Coolant pressure inside engine 1

[jcd06]

It begun when a friend started talking about replacing the stock water pump by an electrical aftermarket pump on his CA18DET engine.

One of the elements in the manufacturers installation guidelines struck me: remove the thermostat.

This brought me to think about what happens inside this liquid system.

In the case of this engine, and I think for most production car engines, the path of the cooling circuit is: pump – engine block – cylinder head – thermostat – radiator – pump.
The thermostat is open when the engine is at normal working temperature.
How much its valve is lifted from its seat depends on the design and the momentary situation. Obviously this will be different at idle or when racing, when freezing outside or in hot weather.
I gathered some figures from a couple of random service manuals and I found values of 8-12mm when heating it in a pot of water.
It is not hard to imagine that the thermostat being open only that little will cause a flow restriction in the coolant path.

Many literature mention the pressure generated inside the engine and the head because the pump is pushing its liquid against the restriction caused by the thermostat.
This pressure is supposed to be beneficial because it further rises the boiling temperature of the coolant.
Especially inside the head, where the coolant runs sometimes very close to the hot exhaust gas, only separated from it by less than a centimetre aluminium wall.
As I didn't find real-life values of this pressure, I decided to measure it myself while I already planned to measure some other engine parameters.
I took advantage of the threaded hole for the dashboard gauge temperature sender in the thermostat housing to plumb-in a pressure transducer.
Link to the graph

The signal was logged and plotted against RPM.
As you can see, pressure went well over 4 bar.
Mind you, this was at around 5°C ambient. When the weather is hot, the thermostat will open further to achieve the same coolant temperature.
When the thermostat opens further, the pressure generated by the pump will decrease.
Also, this is a fairly high-revving, high hp/cc engine.

Anyhow, this 4bar is a rather high value and I must say that it surprised me.
Installing an electric waterpump and removing the thermostat, you lose this advantage completely.

I was under the impression that nowadays some cars are OEM equipped with electric waterpumps.
Am I wrong and are these just secondary pumps for accessories circuits like heating or turbo core?
Or is the pressure that I measured not really necessary and to be considered completely as parasitic in terms of power consumption?
Or are these primary electric water pumps (if they really exist) fitted to low power engines with a much better thermal design of the internals of the cylinder head casting?
After all the CA18DET was a design from the eighties.

Also worth noticing is that the curve is not flattening off at high revs. Am I right to assume that the pump is not (yet) cavitating?

I would appreciate to know your professional opinions about this.
Thank you for sharing your knowledge.

Jean


When we all think alike, no one thinks very much. ---Walter Lippmann

 

[140Airpower]

Compositepro, basically that's true. Zero flow times whatever pressure equals zero power in the system. But, that also means zero cooling. To cool a certain engine making a certain power requires a certain flow, but not necessarily much pressure. A highly open, low resistance system requires the least power for a given amount of cooling.
The CA18DET has high resistance with the thermostat in. I wonder what it would be without the thermostat.

 

[Lou Scannon]

Keep in mind, 100% of the flow is always going through the engine. The state of the thermostat merely determins what fraction of this flow passes through the radiator before returning to the pump inlet.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

hemi, you are talking about bypass, right? I think this depends on the system design. Leaving out the heater or any other auxiliary circuits, it looks like there are 3 possibilities, 1 the Tstat closes the path, stopping circulation; 2 the Tstat bypasses the radiator, maintaining circulation through the engine; 3 the Tstat bypasses the engine/radiator circuit, routing flow back to the pump. In scenario 1, there will be virtually zero flow, high pressure and very low power drain. Scenario 2 should see high flow through the pump and moderate pressure (without the radiator as a restriction) and possibly high power drain. Scenario 3 should see high flow through the pump, but low power drain due to very low pressure. But, the details can change these expectations.

I assumed that for the CA18DET, because of the high pressure, I thought the Tstat being closed stopped or severely restricted the flow like scenario 1. At the left side of the graph were you can see two populations of points, one at a higher pressure than the other, I guessed that was showing the action of the Tstat. However, I'm not familiar with cooling system of that engine.

 

[Lou Scannon]

Oh, I didn't realize scenarios 2 and 3 were possibilities. I had assumed it was "unhealthy" for there to be virtually zero flow through the engine at any time. And I don't understand how the thermostat will be able to sense the actual engine temperature if it is not receiving flow that has gone through the engine.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

hemi, virtually zero flow is not zero. Thermostats I'm familiar with don't seal. Also, they may have a little hole that allows a tiny amount of flow.

 

[patprimmer]

I would call that small hole a small bypass. In my experience I never saw a thermostatically controlled automotive cooling system that did not at least have some bypass.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[ivymike]

You could call that little hole a bypass because it allows coolant to bypass the stat, but what I think of is a radiator bypass wherein the bypass flow is similar to what the rad flow would be.

