Car cooling systems while thermostat closed

[Carbon]

When the thermostat is closed, the water pump continues to pump. Is there some smaller path that carries always coolant around the block bypassing the radiator, or is the bypath path only open above some excess backpressure?

In other words, is the bypass path shunting the radiator flow when the thermostat is open, or is there something to impede the flow thru the bypass and to help the flow thru the radiator?

 

[J_Myler]

That's a good questions. I've always wondered the very same thing

 

[nfs480]

Every time I start my car when it's freezing cold out I always wonder the same thing. I'm assuming there's still flow but it's highly restricted? Is there a pressure release so the water pump is pumping but not generating pressure?

 

[Petrou]

in every water pump and every cooling system there is a bypass. This bypass keeps coolant from entering the radiator and keeps it in the engine and in the heater core. allowing the heater to blow hot and the engine to warm up the water till the thermostat opens then circulates coolant throughout the system. Some have larger bypasses than the others but they do have bypasses in them. and pressure does get built in the process, the pressure is what keeps the water from boiling over in case the thermostat does not open. but then you get overheat issues as well

 

[mechanicx]

Originally Posted By: Carbon
When the thermostat is closed, the water pump continues to pump. Is there some smaller path that carries always coolant around the block bypassing the radiator, or is the bypath path only open above some excess backpressure?


Yes there is always a thermostat bypass that circulates water back to the water pump inlet while the thermostat is closed. This also functions to keep the water circulating and temperatures even throughout the engine. Also coolant circulates through the heater core and loops back to the water pump inlet.

Quote:
In other words, is the bypass path shunting the radiator flow when the thermostat is open, or is there something to impede the flow thru the bypass and to help the flow thru the radiator?


When the thermostat opens it may physically block the thermostat bypass. I don't think this is always the case. Once the thermostat opens coolant start circulating through the radiator and through the bottom radiator hose so they probably effectively shunt the bypass flow some anyway. Actually I think there's internal and external bypasses. the internal has a sprin valve at the waterpump that opens from the pressure when the t-stat is closed, and external the t-stat can block the bypass when t-stat opens.

 

[swalve]

In all cars I've seen, the bypass circuit is just a smaller diameter than the radiator circuit. So when the thermostat opens, the radiator becomes the easiest path for the water to flow.

 

[Eddie]

Water pumps are not positive displacements pumps. You do not need a bypass in the system. Some vehicles have none and some only a small 3/8 inch bypass hose.

 

[mechanicx]

Originally Posted By: swalve
In all cars I've seen, the bypass circuit is just a smaller diameter than the radiator circuit. So when the thermostat opens, the radiator becomes the easiest path for the water to flow.


I think some thermostats have an extra valve on the bottom that opens for the bypass when the thermostat is closed but closes the bypass when the thermostat opens. There's also internal bypass and external bypass. For example the SBC, except for the Vortech, had an extra bypase hole feeding right back to the water inlet side. The BBC and Vortech SBC used an an external hose looped back into top of the waterpump. But I agree that it's mostly a matter of once the thermostat opens, the radiator hose becomes easy path for coolant flow

 

[mechanicx]

Originally Posted By: Eddie
Water pumps are not positive displacements pumps. You do not need a bypass in the system. Some vehicles have none and some only a small 3/8 inch bypass hose.


I don't know if I seen any water pumps that didn't have either an internal or external bypass, although the heater can also function as a water pump bypass. It's true that waterpumps aren't positive displacement and don't need a pressure relief bypass, but that doesn't mean you want the pump cavitating or don't want circulation through the engine to have even temperatures. Those are the two real purposes of a thermostat/water pump bypass, and not pump pressure relief.

 

[440Magnum]

Originally Posted By: Carbon
When the thermostat is closed, the water pump continues to pump. Is there some smaller path that carries always coolant around the block bypassing the radiator, or is the bypath path only open above some excess backpressure?

In other words, is the bypass path shunting the radiator flow when the thermostat is open, or is there something to impede the flow thru the bypass and to help the flow thru the radiator?


The following only applies for "conventional" cooling systems- the new "inlet-side thermostat" and "reverse flow" systems are a big different.

The bypass path is usually always open, and yes it does shunt a fraction of the un-cooled coolant back to the engine all the time. There's a benefit to this- namely it keeps the entire engine at a more uniform temperature. The coolant that goes through the radiator may be cooled VERY far below the optimum operating temperature of the engine. If it were supplied back to the engine undiluted with hot coolant, the lower portion of the engine block would be many degrees colder than the upper block and cylinder heads due to the coolant warming as it passes through the engine (remember we're talking conventional flow directions here). In reality, the hot bypass water is blended with cool radiator water so that the blended return water to the engine is only a few degrees cooler than when it leaves the engine, resulting in a much more uniform temperature across the whole engine, which aids efficiency and reduces wear on internal parts. The water pump, radiator, fan, and bypass are all sized so that even in extremely hot weather, there is adequate total cooling available even with this dilution process. Some systems include an extra part of the thermostat that partially closes the bypass when the thermostat opens fully, but most do not.

Reverse flow and inlet-side thermostats go even further toward making the engine temperature uniform.

 

[SteveSRT8]

Man, I am so glad you are here!

You saved me a lot of 'splaining'!