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Old 05-07-2021, 04:35 PM   #1
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Math Problem

This is more of a fun thread.

I'm trying to determine how large an auxiliary cooler I need.

For google, a diesel engine produces approximately 137, 000 BTUs per gallon of diesel.

The cooler I'm looking at dissipates 67,000 BTUs per hour.

Assuming 5 mi to the gallon at 60 miles an hour I calculate all use 12 gallons in 1 hour.

Multiply the 137,000 by 12 and you get 1,656, 000 BTUs in an hour.

Now I understand that the OEM cooling system is large and dissipating a huge amount of BTUs. The bus frame / body has a bit of a heat sink value. There's also simple radiant.dissipation.

Yet, when pulling up a grade in the heat at a reduced mile per gallon, the amount of BTUs generated per hour is going to increase.

So do you think my math is correct?

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Old 05-07-2021, 05:32 PM   #2
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The rule of thumb is 1/3 of the btu goes to move the vehicle, 1/3 goes out the exhaust, and the last 1/3 goes to the cooling system. Therefore we only need to be concerned about 1/3 of the btu's burned to run the power train. Most of the heat that you will generate will be from the torque converter when it is unlocked. The rear axle generates heat from that 1/3 and the other internals in the transmission generate heat from that 1/3 also. Those are not exact numbers but it is close enough for hand grenades and horse shoes. Running down the road in lockup those btu numbers are going to be close to linear based on load. It is when the transmission goes into unlock it becomes common core math. It is commonly held that unlocked is very inefficient but, while unlocked you also get torque multiplication. The gear spacing on the AT 545 and MT 643 are about 40% splits and the torque converter overlap is a big help in acceleration. About 1.7:1 torque multiplication is a big help when starting out. That makes a 500 ft lb engine about 850 ft lb while starting and tapers back as you begin moving. When you add cooling you are just extending the length of time that you can operate under adverse conditions. Your bus should run close to 10 mpg at 60 mph and 5-7 mpg pushing it hard with a toad on the back.
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Old 05-07-2021, 05:39 PM   #3
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Then we come to the real math problem. How did the seller of the oil cooler calculate his product’s BTU rating? How hot was the oil and how hot was the air? How many cfm of air did they push through it?
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Old 05-07-2021, 05:43 PM   #4
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Btu

About 33%out the exhaust. 33%goes out the crankshaft. 33% into the cooling system. Some energy is recovered by the turbocharger. There are hundreds of items that absorb energy. Tires. They get warm and shed heat into the air. Wheel bearings drive shaft u joints. Alternator. Power steering..... the transmission manufacturer should be able to provide information on cooler size.

I think the way route fluid flow is through oil to air cooler first, then through engine coolant/oil cooler then back to the transmission.

It is okay to over cool the oil, then get it back up to around 180-200 degrees. I do not consider oil coming out of the trans “too hot” till it hits 230-240.

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Old 05-07-2021, 06:48 PM   #5
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So awesome you both know this and share it.

I love learning this kind of stuff. It really helps me puzzle together how my entire bus works and how to think of how one thing affects another.

I totally didn't think about heat as energy, just as something to combat. The minute I started to read your responses I was like...oh yeah, heat = energy.

So, in the estimate that diesel produces ~137k BTUs, that includes all the heat generated in making the bus move. Tires going around, drive shaft going around, crank going around, etc.. Yes??

Of all that heat, only a fraction needs to be cooled through a cooling system. Google says it takes .24 BTU to change one pound of air one degree, so 1btu = 4 degrees Fahrenheit. So if the engine produces 1/3 of 1.6M BTU, that's 533k BTU * .25 = ~132 degrees Fahrenheit that the cooling system has to deal with.

The balance of the heat generated, the lubricating manufacturers formula for their product to best operate within and the size of the cooling system is a great balancing act.

Since the aux cooler I'm looking at supposedly dissipates 67k BTU, that's ~17 degrees of heat.

My research says lubrication experts estimate a 20 degree drop in temperature will make the fluid last twice as long. So, 17 degrees would be pretty good, if that is truly what it can do. So, let's say it's more likely 10 degrees.

s2, I think you got 5 degrees in not so great of conditions from your trans cooler?

Am I making sense?
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Old 05-07-2021, 08:14 PM   #6
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On a hot day I was able to lower the sump temp 20-25 degrees. Griffin did not have a rating other than the fan cfm spal fans are well made and perform well.
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Old 05-08-2021, 09:21 AM   #7
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I'd agree with William about 230-240 degrees being as hot as you want to see oil running. I was lucky that the cooling system in my Isuzu powered rig is large enough for any load I've come up with. However, since I wanted to install my Yamaha inverter generator in an enclosed "sound proof" drawer under the bus I looked into what the various oil mfg's had to say about max oil temps and found the info hard to find with 230-240 degrees being about max. I decided to error on the cool safe with my pricy genny and set it up with a pair of 200 degree kill switchs on the oil. So far even in 100 degree weather the combination pusher and puller fan seem to be doing the job as the genny hasn't ever shut down because of over heating.
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