View Full Version : Minimum cylinder displacement?

8th Oct 2008, 17:02
Air and fuel molecules do not scale well with the size of engine. Which means that flames are stopped by things like fine metal meshes.

How small can piston displacement be so that the engine still works?

8th Oct 2008, 17:24
You can go to about 2cc and still have a reliable, working engine (see model engines) but it would be much use on anything with a wing span of over a metre.

The problem isn't with the air molecules but with the integrity of the internal components. The lighter they are, the more likely to break. At the other end of the scale, if they are heavier it will sap engine power and require a larger displacement just to turn over or idle.

I'm sure you could go smaller if you had the technology or inclination but what would be the point? Everyone knows that with engines bigger is better!

Sorry, slipped into petrol-head mode then...

8th Oct 2008, 17:37
I own a model engine, named 'Merlin', which is 0.76cc displacement.

It has a stroke of almost exactly 1cm, and runs beautifully at around 12,000 rpm.

I believe there is another model, 'Dart', of 0.5cc displacement.

As these were commercially made, I am sure there are many other engines, some experimental, which are very much smaller than this.

8th Oct 2008, 18:11
I know this is not anywhere near the smallest, but I used to have a Rossi 21. That's 3.5cc It revved over 30,000 rpm-I think 34,000 was a possibility with nitro fuel and a few mods.

Also-as it was a boat engine- it got to max revs, and there it stayed!

Most impressive!

8th Oct 2008, 18:15
I'm sure you could go smaller if you had the technology or inclination but what would be the point? Everyone knows that with engines bigger is better!

What about - get more power out of given displacement and weight?

The stresses in moving parts depend on acceleration. Which means that if your individual pistons are smaller and have smaller stroke, you could run the engine at a faster rpm, and get more power out of a large number of smaller cylinders than you could have got from fewer and individually larger cylinders.

The biggest infernal combustion engine produces 115 000 horsepowers, and has 14 cylinders (in a straight row). But the largest mass produced plane with infernal combustion engines had IIRC something like 26 000 horsepowers, produced by 168 pistons distributed between 6 engines. Presumably for a reason.

diesel addict
8th Oct 2008, 18:20
The two smallest production engines I can recall are the Cox TD 010 glowplug - which had a swept volume of 0.01 cubic inches and the D.C. Bambi diesel which displaced 0.15cc. I have owned and flown both in the passing of fifty years aeromodelling.....

Anorak off - go! shambles back to the balsa dust....

8th Oct 2008, 18:24
In my younger day we ran tiny little glowplug engines; the smallest made by Cox.

Here's a clue as to why they went out of fashion:

YouTube - Smallest production engine in the world .010cu in 0.33cc (http://uk.youtube.com/watch?v=2CmI3kp4OfQ&NR=1)

8th Oct 2008, 18:52
And then there's the other end of the spectrum - How big? (http://people.bath.ac.uk/ccsshb/12cyl/)

8th Oct 2008, 21:44
Worlds Biggest [email protected] (http://www.vincelewis.net/bigengine.html)

Worlds Smallest Engines (http://www.vincelewis.net/smallengine.html)
(ok, it's a wankel, but it's still internal combustion)

10th Oct 2008, 03:57
That's an interesting concept, what size of atomised droplet of fuel is just too big? In jet aircraft there must be a problem too, so-called flame holders I'm led to believe induce a carefully induced localised turbulence allowing the flame to "burn back" onto the burner and be stable and not lift-off and blow away. Is there a size restriction for that as well? Also might it not be the case that it just gets too difficult to practically control rich and weak extinction in engines that are very small.