bypass circuit

[robbio c]

hi guys and gulls can anyone explain to me what a bypass circuit is on the engine cooling system thanks

[pete.i]

hi guys and gulls can anyone explain to me what a bypass circuit is on the engine cooling system thanks

 

 

I did reply to this but it got lost dunno why or how. Sometimes the way this forum works is as much a mystery as engines and electrics LOL.

 

Anyway I ramble. I was discussing this very topic today with the mechanic where I am moored and this is how he described it.

 

The thermostat is closed when the engine is first started and the coolant is fed around the engine only bypassing the radiator. This allows the coolant to reach the engines operating temperature quickly. When the coolant has reached the engine optimum operating temperature the thermostat opens and the coolant is then circulated via the radiator so keeping the coolant at the optimum operating temperature of the engine by cooling the coolant using the radiator. If the temperature drops below the optimum operating temperature of the engine then the thermostat closes again so bypassing the cooling effect of the radiator untill the coolant reaches the optimum engine operating temperature again. That cycle repeats itself so keeping the engine at an optimum temperature.

 

Well thats how it was explained to me anyway. If anyone knows different then I am up to being enlightened.

 

Cheers

 

Pete

 

Umm so the thermostat is the bypass circuit or at least the bypass device. The rest of the cooling cirtcuit is the same whether the radiator is bypassed or not. I think.

 

LOL I see Tony Brooks is reading this so we will get a expert explanation soon I hope.

Edited November 22, 2011 by pete.i

[Tony Brooks]

I did reply to this but it got lost dunno why or how. Sometimes the way this forum works is as much a mystery as engines and electrics LOL.

 

Anyway I ramble. I was discussing this very topic today with the mechanic where I am moored and this is how he described it.

 

The thermostat is closed when the engine is first started and the coolant is fed around the engine only bypassing the radiator. This allows the coolant to reach the engines operating temperature quickly. When the coolant has reached the engine optimum operating temperature the thermostat opens and the coolant is then circulated via the radiator so keeping the coolant at the optimum operating temperature of the engine by cooling the coolant using the radiator. If the temperature drops below the optimum operating temperature of the engine then the thermostat closes again so bypassing the cooling effect of the radiator untill the coolant reaches the optimum engine operating temperature again. That cycle repeats itself so keeping the engine at an optimum temperature.

 

Well thats how it was explained to me anyway. If anyone knows different then I am up to being enlightened.

 

Cheers

 

Pete

 

 

This is not a dig Pete.

 

That is a fair description of the operation of a "standard" thermostatic cooling circuit, the bypass system has an addition.

 

While an engine is warming up and the thermostat is closed it is necessary to provide some circulation through the engine because certain parts have to remove far more heat than others (like the area around the exhaust valve seat). If we think about a BMC A series engine (mini sprite midget etc.) for a moment there was a small length of 1/2" hose between the bottom of the thermostat housing and the water pump and similar hoses or drillings could be found on most engines of that vintage. This hose provided a bypass to the thermostat so coolant could still circulate around the engine even though the thermostat was shut (ignoring the breather/giggle pin hole in the thermostat valve itself). The heater circuit (calorifier in our case) also provided a degree of bypass. This bypass is not shut off when the thermostat opens so some hot coolant can bypass the radiator/skin tank.

 

When engines were developed to produce more power (heat) and other factors restricted the radiator size the water bypassing the thermostat became something of a problem because it reduced the cooling capacity of the system. This was overcome by a new design of thermostat (of which there have been two but as far as I know only one remains) that has a large washer like object hanging below the actual thermostat. When the thermostat opens this drops down to cover the bypass port so all the coolant flow has to flow through the radiator/skin tank.

 

This is why Tim and myself do not believe that the Redshaw bypass thermostat housing will work properly with the modern bypass thermostat, apparently what is supplied. It would work with the obsolescent typer where the bypass port was on the side of the housing and sealed by a moving sleeve

 

You are unlikely to get a modern bypass thermostat into a "normal" thermostat housing but you could get a "normal" thermostat into a bypass housing. This would not seal the bypass port so the engine may well run hotter or overheat.

 

I did reply to this but it got lost dunno why or how. Sometimes the way this forum works is as much a mystery as engines and electrics LOL.

 

Anyway I ramble. I was discussing this very topic today with the mechanic where I am moored and this is how he described it.

 

The thermostat is closed when the engine is first started and the coolant is fed around the engine only bypassing the radiator. This allows the coolant to reach the engines operating temperature quickly. When the coolant has reached the engine optimum operating temperature the thermostat opens and the coolant is then circulated via the radiator so keeping the coolant at the optimum operating temperature of the engine by cooling the coolant using the radiator. If the temperature drops below the optimum operating temperature of the engine then the thermostat closes again so bypassing the cooling effect of the radiator untill the coolant reaches the optimum engine operating temperature again. That cycle repeats itself so keeping the engine at an optimum temperature.

 

Well thats how it was explained to me anyway. If anyone knows different then I am up to being enlightened.

 

Cheers

 

Pete

 

Umm so the thermostat is the bypass circuit or at least the bypass device. The rest of the cooling cirtcuit is the same whether the radiator is bypassed or not. I think.

