Small Engines

Summary

One of the interesting things to contemplate is the scale of the internal combustion engine. It’s a very human scale device; pistons the size of fists, Valves about as wide as knuckles. It’s the kind of thing a man with normal sized machine tools can make. Most internal combustion engines in the world are on this human scale. The ideas came about from the very human business of making cannons and pumps for coal mines, so no real surprise at this. There are also fairly large ones driving cargo ships with pistons which are about a yard in diameter. Those are about as big as they get: half meter to a meter in diameter pistons and something around that size has been in existence for about a century (along with similarly sized steam engines they evolved from). Cars with fist sized pistons have a thermodynamic efficiency of around 25%, maybe 35% on a good day. The thing with manhole size pistons hits 50% and is able to burn tar-like bunker fuel. The more important prime mover is the turbine. For gas turbines, the turbine blade is of a similarly human length scale: the things that convert heat into motion are single crystals of nickel-superalloys which are a few inches long; about 6 inches long -not real different in scale from car or marine engine pistons. Steam turbine blades are made of less exotic materials and are considerably longer; maybe a few feet long -just like the old timey big piston steam engines. If we ever switch to supercritical CO2 turbines, the blades will be much smaller -back to gas turbine size or smaller. There are lots of reasons for this, but the primary reason is people are people sized and tend to make things out of parts on people scales. If you start thinking about other length scales, things get very different. For the same reasons you can’t just make a lathe very small and expect it to function similarly, you can’t make an efficient heat engine very small and expect it to work the same way. For example, the surface area to volume ratio in sma...