dave-in-england
Junior Member
- Joined
- Aug 30, 2012
- Messages
- 88
- Reaction score
- 20
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
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
Rather embarrassingly... 
