DOUBLE-ACTING STEAM ENGINE
SLIDER-CRANK · SLIDE VALVE · WATT GOVERNOR · LIVE PHYSICS
indicator diagram — P vs V
drag to orbit · pinch / ⌘-scroll to zoom
The flyball governor trims the throttle to hold a steady speed. Switch it off, then change the load, to see why every engine needed one.
How a steam engine works
- Steam pushes the piston. High-pressure steam from the boiler is let into one end of the cylinder; because the engine is double-acting, the other end is opened to exhaust at the same time, so steam drives the piston on every stroke, both ways.
- The slide valve times it. A D-shaped valve, driven by an eccentric set about 90° ahead of the crank, slides back and forth to uncover each port at the right instant — admission, then cutoff, then release to exhaust, then compression.
- Expansion does free work. After cutoff the trapped steam keeps pushing as it expands, its pressure falling like P ∝ 1/V. Cutting off earlier wrings more work from each kilogram of steam — that is the whole point of expansive working. Watch the indicator diagram change as you move the cutoff.
- Crank and flywheel. The connecting rod turns the piston's straight push into rotation, and the heavy flywheel stores energy through the dead points where the crank can make no torque, smoothing the pulses into steady spin: I·dω/dt = τsteam − τload.
- The governor holds the speed. Two spinning flyballs rise as the engine speeds up (cosα = g/Ω²ℓ); through a linkage they close the throttle, cutting the steam — a self-correcting feedback loop. Turn it off and change the load: the engine races or stalls. James Watt's 1788 governor was one of the first automatic controllers ever built.
The indicator diagram at top-right plots cylinder pressure against volume — the very instrument Watt used to tune his engines. The area inside the loop is the work done per stroke; multiply by strokes-per-second and you have the power.
Try: drop the cutoff to 40% and watch the expansion curve stretch · switch the governor off and yank the load up until it stalls · shrink the flywheel and watch the speed surge between power strokes.
Try: drop the cutoff to 40% and watch the expansion curve stretch · switch the governor off and yank the load up until it stalls · shrink the flywheel and watch the speed surge between power strokes.
Switch the governor · drag the load · drag to orbit · X-ray · ⌘/Ctrl + scroll to zoom