SVX Engine cooling "Again & Again"

[SilverSpear]

Originally Posted by Dessertrunner

I think there are a number of "DON"T KNOWS" which is also a concerns and a number of "DON'T UNDERSTANDS". For example:-
:- What is the real water flow rate at the different revs, friction in water is the square of which in simple terms means double the flow from 100lpm to 200lpm and the amount of friction goes up by 4 times. This means the opening up of the coolent flow pasages becomes more important such as Rally Bob suggested. We need to run some trial to figure this out.
:- If there is restrictions were are they and how do we find and fix?
:- Why does Subaru on most of there higher performance engines use oil coolers, correct me if I am wrong Rally Bob but I thought you guys installed one on the engine you built.
:- How we could orderly develop a test plan to conduct orderly trials to find the soluation.

Maybe the best way forward is to develop a list of things we need to know to fix the problem so as we are not clutching at straws.

Here is my first suggestion.
1 What are the coolent flow rates at different revs.
2 What is the tempreture the oil is running at and what should it be.

Tony

Tony, I still remember a thread talking about the water pump flow at different levels. Tried to look for it, but no luck so far, I will keep trying.

But what I remember is that the maximum flow that the stock pump is capable of was 320lpm... I think someone was trying to compare it with the electric pumps or something...

I still remember clearly three figures, of which one of them was the 320.

[Dessertrunner]

Dan I think I am the one that wrote it and the 320lpm refered to the current 6 cylinder.
tony

[shotgunslade]

Trevor and Harvey:

I am sorry to say that on the one thing you agree on over these many years, you are wrong. For heat flux across any fluidic heat exchanger, the issue of variation of velocity and mass flow on either side of the heat exchanger involves three important interrelated variables. These are the residency time, that you have referenced as being critical, the other two are the thermal resistance of the boundary layer and the logarithmic mean temperature differential between the fluids.

If you look at the graph of heat transfer across the exchanger with variation of mass flow on one side, what you will always see is a drop in total heat transfer with the drop in mass flow. You will probably see a bump along the way as the fluid flow transitions from turbulent flow to laminar flow. In that bump, heat transfer will drop much faster than the rate at which water flow is decreasing. As the mass flow increases, heat transfer will approach an asymptotic value. What happens is that, as mass flow decreases, the temperature of the fluids leaving the exchanger achieve a closer approach. That approach is measured differently depending upon whether the heat exchanger is counterflow, parallel flow or cross flow. This closer approach reduces the logarithmic mean temperature differential across the exchanger, reducing total heat flux.

We often use air to water heat exchangers with very low face velocity on the air side to get a very close approach of the temperature of the leaving air to the entering water (counterflow exchanger), however, in no way does the reduction in air flow result in greater heat flux.

As you increase the water flow from a very low value, two things happen. The first is that slowly the boundary layer at the water side of the coil erodes, provding a closer approach of the exterior radiator tube temperature to that of the water inside. The second thing that happens is that the temperature drop of the water decreases as it goes through the radiator. The water has less residency time, so each unit mass of water loses less heat. But, remember that the mass flow rate is increasing, so does the mass flow rate increase faster than does the drop in heat loss per unit mass of water? The answer is yes and here's why.

Look at the radiator from the air side. The air doesn't know anything about how much water is flowing on the other side. It just knows that it is passing a hot surface and is getting warmed up. With constant air flow and air temperature, the only variable affecting heat transfer is the temperature of the radiator. Look what happens when water flow rate is reduced. The temperature of the water leaving the radiator is colder, so the logarithmitc mean temperature differential of the radiator is reduced, and the boundary layer has a greater resistance, further decreasing the wall temperature of the radiator. From the standpoint of the airside, reduction in water flow means a colder radiator, meaning less heat transfer.

So you may say this is only theory, but I can show you an unlimited number of instrumented coil tests that show that reduction in water flow for a water to air heat exchanger results in reduced total heat flux.

