| Author |
Message |
James Selevan (Jselevan)
New member
Username: Jselevan
Post Number: 27
Registered: 6-2002
| | Posted on Tuesday, July 09, 2002 - 7:48 pm: | |
The Celestial Design Committee has ruled - The thermostat stays!
Gentlemen, I find this most interesting as conclusions, right or wrong, are at least justified with physics and engineering principles. I prefer this to "folklore" and "seat of the pants" approaches. There IS a right answer. We all know what the answer is, we simply can't agree on it. (Just kidding).
By the way, I did get the air pump, clutch, and large bracket/shield out of my Dino without removing the fuel tank. It required removal of the brackets, oil heat exchanger, etc., etc., etc. But at least it's out. Now I am seeking a European model bracket for my alternator. Does anyone know where I might find one?
Thanks for the interesting engineering discussion. This is what I enjoy about Ferrari Chat. Sorry if it is less interesting to others.
Jim Selevan |
Bill Sebestyen (Bill308)
Member
Username: Bill308
Post Number: 269
Registered: 5-2001
| | Posted on Tuesday, July 09, 2002 - 6:43 pm: | |
Harry,
There are too many engineers on this thread. Seriously though, I think your logic is pretty sound except for:
"But then the same would happen in the engine cooling channels and the water wetter mixture would get hotter than water."
I believe this should read:
But then the same would happen in the engine cooling channels and the water wetter mixture would be cooler than a water or water/antifreeze mixture.
If the radiator tube temperatures are fixed, because we haven't altered the fin to air interface, then all temperatures can be referenced to the tube temperatures. If the convection processes are more efficient at both the engine end and the radiator tube ends, the engine itself will be cooler and the coolant temperature will also be cooler. Also, the Water Wetter folks specifically assume there is some localized boiling, probably in the vicinity of the combustion chamber, especially near the exhaust valves. The mechanism they cite is that since Water Wetter lowers the surface tension of the coolant, vapor bubbles, a local insulator, are released more easily, resulting in more efficient cooling.
The fact is, this is a pretty complicated problem and lots of secondary effects are being ignored for simplicity. Thermodynamics says the engine becomes more efficient a higher temperatures. Until we start using super lubricants and begin producing ceramic parts, I don't think we will be able to operate at temperatures much higher than we are today. And what about heating of the intake charge, which lowers the air density and lowers volumetric efficiency? Doesn't heat also reduce oil viscosity, thus lowering losses? This problem is not unique to piston engines. Enormous sums are spent engineering and manufacturing turbines to run at higher temperatures where an increase in efficiency means larger profits for the airlines. To accomplish this, engine makers use single crystal alloys to maintain the strength of turbine and probably last stage compressor blades.
I think I�m starting to get a headache from this discussion. It�s been fun though. Remember James, who asked if it would be wise/unwise to run his Dino engine without a cooling system thermostat?
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Harry (Harry)
New member
Username: Harry
Post Number: 16
Registered: 3-2002
| | Posted on Tuesday, July 09, 2002 - 12:59 pm: | |
Good thread. Thought a little about it and could not really get everything clear. Even if it may be boring for some readers, some more points:
Bill said:
"For the Water Wetter case, the engine would again start at 25 C, warm up to 85 C, where the thermostat begins to open, and stabilize at a lower temperature, say 90 C water temperature, where the thermostat may or may not be fully opened. You have more cooling capacity in reserve for this scenario and you're running about 10C cooler."
Assumption for the following points: Constant engine load at constant speed.
