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Old 01-04-2013, 05:05   #16
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Re: Coolant overflow expansion tank for sailboat diesel engine

The OP stated that his boat was not originally equipped with an expansion tank, therefore the two extra hose system isn't applicable. His is the "old fashioned" kind of cooling system where the heat exchanger takes the place of a radiator in a typical car. Those systems will sometimes have a single "bypass hose" to assist bleeding air from the system or a small bleed hole in the thermostat that performs the same function.

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Old 01-04-2013, 05:26   #17
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Re: Coolant overflow expansion tank for sailboat diesel engine

Originally Posted by mrm View Post
You missed the point completely.

Two hoses is not about circulation, it is about bleeding air from the circuit fast. Resulting circulation is just a byproduct (useful though, for example when adding antifreeze concentrate).

Study this drawing if you like: 12v Pages - The Engine - Component Locations

You will find this design under the hood of most cars, at least in Europe.
Actually, this is generally a bad idea, and I have 30 years of heat transfer expereince and a factory recall (Cummins bus installation) to suport that opinion.

The 2-hose circulation system increases the amount of oxygen transfer in the tank and increases both corrosion and hose deterioration/cloging unless the expansion tank is sealed and presurized (it is on the cars you refference). Additionally, the hose locations must be carefully located with the entire system design considered. As a retrofit, there are more ways to do this wrong than to do this right. If the system was designed with a single cap, odds are that is the correct point to bleed air.

As a compromise, if air binding of the heat exchanger is a persistant problem (generally because there is a leak somewhere which is a better fix), a valve can be installed in the inlet line, opened only during start-up when coolant is changed; this is common in heat transfer systems.

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Old 01-04-2013, 05:40   #18
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Re: Coolant overflow expansion tank for sailboat diesel engine

Seems this is getting more complicated than it needs to be: Original pressurized system has a spring cap which lifts and lets coolant flow through the hose that leads to the engine pan when the system pressure is too high, spilling coolant into the pan. Run hose to a "recovery" tank which will store the coolant which has been purged from the system. No recovery when it cools back down, but very simple. I have done this on both my Perkins as the PO let coolant go to the bilge.

If you want to recover the coolant, pour it from tank back into engine.
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Old 01-04-2013, 07:08   #19

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Re: Coolant overflow expansion tank for sailboat diesel engine

mrm, I got the point but I don't think it has any validity from an engineering viewpoint.

You say the extra hose will help bleed the air quickly, but you only have to bleed air after the fluid has been changed, and then only a small amount if someone hasn't purged the system properly. As the fluid heats up, the air should purge very quickly through the one standard hose. Then for the next several years, that second hose is useless because there's no more air needing purging.

Maybe cars in the EU are switching to this. Mine still has just one hose, and "older cheaper simpler" has always kept it purged and working just fine.

A friend of mine has a "new, improved, highly engineered by the brightest minds" BMW. BMW relocated the battery to the trunk, and left an empty battery box under the hood. And added a vent hose, to vent battery acid under the car, instead of using a sealed battery, as Mazda did. Better performance and weight balance than the old batteyr under the hood? Perhaps. But WAY more complicated, for very little payback. And they made accessing the spare tire into a royal circus as well.

Sometimes, really, the engineers just need to be restrained.
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Old 01-04-2013, 07:53   #20
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Re: Coolant overflow expansion tank for sailboat diesel engine

@thinwater: yes, fully agree with point about oxygen transfer, _but_ I have never seen a design like that unsealed and unpressurized. In normal operation this design is completely sealed. Safety valves (yes, there are two of them) are built into the expansion tank cap, but they remain closed under normal operating conditions.

@hellosailor: then let me respectfully disagree. I see this design as technically superior and user friendly. For a small additional cost of a tiny hose you get fully automatic purge when filling and no oxygen transfer when running. Plus, higher boiling temps due to pressure. EU car makers made that switch decades ago. I will have to go and check (once snow melts), but I think VP in our Bavaria is also configured that way.

@all: I was under the impression that OP considered going to a pressurized system, therefore I offered some advice. If he only wants an unpressurized overflow tank, this is of course redundant..


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coolant, diesel, engine, sailboat

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