On the engine side, the temperature differential betwen the combustion chamber and the coolant is very high, but if you follow the same logic, you will see that increasing the mass flow rate of the water will increases the temperature differential of the water as it flows through the engine, decreasing the logarithmic mean temperature differential between the hot bits of the engine and the fluid. Thuis would tend to decrease the heat transfer to the fluid. But the heat generation of the engine can be considered to be constant at a given operating condition. In order to make up for the fact that the average temerpature of the water going through the engine is higher (due to less water flow), the engine will run hotter.

Conclusion, more coolant flow through the engine results in a cooler engine, in most circumstances, and that is a good thing.

Originally Posted by Trevor

I have been pushing the speed of the coolant flow for quite a long time, over years in fact, with negative response. It is good that Harvey and I agree on this, and more so emphasis this as a being valid for early consideration.

There is no necessity for changing the pump with all the subsequent alterations. Reducing the efficiency will do the trick and is a well proven technique. It will be pointed out that this will also effect flow at all levels of engine speed, but at lower RPM the loading and heat delivery is less intense, and has not in other applications proven a problem.

Speed of flow is now identified as possible problem, therefore some restriction in the right leg will assist rather than detract. Direct cost is nil.

Drilling or reducing pump the rotor is easy and the cost can not exceed the value of a used pump.

P.S. Flow can also be reduced by fitting a restrictor within the thermostat housing. This is worth a try as a first experiment, as so little is involved. However the pump mod. provides a reduced performance somewhat in line with RPM.

As an experiment the result of an increased flow can be ascertained by doing without the thermostat. Some have done this and wondered why a negative result was the outcome, and this incidentally proves the point regarding water possibly flowing too fast.

The combined labour cost, is nowhere near any of the other suggested methods. This combination should be top of the list, even if considered as only an experiment. However it is very important that each alteration should be done and tested separately for individual evaluation, in line with correct experimental practice.

[dynomatt]

This has been posted before, but I'll put it up again.

EG33 pump performance from manual

@ 760rpm = 20l (5.3US gal) per minute
@ 3000rpm = 100l (26.4US gal) per minute
@ 6000rpm = 200l (52.8US gal) per minute

From 8 impeller vanes at 76mm diameter.

Exactly the same performance as the EJ22 which presumeably generates 33% less heat...that's not right surely. Also, coolant capacity is 7.0l in the EG33 and 7.2l in the EJ20 turbo, from the same pump, that doesn't have coolant problems.

EZ30 out of the 2004 spec Legacy manual

@ 5500rpm = 320l (84.5US gal) per minute

From 6 impeller vanes at 73.2mm diameter. Cooling capacity 7.8l.

I'd love to get the same specs from the 3.0 out of the RB.

From this...one could ascertain that Subaru believed a higher flow made for a better result. Clearly more capacity also plays a role.

Honda S2000...arguably a high performance NA flows 176l per minute at 6000rpm...but it's a 2.0l four cylinder

Matt

[oab_au]

Well Dan if you are OK with the increased water flow, not being a problem, it is your engine. We only have to fix the water outlet junction, to cure the problem.

Harvey.

[Trevor]

Originally Posted by shotgunslade

Trevor and Harvey:

I am sorry to say that on the one thing you agree on over these many years, you are wrong. ---------------- Conclusion, more coolant flow through the engine results in a cooler engine, in most circumstances, and that is a good thing.

As I see it, your theory is based solely on mass coolant flow and that the logic in respect of cooling equates exactly with that for heating. No account is taken in respect of the vast difference in relevant surface areas. The large coolant area / cooling air area incorporated in a radiator, or heat exchanger, does not equate with heating factors as exist within an engine.

What is also important is that it does not appear that a factor covering residency time, in relation contact with the heating/cooling areas is included within your explanation. Flow speed/flow rate against restriction, producing pressure within the heating cycle, but not the cooling phase, must also be considered.

At this point not able to agree with Harvey, I can not accept your explanation.

By the way, I am awaiting with interest, a reply to my PM regarding centrifugal pumps.

