Fan Relay Next?

[zcacogp]

There is a small thermocycle when the engine is off, but it is just that - small.

It does feed into another idea I had a while ago; an electrically-driven constant-speed water pump. The current design of water pump on every car I have looked at is such that its speed is directly related to engine speed. Most engine cooling needs to be done at low speed as there is less air flowing past the engine, therefore the water pump needs to be circulating at least enough water for the worst-case cooling scenario when idling (approx 1000rpm). This means that it will be circulating several times as much water when running at high revs (approx 6000rpm = six times as much water) despite there being no greater need for cooling. This will equate to a huge drain on engine power at high speeds.

A water pump that turns at a constant speed would overcome this. And if you were to do this electrically, you could keep it running after the engine has stopped to carry on cooling. (More relevant for turbos, perhaps.)


Oli.

 

[pegasus]

Hi there, you are mixing engine cooling with air with air going past the engine,the heat from combustion is transferred to coolant, it is the coolant that has the energy transfered to the air via the radiator,granted some heat is transferred directly to the air via the engine sump and block etc but the real heat is dissipated to the air via the coolant going theough the radiator. The amount of heat increases with revs and load which works out really well with the circulation rate ,the real waste is when you force cool the rad (hence the introduction of electric fans), when not needed, the set up that porsche has is a bit of icing on the cake, if youf fan did not run when you stopped you would probably not notice the difference but the designers must have had a reason for building this function in,just my thoughts,all the best John.

 

[peanut]

ORIGINAL: pegasus

Hi there, you are mixing engine cooling with air with air going past the engine,the heat from combustion is transferred to coolant, it is the coolant that has the energy transfered to the air via the radiator,granted some heat is transferred directly to the air via the engine sump and block etc but the real heat is dissipated to the air via the coolant going theough the radiator. The amount of heat increases with revs and load which works out really well with the circulation rate ,the real waste is when you force cool the rad (hence the introduction of electric fans), when not needed, the set up that porsche has is a bit of icing on the cake, if youf fan did not run when you stopped you would probably not notice the difference but the designers must have had a reason for building this function in,just my thoughts,all the best John.

I wouldn't have thought that Porsche added a double fan with two speed control on the S2 and turbo models just for fun []

Try sitting in static traffic for more than 15 minutes with your fan fuse removed on idle and see what happens to the engine coolant temperature especially if you have just done some spirited driving.

The fans are there to carry heat away from the surface of the radiator by drawing cooler air from the grille through the radiator. Cooling the engine block with the fans is secondary . As I understood Oli's observation he was referring to increasing coolant circulation with an electric pump in conjunction with a fan to take coolant heat away from the radiator as per a convention cooling system

 

[pegasus]

I am getting confused here he was talking about a constant speed waterpump,that cannot work as heat is propoptional to power/load /revs ie the faster you turn your engine the more power /heat you get. I f you read what I said about the icing on the cake I was talking about the way porsche has the fans run when the engine is turned off nothing to do with the cooling required when sitting in traffic,maybe I read it wrong,all the best John.

 

[peanut]

I guess we should ask Oli what he meant lol[]

I took it that he meant instead of having a mechanical water pump running at engine speed use an external electric pump . Sounds like a good idea to me and much cheaper . I assume it would be electronically contolled so that water circulation speed met demand[]

incidentally your engine speed /heat equation doesn't work as it doesn't factor in surrounding air temperature and air change rate ie convection and radiation

 

[pegasus]

To be quite honest if you have a variable electric water pump to replace a variable mechanical pump you would have moch of a muchness.mechanical pumps do not use a lot of power,you only have to look at the pumping volume of portable pumps driven by small honda engines to see that,like I said the big power waster used to be driving a fixed fan but the advent of electric fans got round that,all the best John.

 

[zcacogp]

I was actually thinking about a constant speed pump, that kept the water going at the same rate regardless of engine load. Although you could have it electronically controlled such that it met demand.

I was working on the basis that in the worst case scenario (very hot day, car sat in traffic), the waterpump has to cope. Given the usual arrangement, flow is proportional to engine speed, therefore in the worst case scenario then it copes at minimum flow rates. When the car is moving, there is more cooling available (more air flow through the radiator) BUT the pump is also working harder; unnecessarily hard.

I guess it is a slightly confused argument as it is confusing water flow with rate of cooling. In the worst case scenario mentioned above, cooling is at it's worst (smaller temperature differential between coolant and air, and little air flow). Is the rate of water flow relevant to this? I'm not sure. It perhaps is, but perhaps is not as crucial as the air flow.

I guess you could consider the situation as being an engine producing heat, and a radiator getting rid of it. The water flow is the rate of transport between the two. If there is more heat being produced (as the engine revs increase - and that's another point, is the heat produced directly proportional to the engine revs, given the thermodynamics of controlled expansion in the engine?) then does it necessarily need to have the transport system running more quickly (i.e. the water flow rate increased). If it is not necessary then it is surely not desirable as it is inefficient and power-sapping.

Perhaps I should stop now. I fear I am making even less sense than usual (and I have not had any alcohol to drink yet!)


Oli.

Edited to amend a daft error! Change in italics. Thanks John/Pegasus.

 

[pegasus]

Hi Oli "is the heat produced directly proportional to the heat produced", time to call it a day,all the best John.

 

[zcacogp]

Ah. Oh. Hmmm. Told you I was making less sense than usual!

Original message amended.


Oli.

 

[pegasus]

Its nice to have some sort of diversion,the day I've had ,unbelievable,have a look at my post on rear camber,I give up with all this, it's time to go back to sea I think,all the best John.