Fact/Fiction- cooling

[Tommd55 (Tom D)]

@Robert_Wilkinson

Basically, no

@Michael_Frank

Faster is better.

@Robert_Wilkinson

The more/better air flow issue is IMO virtually identical to the coolant flow issue.

@Kirbert

No. That’s an old wives’ tale.

I agree with the above. But old tales do not die. (I also know most of what I am going to say has probably already been said by others in some manner.)
Robert makes a valid point about increasing flow will increase pressure and heat. True, but I am going to discount that in this discussion because I believe for any practical engine, we would be below any levels this would come into play, in contrast to the extreme heat generation of a hydraulic system operating at 1000+ psi.
If this were an open system, then I believe the answer would be different. That is, if hot water was poured through a radiator ONE time, then out on the floor, slow would cool more. But this is a closed system. Once the water is through the radiator, it goes back to the engine to pick up more heat.
IF the flow was very slow, the radiator would potentially totally cool the water. But, at such as slow rate, the heat would not be moved away from the engine quickly enough, and it would overheat. On the other hand, the faster the flow, the more the water temperature of the water would be the same throughout the system. And if it were the same through out the system, the water exiting the radiator would be higher than at a slow flow rate, but the water exiting the engine would be cooler than at a slow rate, thus I believe overall better temperature gradients for the overall system.

@inlinesix

One of the purposes of the thermostat is to reduce flow for that purpose.

I believe this is only true if the design of the system allows removing the thermostat to cause the flow to go the bypass route and not the radiator, which some engine designs do. It would be interesting to know what the Austrian did totally. Changing flow patterns through the engine head and block clearly effect cooling. I believe much of the too fast is bad story is based on pump cavitation, as Doug said.

Another potentially interesting question. If you agree with faster (to a point) is better, but then if as I said, faster flow means the overall water temperature becomes the same throughout, then the water leaving the radiator is about the same as the water entering the radiator. And so, indeed it “appears” that little heat was lost. We are often conditioned to think we want a large temperature drop across the radiator. So do we? And if so, then how is the heat being “lost” when the flow is greatest?
Tom

[Michael_Frank]

@Tommd55

Another potentially interesting question. If you agree with faster (to a point) is better, but then if as I said, faster flow means the overall water temperature becomes the same throughout, then the water leaving the radiator is about the same as the water entering the radiator. And so, indeed it “appears” that little heat was lost. We are often conditioned to think we want a large temperature drop across the radiator. So do we? And if so, then how is the heat being “lost” when the flow is greatest?

The fallacy implied in your question is that the engine’s heat production is infinite. It’s not.

The heat being lost can be objectively measured by the number of calories transferred into the airflow…the temperature rise of the output air vs the input air is easily measured, as is the mass airflow. This will be maximized when water flow is sufficient to keep the entire radiator core ‘hot’. If inlet temperature was exactly the same as outlet temperature, you would still be rejecting enormous amounts of heat, but you would have the very odd condition where the heat produced by the motor equaled or exceeded the maximum heat that could be rejected by the radiator. That should never be possible because the cooling capacity of the radiator is greater than the heat production of the motor by design. So as a practical matter, the return water will always be cooler than the engine.

And as for the assertion that led us here…The sole purpose of the thermostat is to increase operating temperature to some pre-determined minimum. It isn’t thereto “slow the flow”. In fact, most thermostat caps are designed to enhance the venturi effect. And since ALL engines have bypass systems, the thermostat can only redirect flow.

[Robert_Wilkinson]

@Michael_Frank

If inlet temperature was exactly the same as outlet temperature, you would still be rejecting enormous amounts of heat, but you would have the very odd condition where the heat produced by the motor equaled or exceeded the maximum heat that could be rejected by the radiator.

Not sure where your coming from here.

Anything that generates heat will rise in temperature until the temperature gradient for conduction, convection and radiation to the ambient becomes sufficient for the heat to be dissipated. This includes a water-cooled engine connected to a radiator. So (ignoring melting or seizure of parts, boiling and excess pressure etc) any radiator (or no radiator) will cool any engine–the only variable is how hot the system gets before heat generation and heat dissipation to the ambient are in equilibrium.

Quite separate from that point is that higher coolant flow rates will allow any radiator to be more efficient, by allowing all of the fins to reach the actual temperature of the engine.

@Tommd55

If this were an open system, then I believe the answer would be different. That is, if hot water was poured through a radiator ONE time, then out on the floor, slow would cool more

No, slow always cools less, because allowing the water to cool decreases the temp gradient to ambient as the water flows through the radiator.

“Cooling” in this context refers to heat removal (not temperature drop) in a steady state. The engine is “warmed up” and generating so much heat per unit time. That’s power, expressed in watts, calories per second or BTU/hour for example. So by more cooling we mean heat removal per unit time.

