volvo 2003 thermostat

[RIBW]

Looking at pictures in catalogues etc I noticed that the one for the seawater cooled engine did not appear to have the secondary valve, whereas the one for the freshwater cooled version did. Dunno why.

I took my spare thermostat out this morning and took a few photos which I hope will clarify things. I also took it to my local auto parts man who has been there since I was a boy and usually has an answer. On this occasion, he was stumped.
The plunger is ~13mm diameter. The flange differs from those on Wiki in that it is towards the other end of the body. It is ~54mm diameter. The 'body metal' is ~43mm diameter and unit's overall height (closed ) is 40mm.

The thermostat housing has another port which is used, I understand, to connect the header tank on a fresh water cooled system - this might explain the FW version double flange?

The photos are Top, Bottom closed, Bottom open and engine mating.

Cheers
Bob

 

[RIBW]

This type of thermostat opens the coolant supply from the engine while simultaneously closing the bypass. It has a plate on the inboard end for this purpose.

I am confused and would welcome clarification.
Vyv & Vic seem to agree that a 'bypass' is closed. My understanding is that the 'bypass' is a "Distribution Tube" (From the VP parts list) and I thought Vyv agreed with that.
Quote from Vyv "9 Feb 2013, Can anyone inform me precisely how the coolant system on a raw-water cooled Volvo 2000 engine works? I am aware that when [the engine is] cold, water is pumped through the perforated tube inside the cylinder head and out through the double right-angle pipe to the manifold. As this is the bypass flow.......".

The perforated tube goes front to back on the engine (looking at the workshop manual and parts list) and I don't detect any way the bypass can be interrupted, especially as the interruption would have to happen at the rear of the engine as the perforations in the tube are the only way I can see raw water getting into where it needs to be.

On the other hand, the plunger in my photos above obviously enters the orifice in the mating view and partially impedes any flow in that orifice. Is there a second bypass? What am I not understanding? Apologies if I am being thick.

TIA
Bob

 

[vyv_cox]

It seems you are relating the current thread about Volvo raw water cooled thermostats to a bit of a thread drift about Yanmar ones. Yanmar and Bukh have a seawater supply hose that either circulates through the engine once the thermostat is open, or bypasses the engine if it is closed. Their thermostats are at the end of the flow route.

Volvo's system remains something of a mystery, apparently relying on a change in flow rate when the pressure in the bypass reduces when the thermostat opens. Volvo thermostats are at the front of the engine which makes it difficult for me to understand their workings. I don't pretend to be a Volvo expert but I have asked many who do and nobody can give me a truly satisfactory explanation.

Edit: should have explained better. Whereas on Yanmar and Bukh the flow is either bypass or engine, on the Volvo the perforated tube is always open to the manifold. When the thermostat opens hot water flows through the external pipe, again to the manifold. Why it should choose to do so is the question.

 

[RIBW]

A. I don't pretend to be a Volvo expert but I have asked many who do and nobody can give me a truly satisfactory explanation.........

B.On the Volvo the perforated tube is always open to the manifold. When the thermostat opens hot water flows through the external pipe, again to the manifold. Why it should choose to do so is the question.

Thanks very much Vyv.

A. Here's hoping for a Volvo expert? Anyone? Particularly, where does that orifice lead if anywhere?

B. I am happy with the 'Why' - it is simply pressure. The seawater pump creates pressure and the the flow in each route (bypass and cooling) is determined by the relative impedances of the routes. The final double right-angle pipe to the manifold in the bypass is quite small and therefore high impedance, giving ample opportunity for the low impedance cooling route to take a fair share of the water. If we were talking electrics, it would be a shunt as applied to an ammeter.

Cheers
Bob

 

[vyv_cox]

B. I am happy with the 'Why' - it is simply pressure. The seawater pump creates pressure and the the flow in each route (bypass and cooling) is determined by the relative impedances of the routes. The final double right-angle pipe to the manifold in the bypass is quite small and therefore high impedance, giving ample opportunity for the low impedance cooling route to take a fair share of the water. If we were talking electrics, it would be a shunt as applied to an ammeter.

Cheers
Bob

That's the answer most people come up with. The problem is that we are talking about a positive displacement pump, which cannot be throttled downstream unlike a centrifugal one. When the engine is warming up the pump produces the same flow into one pipe as it does when hot into two. The pressure in the cold engine and associated pipework must therefore increase considerably, most probably causing accelerated wear to the pump vanes and seal. I can't help thinking that Volvo must have better designers than that and there has to be a better explanation.

 

[RIBW]

That's the answer most people come up with. The problem is that we are talking about a positive displacement pump, which cannot be throttled downstream unlike a centrifugal one. When the engine is warming up the pump produces the same flow into one pipe as it does when hot into two. The pressure in the cold engine and associated pipework must therefore increase considerably, most probably causing accelerated wear to the pump vanes and seal. I can't help thinking that Volvo must have better designers than that and there has to be a better explanation.

Vyv,
Thanks for that clarification - so, we have a constant flow type pump, which means that the constant flow will divide according to the impedances of the bypass and open thermostat cooling circuit unless there is another route (e.g. a bleed channel through the orifice that gets filled by the thermostat when hot) which would address your concern (- "most probably causing accelerated wear to the pump vanes and seal"). Could the plunger in orifice have a role here? I don't have any cause at the moment to take the head off my spare engine so is anyone about to do an overhaul? Any volunteers to investigate?

Cheers
Bob