914/RS: Re-build and Re-design: Engine and Cooling Options

[wndsrfr]

Decisions made...cutting the cage and training the flares. No brainer in person...thanks for the thoughts.

Tony

The trim line looks to me to be more aggressive than the "traditional" one above....

Meaning too much in the middle area & not enough French curve at the bottom.....I like curvy bottoms...

[worn]

I walked the pits of the Walter Mitty Classic at Road Atlanta last year and took a bunch of pictures. All late-model mid-engined prototypes used Water-to-oil heat exchangers in lieu of oil-to-air.

I like your posts and builds. I figure if you aren't a Tygaboy you must be his cousin.

I am trying to understand the physics here. The heat goes from the oil to the water to the air. How is this more efficient than oil to air direct? Sooner or later you are cooling stuff with ambient air moving across as big a surface as you can afford to have. The possibilities that I can think of are A) it easier to produce a larger water radiator than an oil radiator, or B) the heat transfer from oil to water and/or water to air adds up to better than oil to air - though I cannot figure why that would be true. If you could use the phase change from liquid water to steam, it would produce tremendous heat absorption. But only as long as you have water left, so I don't think that is it. Water has a higher heat capacity, so you can get more heat transfer with less flow with water.

How does this work? I have to believe that these cars are built by people who know their physics a lot better than I do.

[worn]

Now here is the big question..input is welcome. One of the visual highlights of a Shelby 427 are tapering/cutting down and in rear fenders. By transferring the front flare to the rear we grafted in the shape as well and it now has a squred off base.

Big question: Do I cut the flare back to be more representative of an original tapered style or maintain the cars Rebel status all the way to the end - and stay square? Pictures that clearly explain what I am referring too...

Traditional rear


or Squared off flare

FWIW, I like the reverse curve on the rear of the Cobra at top. I can't see how you are going to come away with anything but a great looking car in the end though.

[jd74914]

I walked the pits of the Walter Mitty Classic at Road Atlanta last year and took a bunch of pictures. All late-model mid-engined prototypes used Water-to-oil heat exchangers in lieu of oil-to-air.

I am trying to understand the physics here. The heat goes from the oil to the water to the air. How is this more efficient than oil to air direct? Sooner or later you are cooling stuff with ambient air moving across as big a surface as you can afford to have. The possibilities that I can think of are A) it easier to produce a larger water radiator than an oil radiator, or B) the heat transfer from oil to water and/or water to air adds up to better than oil to air - though I cannot figure why that would be true. If you could use the phase change from liquid water to steam, it would produce tremendous heat absorption. But only as long as you have water left, so I don't think that is it. Water has a higher heat capacity, so you can get more heat transfer with less flow with water.

How does this work? I have to believe that these cars are built by people who know their physics a lot better than I do.

It's not more efficient since you have an intermediate transport process increasing entropy. There can be a few benefits:
-You don't need to run oil lines all over the place so packaging is easier. I'm assuming they use engine cooling water for oil cooling (so no separate system) too like most bikes.
-Plumbing weight is greatly reduced since you now can get rid of oil lines and a small increase in water line OD results in little overall weight increase. Oil-to-water heat exchangers are also likely pretty light since you don't need much heat transfer area due to higher overall heat transfer coefficients.
-You can also better optimize your oil pump sizing since you don't need to worry about a viscosity change of 2+ orders of magnitude which greatly increases cold line pressure drop-a big deal with really long, essentially laminar flow, lines.
-Possibly better temperature control, especially if you're using a booster pump on the water side to control flow through the heat exchanger.

[Cracker]

Worn - The water to oil heat exchangers are more efficient...a unit measuring roughly a foot long (like my cooler) has the equivalent capacity of a 40-row oil to air cooler. The other benefit is what was pointed out in the last post - it is far easier to plumb oil to a cooler located in the rear of the chassis than to the front. All of this improved energy loss doesn't come free...my entire cooling system from lines, junction boxes, radiator and an improved shroud is all being replaced.

Another big difference between the two systems is the oil being cooled will be off of the scavenge circuit - not the pressure side. It will move slowly enough to give the cooler/coolant a better chance to pull the heat from the oil. I will probably then pass either the coolant or the oil through an liquid to air cooler with a fan pack after the first cooler. The coolant will rejoin the engines heading towards the radiator - the oil will then go the dry sump tank waiting to be sucked into the engine.

I do not pretend for a moment to know the scientific explanations or basis but having gone through what doesn't work - I have learned quite a bit. Thanks for the thoughts...

Tony

[Dave_Darling]

Another interesting thing is that a water/oil heat exchanger can actually help get the oil up to operating temperature faster! A pretty neat side benefit, IMHO.

--DD

[Cracker]

I hear you John...I worked on adding more radius to the lower third of the flare. Either way, we'll make progress - working on this again Tuesday evening - maybe. Thanks for the input.

