Coolant & Air Flow Rate: Finding the Car's Rates

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Discussion Overview

The discussion revolves around methods for calculating the coolant flow rate and air flow rate through a car's radiator, focusing on theoretical approaches and practical applications for a specific engine type. Participants explore various equations and assumptions related to engine power, temperature differentials, and airflow dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest dividing engine power in bhp by 3 to estimate coolant flow rate in L/min, while others seek methods to calculate airflow through the radiator at specific car speeds.
  • One participant references a graph correlating coolant heat flow rate with brake horsepower, proposing to use this to compute heat flow needed for cooling based on engine power.
  • Assumptions about water temperatures entering and exiting the radiator are made, with a delta of 20°C used in calculations.
  • Another participant mentions the complexity of calculating air flow through the radiator due to multiple variables, including air temperature and speed, and suggests using convection principles to estimate airflow rates.
  • A participant shares details about their specific engine (796cc, 3-cylinder, 37 bhp) and expresses a desire to calculate the radiator size needed for cooling, which requires knowing the air flow rate.
  • Participants discuss the need for approximations in calculations due to varying factors affecting air speed and heat transfer efficiency.
  • There is mention of using the Dittus-Boelter equation for calculating heat transfer, but uncertainty remains regarding how to accurately determine the maximum power RPMs for the engine.

Areas of Agreement / Disagreement

Participants express various methods and assumptions for calculating coolant and air flow rates, but no consensus is reached on a definitive approach or solution. Multiple competing views and uncertainties regarding the calculations persist throughout the discussion.

Contextual Notes

Limitations include the dependence on specific engine characteristics, assumptions about temperature differentials, and the variability of airflow conditions. The discussion highlights the challenges in obtaining precise measurements and the need for approximations in calculations.

Who May Find This Useful

This discussion may be useful for automotive engineers, students working on vehicle cooling systems, and enthusiasts interested in thermal management in engines.

meLokesh
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Does anyone have an idea about how to find the coolant flow rate in the car?? Any theoretical method..
And the volume flow rate of air through the radiator?
 
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I read somewhere that..what someone usually does is that he divides the engine power in bhp by 3 & the result is coolant flow rate in L/min. e.g. 300 bhp engine would have 100 L/min of coolant flow rate...
but how to calculate the airflow through the radiator core at a given car speed...
suggest something please..i've got just few hours...

i mean, I'm not going to die:biggrin:.. i have to submit my calculations for the car cooling system within a couple of hours..:cry:
 
Attached is a graph that has the coolant heat flow rate divided by brake HP. I took the revs that make the most power (from dyno test) and got a value (0.95). It depends on the engine type as seen on the right corner.
Once you have the value place your engines brake Power (in KW) and compute the heat flow needed to remove (Q dot = value X power [KW]).
Now let's make a few assumptions:
The water temp entering the radiator : 100 degrees C.
The water temp exiting the radiator : 80 degrees C.

The delta= 20 C.

from the famous equation: Qdot=mDot X Cp X delta T

You can isolate the mdot and calculate it.
heat.png

The graph is from an auto engineering book.. I don't remember which one.
 
berko1 said:
Attached is a graph that has the coolant heat flow rate divided by brake HP. I took the revs that make the most power (from dyno test) and got a value (0.95). It depends on the engine type as seen on the right corner.
Once you have the value place your engines brake Power (in KW) and compute the heat flow needed to remove (Q dot = value X power [KW]).
Now let's make a few assumptions:
The water temp entering the radiator : 100 degrees C.
The water temp exiting the radiator : 80 degrees C.

The delta= 20 C.

from the famous equation: Qdot=mDot X Cp X delta T

You can isolate the mdot and calculate it.


heat.png

The graph is from an auto engineering book.. I don't remember which one.

where to get this graph for my engine??
& what about the air flow rate through the radiator..
 
This graph covers most cars. The paragraph written on the upper right side states what letter represents what type of engine.
Each engine type has a line or a range and according to your Engine Speed you derive the coolant heat flow/ brake power ratio.
Using this ratio- follow the instructions in my first reply and you can get a water flow rate.

What type of engine do you have and how much HP does it have? Do you know when it reaches its peak power (REVS) ?

About the air flow through the radiator, this is a pretty complex question because there are a lot of variables, from air temp, air speed, contact area, shapes and more..

To try to make things simple:
From my little experience i assumed that the water that enters the radiator is about 100 C and the water exiting is about 80 C. (delta T= 20). Next assumption was that all the heat lost from the water was dissipated to the air via convection (no conduction). Now just calculate the heat loss rate (Q dot) for the water, contact area with the air and the find the h (convection coefficient). With this h you can find the air speed. You will need some basic heat transfer knowledge , Reynolds, Nu, GrXPr and so on..
 
mine is an 796cc, 3 cylinder, 37 bhp (max at 5000 rpm) petrol engine.
and for the air flow speed..i could do what you suggested..probably using DittusBoelter Equation, but I wanted to calculate the radiator size needed for the engine cooling...so I need to know the air flow rate (i.e. volume flow rate) to calculate the contact area.
 
hmmm. What are you building? perhaps you can calculate the speed of your vehicle and determine the air velocity through that?

Then just calculate the size needed for sufficient heat transfer with all the values you know.
This will give you a good ground for an estimated size.
 
this is for the SUPRA FSAE car...
i know the avg. max speed the car going to be driven, but air speed through the radiator core will be different due to all the contact area & flow hindrances...wouldn't that matter in the calculations??
i m a little inexperienced:redface:..do you know how to get the revs that make most power?:confused:
 
That would matter but what you need is a close estimation, take all those things to consideration and get a close approximation. I think that getting a 100 percent accurate calculation is very difficult because of all these factors that constantly change and depend on lots of other constantly changing factors.

I got the max power revs from a dyno test. Maybe you can find a detailed spec sheet for your engine, usually its data that is pretty common.
 
  • #10
berko1 said:
I got the max power revs from a dyno test. Maybe you can find a detailed spec sheet for your engine, usually its data that is pretty common.

I could not find the specsheet:cry:..& the dyno test here is a far possibility..any other way out?
 

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