In the Groove

[dave-in-england]

Hi All,


I have always been interested in the reason why the fins on small STATIONARY engines are made just the same as those on moving engines.

Horizontal cylinder fins on motorcycles are obviously horizontal so that the passing air
rapidly clears out the heat between the roots of the fins, keeping the engine cool.

But what about those model stationary engines, running on a bench in your shed, or kitchen sometimes ?



Without any passing air to take the heat away, the horizontal fins actually hold the heat
between the roots of the fins, and restrict the movement of rising hot air while creating a series of annular hot-spots
around and along the length of the cylinder.

Maybe it is an accepted tradition that they have always been made that way.

Sure I know that the model engine is not going to be hammered taking you to work and back,
but it still gets hot, very quickly, and I wondered if vertical fins would be a more efficient way for cooling these model engines.


So I made up some computer models of some variations to see if it would make any difference.


Picture 1

The first picture shows a mild steel cylinder of 60mm diameter and 100 mm long, as a starting reference as a
substitute for an engine cylinder.

This plain smooth cylinder has a surface area of 24500 Sq mm.
This area includes both of the end faces, which I have left plain with out any cylinder bore.
It has a mass of 2200 grammes.


Picture 2

I made 4mm wide horizontal grooves with 4mm spacing and an actual cut depth of 12mm from the outside diameter of the cylinder.

This grooved cylinder gives a surface area of 64300 + Sq mm, which is over 260 % larger area than that of the plain cylinder surface.

It also reduced the mass of the cylinder to approx. 1500 grammes.


Picture 3

I have now made the horizontal grooves 2mm wide and the fins 2mm wide, with the same 12mm cut depth.

This increases the surface area to 107600 Sq mm, which is over 430% more cooling surface area than the plain cylinder, but still about the same 1500 grammes mass as the 4mm wide groove version.


Picture 4

Next I make a cylinder with 23 vertical grooves of 4mm wide, 4mm spacing and 12mm cut depth.

This cylinder version has a surface area of 74100 Sq mm.
This is over 300 % increase in surface area relative to the plain cylinder.



Picture 5

Here the vertical grooves are 2mm wide and the fins 2mm wide, still at 12mm cut depth.

This gives a surface area of 129400 Sq mm, almost 530% of the plain cylinder area.

The mass is also slightly less than that of the two horizontal fin cylinders.


I expect that making even thinner fins and thinner spacings will eventually reach the end point of diminished returns,
but it does show that vertical fins on a comparable size cylinder gives a larger surface area cooling option.

The conclusion is that using thin vertical fins gives a lighter cylinder with the most cooling area.

Also, the heat of the cylinder does not get trapped between these vertical fins, it has an easy rising path without any restrictions.

However, I can see that actually making the vertical slots on a milling machine is going to be a long and laborious job with
a long series tapered milling cutter !

Dave

 

[Sshire]

Dave
Very interesting. Your analysis and calculations make for a compelling argument.
Milling the slots with the piece oriented with the long axis as "Y" in a SuperSpacer or spin indexer does not seem to be a problem. What is the reason for a tapered cutter? I'm missing that one.

 

[Paulsv]

Yes, very interesting. A couple questions occur to me: Are thin fins as effective at transferring heat away from the center of the cylinder as thicker fins? For instance, if you have two thin fins, they will have almost twice the surface area as one thicker fin of the same size, but will they be as effective at moving heat from the center of the cylinder to the surface of the fin as one thicker fin the same size as the two thinner fins? How much of a restriction do you really have with horizontal as opposed to vertical fins? The horizontal fins are open to the air around the entire perimeter of the groove, and presumably there is convective air movement all around that perimeter. Another way of asking the same question is: if you have fins such as in your picture 2, will the cylinder cool appreciable better if the cylinder itself is mounted horizontally on the engine, as opposed to if the cylinder is mounted vertically on the engine?

 

[dave-in-england]

What is the reason for a tapered cutter? I'm missing that one.


Hi Stan, thanks for your comment !

If the slot was cut parallel all the way down to the bottom of the groove, the thickness of the fin would be much reduced where it meets the actual solid part of the cylinder at the root.

A taper cutter would be needed to keep the fin the same thickness all the way down.

Not too important for thick fins, but this would apply increasingly as the fins are made thinner.

Here is an extreme example......where they are in danger of snapping off.

dave

 

[dave-in-england]

Yes, very interesting. A couple questions occur to me: Are thin fins as effective at transferring heat away from the center of the cylinder as thicker fins? For instance, if you have two thin fins, they will have almost twice the surface area as one thicker fin of the same size, but will they be as effective at moving heat from the center of the cylinder to the surface of the fin as one thicker fin the same size as the two thinner fins? How much of a restriction do you really have with horizontal as opposed to vertical fins? The horizontal fins are open to the air around the entire perimeter of the groove, and presumably there is convective air movement all around that perimeter. Another way of asking the same question is: if you have fins such as in your picture 2, will the cylinder cool appreciable better if the cylinder itself is mounted horizontally on the engine, as opposed to if the cylinder is mounted vertically on the engine?