[shotgunslade]

Actually I did refer to residency time when I said that we often use very low face velocity on a coil to achieve a very close approach of the leaving air to the entering water temeprature. The phenomenon that enables that to occur is increased residency time.

The most important thing about a heat exchanger is that there is a heat balance on each side of the exchanger. Residency time for the water enables the water to achieve a closer approach temperature to the air. The only impact that water residency time has on heat transfer to the air is that it lowers the logarithmic mean temperature difference between the air and the water.

On the airside, heat transfer can be chacterized by Q = U * A * (Delta T) where Delta T is the logarithmic mean temperature difference between the air and water. U is the heat conductance across the heat exchanger. A is the area of exchange surface. Of course, the lower the water mass flow through the heat exchanger the lower is the leaving water temperature, and thus, the lower is the average temperature of the water and the lower is average temperature differential between air and water, and thus the lower is the total heat transfer.

It is very simple physics. It is the same on the water side as it is on the airside. You can't tell me that when the car is overheating, you can ever improve the problem by reducing airflow through the radiator.

[svxistentialist]

Trevor, I think Dan's explanation on heat "flux" is more about heat transfer in an air-water interface, this seems to be quite a correct analysis to me but misses the point that is relevant to the problem here.

It's not about how fast the rad can lose heat [ I'm not talking about degrees heat here, I'm talking about mass of heat] it is more about how evenly we can ship heat from both the left and right block of cylinders.

So we have deduced the right block ships heat and gets by OK, but the left block does not. The left block would be expected to be a mirror image of the right, but we now know that heat is not shipped away from the left block as fast or as effectively as from the right. More particularly this becomes a problem at sustained higher revs, when consistently more heat is generated.

Dan I accept your analysis concerning heat flow, in particular the relevance of heat dumping over an air water interface if the mass of coolant circulating per minute is increased. What is most at issue here is the rate of heat loss is different between the two cylinder banks before the coolant gets to the rad. More cooling fluid is circulating in the right bank compared to the left bank. Trevor's suggested solution is to restrict fluid flow in the right bank in order to promote better circulation and equalise heat loss in both cylinder banks.

This is totally different from reducing the fluid flow through the rad as you seem to be arguing against. The presumption is that the same amount of heated fluid will pass through the rad, losing the same amount of heat, but restricting flow in the right cylinders [that currently "own" the path of least resistance to the pump] will allow increased circulation of fluid and corresponding increased collection of and dumping of heat from the problematic left bank of cylinders will keep the heat transfer from the two sides of the engine balanced.

The laws of physics in relation to fluid dynamics, heat transfer and laminar flow will apply here regardless. Our mission here is to ensure that all heat generated in either bank is uniformly and evenly shipped via the pump or pumps to a radiator sufficiently efficient at losing this heat to the passing air movement.

The above sounds long winded, but that's the only way I can describe the concept of even heat movement we need. Please forgive me if it sounds pedantic.

Joe

[shotgunslade]

Joe:

I absolutely agree with you. The left cylinder bank is getting shortchanged on coolant flow, possibly because of the restriction that YT found. My previous post concerning balancing current in parallel circuits directly addresses that issue.

7. Trevor is correct that the easiest way to balance current flow in the 2 legs of a parallel circuit (it's easier to think about this using the electrical analogy) is to add resistance to the leg with the smaller resistance, however that will add to the overall resistance of the circuit, requiring greater voltage (pressure from the water pump) to get the same amount of current (coolant flow). A better course is to reduce the resistance on the leg with the larger resistance to the extent easily achievable and then afterwards, increase resistance on the other leg to balance current flow.
8. Remember that the current (water flow) balance doesn't have to be perfect, only that the flow through the high resistance leg (left cylinder bank) is adequate to avoid local wire meltdown (coolant boiling).

Adding some resistance on the right bank after you have done everything you can to reduce restriction on the left bank might somewhat reduce overall flow, but it could cure our problem by shifting some flow from the right bank to the left bank.

