cooling system design?

[fueliefan]

I was wondering if anyone can recommend a text for vehicle cooling system design. I'm looking for a book that covers:

-general discussion of system design
-the critical design points (idle, speed on grade, others?)
-acceptable radiator performance estimates (hard data is hard to come by if you don't work for a major OEM)
-acceptable airflow estimates when the rest of the vehicle is still little more than a sheet of paper
-acceptable drive cycle average power dissipation requirements
-typical assumed worst case conditions
-advanced electric pump/fan/thermostat control strategies

I'm already looking to purchase "compact heat exchangers" by kayes and London but I'm not sure how well their data will apply to modern automotive radiators.

I've done a couple cooling systems but they've been pretty heavily based on similitude (ie, use the same radiator as another vehicle of roughly the same power, or even bigger to be safe). However, our next vehicle is extremely space challenged so we are looking to minimize component size wherever possible. Obviously reliability is still factor #1, but we can no longer afford to grossly oversize as has been done in the past.

I am an engineer and am not afraid of books that are not a "friendly read."

 

[IceStationZebra]

I seriously doubt that you will find one text to answer all of these questions. I would think that SAE has some applicable papers though. If you have specific questions someone here will probably be able to help.

My 2 cents.
- general discussion of system design
If you have done "a couple of cooling systems" already I' m not sure what you expect to get out of this? Be more specific.

- the critical design points (idle, speed on grade, others?)
Depends on your application - which you didn't mention. (truck, sports car, family sedan, OTR truck?)

- acceptable radiator performance estimates
If you have any volume your radiator supplier should be willing to help. Otherwise try taking some data on your past projects. A data logger that records 6 thermocouples is not that expensive. I've gotten stationary air flow measurements with a cardboard box ducting all the air from the radiator and an anemometer.

-acceptable airflow estimates when the rest of the vehicle is still little more than a sheet of paper
Can't help here. The only thing I can think of is CFD. My hunch is that CFD could get you in the ballpark but design details would affect your numbers +/- 25%.

-acceptable drive cycle average power dissipation requirements
I doubt you will find any of this data unless you have a contact in the industry. The best numbers would come from benchmarking the competition with actual data (refer to data logger above)

-typical assumed worst case conditions
First this depends on your application and geographic sales region. Beyond that I know Cummins (and probably others) assume in off-road applications that you are only using water for a coolant and have a bad radiator cap (i.e. no system pressure and a max top tank temp of 212°F) If this is a heavy duty application you might also add a safety factor to cover production variances, system degradation/corrosion, partially plugged radiator (think bugs, mud, paint, plant matter, etc.). I've also seen a couple of radiators with 30% of the fins bent over due to careless use of a high pressure sprayer to wash the vehicle.

- advanced electric pump/fan/thermostat control strategies
Do you even need a thermostat if you are using an electric pump? This seems redundant. I would think that you will want the engine to get to operating temp ASAP, then hold it there. As for more advanced strategies it really depends on what kind of vehicle you have. You will probably also find a sweet spot with a range of pump flows where the cylinder-to-cylinder temp variations are minimized with the benefit being emissions and friction reduction, but that will depend on your specific engine config and plumbing.

ISZ

 

[Lou Scannon]

As I recall there is a decent treatment of cooling system design in The Diesel Engine Handbook by Challen & Baranescu.

 

[Bribyk]

Proper airflow and use of ducting/shrouds seems to be the key. Overheating is the second most common failure in Formula SAE competition but the radiator sizes that teams use can very greatly for the same engine/power. I've seen small electric fan-induced setups work fine while teams with large radiators in open airflow overheated. Cooling seems to be a black art in the series that very few of the young engineers understand or test properly. I had neither the time nor resources so I fit the largest rad and fan I could within the space constraints and fashioned a good, sealed shroud. Track testing confirmed that it was adequate but we didn't collect the flow/temp data to determine if it was oversized or by how much. If you don't have a good duty cycle for your engine, all your calculations will be based on an assumption anyway.

 

[reidh]

I don't know where you are located, but Valerie Nelson and Jack Williams (they both used to work for Ford in the cooling system design dept.) teach an SAE course in the USA on cooling system design that may be of help. I can't vouch for their course, but having worked with both of them, they can probably answer many of your questions if you were to take the course.

-Reidh

 

[fueliefan]

I saw that course, it looked like exactly the info i was looking for, but I'm way out in CA.

Thanks ISZ for your feedback, especially the worst case ideas. The application is combined on and off-road. I have a pretty good feel for the on-road points, but off-road i'm not so sure about. As for the electric pump/thermostat, i recall seeing a system using both in conjunction. i believe the idea was that you could then circulate "warm" water to the passenger compartment heater core and still prevent flow through the radiator. There may be other advantages as well.

 

[IceStationZebra]

electric pump - I forgot about the heater, so I guess the thermostat makes sense. EMP has several papers available for download that you might find interesting.

http://www.emp-corp.com/techLibrary/

worst case - Seeing as you are going off-road I assume that you will be operating at very low ground speeds or stationary. That being the case you have to account for wind direction.

radiator fins - Beware the kind made by punching "flaps." They are rarely used off-road because they can easily become plugged and can be difficult to clean.(don't use louvers like this

http://www.freepatentsonline.com/6968891.html)

With off-road equipment you don't get to use vehicle velocity to push air through the radiator so tweaking the fan becomes more important. Assuming you have an engine driven fan:
- Try to keep the fan away from the radiator. If sucker fans are too close to the radiator they tend to only pull air where the fan blade pass. (think dough nut shape) The center of the fan and corners of the radiator become dead zones. Cummins recommends 2-4", my other suppliers don't get that specific.
- Too close to the engine can also restrict flow if ancillary equipment is blocking the flow. On one project we picked up significant air flow by tucking the alternator closer to the engine. (enough to pass ;-) )
- Ideally the fan shroud should be smooth with no sharp corners in the air stream. Check out this web site for some good examples. If you fab the shroud out of sheet metal at least put miters on the corners were the shroud meets the radiator - this helps keep the air moving in the corners.

http://www.mahaffeymotorsports.com/FanShrouds.htm

- I have never seen a fan clutch on purely off-road machines, not sure about your application though. If roading is limited you probably don't need it as it is mainly a fuel saving item. I believe that off-road doesn't use them because of extra cost and they are usually air flow limited (and the clutch does have some slip)
- Keep the tip clearance at a minimum. The tighter you can keep it the better your airflow. Many large trucks/buses put a ring shroud on the engine itself to keep the gap small. Just make sure the fan doesn't hit when the engine rolls over at max torque.
- Don't put extra coolers on only one side of the radiator. The uneven pressure differential can cause fans to flex and fatigue.
- Fan spacer optimization has gotten me 10-15% more flow every time. And don't take the fan engagement "rules" as gospel. The rule-of-thumb for sucker fans is 2/3 of the blade in the shroud, 1/3 out - but I have found in several optimized applications that the fans worked best with 1/3 in, 2/3 out. It is worth experimenting with this. I run a 4 foot duct the same size as the radiator and take air flow readings 1 foot ahead of the radiator at low idle and high idle. Change the fan spacer and do it again. This is also a good way to test different fans. It is more precise than trying to measure 1 or 2 degrees difference in top tank temperatures.

ISZ

 

[Bribyk]

You'll want to leave a little more clearance between the fan blades and the rad/shroud for offroad use. The body and chassis will flex more and a sucker fan will tend to pull towards the rad if it gets into water. I don't know if you're going this offroad though...