Engine Coolant Heat Dissipation Calculations

[Chestermiller]

Okay. So that means its the coolant energy that is higher than the actual number. And since we have everything that is makes sense, we are down to changing the coolant flow rate?

Yes. The heat of combustion is an absolute indicator of the maximum amount of energy available, so, to me, that means that we are down to changing the coolant flow rate.

The maximum horsepower that the car engine/gasoline is capable of delivering is measured on a dynomometer with the engine red lined in low gear and a huge torque applied to the wheels. Under these conditions, we are expecting a much much greater consumption of gasoline and a much greater measured mechanical power, on the order of 100's of kW. And the combustion energy will have to be substantially greater than this. And the energy removed by the radiator will also have to be high (if the car is run under these conditions for any significant interval of time). So 70 kW for the radiator cooling is not unreasonable under extreme conditions. But, under ordinary cruising conditions, it must at least be much less than the total energy delivered by the gasoline.

Chet

 

[Jay_]

Okay, I got a reference for the flow rate and it works. I get 31.2938% of the total fuel as the coolant heat energy, which I think is consistent with literature. From this link : http://talk.newagtalk.com/forums/thread-view.asp?tid=349481&DisplayType=flat&setCookie=1

The post by user Gerald J. I also got a pdf copy of the book Mark's Standard Handbook for Mechanical Engineers, so I will cite that in the report.

He mentioned it says 2 gallons/bhp-hour for large engines, to 4 gallons/bhp-hour for small engines. I took the middle number, and flow rate as 3 gallons/bhp-hour. This equals 0.05 gallons/minute/bhp, which is much like the constant we were trying to adjust earlier.

 

[Jay_]

However, he posted saying this is for a 90 degF (or 32 degC) rise. Should I change the value based on delT?

 

[Chestermiller]

To orient yourself on this (i.e., get yourself in the ballpark), you need to do a calculation where the temperature rise is about 15C, and the rate of heat removal is about 1/3 the rate of energy delivery by the gasoline. In your test, the average rate of energy delivery by the gasoline was about 41 kW. So the rate of heat removal by the radiator was about 13.5 kW. This would break down to about 0.25 kg/s through the radiator (0.25 L/s). This translates into about 4 gpm of coolant flow through the radiator (or 6 gpm with a 10 C temperature rise).

 

[Jay_]

As of now, in my city drive cycle the calculations seem correct. I get average coolant power = 12.3115 kW, fuel_power = 39.3417 kW, so its 31.29%

However, for my highway driving, I get coolant power = 11.78 kW, fuel_power = 64.70 kW, so its just 18.61% Does this make sense?

I had to estimate the radiator cool side temperature, and I put it as 80 deg C for both cases. Should I reduce it in the highway drive cycle?

Because this link seems to indicate that. See the temperature graph: http://www.bimmerfest.com/forums/showthread.php?t=499204 In the "drive on highway" part the radiator cool side temperature is much lower. So question is, should I lower it in my code as well. Maybe take it as 70 degC, instead of 80 degC like in the city drive?

Here are the percentages for various values of Tcold in the highway drive:
Deg C Coolant heat as percent of total fuel burnt

80 -- 18.16%
75 -- 22.60%
70 -- 27.05%
65 -- 31.49%
60 -- 35.94%

 

[Chestermiller]

Hey Jay. This is starting to shape up now. Good going.

I thought you were measuring the coolant temperatures at the inlet and outlet of the radiator. Is this not the case? If not, you now have two highly uncertain parameters involved in the heat load calculation, namely, the coolant flow rate and the coolant temperature out of the radiator. Please tell me you can measure the coolant temperature coming out of the radiator.

Chet

 

[Jay_]

I can't. But I have that graph and I am getting it from literature. I showed it to my professor, he said its okay since we are also out of time. Now, what I would like to know is based on automotive engineering:

1. Does that graph in the previous link make sense? Specifically, is the temperature of the cool side less in a highway drive as shown in the link above?
2. If not, what values would be good for the cool side temperatures? 80 deg C works well for the city drive.

 

[Chestermiller]

I can't. But I have that graph and I am getting it from literature. I showed it to my professor, he said its okay since we are also out of time. Now, what I would like to know is based on automotive engineering:

1. Does that graph in the previous link make sense? Specifically, is the temperature of the cool side less in a highway drive as shown in the link above?
2. If not, what values would be good for the cool side temperatures? 80 deg C works well for the city drive.

That graph in the previous link makes some sense. The lower the coolant flow rate through the radiator, the lower the outlet temperature is going to be (compared to the inlet temperature). Do you expect the coolant flow rate to be lower during highway driving, and do you expect the inlet temperature to be about the same as for city driving?

Chet

 

[Jay_]

My professor said I needn't assume any difference in the outlet temperatures, so I went with that. I also assumed the coolant flow to be based on the bhp, which depends on the rpm. So depending on that I don't get much of a difference.

 

[Chestermiller]

Okay. He's the boss.

Chet