 

LOL I see Tony Brooks is reading this so we will get a expert explanation soon I hope.

 

 

Pete - cut the expert thing please. I fully subscribe to the has been little drip thing and I KNOW there are far better mechanics than me out there. Maybe I can explain but that does not make an expert.

[pete.i]

Pete - cut the expert thing please. I fully subscribe to the has been little drip thing and I KNOW there are far better mechanics than me out there. Maybe I can explain but that does not make an expert.

 

Hi

 

Well in comparison to me and, no doubt many others, you are an expert on these matters. that remark was meant sincerely even though I did put a "LOL" in at the front of it. The LOL (Laugh Out Loud in internet parlance) was in respect of the fact that I saw you were reading the thread and in no way was meant to add belittleness to my "expert" remark. Unfortunately due to the lack of any sort of inflexions in the way things are written/said especially on forums it did appear that that was what I was doing. Being an experienced internet and forums user I should have realised that when I wrote it. I aplogise if that caused offence and as I said what I said was said in all sincerity.

[Tony Brooks]

Pete - cut the expert thing please. I fully subscribe to the has been little drip thing and I KNOW there are far better mechanics than me out there. Maybe I can explain but that does not make an expert.

 

 

Hi

 

Well in comparison to me and, no doubt many others, you are an expert on these matters. that remark was meant sincerely even though I did put a "LOL" in at the front of it. The LOL (Laugh Out Loud in internet parlance) was in respect of the fact that I saw you were reading the thread and in no way was meant to add belittleness to my "expert" remark. Unfortunately due to the lack of any sort of inflexions in the way things are written/said especially on forums it did appear that that was what I was doing. Being an experienced internet and forums user I should have realised that when I wrote it. I aplogise if that caused offence and as I said what I said was said in all sincerity.

 

 

I did not take offence and maybe I should have added a smiley but I get very uncomfortable when called an expert. I call Harry Bathgate who designed the BMC A & B series engines long before computers a real expert - they only lasted about 45 years in production

Edited November 22, 2011 by Tony Brooks

[WotEver]

I did not take offence and maybe I should have added a smiley but I get very uncomfortable when called an expert. I call Harry Bathgate who designed the BMC A & B series engines long before computers a real expert - they only lasted about 45 years in production

Tony, compared to most of us on here you're an expert. Under the circumstances, with Harry Bathgate not available to us, you'll do

 

(Yet another) Tony

[Pretty Funked Up]

I was going to put a bypass circuit refers to a ciruit that bypasses the thermostat.

So that there is still circulation even when its closed.

 

Thought better of it as my knowledge is from cars and i was expecting it to turn out to be another raw water cooled jobbie.

lol.

 

 

Tony's explaination is superior in every way

[WotEver]

Tony's explaination is superior in every way

Yours is more succinct

[Neil2]

I didn't know that modern thermostats shut off the bypass circuit and I'm grateful to Tony for the explanation. I now understand why my "modern" Honda civic was more affected by a stuck open thermostat than my old mini.

 

If I have understood correctly then, on our elderly BMC diesels some of the coolant still bypasses the skin tank circuit even when the thermostat opens, ie the bypass circuit is still "active". Given that some of us are concerned about the effectiveness of our cooling systems, in my case because the skin tank is too small, I wonder if there is any way an old BMC can be modified to close off the bypass circuit? Or would this not be a good idea for some reason.

[NBMike]

Snip < on our elderly BMC diesels some of the coolant still bypasses the skin tank circuit even when the thermostat opens, ie the bypass circuit is still "active".>

 

 

This is exactly the problem I find with my Petter PJ3W and the Redshaw thermostat housing. I think I might investigate smileypete's idea of a thermostatic mixing valve on the bypass circuit, ans if that doesn't do it I'll try the extra pump on the oil cooler route.

[Tony Brooks]

I didn't know that modern thermostats shut off the bypass circuit and I'm grateful to Tony for the explanation. I now understand why my "modern" Honda civic was more affected by a stuck open thermostat than my old mini.

 

If I have understood correctly then, on our elderly BMC diesels some of the coolant still bypasses the skin tank circuit even when the thermostat opens, ie the bypass circuit is still "active". Given that some of us are concerned about the effectiveness of our cooling systems, in my case because the skin tank is too small, I wonder if there is any way an old BMC can be modified to close off the bypass circuit? Or would this not be a good idea for some reason.

 

 

I am afraid its over 40 years since I had a 1.5 apart and I can not remember exactly how the permanent bypass is arranged - its not a hose and it may not even use one and just rely upon the small circulation via the "giggle pin" hole in the thermostat. If there is a bypass my feeling is that as its cast into the block it would be all but impossible to modify and it would not be a good idea to permanently block it because early use of full power just might cause localised boiling in the engine.

 

Localised boiling is when water on those hot spots I mentioned boils. The gauge will read pretty much normal but water might be ejected from the system. This allows the hot spots to fur up which in turn provides thermal insulation which leads to localised cracking. (Hence the move away from direct raw water cooled marinised engines which were difficult to run under pressure).

[robbio c]

hi all thanks a lot for all your replies.it has been well explained.