The engine produces a certain amount of heat which has to be tranfered to the environment. As a rule of thumb, 100% fuel energy is converted into 33% of mechanical power, 33% exhaust losses and 33% internal losses. The internal losses heat up the engine (just theory: if you assume you would not cool but insulate the engine, it would heat up to the combustion temperature and you would save the 33% internal losses). In reality you want the engine to run in a certain temperature range, therefore you cool the engine with water (and to some extend with oil). The water needs to get rid of the heat it picks up in the engine (with about 100kW power output which is a constant speed of about 200kph, this reaches 100kW cooling power!). The only place it can transfer the heat is the radiator. In the radiator there is a heat transfer from fluid to tube and then from tube to air. The amount of heat which can be transfered follows the basic formula:
Heat flow = alpha x suface x delta T
alpha = heat transfer coefficient
delta T = temperature difference
Now you say that the temperature of the coolant can be 10 degC lower using some water wetter mixture. At an outside temperature of 20 degC your delta T is then (90degC-20degC)=70K instead of (100degC-20degC)=80K what means you loose 12% cooling capacity on this interface. As the system tries to get in balance again, the temperature of the coolant will raise until enough heat can be transfered through this interface.
Only solution: the heat transfer from fluid to tube is much better with the additive in the water and so the tube temperature can be the same even though the water temperature is lower. But then the same would happen in the engine cooling channels and the water wetter mixture would get hotter than water. Only solution now: water wetter mixture has a higher heat capacity per volume unit, but this is normally directly related to the density of the fluid. And higher density means higher pumping power required, which directly reduces the engine power output.
This would mean that the ideal coolant temperature/engine temperature is close to the max operating temperature of the sensible engine parts/engine oil (remember also the thermodynamics) and low enough to avoid evaporation of the coolant on hot spots.
If this is all correct, using water wetter would mean you reduce your power output and efficiency of the engine by a certain amount (have no data for the cooling system to calculate it).
Am I wrong?
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'75 308 GT4 (Peter)
Intermediate Member
Username: Peter
Post Number: 1898
Registered: 12-2000
| | Posted on Tuesday, July 09, 2002 - 12:30 am: | |
Thanks for the answer Bill.
Excellent thread guys, this is the kind of high-quality content that's too far-and-few-in-between, here in "Tech Q&A". |
Bill Sebestyen (Bill308)
Member
Username: Bill308
Post Number: 266
Registered: 5-2001
| | Posted on Monday, July 08, 2002 - 5:56 pm: | |
James,
I concur with your thoughts. I don't see any significant disagreement here. My point is that you really want an engine to run at as constant a temperature as possible. Heat affects the working clerances, rings, bearings, gears, and valve guides. A cold engine might pass a little oil past the rings, but when up to temperature, seal quite well. Oil lubricates better when up to a normal working temperature. This is why drivers are cautioned to let the engine come up to temperature before it is put under heavy load. While material properties my not change significantly over the working range, they will expand or contract proportionally to the material's alpha, or coefficient of expansion. This affects the working clearancs, which in the case of rings, affects their ability to seal. In an ideal situation, an engine would run at a constant temperature, the hotter the better from a thermdynamic standpoint. But there are other considerations like oil break down and weakening of piston crowns, so the cooling system is disigned to limit ultimate temperatures. It is in my opinion, that most wear occurs during the cold to hot transition. The more that this effect can be limited, the better from a wear stand point. |
James Selevan (Jselevan)
New member
Username: Jselevan
Post Number: 26
Registered: 6-2002
| | Posted on Monday, July 08, 2002 - 4:37 pm: | |
Bill - the thermostat, in effect, represents a feedback control system where the error signal is the water temperature. Thus, no matter what magic fluid you put in the cooling system, the thermostat will either open, close, or partially open to maintain a range of temperature. This temperature range includes a "hysteresis" resulting from the characteristics of the thermostat (it does not close at the same temperature that it opens).
In addition, from a purely thermodynamic perspective, you want the engine to run hot. The more heat that is lost to the coolant, the less heat available for adiabatic expansion in the combustion chamber. Thus, by cooling the engine, one reduces its thermodynamic efficiency. After all, an internal combustion engine is no more than a heat pump, converting heat to mechanical work. The more air:gas combination (14:1) that can flow through the engine in unit time, the more heat generated and thereby the more work that can be performed. This, of course, has to be balanced with the consequences of heat to rubber seals, metal to metal clearances, oil characteristics, etc.
I submit that our (appropriate) concern for controlling engine temperature is unfounded in the mechanical engineering of the engine. As long as metal to metal clearance is not compromised, the engine does not care whether running at 180 F or 240 F. It may bother the driver, but not the metallurgist.