Tony

Meaning too much in the middle area & not enough French curve at the bottom.....I like curvy bottoms...

[jd74914]

Another big difference between the two systems is the oil being cooled will be off of the scavenge circuit - not the pressure side. It will move slowly enough to give the cooler/coolant a better chance to pull the heat from the oil.

Sorry, but this is very incorrect. Slower speeds do not allow more heat transfer. You're actually decreasing the internal convention coefficients of your cooler (reducing it's overall "efficiency"), so you need more temperature difference to transfer the same amount of energy. The perceived "slower=colder" is because now you have less fluid mass to cool, so the energy balance works out that the outgoing fluid is cooler (assume the two flows are going in opposite directions).

I will probably then pass either the coolant or the oil through an liquid to air cooler with a fan pack after the first cooler. The coolant will rejoin the engines heading towards the radiator - the oil will then go the dry sump tank waiting to be sucked into the engine.

I do not pretend for a moment to know the scientific explanations or basis but having gone through what doesn't work - I have learned quite a bit. Thanks for the thoughts...

If you are going to add a second air-to-fluid cooler, I would do it on the oil side. Putting it on the water side will be a less efficient use of components since now you'll be lowering the water temperature into your radiator, lower temperature difference between the radiator inlet and air inlet, effectively reducing heat transfer rates. It's just not the most efficient use of energy. On the oil side you can use it for fine temperature control, depending on what oil and and water temperatures you like. I wouldn't be surprised if some GT cars are built like this because water temperatures in their pressurized coolant systems are probably much higher than desired oil temperatures.

Just something to think about. (IMG:style_emoticons/default/smile.gif)

[jd74914]

I hear you John...I worked on adding more radius to the lower third of the flare. Either way, we'll make progress - working on this again Tuesday evening - maybe. Thanks for the input.

Tony

Meaning too much in the middle area & not enough French curve at the bottom.....I like curvy bottoms...

With a bit more curve that is going to look fantastic! (IMG:style_emoticons/default/drooley.gif)

[Cracker]

Interesting. Aviad and ARE who both contributed to the construction of my dry sump system gave that "rationale" as to why they wanted me to cool the scavenge circuit. My existing cooling system is inadequate (IMO) to add the W2O cooler as it sits currently...time will tell which method is more effective.

Tony

Another big difference between the two systems is the oil being cooled will be off of the scavenge circuit - not the pressure side. It will move slowly enough to give the cooler/coolant a better chance to pull the heat from the oil.

Sorry, but this is very incorrect. Slower speeds do not allow more heat transfer. You're actually decreasing the internal convention coefficients of your cooler (reducing it's overall "efficiency"), so you need more temperature difference to transfer the same amount of energy. The perceived "slower=colder" is because now you have less fluid mass to cool, so the energy balance works out that the outgoing fluid is cooler (assume the two flows are going in opposite directions).

[worn]

[quote name='Cracker' date='Oct 28 2017, 07:52 AM' post='2542805']
Interesting. Aviad and ARE who both contributed to the construction of my dry sump system gave that "rationale" as to why they wanted me to cool the scavenge circuit. My existing cooling system is inadequate (IMO) to add the W2O cooler as it sits currently...time will tell which method is more effective.

Tony

[quote name='jd74914' post='2542797' date='Oct 28 2017, 11:12 AM']
You see the slower so it has time to cool story bandied about in the hot rod magazines too. It is not correct physically, but it sounds right. A lot of people who know a huge amount about engines aren't like me trying to figure things out going from college class to reality. As everyone knows, reality can turn around and bite you.

Given the somewhat tragic nature of your dilemma it is nice to see you looking at it from a bluer skies ahead perspective. Courage there.

[Cracker]

With all due respect...nothing tragic occurred. The car will be improved as a result of this event - all is good.

For those who enjoy the Cobra project...I just spent a good amount of time working on trying to get the flares sorted. I had a "duh" moment after a few free-hand attempts! it occurred to me that I simply needed to make a template of the front fenders matching radius and apply it to the rear flares. Bingo! I also lowered the ride height a smidge and with every refinement, the car just keeps getting better and better - at least in person it does!

PS: I hope this enough of a "curvy bottom" for you John! (IMG:style_emoticons/default/biggrin.gif)

Tony

[Chris914n6]

Another big difference between the two systems is the oil being cooled will be off of the scavenge circuit - not the pressure side. It will move slowly enough to give the cooler/coolant a better chance to pull the heat from the oil. I will probably then pass either the coolant or the oil through an liquid to air cooler with a fan pack after the first cooler. The coolant will rejoin the engines heading towards the radiator - the oil will then go the dry sump tank waiting to be sucked into the engine.

Won't matter as the volume will be the same. "Slower" would be a higher capacity heat exchanger, as it stays in the cooler longer.

