You are basically asking what is the temperature when it starts moving and then what is it for 42mm length? I can only think that that is what is described as “thermostat operating range”.
When I had them all in the pan of hot water all those years ago, it was my impression that they started opening quite slowly, then moved a lot and very quickly once a critical temperature was reached and all of the wax was melted.
Of note also is that they had a spring loaded foot. That implies that the designer anticipated their carrying on opening once the bypass was already fully closed off.
Any time that the temperature is above the initial opening temperature, but the extended length is still below 42mm (?) then you must have flow through both ports.
Interestingly, but fatter John’s extra washer is, the more rapid and binary the outcome will be, so long as it doesn’t exceed Kirby’s “both shut off at the same time” length, which would indeed be a disaster.
Yes…I am interested in the operating philosophy. I also noted the spring under the bypass “blocker” so I wonder if the bypass is a way to even out the engine temperature on warm up and then either take it out of circuit completely and leave temperature control to the thermostat radiator and pump circuit…or bring the bypass back in if the water gets too cool…a bit fussy for my liking…I would think the cooling system should be able to function without the complication of introducing a bypass…which is just for stability during warm up…but it all depends on the design of the thermostat.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
43
You think it’d be a better idea to deadhead the pump when the engine is cool?
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
44
Perhaps an interesting discussion, but definitely overthinking. There are only two things that are important: 1) When cold the passage to the radiator should be closed and the bypass open, and 2) when too hot the passage to the radiator should be open and the bypass closed. What happens in between is irrelevant.
The purpose of the by-pass is to prevent any possibility of pump cavitation during the warm up process, and thus eliminate any possibility of air pockets being created in the cooling system.
Air pockets can create hot spots in the head which are extremely detrimental to alloy heads. Cast iron heads are less effected by this but can still crack if extreme temps are reached.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
47
It’s not controlled. Whatever output the water pump generates, that’s how much flow goes through the engine.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
48
It probably does that, yes. However, I’d describe its key function as making sure the thermostats respond to heat within the engine. Without the bypass, it’d be possible for the engine to get really hot while the thermostats are still cold. Just hoping enough heat will manage to conduct its way through stationary coolant to the thermostats so they open up before the valve seats drop is iffy.
I think most other cars have the thermostat right up on top of the block itself so heat will rise directly to the thermostat and open it up. Hence they don’t need to ensure flow during warmup. And they don’t seem to have a problem with pump cavitation.
No…when it is warming up the bypass is in use to ensure even flow through the engine block. When it is in the thermostat operating range the thermostats move in response to temperature …and I assume the bypass is closed by the flap which has a spring on it to allow more movement of the thermostat while the flap is closed…implying that the bypass stays out of it and the thermostat increases and decreases the pressure on the pump …and the coolant flow rate increases or decreases …and at a much greater rate ( a “square” relationship…so half the pressure is 4 time the flow…) so the thermostats have a good chance of keeping the temperature under some sort of control.
Marek’s plots clearly show an underdamped control system at work…where the small amount of over or undershoot is of no consequence to the running of the engine. I suspect the designers were forced to use relatively high flow rates to extract the heat from a reduced frontal area of cooling because of the styling of the Jags…355litres/minute at 6000rpm …that’s 6 Jugs of beer every second!!!
CRAP!
So you are telling me that the majority of engines prior to alloy ones that had no by-pass system, the thermostats didn’t control the water flow?
The water “pump” is not a positive displacement pump, it is an agitator, it only delivers the amount of flow that the thermostats allow.
So why will a water pump collapse a lower(suction) hose if it is still pliable and soft unless it has a coil spring in it, only suction would do that not agitation. A big steel block would tolerate pulsed flow with a on/off flow that a stat would produce. A more tempature critical alloy engine might require continuous flow that a bypass/radiator flow system would offer. I think the V12 pump is always moving fluid it just varies between in the block or out to the radiator depending on the stats, with a opening/closing stat you would flow through both the bypass and rad. And the pump will pull more fluid through the bypass if the stat is single action as I experienced for two years when my 2+2 had the single action 165f stats that I am sure stayed full open all the time when warm.