Good point !

The heat transfer by conduction depends on what the cylinder material consists of and the thickness, but even that probably makes a negligible difference in this case because the heat will still be contained within the metal, .... it is only the air currents which take the heat away.


If the engine is in a static position, there is only the fact that heat rises which would draw in cooler air from below.

With the horizontal fins, the cooling air never gets right down to the root of the fins, where it is needed most.

Seems to me that deep horizontal fins clog themselves up with non-moving hot air, because there is no incentive for it to go anywhere, but of course if that cylinder was then laid horizontal, there would be a lot less resistance against the rising heat because the fins are now pointing in the same direction that the hot air wants to go, allowing cooler air to be drawn up across the wider faces of the lower fins.



Dave

 

[petertha]

Interesting topic. It would be neat if someone on the forum worked with, or had access to heat simulation plug-ins like what I think is available for Solidworks.
https://forum.solidworks.com/community/solidworks_simulation/flow_simulation/blog/2010/07/19/why-sw-flow-simulation-is-the-best-tool-for-thermal-analysis

ie. draw up those different cylinder designs, specify the material, impose a combustion temperature at the top & observe the temperature profile accross the cylinder at time duration. Iv'e seen demo's of this applied to heat sinks & piping, its pretty cool.

Related to a cylinder, I wonder how much of the lower portion finnings matter as much? Most of the heat is in the top head area & helps explain different (head) finning orientations & material selections. This V8 pic kind of depicts this, although the cooler lower portion is likely because of circulating cooling (radiator) exchager fluid.

 

[dave-in-england]

Interesting topic. It would be neat if someone on the forum worked with, or had access to heat simulation plug-ins like what I think is available for Solidworks.
https://forum.solidworks.com/community/solidworks_simulation/flow_simulation/blog/2010/07/19/why-sw-flow-simulation-is-the-best-tool-for-thermal-analysis

ie. draw up those different cylinder designs, specify the material, impose a combustion temperature at the top & observe the temperature profile accross the cylinder at time duration. Iv'e seen demo's of this applied to heat sinks & piping, its pretty cool.

Related to a cylinder, I wonder how much of the lower portion finnings matter as much? Most of the heat is in the top head area & helps explain different (head) finning orientations & material selections. This V8 pic kind of depicts this, although the cooler lower portion is likely because of circulating cooling (radiator) exchager fluid.

Yes....... Rather embarrassingly... I am using
Solidworks 2012 premium....with all that thermal simulation stuff built in.

But that is in the HEAVY end of the program that I have not delved into yet and have been avoiding !!!

This may be a good incentive to start. Ha Ha..

 

[Jasonb]

Well the theory may be OK but try cutting those thin vertically slotted fins with a tapered cutter. Not too bad for the first cut but the remaining cuts will tend to bend the previous fin into the gap on the other side. Even with circular groves there comes a time when the fin gets too thin and bends.

Also as the majority of stationary model IC engines use a horizontal cylinder the usual circular groves are orientated the right way and far easier to cut with a parting tool.

You can also drill through the radial grooves for cylinder studs, if you tried that with the vertical fins you would loose a fin or two. See how the cylinder in the attached PDF has a shallower slot at the top so there is some thickness for the head screws and a flange at the bottom to mount it to the crankcase, you could not easily cut the vertical fins on a usable cylinder.

And should the builder wish to add a cooling fan to the crankshaft or rely on propwash then the circular grooves will also be in the right orientation for the flow of air.

J

View attachment cylinder.pdf

 

[petertha]

I am using Solidworks 2012 premium....with all that thermal simulation stuff built in.... This may be a good incentive to start. Ha Ha..

Aha! Sweet victory. You have fallen into my snare! Now we have uncovered someone with industrial strength tools to help answer these tricky model IC questions! (Just kidding, it would be useful to see what you come up with... especially with a head popped on the cyl assembly).

 

[Sshire]

Or
Actually make 3 test pieces. The first untouched, the second with horizontal fins, the 3rd with vertical. Put all 3 in the oven to heat. Take them out and record surface temps at some regular interval.

 

[Wizard69]

A very interesting post.

Hi All,

I have always been interested in the reason why the fins on small STATIONARY engines are made just the same as those on moving engines.

Many of these engines are built from castings. The common approach would not allow for vertical fins.

Horizontal cylinder fins on motorcycles are obviously horizontal so that the passing air
rapidly clears out the heat between the roots of the fins, keeping the engine cool.