Say, in the open system, your 10 Kg of water is at 100 deg C and it’s a cold day–ambient 0 deg C. Say you pour it through slowly–it takes 100 seconds, and the water coming out is 20 deg C. Now we know that the top fins had a gradient of 100 deg to the ambient, but the bottom ones had only 20–they were cooling 5X less efficiently as the top ones. So you lost 10Kg X 80 deg or 800 Kilocalories of heat in 100 sec, or 8 Kcal per sec.

Now say you pour it through fast, say 5 seconds. It hardly cools off at all, going from 100 deg to 92 deg. That’s 10 KG X 8 deg, or 80 Kcal. but it does so in 5 sec, or 16 Kcal per sec.

[inlinesix (Terry Sturgeon)]

This is a very interesting discussion about theory, some of which may have been proven by actual experimentation, or years of experience by smart observant people with different cooling systems, but here’s the reality - an E Types’ cooling system, in proper working condition, including block and rad cleanliness, proper thermostat, appropriate ignition timing, pressurization, etc is adequate to cool the engine appropriately, under virtually all conditions encountered, EXCEPT for the issue of proper airflow through the rad and engine compartment, at slow speeds in Ser 1 cars. You can easily fix that issue with a properly constituted fan, and it’s the simplest, cheapest method. Moving more water won’t help here, nor will removing the thermostat. The Ser II cooling system is better IMO because of the twin fans and the downflow rad., but it had to be because of the pollution control issues it had to meet. My only caveat - I express no opinion with adequacy on A/C equipped cars as I have zero experience with them.

[Nick_53_XK120_OTS]

thanks Robert…for putting “time” into the variables of cooling. I think of the riddle of which cools your piping hot coffee faster.: .put the cool cream in first…or let the hot coffee sit, then put in the cream? Since thermostat removal was mentioned…I will say again…Do not remove the thermostat thinking it will help your cooling. I Won’t dive into it all here…see the thermostat article on the XK forum…but once the thermostat is open…say at 74C…it is open. For temps above that…it is open. (open direct to radiator, closed to direct to the engine with bypass of radiator) The physical stat body itself is not a restriction to flow BEYOND that which was intended…as part of the system design, (duh…ya think for 100 years the engineers did not consider that) and as Michael mentions venturi effect is part of the system design. Nick

[Lost_Horizon (Dana)]

Robert has the right perspective on all this…

Since the ‘Heat In’ is the assumed fuel flow in steady state minus Work by the Engine, that’s how the radiator and engine bay get designed. The Radiator has three main variables; viz coolant flow, air flow and coefficients of transfer.

The cooling system is, as Robert points out, a net heat flux problem, On the heat supply side, there is the engine. On the rejection side, it is primarily the radiator, direct block radiation, convection and exhaust. When the rejection side of the equations are less than the supply, the mean temperature of the block coolant will increase until the positive and negative flux are in balance. That point is the steady state. The radiator interfaces with two main mass flux flows, one being the coolant, the other, the air.

At steady state, the heat rejection to the radiator plus direct block convection and radiation and the exhaust will equal the heat generated by the engine. In the simplest of terms, the heat input to the system at the coolant can be determined as the product of mass coolant flow through the engine times the delta temperature between supply and return from and to the radiator (plus the direct transfer by convection+radiation from the block and out the exhaust).

At the radiator, the airflow mass flow times the delta air temperature integrated in and out across the surface area and incidental radiation equals the net rejection of heat for that component.

So, to lower the returning coolant temperature from the engine in steady state for a fixed coolant mass flow from the water pump to the radiator, you have to lower the radiator supply coolant temperature back to the engine, to maintain the same delta for the equivalent heat to be carried away (Mass*delta-temp).

Your choices on that side of the system are to increase the heat transfer flux to the air in some fashion. You can increase the air mass throughput by increasing radiator surface area, but unless you increase fan size to keep the mass of air flowing through the radiator at least equal to the original flux rate per unit area, you might not gain much.

If you increase the primary coolant flow, you could gain some radiator and coolant efficiencies across the various surfaces by getting a better average delta temp across the radiator surface area for the total air flux to carry away the heat. However, there are limits and diminishing returns for this approach, given that the original designers usually push the limits of turbulence, flow and cross sectional balance for their designs in the first place.

The key is that the all heat the engine produces must get hauled away, or the steady state temperature will go up. In the simplest of terms, Heat In (Engine) = Heat Out (Radiator++Exhaust + Convection)

[Tommd55 (Tom D)]

@Michael_Frank

fallacy implied in your question

No fallacy implied, just a question to raise thought.

