The car engine
How does a spark turn into a turning wheel?
The question
A car engine burns fuel, which makes heat. But the wheels need to turn. How does a bang in a cylinder become smooth turning?
Have a guess, then show the answer
Each cylinder runs four strokes over two turns of the crankshaft. In, squeeze, bang, out. Only the bang, the power stroke, pushes. With four cylinders taking turns in the order 1-3-4-2, one of them is always pushing, so at 3000 rpm the crank gets 100 pushes a second.
Play
Turn the crank, then add cylinders
This is a cylinder cut open from the side, slowed down 50 times. Drag the crank round, or set the RPM. Watch the valves, and the spark at the top. Then switch to four cylinders and see them take turns.
Intake. The piston goes down and pulls in air and fuel. One power stroke every two turns: 25 a second at 3000 rpm.
Drag the crank round clockwise, use the slider and buttons, or focus the crank and press the right arrow.
What's happening
- 1
Intake. The piston moves down with the intake valve open, pulling in air and fuel.
- 2
Compression. Both valves shut, and the piston comes back up, squeezing the mix into a small space.
- 3
Power. At the top, the spark plug fires. The burning mix gets very hot, expands, and pushes the piston down hard. This is the only stroke that pushes.
- 4
Exhaust. The exhaust valve opens and the piston comes up again, pushing the burnt gas out. Then it starts over. Four strokes, two turns of the crank.
- 5
One cylinder pushes once every two turns. Four, firing 1-3-4-2, push every half turn. At 3000 rpm that is 100 power strokes a second.
The maths
Each cylinder fires once every two turns, so
One cylinder at the same speed gives 25 a second, which is why a single-cylinder engine shakes so much more.
Simplified, and sources
The drawing is simplified and slowed down 50 times, and the cylinders are drawn side by side on one shaft with the counterweights left out. Each valve is open for exactly one stroke and the spark comes exactly at top dead centre; real engines open the valves early and close them late, and fire a little before the top. The colours for the gas are only labels, not temperatures. The firing order 1-3-4-2 is the common one for inline four-cylinder engines, but some use 1-2-4-3.