My thoughts.
Jim Selevan |
Bill Sebestyen (Bill308)
Member
Username: Bill308
Post Number: 265
Registered: 5-2001
| | Posted on Monday, July 08, 2002 - 2:32 pm: | |
Peter,
Tnats a good question. Lets look a 2-scenarios.
An engine running on an antifreeze/water mixture, may be started in a 25 degrees C ambient, warm up to 85 C, where the thermostat begins to open, and stabilize at 100 C water temperature, with the thermostat fully open.
For the Water Wetter case, the engine would again start at 25 C, warm up to 85 C, where the thermostat begins to open, and stabilize at a lower temperature, say 90 C water temperature, where the thermostat may or may not be fully opened. You have more cooling capacity in reserve for this scenario and you're running about 10C cooler.
I submit that there is not a lot of difference in normal running at a nominal 90C or 100C. There are situations where you will exceed these temperatures on the high and low side during normal operation. Still, if the bulk of your driving is in the 90 to 100C, rather than the 25-100C range, you will have gone a long way in helping your engine live a long and healthy life IMHO.
I think in the final analysis, Water Wetter doesn't make a lot of difference from a normal running temperature standpoint. The thermostat rules for anything other that a short run or very hot conditions. Unless ambient temperatures are really cold, the engine will always warm up to the point where the thermostat opens. If you're doing short runs, you may never get to normal operating temperatures. Under hot conditions, you will warm up beyond the point where the thermostat opens and run at the cooling capacity of your system. When running in this regime, an increase in heat output will increase engine temperatures a proportional amount. Fortunately, most cooling systems in good repair, have sufficient reserve capacity so this is not a real problem. |
'75 308 GT4 (Peter)
Intermediate Member
Username: Peter
Post Number: 1897
Registered: 12-2000
| | Posted on Sunday, July 07, 2002 - 11:24 pm: | |
Bill: "What you really want is for the engine to heat up to operating temperature quickly and then remain in a narrow temperature band. Engine clearances will be more stable under these conditions, reducing wear and probably running better."
Using this logic, what about those people using additives like the Water Wetter which reduce temps? Does it take it out of the proper running temperature band by running too cool? |
James Selevan (Jselevan)
New member
Username: Jselevan
Post Number: 25
Registered: 6-2002
| | Posted on Sunday, July 07, 2002 - 10:36 pm: | |
Bill - great discussion and explanation. Thanks. Looks like the thermostat stays right where it belongs - in the car and not in the garage.
Jim Selevan |
Bill Sebestyen (Bill308)
Member
Username: Bill308
Post Number: 263
Registered: 5-2001
| | Posted on Sunday, July 07, 2002 - 8:59 pm: | |
Increasing coolant flow rate will increase the convection coefficient, for both the engine-to-coolant interface and for the coolant-to-radiator interface. This means the engine will run cooler, all else being the same. The down side is there will be larger pumping losses and higher loads on the water pump belt, bearings, and seals.
My 308 owner's manual specifies that without a thermostat fitted, coolant will bypass the radiator circuit and primarily recirculate within the engine. You could break the thermostat open or fabricate something to accomplish this same goal, but IMHO, this would be a questionable goal. The results would be the engine would take much longer to heat up. What you really want is for the engine to heat up to operating temperature quickly and then remain in a narrow temperature band. Engine clearances will be more stable under these conditions, reducing wear and probably running better.
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James Selevan (Jselevan)
New member
Username: Jselevan
Post Number: 24
Registered: 6-2002
| | Posted on Sunday, July 07, 2002 - 12:38 pm: | |
Steve, thank you for your suggestion that the thermostat both opens and closes ports. As I examine this Dino thermostat housing, however, it appears to simply open a passage without closing one. But the concept is important and must be taken into account.
Joe, I examined Evans Web site (http://www.evanscooling.com/main21.htm) and found it very interesting. I have to digest this for a moment, and separate the hype from the physics, but the concept is sound. The unknown to me is what the boiling point of a 50:50 mix of water and ethylean glycol will be at 14.5 psi (above atmospheric). If it exceeds the 350 degrees achieved by Evans, then their is not as dramatic an advantage to Evans (save the viscosity and the low pressure issue). More to come. Thanks.