[SVXRide]

Dan,
thanks for taking me back to Junior year Heat Transfer and Fluid Mechanics classes. Haven't been there for a while
-Bill (BSME, class of '83)

[Johnybeas]

damn engineers.... blah blah blah just figure it out already so us dum dums who don't live breath and eat calculus and physics can be happier

[Trevor]

Originally Posted by svxistentialist

Trevor, I think Dan's explanation on heat "flux" is more about heat transfer in an air-water interface, this seems to be quite a correct analysis to me but misses the point that is relevant to the problem here.

It's not about how fast the rad can lose heat [ I'm not talking about degrees heat here, I'm talking about mass of heat] it is more about how evenly we can ship heat from both the left and right block of cylinders.

So we have deduced the right block ships heat and gets by OK, but the left block does not. Joe

Joe,

The radiator was only brought into the picture, as a means of comparison in order to prove a theory, but the point is still open to discussion, having not been covered.

The point at issue, was my suggestion that the speed of flow, can have a negative effect in respect of the transfer of heat within the engine. There is and has been no suggestion that the radiator is at fault. The question of balancing the flow in the two legs, is also separate issue.

We must stop this ring a ring a roses approach, and stick to each individual relevant point, as it comes up.

Cheers, Trevor.

[svxistentialist]

Originally Posted by Trevor

Joe,

The radiator was only brought into the picture, as a means of comparison in order to prove a theory, but the point is still open to discussion, having not been covered.

The point at issue, was my suggestion that the speed of flow, can have a negative effect in respect of the transfer of heat within the engine. There is and has been no suggestion that the radiator is at fault. The question of balancing the flow in the two legs, is also separate issue.

We must stop this ring a ring a roses approach, and stick to each individual relevant point, as it comes up.

Cheers, Trevor.

The radiator was brought into the picture in Dan's two posts #78 and #82, and I directly addressed that matter in my opening two paragraphs. Dan was theorising on how much heat would be lost if the flow rate to the rad was reduced.

I don't agree with your hypothesis that this thread is a "ring of roses". This thread is working excellently as a think tank, different people, engineers and otherwise, putting forward possible solutions.

It is educational and it is a model for owners as to the process for thinking through problems and applying solutions. It is surprising what progress can be made when people collaborate to find solutions, instead of the petty bickering about points that sometimes goes on here.

Joe

[SilverSpear]

Now I have read everything in this thread, understood all points of views except Dan's. I can do that though since myself I am a science/finance person but really this will take time to grasp and for the formulas and stuff... I will do that later

I was looking at the pictures of Sov13t's engine and I see the water taps in the right and left blocks. They seem identical in diameter. Now so far, I think the weakness in our engines is also the water pump, I think it is small for that engine (accoding to Matt).

Above to what YT and Tony are doing, I think we should look for someone who can modify the water pump with more impellers and we need to PORT the left block to make way for more coolant flow.

PS: Tev, I know that you would prefer to make the right tap smaller, or put something to limit the flow though...

Am I making sense?

[shotgunslade]

Trevor, I did address the engine side.

On the engine side, the temperature differential betwen the combustion chamber and the coolant is very high, but if you follow the same logic, you will see that increasing the mass flow rate of the water will increases the temperature differential of the water as it flows through the engine, decreasing the logarithmic mean temperature differential between the hot bits of the engine and the fluid. Thuis would tend to decrease the heat transfer to the fluid. But the heat generation of the engine can be considered to be constant at a given operating condition. In order to make up for the fact that the average temerpature of the water going through the engine is higher (due to less water flow), the engine will run hotter.

I absolutely agree that low coolant flow through the left cylinder bank seems to be our culprit for local boiling. Overall, for nearly stock engines, the stock radiator or the PWR radiator seem to be adequate to maintain the average coolant temperature at an acceptable value. Hopefully my new engine built by YT will address the low flow issue on the left cylinder bank that we suspect as the culprit for our problems.