Don’t want to fan the flames any higher.
There is an electric pump on the coupe V12 engine that is claimed to pump 80L/min.
That is recommended for 5L V8s. I feel sure the 80L/min is quoted for a typical installation where the pump has to drive coolant through the resistance of the engine, hoses and radiator.
The coupe is never used in a race so the coolant is never expected to handle the maximum heat dissipation of full bore at 6000RPM.
No matter what kind of pump, it has a set of characteristic performance curves
On the graph you have a curve of flow rate against RPM. For any RPM the maximum flow rate is when the pump is “open circuit” with no restriction at inlet or outlet. That gives the best curve. For various levels of restriction, as it increases, you get curves showing less flow at any RPM.
The V12 mechanical pump will have a set of curves, and at any time the flow rate will be predetermined by one of those curves. The curve to use will be set by the restriction in force at the time, and the flow rate follows that curve for all values of RPM.
Over the operating range of the engine the restriction varies with temperature and consequent thermostat opening. Hence the pump performance will be working on a number of different curves.
When the engine is cold the coolant flow bypasses the radiator and should be on a curve giving maximum flow.
The same applies to the electric pump, but not quite. The electric pump will always try to run at its maximum speed which is determined by by the applied voltage. However, as restriction increases it will slow down to some extent but in general will run over a narrow RPM range, unlike the mechanical pump.
I’ve never said it doesn’t create pressure. There has to be a pressure differential across the inlet and outlet, otherwise their will be no flow.
But with the thermostats closed there is no flow unless you have a by-pass system that allows it.
A thermostat has two functions, flow control and temp control. Maximum cooling occurs at the lowest flow as it allows more heat rejection for the same time.
This why when you remove the thermostats, the engine is more likely to boil.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
55
That sounds simplistic to the point of inaccuracy. My washing machine has an agitator. A centrifugal pump has an impeller. It generates a pressure differential, not flow. Even if the flow is deadheaded, the centrifugal pump will still be generating a pressure differential. The pressure differential created is a function of the density of the fluid, the OD of the impeller, and the RPM of the impeller. What flow results is a function of the flow resistance of the circuit; the less resistance, the more flow.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
56
In general the higher the flow rate of coolant the more ability to remove heat from the coolant into the air through the medium of the radiator. You also need air flow through the radiator to remove heat from the coolant. This probably involves a curve which is an asymptote, whereby at zero coolant flow you get zero heat transfer to the air and as the coolant flow increases you get decent transfer but the rate of that transfer increase falls off as the coolant flow keeps increasing. You get to stage that no matter how much faster the coolant flows, even if it is supersonic, the airflow will not be enough to remove any more heat.
The art of heat transfer is not my expertise, but I can see there is a sweet spot where the relative flows of air and coolant give best result. You cannot just add more rows of tubes to the radiator to get better cooling, since that restricts airflow and is counter productive.
If someone can come up with the right type of flow sensors of the correct diameter, I’ll fit them and datalog that. Then we can observe how flow is correlated to rpm and what the split at the thermostats is under various conditions. You’ll also see whether the spring loaded foot has any bearing on the outcome, even if that is just for philosophical interest.
There are a few “urban myths” floating around.
The jag water pump,is a centrifugal pump…it is not a positive displacement pump it is a centrifugal pump. It will have a characteristic curve which essentially is a plot of flow on the bottom axis versus pressure on the veritical axis…and it will flow different litres/second depending on the restriction on the outlet of the pump…which is the discharge pressure.You can block the outlet and it will not destroy itself like a positive displacement pump will…it will flow different amounts depending on the restriction . Most pumps are rated at a fixed speed…but the XJS pump will have a series of pressure vs flow curves at each 500 rpm so the designers can investigate the envelope of performance.
The rate of heat removal in all heat exchange systems is BETTER with increased velocity of fluid being exchanged. This is a function of a critical number called Reynolds Number…which is a function of velocity…so higher velocity equals higher heat transfer.
The most important factor is surface area and speed of fluid …whether is is coolant in the radiator or air thru the fins. Armed with this knowledge the behaviour of the Jag cooling system can be analysed and understood.