@Michael_Frank

And as for the assertion that led us here…The sole purpose of the thermostat is to increase operating temperature to some pre-determined minimum.

I do not believe I ever made that assertion. The original post/question for thought never mentioned a thermostat. It questioned whether water could go too fast through a radiator that it would not lose enough heat. I believe you agreed that fast flow is not a problem.
Also, you already know ALL engines do not have bypass systems- all engines do not have thermostats. Thank you for all your input.
Tom

[Wiggles (Paul Wigton)]

@Tommd55

ALL engines do not have bypass systems- all engines do not have thermostats.

My DKWs wouldn’t recognize them…Or, a water pump!

[Nickolas (Nick Saltarelli)]

@inlinesix

The Ser II cooling system is better IMO because of the twin fans and the downflow rad., but it had to be because of the pollution control issues it had to meet.

Agreed. But to split hairs just a bit the cooling system used in Series 2 cars was first used in the 1968 Series 1.5 in all markets including those outside the US that received triple carb cars. It is entirely likely the US Federal Regulations triggered a wholesale change in all markets but it’s fair to say non-emissions-controlled engines benefited just as much. But what distinguishes the Series 2 cooling system as the best of all the XK-engined E-types is the much increased air intake, which some perceive as aesthetically more appealing than the Series 1/1.5 treatment.

[Wiggles (Paul Wigton)]

@Nickolas

which some perceive as aesthetically more appealing than the Series 1/1.5 treatment.

Maybe to them what own’em… and in Moody Blues videos…

[John_Walker (69 FHC. . . $ via PayPal)]

@Nickolas

But what distinguishes the Series 2 cooling system as the best of all the XK-engined E-types is the much increased air intake,

With the car stopped in traffic is the smaller bonnet mouth ( about 120 square inches) constrictive enough to impede airflow to the part of the radiator covered by the fan (231 square inches)?

[Nickolas (Nick Saltarelli)]

Perhaps at a standstill in hot weather the S1 single blade fan doesn’t pull enough air to keep the engine at a stable operating temperature but I find the stock dual fans do the job well, even through the smaller bonnet intake.

[John_Walker (69 FHC. . . $ via PayPal)]

@Nickolas

I find the stock dual fans do the job well, even through the smaller bonnet intake.

That’s my experience as well.

[Tommd55 (Tom D)]

@Robert_Wilkinson

No, slow always cools less,

Robert, not sure I follow this, not sure I am using a similar example, and not sure of your numbers- but I also am not sure what I am saying! Let me give it a try.

If I have the same open system radiator you have, with the same 10Kg of water. at 100 C on the cold day. and I poor the water through the same radiator very slow. Let’s just park it there for 3 hours. Then drain it out. The water is cooled to ambient temp of 0 C. (We drained it before freezing.) By going slow, we cooled it all.

Then, pour it through in 1 second. It cools to 99 C. Virtually none of the heat was removed.

So, slow cools more. In my scenario, I do not care about the rate of heat removed per second. And, just to ask the question first- Should I care about the rate in my scenario?

Next, just out of curiosity, should I assume your numbers are real world? I assume it would depend on the size of the radiator?? For example, if your fast version only cooled to 99 C, then would it be 10Kg X 1 deg, or 10 Kcal in 5 sec or 2 Kcal per sec.

(to be clear on the initial question, I believe we agree that water moving fast in the radiator will not prevent it from giving up heat, unless the conditions are extreme.)
Tom

[John_Walker (69 FHC. . . $ via PayPal)]

I was looking at a study titled "Formula SAE Cooling System Design". The conclusions starting on page 74 may prove helpful.

https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1229&=&context=mesp&=&sei-redir=1&referer=https%3A%2F%2Fwww.bing.com%2Fsearch%3Fq%3Doptimal%2Bautomobile%2Bradiator%2Bdesign%26qs%3DPF%26cvid%3Def57a9165e674e8d8cb0aaa4a3a92c0a%26refig%3D137c931854a84570f939ad249de3d455%26cc%3DUS%26setlang%3Den-US%26plvar%3D0%26first%3D7%26FORM%3DPERE#search="optimal%20automobile%20radiator%20design"

[Michael_Frank]

@Tommd55

If I have the same open system radiator you have, with the same 10Kg of water. at 100 C on the cold day. and I poor the water through the same radiator very slow. Let’s just park it there for 3 hours. Then drain it out. The water is cooled to ambient temp of 0 C. (We drained it before freezing.) By going slow, we cooled it all.

Then, pour it through in 1 second. It cools to 99 C. Virtually none of the heat was removed.

