Too goofy?

jeffcoslacker > wrote in
:


>
> I give up...I was talking about the OP's idea, not current
> systems...like talking to a wall...
>
>



Since nobody has actually answered HLS's original questions, I will try.

Thermosyphon (convection) in a modern engine could not work because
combustion temperatures are about 2,000 degrees, and the engine is meant
to operate at 194F. This excess heat must find its way, quickly, into
the cooling system. A thermosyphon system would operate far too slowly
for the heat generated to be removed before localized overheating
occurred.

As I said in an earlier message, with the thermostat closed, the water
pump constantly circulates and mixes the coolant in the block and head,
sending it round and round again, but ONLY within the engine so as to
minimize heat loss and promote rapid warmup. This keeps the temperature
even throughout the block and head. If you did not circulate the coolant
right from startup, you would risk localized overheating, boiling and
engine damage, even as outlying areas remained cold (including the
temperature sensor).

Thermosyphon is a far less efficient heat-transfer mechanism, since as
the surrounding water heated up, the rate of heat transfer from the
source would slow dramatically, and in any case would be far slower than
it would be under high-flow conditions. Also, the water pump's forced
flow provides the turbulence necessary to help disrupt any boundary
layer that may form and prevent efficient heat transfer.

Modern cooling systems use a high-flow, low thermal transfer
arrangement, taking advantage of the fact that greatest heat transfer
takes place at the largest temperature difference between source and
destination.

Finally, I'm not sure the water pump is much of a drain on engine power.
Or if it is, my guess is that there have already been steps taken to
reduce engine load as much as possible while maintaining necessary flow.
Emissions regs and CAFE would have seen to that.


--
TeGGeR®

wrote:

> Basically, why could not a system be developed that used an
> electromechanical
> clutch coupled to the water pump?

1. Of course it could.

2. It's not worth it (little benefit, more cost, less reliability)

3. I'm sure there's at least one vehicle driving around with one
already.

There are a couple of good reasons against it. Economics and
reliability are probably the best. Also a pump is usually a _big_ flow
restrictor when it's not turning, so you might even need to add a
bypass valve to it.

Main one though is that cooling needs are matched fairly well to rpm,
so having an engine driven pump is already a reasonable first
approximation to what you need. Couple this with the airspeed-sensitive
cooling efficiency of typical radiators and an electrically driven
radiator fan that only puts in an appearance when stationary, and
things are already quite well matched.

If you're after a few HP back at top speed, why not switch from
hydraulic power steering to electric ? Now there's a system that works
well and saves wasted power (if it's Japanese rather than American, at
least).

As to the thermostat, then so what ? You don't need thermostats or
restrictors by some divine law, you just need them in systems designed
around having them. I've got car engines that run cold with no
thermostat, are infamous for running hot with no thermostat, and even
that form an outside 3-way pipe junction (try removing that!) I used
to have one where the thermostat was known for having no effect at all,
but the radiator blind had to be adjusted according to the weather
forecast.

TeGGeR® wrote:

> Thermosyphon (convection) in a modern engine could not work because
> combustion temperatures are about 2,000 degrees, and the engine is meant
> to operate at 194F.

Thermosyphons, and pump-assisted thermosyphons were used in cars for
_years_, well into the '80s. Peak combustion temperatures have also
dropped, if anything.

If there's a single reason why thermosyphons no longer cut it for car
engines, it's to do with mechanical density of cylinders in smaller
blocks with smaller passageways. Flow velocities are increased, and on
many systems the pressure is too. 15psi used to be fairly standard, now
it's often around 30psi.

Irwin Corey wrote:

> A dwarf who clearly couldn't differentiate between either
> enthalpy and entropy or a Reynolds Number and Reynolds
> Aluminum, provides a supercilious and didactic, yet totally
> erroneous "lecture" on Thermodynamics and Fluid Mechanics.

Who mentioned either enthalpy or entropy ?

Besides which, what does it matter ? We're not changing coolant
characteristics or temperature here, just regulating the simplest mass
flow of it and wondering whether that's best done with a valve or a
controlled-speed pump.

wrote in
oups.com:

>
> TeGGeR® wrote:
>
>> Thermosyphon (convection) in a modern engine could not work because
>> combustion temperatures are about 2,000 degrees, and the engine is
>> meant to operate at 194F.
>
> Thermosyphons, and pump-assisted thermosyphons were used in cars for
> _years_, well into the '80s.




"Pump assisted thermosyphon"? Isn't that contradictory?



> Peak combustion temperatures have also
> dropped, if anything.



They have. From about 2,700F to about 2,000F. Nitric oxide emissions are
the reason. NO emissions become significant at about 2,500F.



>
> If there's a single reason why thermosyphons no longer cut it for car
> engines, it's to do with mechanical density of cylinders in smaller
> blocks with smaller passageways. Flow velocities are increased,



As I said...



> and on
> many systems the pressure is too. 15psi used to be fairly standard,
> now it's often around 30psi.
>
>


I'm wondering about your examples. I see 13psi (0.9bar) and 16psi
(1.1bar). 30psi would be 2.0bar. Personally, I've never seen a road-
going automotive cooling system running under that kind of pressure.



--
TeGGeR®

wrote:
> With all the problems we have discussed recently about cooling systems, I
> would like to
> revisit a question that, I believe, was posted some time ago:
>
> Basically, why could not a system be developed that used an
> electromechanical
> clutch coupled to the water pump?


Because you NEVER want to slow down the water flow through the engine.
Modern cooling systems (and even ancient ones, but to a lesser degree)
do not SLOW the flow of coolant when little cooling is needed, they
DIVERT it away from the radiator and circulate it back through the
engine. What this does is make the temperature through the engine more
uniform, because without recirculation some parts of the engine would
overheat and others would be running too cold for maximum efficiency and
minimum wear. Even when maximum cooling IS needed, a lot of heavy-duty
cooling systems still recirculate up to 50% of the total flow volume to
mix with the cool water returning from the radiator so that the engine
isn't cold around the cylinder skirts and boiling in the heads.
Reverse-flow cooling systems achieve similar results, but still use some
blending.

TeGGeR® wrote:

> "Pump assisted thermosyphon"? Isn't that contradictory?

Why? You have a large bore, slow flow system, and it gains a pump too.
Think of all those '50s Brit engines that stayed in production until
the '80s.


> They have. From about 2,700F to about 2,000F. Nitric oxide emissions are
> the reason. NO emissions become significant at about 2,500F.

Depends on the engine - some of the leaner burn engines still have
pretty high temperatures (NOx is indeed a major problem with their
adoption).


> I'm wondering about your examples. I see 13psi (0.9bar) and 16psi
> (1.1bar). 30psi would be 2.0bar. Personally, I've never seen a road-
> going automotive cooling system running under that kind of pressure.

Look at modern mainland Europe. 2 bar is almost commonplace on recent
designs.

wrote in
ups.com:

>
> TeGGeR® wrote:
>
>> "Pump assisted thermosyphon"? Isn't that contradictory?
>
> Why? You have a large bore, slow flow system, and it gains a pump
> too. Think of all those '50s Brit engines that stayed in production
> until the '80s.



Didn't those things overheat all the time? And in any case, we're
talking modern engines designed by people who hadn't had two gallons of
best bitter at the pub the night before.



>
>
>> They have. From about 2,700F to about 2,000F. Nitric oxide emissions
>> are the reason. NO emissions become significant at about 2,500F.
>
> Depends on the engine - some of the leaner burn engines still have
> pretty high temperatures (NOx is indeed a major problem with their
> adoption).




Like I said...

It does not depend on the engine. If combustion temperatures approach
2,500, you start to get vastly increased amounts of NO. Period. All NO
reduction strategies that I'm aware of involve reducing combustion
temperatures.



>
>
>> I'm wondering about your examples. I see 13psi (0.9bar) and 16psi
>> (1.1bar). 30psi would be 2.0bar. Personally, I've never seen a road-
>> going automotive cooling system running under that kind of pressure.
>
> Look at modern mainland Europe. 2 bar is almost commonplace on recent
> designs.
>


Really? I wonder how their rad hoses are built.

--
TeGGeR®

TeGGeR® wrote:

> Didn't those things overheat all the time?

No. Unless people took the thermostats out, as the BL A & B series
engines were one of those where removing the stat completely would
disrupt the waterflow.

> And in any case, we're
> talking modern engines designed by people who hadn't had two gallons of
> best bitter at the pub the night before.

Are we? Like much of Usenet, this group is somewhat US-centric. You
can lecture Europe on engine design when you lose your obsession with
huge low-compression OHV V8s.


> It does not depend on the engine.

Of course combustion temperature depends on the engine! Look at the
trouble Porsche had with the 911 engine, where they had to start
watercooling the heads to get emissions under control. It's not just
the peak temperature that matters here, for volumes of NOx, it's an
integral over the whole volume of the combustion chamber and duration
of combustion.

> All NO
> reduction strategies that I'm aware of involve reducing combustion
> temperatures.

Look at modern Japanese small engines, like the Honda Jazz. Tiny engine
with high compression, lean mixtures, high specific outputs and every
sign of peak combustion temperatures being set to go high -- yet they
keep the emissions down somewhow. I don't know how they do it, but I
guess it's by avoiding localised high temperatures (we're back to
cooling system density again, and the problems of small waterspaces and
making them reach absolutely everywhere).


> > Look at modern mainland Europe. 2 bar is almost commonplace on recent
> > designs.
>
> Really? I wonder how their rad hoses are built.

Same as ever. The limit is in hoses are attached to spigots. If you see
a failure it's nearly always because a hose has blown off, not because
it has split. If you do see a car boiled up by the side of the road
today, it's usually full of teenage chavs wondering why their silicone
dress-up hoses have blown off from their Subaru, when they didn't fit
them carefully enough.