Jim Selevan |
joe saldana (Ironjoe)
Junior Member
Username: Ironjoe
Post Number: 82
Registered: 12-2001
| | Posted on Sunday, July 07, 2002 - 10:54 am: | |
James,Go to google.com and look up evans coolant it will change the way you think about heat transfer forever,Its 22.50 a gallon but no thermostat, n zero to 7 lbs of pressure,no detonation n up to 370 degrees temp.....I love it I will never go back to water base,non-aqua is the cats-ass.... |
Steve Magnusson (91tr)
Member
Username: 91tr
Post Number: 824
Registered: 1-2001
| | Posted on Sunday, July 07, 2002 - 10:17 am: | |
James/Paul -- Regardless of the (interesting) flow speed vs. final fluid temp question, the F thermostat/housing design is a 2-position valve that not only "opens" passages, it also "closes" other passages so physically leaving it out actually decreases the coolant flow thru the radiator(s) and most of the coolant circulation will stay in the engine. You could maybe somehow permanently fix the thermostat plunger in the warm position -- which would always circulate all the coolant thru the radiator(s) -- but I wouldn't (although James if your Dino is still on it's original thermostat or it's 2+ decades old, putting a new one in wouldn't be a sin IMO -- or at least functionally testing the existing one if you're having a problem). I can't recall if it's shown in the Owner's Manuals or only the WSMs, but there's usually a decent schematic showing the hot vs cold coolant flow and how the F thermostat plunger works. |
James Selevan (Jselevan)
New member
Username: Jselevan
Post Number: 22
Registered: 6-2002
| | Posted on Saturday, July 06, 2002 - 11:18 pm: | |
Paul - you raise a VERY interesting issue. At first thought, your suggestion that flow through the system that is too fast will lead to less heat dissipation seems intuitive. However, one must consider the volume of flow, the thermal "mass" of the fluid (water and antifreeze mix), and the equipoise of heat generation (by the engine) and heat dissipation by the radiator. Does the engine produce heat faster than the radiator dissipates it, or the opposite? It is obvious that in steady state, with the thermostat open, heat dissipation equals generation, otherwise operating temperature would continuously rise. Thus, by increasing volume of flow (not just speed), one picks up heat faster and delivers more hot water to the radiator in unit time, but has less time to dissipate heat in the radiator. Hence, the ultimate question: does the radiator have capacity to dissipate more heat than it normally does under steady state? I believe that it does, and therefore, anything that increases volume of flow will lead to lower operating temperature.
Not a simple discussion. There are a few mechanical engineers participating on Ferrarichat. While I did study thermodynamics as part of my Physics and Electrical Engineering training, this is a classic mass flow discussion that chemical engineers and mechanical engineers eat for breakfast. Their input would be appreciated.
Jim Selevan |
Paul Newman (Newman)
Junior Member
Username: Newman
Post Number: 249
Registered: 12-2001
| | Posted on Saturday, July 06, 2002 - 6:26 pm: | |
James, as far as the thermostat goes, I wouldnt run it without one. The thermostat also has an affect on how fast the coolant passes through the rad. Too fast = hotter. If you dont want a thermostat, install a restriction in its place. |
James Selevan (Jselevan)
New member
Username: Jselevan
Post Number: 21
Registered: 6-2002
| | Posted on Saturday, July 06, 2002 - 2:11 pm: | |
I am attacking several issues on my Dino, and appreciate any help available.
1) - can I remove the large alternator - air-pump shield/bracket without removing either the forward exhaust manifold or the engine? I would like to take this equipment off the car and replace with the standard European alternator bracket.
2) How wise/unwise is it to run the Dino without a cooling system thermostat? In a warm climate (Southern California), and with respect for initial oil temperature and low RPM, is it prudent to eliminate this potential point of failure and water-flow restrictor?
3) Can the passenger-side fuel tank be removed without removing the rear suspension?
Your thoughts are appreciated.
Jim Selevan |
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