Worn - The water to oil heat exchangers are more efficient...a unit measuring roughly a foot long (like my cooler) has the equivalent capacity of a 40-row oil to air cooler. The other benefit is what was pointed out in the last post - it is far easier to plumb oil to a cooler located in the rear of the chassis than to the front. All of this improved energy loss doesn't come free...my entire cooling system from lines, junction boxes, radiator and an improved shroud is all being replaced.

The amount of heat being removed will still be the same, doesn't matter if it is transferred by water or oil. Since you hinted it was running hotter than you would like I'd focus on the radiator and leave the second cooler out until you find the radiator insufficient.

[Cracker]

The volume is not necessarily the same - the two circuits operate on completely different pumps and pick-up sources. I just do not understand where you are drawing that parallel from...we are installing what should prove to be a far more efficient radiator.

T

[bulitt]

Flairs look Great Tony!!!

[jd74914]

Interesting. Aviad and ARE who both contributed to the construction of my dry sump system gave that "rationale" as to why they wanted me to cool the scavenge circuit. My existing cooling system is inadequate (IMO) to add the W2O cooler as it sits currently...time will tell which method is more effective.

I'm surprised they didn't recommend the pressure side going something like tank>pressure pump>filter>oil cooler. This would give you the least aerated oil which would both help overall parasitic pumping loses since you wouldn't be forcing oil+air bubbles through a cooler and likely improve heat transfer. I'd be a bit worried about air collecting at a high point and effectively blocking some of the channels.

They definitely have more practical experience there, it's just interesting. Not what I would have expected for sure. I've only designed one dry sump system (it did work really well) and was not super impressed with the level of design data available from the pump manufacturer or heat exchanger manufacturers. Neither Aviad or Armstrong have much actual data on their sites. It looks like a lot of kits which culminated from BTDT. I'm not trying to say they are not very good at what they do, but its just very different than what you see when buying aerospace or industrial parts.

Slightly tangent, but the automotive aftermarket is a funny place. To date the only radiator/intercooler supplier I've talked to who can offer real engineering data is Bell-they actually deal with mass flows and energy dissipation requirements, not "HP". But maybe that's because of Gerhard Schruf?

The volume is not necessarily the same - the two circuits operate on completely different pumps and pick-up sources. I just do not understand where you are drawing that parallel from...we are installing what should prove to be a far more efficient radiator.

Sorry, confused there. Maybe I'm misunderstanding everything? I assume you have a number of pickup scavenge pumps>single outlet>tank and then a tank> pressure pump>engine. Then the coolant side uses a single pump/radiator with branch line through oil cooler?

You see the slower so it has time to cool story bandied about in the hot rod magazines too. It is not correct physically, but it sounds right. A lot of people who know a huge amount about engines aren't like me trying to figure things out going from college class to reality. As everyone knows, reality can turn around and bite you.

The slower thing just kills me-it's everywhere.

For sure, reality is tough. When you look at coupled systems, not a lot of stuff is very straightforward and sometimes it 'appears' to go against conventional physics. I think that's where some of these wives tales come into play. Then when you model the whole thing, the math shows deviation from physical expectations are caused by a bunch of factors acting against each other, etc.

Sorry to if I'm coming off as an armchair-internet-know-it-all. Just trying to share some of my engineering perspective and seeing what other people are doing is interesting. I've spent close to 10 years now designing/troubleshooting thermal management systems for power plants and high end aerospace/marine test systems and am almost finished with my PhD which focuses on aerospace combustion/heat transfer/fluids systems so I like to think I have some relevant experience.

[jd74914]

Flairs look Great Tony!!!

(IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif)

That new curve is perfect!! (IMG:style_emoticons/default/drooley.gif)

[Cracker]

Thanks guys! When you finally get it right - it is obvious!

Jim - Your insight is much appreciated and welcomed...I certainly fall under the often described (by crews doing the real work) as the "the dummy behind the wheel"! (IMG:style_emoticons/default/biggrin.gif)

Tony

Flairs look Great Tony!!!

(IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif)

That new curve is perfect!! (IMG:style_emoticons/default/drooley.gif)

[Chris914n6]

The volume is not necessarily the same - the two circuits operate on completely different pumps and pick-up sources. I just do not understand where you are drawing that parallel from...we are installing what should prove to be a far more efficient radiator.

T

Because it's a fixed amount of oil in a closed circuit and I'm assuming you will be cooling 100% of the pumped oil.
I think you're over thinking it.

Fast in a little tube cools better than slow in a big tube when capacity is equal.

[Cracker]

...you are just not tracking with how my system is plumbed and functions (it appears) (IMG:style_emoticons/default/biggrin.gif)

Tony

[quote name='Chris914n6' date='Oct 28 2017, 05:09 PM'
[/quote]
I think you're over thinking it.
[/quote]