk of over heating.

- back on topic

[T-Cut]

The complete thread should be read so we don't go around the old ground.

TC

[Trevor1975]

Well I don't know if this helps, but I looked through the cars MOT history and in 2008 it was idling at 85 degrees on emissions test, in 2010 at 82 degrees and now it stays at 75-76 degrees (IPK 7.0) very consistently.

whether there is a error between these bits of info I don't know but it would seem my thermostat has gradually lost 10 degrees in the last 5 years....