Coolant volume vs. displacement or HP

[johnlaudano]

I’m working on an engine design for a liquid cooled inline twin. I’m wondering if anyone has information about how much internal volume and/or internal surface area is needed for the internal water passages. The coolant would be raw pond water and the pump rate could be infinitely adjustable. The displacement would be 62cc’s total. The fuel would be propane and the anticipated load rpm would be 1500-1800. Any thoughts?

 

[BaronJ]

Hi John,

You have posed a difficult question there ! Water, or any coolant for that matter, will adsorb heat as long as the heat source is hotter than the coolant. The speed at which the coolant adsorbs the heat is dependant upon its volume, temperature and the rate of flow. So a slow flow will adsorb more heat in a given time frame. Also the area that is providing the heat source has to be taken into account along with the amount of energy generating the heat. It has been a very long time since I did the maths for this type of system. Basically energy in equals energy out. You will also have to calculate backwards for the heat loss when cooling your coolant down.

Maybe a read of these web sites will help:

https://elementsofheating.wordpress...-heat-a-volume-of-water-in-a-particular-time/

http://nessengr.com/techdata/watercool/watercool.html

At least this will give you a starting point.
Good luck with your engine design.

 

[johnlaudano]

Thanks Baron. Both are good reading. I don't have the required education level to apply it however. Good old seat-of -the-pants reasoning may be in order.
The interesting part is in the article that mentions heat transfer in oil as being better than that of water. Maybe an oil-cooled engine with a radiator would be a viable alternative.
John

 

[ultralight1]

The HKS 700 E light sport aircraft engine uses air cooled cylinders and oil cooled cylinder heads. It has been a production engine available in the U.S. for the last 17 yrs or so.

 

[petertha]

I think water (the typical water/glycol mix for radiator purposes) is the better cooling liquid. There is heat capacity &then there is specific heat. (I hope Ihave this right, been out of school a long time

An object’s heat capacity describes the amount of heat required to change the temperature of that object by a certain amount. Specific heat is the amount of heat required to change the temperature of a substance by one degree (generally °C).

Discusses interplay of cooling medium & thermal conductivity of materials
http://koolance.com/cooling101-heat-transfer

oil vs water vs all kinds of things
https://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html

a lot of what are called oil cooled are combo air+oil in specific applications
https://mechanics.stackexchange.com/questions/33777/oil-cooled-vs-water-cooled

 

[Cogsy]

Water, or any coolant for that matter, will adsorb heat as long as the heat source is hotter than the coolant.

Just a small point here Baron, but it's aBsorb. The process of aDsorption is very different.

There is heat capacity &then there is specific heat. (I hope Ihave this right, been out of school a long time

You confused yourself a little - these are the same quantities and relate to the amount of energy required to raise the temperature of the material by a given amount, and is expressed as units of energy per unit of mass per degree.

I believe the second quantity you were trying to describe is thermal conductivity, which is the energy flow per unit area per degree, so the higher the temperature difference and larger the contact area, the more heat transfer. We need to compare different substances with the same conditions to evaluate effectiveness.

So, in short, the specific heat capacity of water is around 4.2 joules per gram per degree Kelvin (or Celsius) and mineral oil around 1.7 joules per gram per degree. This means if we have the same mass of each substance and add the same amount of heat to them, the oil will increase in temperature by over double the amount the water will.

Looking at thermal conductivity, water has better conductivity by a factor of over treble that of most oils, so even with the increased temperature of the oil due to it's lower specific heat capacity, the water will still get rid of its heat faster (not to mention the higher temp oil will severely reduce its ability to actually remove heat from the engine).

So in really short terms, don't replace the water in your radiator with oil!

 

[Cogsy]

The speed at which the coolant adsorbs the heat is dependant upon its volume, temperature and the rate of flow. So a slow flow will adsorb more heat in a given time frame.

This is only true for the increase in coolant temperature. For removal of heat from the source, the faster the flow the more heat is removed. If you slow down the coolant flow the coolant gets hotter and heat flux reduces due to the smaller temperature differential.

If you've got access to unlimited cold pond water, you can pump it through nice and quick and it will strip away massive amounts of heat, so internal volume won't be overly critical. Most model engines have issues dumping the heat from the fixed volume of coolant rather than getting heat into the coolant itself. You've basically got access to unlimited coolant volume so that won't be an issue for you.

 

[WSMkid]

If you don't mind letting the cat out of the bag, what will this engine be running? If you are using pond water as a coolant have you considered adding a thermostat?
Have you considered air cooling? A proper fan and duct system my be simpler to build than a full "wet" cooling system. Air cooled working engines always bring deutz diesel engines to my mind.

GJ

 

[BaronJ]

Hello Al,

I was never great at spelling Its way over 50 years since I did anything like this, so that gives you an idea of how little I remembered from what was then Chemistry / Physics... I'm glad I went for Electrickery

 

[Mechanicboy]

The aircooled engine will last longer life due short time to heat up the engine to working temperature. The engine with small water jacket will rise fast up to working temperature than the engine with big water jacket. I will select the engine with small water jacket + thermostat to control working temperature.

The normal working temperature is 85-120 degree celsius/185-248 degree Fahrenheit.

 

[johnlaudano]

Gentlemen,
Great discussion and great links. The engine would power another mahogany boat. I feel better about small water jackets now and I like the thermostat idea. Lots more drawing to go.
John

 

[Rustkolector]

John,
In my mind, a fresh water cooling jacket should be more on the generous side rather than the thin side. Thin jackets will foul more quickly if the water clarity is poor. An intake water strainer might also be advisable.

Set up your engine running at the design load and RPM with a summer pond temp intake water supply. Adjust the flow or pump speed to get a 9-12 degree F differential temperature across the engine cooling system. This will provide an optimum coolant flow to cool your engine under the warmest expected operating conditions. A bypass thermostat would be ideal, but I have never seen one for small scale use.

Another issue is that a smooth running 360 degree crankshaft twin cylinder engine will create significant crankcase pressure pulsations and require a check valve type crankcase vent. This will likely trap blow-by moisture in the crankcase making the oil milky. It gets a little better as the rings seat, but doesn’t completely go away. Design for easy crankcase oil draining and flushing with WD-40 after use. A good hot running engine will combat the moisture build-up and gasoline produces a little less moisture in the combustion gases than propane.

Jeff

 

[Cogsy]

Hello Al,

I was never great at spelling Its way over 50 years since I did anything like this, so that gives you an idea of how little I remembered from what was then Chemistry / Physics... I'm glad I went for Electrickery

Hi Baron,

You picked an unlucky word to mis-spell, adsorption is a confusingly similar, but physically quite different process and can be easily confused with absorption, which is why I mentioned it.

I have exactly the opposite problem to you, I'm hopeless with electricity (as evidenced by my occasional self inflicted shocks) but seeing as I'm a physicist, physics and thermodynamics is right in my wheelhouse... each to his own I guess. Have a good one.

 

[johnlaudano]

Jeff, I received your PM. I sent you two replies but can't verify if they were posted. The web site seems to be having a problem.