Heat Science

[JimLill]

I am redoing my oil cooler system, a simple project for a low HP car like my VW but knowing that John's Stohr might need the same, I dub into the science.

How much heat does an engine make? For gasoline you can use the formula Gallons consumed times 126,000 and the the BTU/hr produced.

How many of those BTU turn into power? Use the formula gross HP times 2544 to get the BTU/hr that turned into power.

So making a few assumptions for John's DSR...

He averages 80 mph and get's 8.0 mi/gal, so uses 10 gallons an hour. Thus there were 1,260.000 BTU in that hour.

His engine might make 160 HP peak but let's use an average HP of 120 for that hour or 305,280 BTU used.

That leaves about 900,000 BTU of which goes out the tailpipe, via radiation cooling, or into the oil or water.

The engine is thermally 24% efficient.

More to follow........

[JimLill]

Went back into the setup books for my own VW.....

It uses 6 gal in 50 mins or 7.2 gal/hr = 907,200 BTU

With 115 HP peak, but the pedal to the floor, I'll assume 92 HP avg or 234048 BTU

So about 25% efficient. My SWAG on the DSR must be pretty close.

[JimLill]

Typical Oil Cooler BTU/hr

[JimLill]

The next thing to consider is the desired range of oil temp. 180-210F seems the CV. You want to stay warm enough to drive condensed water out of the oil. IIRC, 240 F is a good max.

[JimLill]

ROT... 35% of heat goes out the exhaust...

[JimLill]

More points of reference:

The size OC I have on my VW is pretty close to what S-B uses on the F-D cars, about 10000 BTU/hr.

My thought is that since you keep the oil cooler than the water, you can size your radiator around the need to keep water at 220F or so (remember, under pressure).

Then you size the oil cooler to keep the oil temp within range, 180-210F.

There seems to be no simple way to figure out the split of heat by analysis alone. For John and his DSR, I'd think that the 10000-15000 range is in the ballpark since a motorcycle engine uses oil was much as water to keep cool.

This 11X7X1.5 is rated at 15,000 BTU/hr

Here is probably better data from Setrab http://www.elephantracing.com/docume...oolerspecs.pdf

[jstecher]

Just wanted to thank you Jim for the calculations here and helping me get the right size cooler picked out. Now I just need to get the car back and start plumbing...everything is sitting in the garage now or on its way from HRPWorld.

[roy]

I took the scientific approach also.
Used the following variable:
What s the biggest most efficient cooler that will fit but also doesn't cost an arm and a leg to buy?
It seems to be working out quite well for the temps over here.

[JimLill]

That approach may work when you have lots of room, or aren't weight constrained. On a pure race car like John's DSR or even my lowly VW, a different approach can be worthwhile.

[Conan]

Good info, Jim. Thanks! I know my H2O temps were running about 205F on the track. What's the normal oil temp relationship? It would make sense that the oil would be hotter (more heat capacity). So I'm guessing my oil temps were 220F - 230F?? Maybe I should put the second oil cooler in?