My mind is foggy here but I seem to remember reading about the engineering challenges in keeping those air cooled motors running at the correct temperature. The problem on a motor cycle is that the cooling isn't even as the front facing half of the cylinder gets cooled far more aggressively than the rest of the cylinder.

But what about those model stationary engines, running on a bench in your shed, or kitchen sometimes ?

Without any passing air to take the heat away, the horizontal fins actually hold the heat
between the roots of the fins, and restrict the movement of rising hot air while creating a series of annular hot-spots
around and along the length of the cylinder.

Many of these engines are designed for aircraft so if you are running them with out a prop isn't that abuse?

Maybe it is an accepted tradition that they have always been made that way.

Often in models it is a matter of recreating the prototype. That is all well and good but the prototypes are often designed for mass production. If the engine is designed for diecasting for example you have to be able to extract it from the die.

On the other hand many of the unique designs the the modeling community has come up with over the years have been designed to be produced on a lathe. For years low cost mills have been unheard of. So if all you have is a lathe it makes sense to design for easy production on that machine

Sure I know that the model engine is not going to be hammered taking you to work and back,
but it still gets hot, very quickly, and I wondered if vertical fins would be a more efficient way for cooling these model engines.

I honestly doubt it would make a huge difference. The only possibility here is that such an arrangement might work well with fan cooling and plenty of duct work.

So I made up some computer models of some variations to see if it would make any difference.

Picture 1

The first picture shows a mild steel cylinder of 60mm diameter and 100 mm long, as a starting reference as a
substitute for an engine cylinder.

This plain smooth cylinder has a surface area of 24500 Sq mm.
This area includes both of the end faces, which I have left plain with out any cylinder bore.
It has a mass of 2200 grammes.

Picture 2

I made 4mm wide horizontal grooves with 4mm spacing and an actual cut depth of 12mm from the outside diameter of the cylinder.

This grooved cylinder gives a surface area of 64300 + Sq mm, which is over 260 % larger area than that of the plain cylinder surface.

It also reduced the mass of the cylinder to approx. 1500 grammes.

Picture 3

I have now made the horizontal grooves 2mm wide and the fins 2mm wide, with the same 12mm cut depth.

This increases the surface area to 107600 Sq mm, which is over 430% more cooling surface area than the plain cylinder, but still about the same 1500 grammes mass as the 4mm wide groove version.

Picture 4

Next I make a cylinder with 23 vertical grooves of 4mm wide, 4mm spacing and 12mm cut depth.

This cylinder version has a surface area of 74100 Sq mm.
This is over 300 % increase in surface area relative to the plain cylinder.

Picture 5

Here the vertical grooves are 2mm wide and the fins 2mm wide, still at 12mm cut depth.

This gives a surface area of 129400 Sq mm, almost 530% of the plain cylinder area.

The mass is also slightly less than that of the two horizontal fin cylinders.

I expect that making even thinner fins and thinner spacings will eventually reach the end point of diminished returns,
but it does show that vertical fins on a comparable size cylinder gives a larger surface area cooling option.

Thin cross sections won't cool as well as thicker cross sections. In the electronics industry power components are mounted on the thickest part of the heat sink. So yeah it is a balancing act, you need area to dissipate the heat but you need cross section to get the heat there.

The conclusion is that using thin vertical fins gives a lighter cylinder with the most cooling area.

The argument can be made that horizontal fins allow the concentration of cooling fins where they do the most good. This leads to the common appearance of cylinders as top heavy tubes.

Also, the heat of the cylinder does not get trapped between these vertical fins, it has an easy rising path without any restrictions.

However, I can see that actually making the vertical slots on a milling machine is going to be a long and laborious job with
a long series tapered milling cutter !

You need to think different here. In this case think like a machinist married to his horizontal mill. Of course this means making your own taper cutter, but it will likely be easier to actually use on a mill. Especially with aluminum. Beyond that even doing this in aluminum might not be ideal, consider machining foam or wax with the goal of casting the shapes you want.

Dave

 

[Goldflash]

I think you also have to take into account what type of Engine you are building . OHV 4 stroke 70-80% of the heat will be in the cylinder head and its more important to look at cooling of the Head . SV 4 stroke most of the heat will be in the area of the exhaust port.
2 stroke is a bit different but again the cylinder head plays a major part in the cooling and again a lot of heat will be in the region of the exhaust port.
Hit and Miss engines go through a cooling cycle with the inlet charge flowing through the cylinder and head to some extent and cooling it.
apart from model aircraft engines which are performing a work load most model engines internal and external combustion are very lightly loaded and run at low rpm and dont generate that much heat .

 

[AussieJimG]

Good onya Stan. That's one thing about technical discussion: in the end you can build it and measure it.

Discussions about angels and heads of pins just keep on going and getting nowhere.

Mind you, I have been in tea rooms where discussions about pop-pop boats have had a distinctly "angelic and pinhead" flavour.

Jim