Engine Cooling

mooman2

Hi there I'm new to the forum, I stumbled across it while trying to find the answer to a question I have. I am currently building an autograss car and have come to the stage of trying to cool the engine which is a 2 stroke V6 outboard engine. Basically what I need to know is how to figure out how much water needs to flow through the engine to keep it at a steady temp of 85⁰C. I have no idea where to start with working this out and your help would be greatly appreciated.

wolram

I think you would have to consider the air flow through the radiator as well as the amount of water, are using electric fans?

brewnog

Well, to calculate this, you need to know how much heat is rejected by the engine.

In your case, I would suggest that you look at the published/expected fuel consumption (worst case), make a guess at what percentage of that (give me more details and I'll help with that) will go to your cooling system, and look at what coolant flow you have available.

mooman2

i do have a basic knowledge of this and can work out the heat rejected (just havent got a value for the fuel consumption right now!) how do i figure out what flow i have available? will i have to measure the passages in the engine?

brewnog

Well do you have a water pump in mind?

mooman2

I would have thought the water pump would be decided on after figuring out the volume that can flow through the engine? sorry if i seem a bit slow but i'm here to learn!

brewnog

Well, does the engine not already have a water pump?

It's a safe bet that the engine will have been designed with sufficient cooling in mind. So if you can find out the water flow that the engine has been designed to use, (or look at the water pump specified for the engine) then you have your flow.

ray b

remember he is useing a outboard boat motor
in its native envro it gets cool to cold water input and dumps it out
so I would up size the pump as input water will be alot hotter in a closed system

I would go with a big pump, big rad, and big fans too
as the stock outboard motor needs alot of cooling help
to convert from cold water input to hot closed water flow
opening up the cooling passages may be needed too

mooman2

there is no water pump on the engine all the auxiliaries have been disguarded. I now know the water needs to flow at 1.8 l/s through the system and the temperature change in the water is 70 degrees C. is there some way i can figure out how much air i need to pass through the radiator/how big a fan i need to cool the water by that much as i dont want to just throw on the biggest fan i can find for the sake of it?

ray b

outboards use a pump in the lower unit

ruff guide use a rad of a equal HP rated car or a size or so larger
you can always block airflow to the rad to bring up temps
but it is hard to make too small a unit work cooler
but I FEAR THE INSIDE COOLING PASSAGES SIZE
as they are sized for cold raw water cooling
and I donot think you mean a 70 C drop in temp but to get the coolant
down to 70 C temp coming out of the rad
as it should be about 100 to 110 C going in or a 30 to 40 C drop
variable speed fans may help to adjust cooling
as may an electric water pump to adjust flow

brewnog

Don't worry about reducing coolant temperatures by having variable flow pumps or fans, just fit a thermostat on the water side and be done with it. Your 1.8 litres/sec flow and 70°C delta T give you a heat rejection of 530W, but it's important to note what conditions these parameters are given under such that you can reject the necessary heat under full load conditions.

russ_watters

Where did you get those numbers? A 70C delta-T is very, very high and very difficult to achieve. I would think you should be looking for something more on the order of 10-20C.