Steam — Laboratorium

p–V indicator diagram

Pressure on the left side of the cylinder versus volume — just as Watt's indicator drew it.

left: — right: —

Work per cycle

0 kJ

Mean eff. press.

0 bar

Indicated power

0 kW

The area inside the loop is the work of one cycle. Live steam fills the cylinder, the slide valve cuts it off, the steam expands, then it exhausts. Turn on the condenser — the bottom line drops below atmospheric pressure and the loop grows.

Challenge

The governor guards the speed

With the Watt governor on, change the load by at least 30 points and wait for the speed to settle. Then turn off the governor (G) and do the same.

with governor: —without governor: —

Well done! With the governor the speed barely budged; without it, it ran away. That's negative feedback: the engine measures its own speed and adjusts the steam supply.

Red zone

Build up the fire, close the throttle part way and bring the pressure into the gauge's red zone. What will the safety valve do?

pressure: —

Well done! The steam lifted the weighted valve disc and escaped into the air — the pressure stopped rising. In the 19th century, jammed valves ended in boiler explosions.

Thrifty as Watt

Reach an efficiency of at least 6% with at least 15 kW of shaft power and hold it for a few seconds. Hint: use the condenser and run close to full load.

now: —

Well done! The condenser lowered the back pressure, so every kilogram of steam does more work — exactly the idea that made Watt famous.

Did you know…?

“Horsepower” was James Watt's marketing trick. He defined and popularized the unit so customers could compare his engine with the horses it replaced. The unit of power was later named after him — the watt (1 metric hp ≈ 735.5 W).

The centrifugal governor gave birth to control theory. In his paper “On Governors” (1868), James Clerk Maxwell explained mathematically why such governors sometimes start to “hunt” — and how to prevent it.

Steam still powers the world. Roughly half of the world's electricity today comes from steam turbines — in coal, nuclear and combined-cycle gas power plants. The only change is that the steam spins a rotor instead of pushing a piston.

01 · boiler

Fire becomes pressure

Flue gases from the firebox flow through fire tubes in the water and give it their heat. In a closed boiler the steam can't escape, so the pressure rises — and with it the boiling point: at 8 bar, water boils only at about 175 °C.

02 · slide valve

Steam from the left, then from the right

An eccentric on the shaft moves the slide valve. It admits live steam to one side of the piston while connecting the other side to the exhaust. Thanks to the lap, the valve cuts off the supply before the end of the stroke — from then on the steam works by expanding.

03 · feedback

A machine that watches its own speed

The piston's force is uneven — at the dead centers it doesn't turn the shaft at all. The flywheel stores energy and smooths the motion. The Watt governor measures the speed: when the engine speeds up, the balls swing outward and close the steam valve.

04 · condenser

Cold water, more power

Instead of escaping up the chimney, the exhaust steam goes into the condenser and turns back into water, taking up over a thousand times less space. This creates a vacuum, so the piston doesn't have to push steam out against the atmosphere. Watt came up with this in 1765.

Steam engine — the steam that changed the world

Instruments