# How a Steam Locomotive Works

How a steam locomotive turns coal fire into steam and then into the turning of its driving wheels, from firebox and boiler to chuffing chimney\.

By BigAl

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/0ljzn8hlHxvw)

## Transcript

### 1. A machine that breathes steam

A steam locomotive is a power station on wheels\. It burns coal to boil water, and the steam pushes the wheels round\. Follow the heat and the steam from the fire to the rails\.

Drag to turn the view\. This express engine is modelled on a 1920s British design with 2\-metre driving wheels\.

### 2. Fire in the firebox

At the back of the boiler, by the cab, is the firebox\. The fireman shovels coal through a door onto a grate, and air rising through the grate keeps it burning fiercely\. Water fills the walls all around the fire, soaking up its heat\.

The front half of the engine is cut away so you can see inside\.

### 3. Boiling water into steam

The hot gases from the fire rush forward through dozens of fire tubes\. The tubes run through the boiler, a long tank full of water\. Heat passes through the tube walls and the water boils\.

Lots of thin tubes give the hot gas a huge surface to warm the water through\.

### 4. Open the regulator

Steam gathers in the dome on top of the boiler, held at high pressure\. When the driver opens the regulator, a valve in the dome, steam rushes along the steam pipe to the cylinders\. Press the button to open it\.

Boiler pressure on engines like this was 180 to 250 pounds per square inch: more than twelve times the push of the air around us\.

### 5. Steam pushes the piston

Inside each cylinder is a piston, a snug disc that can slide\. A sliding valve lets steam in behind the piston to push it one way, then in front to push it back\. The white cloud shows which side the steam is pushing\.

Because steam pushes on both sides in turn, each cylinder gives two pushes every time the wheels go round\.

### 6. Rods turn the wheels

The piston rod drives a crosshead along its guides\. The connecting rod turns the back\-and\-forth push into a turn of the middle driving wheel, like your leg on a bicycle pedal\. Coupling rods share the push with the other driving wheels\.

The cranks on the two sides are a quarter\-turn apart, so one side can always push and the engine never gets stuck\.

### 7. Chuff\! Steam up the chimney

Used steam leaves the cylinders through the blast pipe and shoots up the chimney\. Each puff sucks air through the fire behind it, so the faster the engine runs, the harder the fire burns\. That rhythm is the famous chuff\.

With two cylinders, you hear four chuffs for every turn of the driving wheels\.

### 8. Water and coal on board

The tender behind the engine carries the coal and water\. A big express tender held about 8 tonnes of coal and 5,000 gallons of water\. Now you drive: use the regulator slider to set the speed\.

Fire, water, steam, piston, rods, wheels: heat turned into motion\.