Small Engines
3 days ago
- #Heat Engines
- #Energy Storage
- #Scale Challenges
- Internal combustion engines are human-scale devices with pistons the size of fists and valves as wide as knuckles, evolved from cannons and pumps.
- Large ship engines with pistons up to a meter in diameter achieve up to 50% thermodynamic efficiency, compared to 25-35% for car engines.
- Turbines, like gas turbines with nickel-superalloy blades a few inches long, are key prime movers; supercritical CO₂ turbines could have even smaller blades.
- Scaling down heat engines is challenging due to unfavorable surface area-to-volume ratios, combustion instability, and issues with fuel droplet size relative to bore size.
- Hydrocarbons store more energy by weight and volume than current batteries (e.g., kerosene at 40,000 kJ/kg vs. lithium batteries at 1,200 kJ/kg), making small engines desirable for applications like insect-sized drones.
- Small engines face difficulties like maintaining large temperature gradients over millimeters, flame quenching distances preventing propagation, and piston ring geometry issues.
- Higher RPMs in small engines can reduce combustion efficiency due to slower flame propagation, and thermal conductance scales differently at small sizes.
- Alternative approaches include thermoacoustic engines (using sound waves as pistons), thermoacoustic ratchets, pyroelectric materials, and vapor bubbles, leveraging new physics for small prime movers.
- Tinkering with small engines is accessible, inspired by ideas like off-grid energy generation and historical model engines, though efficiency has often been neglected in favor of functionality.