WARNING
The following post contains facts, lots of them, and most totally uninteresting and irrelevant to your enjoyment of the MG F/TF
There is no benefit in reading any of this!
If you value your time do not read any further:grin:
If you do read further do not blame me for loosing your sanity and the will to live!
i dont want to get drawn into an non beneficial argument again . its why i removed the topic i started before, that i and Sundance contributed too.
Hey!
Don't forget I contributed to it also.
I put a lot of effort into trying to help you understand what you were observing. You deleted my posts without consulting me at the same time.
There was a lot of interesting information in it, but you deleted it because:-
1 You didn't understand it
2 You didn't agree with it
3 You could
I promise not to go on about absolute pressure and gauge pressure :wink:
Just got to comment on so many errors!
at normal operating pressure, is a point beyond which the cap begins to open.. it begins to bleed.. it allows expansion of the coolant.. its normal.. the pressure is maintained at that point..
If the cap starts to vent, then the cooling system pressure is too high. The cap opens due to excessive pressure. At normal operating pressure the cap will not open for any reason!
BUT... if the pressure cap has opened at all, ( IT DOES ) then the mass in the system has changed.. its not as big as it was... both air,, and a very small amount of coolant escape when it does..
Why would it open unless the cooling system has gone into fault condition and over-pressurised?
the point the cap does that isnt the same for every single cap... its a median point..
Unless the cap is faulty, the range of operating pressures should be very small (1-2 psi max). Mass production methods and statistical process control result in high repeatability.
and it affected by the coolant pump its self.. the pump can push the cap to point of opening pressure even when the coolant is cold...
In a closed system, a pump will only generate pressure where the fluid movement is restricted. Where a restricted flow opens up into a larger chamber (such as the expansion bottle), the pressure will fall. To establish what the system pressure on the filler cap is, empirical measurements will need to be made in the expansion tank, not on the hoses that feed it. How have you kept the coolant cold while revving the engine enough to activate the pressure relief valve in the cap.
and.. its very likely that there other hose joints that breathe in the same way... let air in, and push fluid out in tiny amounts.. under particular circumstances... the whole system is unable to be completely sealed, even if one cant see any obvious change in the coolant level... i would say your not looking hard enough...
You mean if the system is leaking. We have established that a well maintained cooling system does not leak, and should not leak.
i made a video of it doing that.. just by revvimg the engine.. and watching the calibrated pressure gauge..
What was the pressure gauge calibrated against??
As above you were observing the pressure build up due to pushing coolant through restricted pathways in a closed system. The pump can not increase the mean closed system pressure. If the pressure increases at one point, a reduction in pressure will be observed somewhere else (inlet to pump, or where the restricted passageways open up into less restricted passageways). Closed system fluid dynamics are not the same as open system fluid dynamics mainly because atmospheric pressure is taken out of the equations.
if you bought say five caps and measured the point of opening on them like i and Sundance have done.. its unlikey that any of the five would open or close at the same pressure..
What is the manufacturing permitted tolerance on the pressure caps. When Rover made them they would have been part of a strict SPC (Statistical Process control) to ensure the parts were all (or an acceptable percentage) produced within specified tolerances. Can this be said of aftermarket caps?
:hysteria:
as the hot coolant cools... and the mass inside the system has changed, then as it cools to prevent a difference between outside and inside pressure ( a vacuum ) the cap lets air in.. ( IT DOES)
Why would the mass change, other than if you have had a loss of coolant? If the coolant system is working well, then after an initial loss of coolant, due to over filling, an equilibrium will develop, where the volume increases due to a rise in temperature, then drops back to the original volume when cooled down again. The volume changes, but the mass stays constant, unless you have a leak. The cooling system is designed to accommodate the change in volume (hence the expansion tank) under normal condition.
(science bit warning)
The pressure in the system changes partially due to the increase in water volume as it heats up (its density decreases with temperature) but mainly due to the release of dissolved gasses within the water. This is why the pressure increases before the coolant reaches the phase transition condition (boiling point).
at equilibrium, the outside pressure and inside system pressure, will be the same... the cap enables that to happen..
That is what equilibrium means!, but the cap does not enable this, unless there has been a change in the coolant mass due to a leak.
its not a sealed closed system.. one can like Sundance and i have done.. use gauges to measure what happens.. its how i know the pump speed affects the system pressure regardless of temp.
Please show me your experimentation and results that demonstrates that it is not a sealed closed system? How are you measuring the change in coolant volume and mass, or the ingress or regress of air from the system through the cap. The pressure changes you see are due to the restrictions in the system as the flow increases (explained by fluid dynamics). However the overall average system pressure will not be changed!
the amount of coolant that can leave the system at any time except at total cap venting.. is small.. hardly noticeable.. but it does occur over time, the venting is humid.. its full of water vapour.
So over time it is necessary to keep re-filling the coolant system up? If you have to then you have a leak. Presumably a very small leak that you find manageable, but a leak non the less!.
when the cap does fully vent - open completely, its very sudden and dramatic... it might only click fully open for a second or less.. but because the system is fully venting under pressure the pressure loss in that one second is huge... its like opening an air line or the tyre valve on a tyre.. ive done that test on purpose.. to see what happens, at temps below the normal boiling point... i dont reccomend trying it unless you have good calibrated equipment to know what will happen,
That is what the cap is designed to do. In a catastrophic fail condition, the cap vents excess pressure to prevent a hose bursting, or blowing off, which could cause injury or damage elswhere. Imagine if one of the heater matrix hoses burst near your feet. The cooling system will be designed to survive something like 2-3 bar, but the pressure cap will relieve the pressure well before the pressure gets this high.
This is the pressure caps sole function...to prevent excessive cooling system pressure.
The temperature of the coolant doesnt change so fast.. one doesnt loose three or four degrees of temp in that time.. it remains the same.. so.. that hot coolant now boils vigorously.. but one can loose over 5ps pressure in that time.. less than a second.. ive seen it.
Finally a point we can agree on:rocker:
so.. i will just say.. in normal operating conditions.. the cap breathes.. it lets pressure in and out... its not fully closed or fully open.
I knew that wouldn't last :frown:
i did contemplate, when Sundaance and i were considering these effects of pressure, if i could make an alteration to the top cap. to allow its breathing to vent into an external container, so i could see the effect of that breathing into a pipe that went to a separate container with the end of the pipe submersed in coolant in that external chamber... so i could see it.. BUT.. its not an easy thing to do... one would need a way of enclosing the cap completely with an external pipe into a second chamber. its laboratory stuff... not everyday motoring..
This is the way many cars operated in the 70's and 80,s. There were two seals on the cap. One at the top, and one on the spring loaded pressure disk. As the system pressure rose the pressure relieved past the first seal, and was contained by the second seal. A small pipe directed the water vapour into an external header tank when it would condense. As the system cooled again, another valve opened in the cap to allow the water to be sucked back into the cooling system.
I think my 1970 MGB used this system or was it my fiat Strada :| No it might have been my MKII ford consul, or was it the herald. Dont think it was the P6, but it could have been. I know for certain it wasnt my Citroen GS!