All the cars' thermostats Ive seen have been the dribbling "bypass" kind at the stat...but I think many had a pencil-sized bypass somewhere in/near the stat housing to allow a bit more flow thru the block.

 

[patprimmer]

Most I saw had a 5/8" hose from the outlet side of the head or vicinity to the inlet to the water pump or vicinity, which makes a about a 1/2" dia or slightly smaller inside the fitting as the bypass flow controller. Still somewhat smaller than a radiators flow I think, like probably 10 or 20% of radiator flow capacity at a real wild guess.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[140Airpower]

I think some older systems had only a blocking thermostat that dribbled some flow and the heater circuit that bypassed the Tstat was controlled by a manual valve. This system did not allow significant flow through the block when the Tstat was closed and the heater off.

 

[patprimmer]

I never saw one where the heater was the only bypass. Water heating to the inlet manifold was common and was in effect a bypass. On cars with exhaust gas heated inlet manifolds, their was normally a loop to bypass. I thought the dribbler was more about venting air traps.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

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[140Airpower]

Pat, I think that is correct. Venting air is an issue. SBCs did not have water in the manifold. I think details have varied all over the map, but I think full circulation through the block with the tstat closed was not common in the past. Don't know about now.
These details are interesting when considering the power requirement of the water pump and converting to electric drive.

 

[Lou Scannon]

Interesting discussion. BTW, I meant to say in my post above, I didn't realize that scenarios 1 and 3 were possibilites.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

I said the SBC did not have water in the manifold. I meant no water heating. There actually were water passages from both heads to the Tstat housing, but this was not functional as a bypass path.

 

[PJGD]

As the diversity of opinion in this thread suggests, cooling system design is not completely intuitive, and is in fact quite complex.

I would recommend taking a look at this book from SAE:

http://books.sae.org/book-sp-1541/

Also, it may well be worth downloading this free software with which you can model your system and perhaps use it to point you in the right direction:

http://www.aft.com/psim

If you try it, let us know how you get on.

PJGD

 

[Tmoose]

A seeping thermostat would only circulate water thru the radiator.


Factory Manuals in the 50s and 60s show and describe specific circuits provided for continuous circulation in addition to the heater circuit (which was sometimes optional in those days) .

From 1958 (Ford) Edsel shop manual, 261-332 V8 engine cooling system description.
"If it (the thermostat) is closed a small portion of the coolant is returned to the water pump for recirculation.
In the 6 cylinder engine cooling system description the Edsel manual is more specific - "a small portion of the coolant is bypassed thru a pipe which returns the coolant to the water pump for recirculation. "

I'd bet there still are still such circuits today.

Uniform cooling would seem impossible if relying on conductive or thermosiphon heating of the thermostat.

 

[140Airpower]

Tmoose, this is very helpful research. I think we could consider 50 year old systems as basic. Thermostats without dribble holes leak a little, but I think the holes are necessary to clear air. Also, a small amount of flow through the radiator helps to thaw it out in freezing weather while the engine is warming instead of just dumping hot water into it once the Tstat is open. I heard it's a cure for radiator cracking.
The Slant 6 diagram implies the small bypass flow is through the block.

 

[patprimmer]

I would have thought antifreeze was the required method of preventing radiator cracks as any freezing would crack it regardless of thawing method.

The bypass as I understand it is to:-

1) Provide water heated by the block and head to the thermostat heat sensitive side to cause it to open as the coolant in the head and block reach operating temperatures.

2) To provide water to the pump so it does not run in a permanent stall or state of cavitation which might damage the vanes through cavitation or damage the seals/bearings through overheating and lack of lubrication.


Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[140Airpower]

Pat, my bad -not actually frozen, just ice cold. Cracking due to thermal shock does not require a frozen radiator. I guess that suddenly running hot water through a cold radiator is never good for it's longevity.

 

[BigClive]

Jcd06 - I wouldn't worry too much about removing the thermostat - it will not instantly destroy the engine. Most of the cars I own are older than 10 years (one is 23). I find no thermostat improves the cooling and causes much less pressure stress on the various engine and heater hoses etc. Despite dire warnings from my garage mechanic about upsetting the computer and the higher water flow eroding the engine's innards it has no ill effects that I have noticed. I should point out that I live in a fairly warm area in Oz where the summer temp can be 40 degrees C and the winter min is 10-15 degrees or so. In an icy area I think you would need the thermostat.
Even wihout the thermostat the electric radiator fan or a thermostatic mechanical fan act as a fairly effective pseudo thermostat.

 

[Lou Scannon]

I think it's pretty well understood that removing the thermostat slows down the warmup process, and will result in the coolant temperature being lower than intended, most of the time (whether that is a good thing or not).
A consequence of this, apart from effect on fuel, ignition, and other parameters (such as idle speed), is an increased tendency for sludge formation in the oil. Higher fuel consumption is predicted also, both due to operating parameters, as well as higher oil viscosity, and higher heat rejection to the coolant. As you say, in tropical Oz, these may not be significant issues.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz