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#21
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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® |
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#22
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Too goofy?
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#23
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Too goofy?
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. |
#24
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Too goofy?
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. |
#25
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Too goofy?
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#26
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Too goofy?
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#27
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Too goofy?
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. |
#28
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Too goofy?
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#29
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Too goofy?
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. |
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