Thanks, TiPi.
Okay, here's something for folks to throw rocks at:
Does the higher ratio of surface area-to-volume of smaller air-cooled cylinders allow them to burn proportionally more fuel/produce more HP while staying within CHT limits?
TiPi's chart above shows the relatively high power density (HP/swept volume) these industrial engines produce (even in stock form) without overheating. They do have the advantage of fan-forced air, but the amount of air moved, and the static pressure available to move it, does not appear to be much different than the conditions aero engine ducting typically provides at 60 kts. Moreover, some of the industrial engines are powering aircraft now, without the stock fans, and not overheating despite power densities that are higher than aircraft engines. Their fins are >not< numerous, big, fine, or designed with the attention to airflow as we see in a Lycoming or Continental jug, but they seem to work and CHTs are reasonable.
Is this because the cylinders are small?
As a cylinder increases in size, the volume increases as a function of the
square of the radius, while the surface area only increases linearly with the radius. So, bigger cylinders have proportionally less surface area, per unit volume, than small cylinders. If HP (and therefore, waste combustion heat gained by the cylinder) is a function of displacement, but cooling is a function of the surface area of the cylinder, cylinder head, and the underside of the piston, it would make sense that smaller cylinders can burn more fuel (and produce more power) per unit volume while staying within CHT limits.
Some examples:
.....................................Swept Volume....Surface area...HP per....Surface area...power density
Engine.........bore....stroke.... per cyl ............per cyl........cylinder...per volume........(HP/cc)
Lyc O-360....130mm...111mm...1473cc............453cm2.........45..........0.31...............0.031Lyc O-235C1......111mm...98.4mm...952cc............343cm2.........29..........0.36...............0.030
Cont O-200..103mm....98mm.....823cc............319cm2.........25..........0.39...............0.030
VW 2180cc....92mm....82mm.....544cc............237cm2.........19..........0.44...............0.034
B&S 810cc.....85mm...73mm.....412cc.............195cm2.........16*.........0.47...............0.039
* HP claimed for SE-33 engine
The trend is: smaller cylinders appear to allow higher power densities.
But:
1) The big change in power densities also coincides with the "certified" vs "noncertified" change. So, it could result from certified engine makers rating their products more conservatively.
2) The same increased surface area-to-volume ratio that allows small cylinders to get better cooling per unit of fuel burned, would presumably also work the other way, allowing the cylinder to take on more heat from the hot combustion gasses. I don't think we can have one without the other.
I'm sure nothing here is new, but I'm not a heat engine expert (obviously!

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