It’s not a problem of cooling 10KG of water to room temperature. It’s a problem of a creating a continuously recirculating flow. So sure, you could leave the water in a tank for three hours and it would cool to ambient temperature. But engine cooling isn’t a simple matter of rejecting X calories. It’s a question of establishing a continuous process that transfers X calories/minute. The hotter you can get the radiator, the bigger X will be, because heat transfers at a faster rate when the temperature differential between the fins and air is higher. So lets see how this looks:

Let’s think about this from the air side. Cool air enters the front of the core, absorbs heat, and hot air exits the rear. Raising air temperature is the primary objective: the hotter the exit air, the more heat is being removed from the system. But air entering the at bottom of the core encounters lower temperatures, and can’t remove as much heat as air entering the top. If you can push water through the core faster, more of the core stays hot, and more heat is removed. By contrast, if the flow is so slow that the exiting water is at ambient temperatures, the bottom of the radiator is doing nothing useful.

[MattFurness]

@John_Walker

I was looking at a study titled "Formula SAE Cooling System Design". The conclusions starting on page 74 may prove helpful.

I was interested enough to have a skim read through this report because this stuff is of interest to me…but the disclaimer at the front is well advised. Acceptance does not imply technical accuracy or reliability. This is an undergraduate student effort.
The general principles are from the textbooks but there are some misguided assumptions that should have been picked up by the lecturer.
One glaring error is the assumption that the flowrate of the pump connected to the crankshaft should follow the power output curve of the engine…complete with plotted curve to reflect this. A centrifugal water pump has a completely different characteristic curve than an internal combustion engine …and will simply use a small fraction of the engine power to pump water regardless of the shape of the engine power curve… the students should have been appropriated guided.
Still…the effect of damaged fins and related fin information was interesting.

[PeterCrespin]

@John_Walker

I find the stock dual fans do the job well, even through the smaller bonnet intake.

Much (most?) of the extra mouth size has no effect on radiator airflow. The extra area feeds the side ducts and the substantially larger motif bar deducts a little.

Indeed, the mesh stone guard opening, which forms the final gateway controlling throughput, is effectively identical for S1 and S2. There is an increased vertical opening size which presumably helps, but it would be interesting to how much, if at all, the cross-sectional area differed at the choke point(s) controlling radiator air flow.

A big thanks to Robert, Mike and Matt et al for a high-value thread.

[Kirbert (Author of the Book, former owner of an '83 XJ-S H.E.)]

@John_Walker

is the smaller bonnet mouth ( about 120 square inches) constrictive enough to impede airflow to the part of the radiator covered by the fan (231 square inches)?

Heh. This is another area where engineering is superceding myth. The 427 Cobra had a vastly enlarged cowl opening compared to the 289 Cobra, evidently on the belief that the bigger engine required that much more cooling air. But one must remember that the radiator core is quite restrictive; somebody should probably do a comparison to an orifice to quantify the restriction. I’ll bet an open hole half the size of a radiator core would flow more. In any case, it has been well demonstrated that remarkably small grille openings will draw enough air to keep an engine cool. After the Cobra proved too un-aerodynamic to win at LeMans, it was rebodied to create the Cobra Daytona Coupe – with a little tiny intake at the front. 1990’s Camaros have a great big grille that’s actually completely blocked off, 100% of the air for the radiator is drawn in from under the car. The original Mini had a fake front grille, drew all its cooling air from under the left front wheel well. Many, many modern cars including the Ford Focus have large fake grilles with only small openings actually admitting air to the radiator. All of this is because air intake into the engine compartment is a huge component of aerodynamic drag; making it smaller makes the car go faster and get better fuel economy.

[Tommd55 (Tom D)]

@Michael_Frank

But engine cooling isn’t a simple matter of rejecting X calories.

Michael, you are not reading what I wrote- my “open” system is open because it is NOT involving an engine, it is NOT involving continuous heat generation. My example is water in a radiator, it has nothing to do with the heat generated by an engine. That is why I initially used that example many posts ago to show that an “open” system without an engine that continues to generate heat is NOT the same as a closed system that DOES generate continuous heat. On one hand you are telling me I am incorrect, and then you go on to agree with me!

Edit: Let me continue. The basic premise of my original post is that I have heard many automotive people, especially racers, say that if water goes through the radiator too fast, it does not have enough time to give up its heat. They say it most go slower to have the time to give up heat. I made to post to help correct that thought. From what I read of your post, you and Robert (and some others) do agree with that point, and have given some great information to back that up. But, to many, it still seems to make sense that if the water stays in the radiator longer, it will cool more. That is why I separated a “closed” system, one with an engine that continually generates more heat from an “open” system that only has to cool the water once time. (Although Robert seemed to disagree with me on the “open” system, and so I am still waiting for his response to see what I may be missing there.) But again, on the “closed” system, on the system that continually generates heat, we do seem to totally agree. Thank you for all your contributions.
Tom