The piston is the valve
Windows are cut into the cylinder wall. As the piston moves, it covers and uncovers them by itself. The exhaust opens first and closes last, because its upper edge sits higher than the edge of the transfer ports.
Technology · Two-stroke engine
The two-stroke engine — power on every turn
The whole cycle — intake, compression, combustion and exhaust — fits into two strokes of the piston, that is, one turn of the crankshaft. There are no valves and no camshaft: the piston itself is the valve, and the crankcase is the pump. Watch what happens above and below the piston at the same time.
fresh mixture
compressed
combustion
exhaust gas
oil droplets (1:50)
Drag the flywheel to turn the crankshaft
Shortcuts: Space start / pause · ← → step 15° · + − speed · L labels · C compare with a four-stroke
Engine parts — hover, tap or pick one
Hover over any part of the cutaway (or tap it) to see its name and what it does. The numbers match the markers on the drawing in the mobile view.
Two-stroke vs. four-stroke
Same crank angle, two different engines. Turn on the comparison and count the power strokes.
Two-stroke
a power stroke every revolution
Four-stroke
a power stroke every other revolution
power strokes
power strokes
Crankshaft revolutions: 0
Two-stroke: 1 power stroke per revolution · Four-stroke: 1 power stroke per 2 revolutions
| Feature | Two-stroke | Four-stroke |
|---|---|---|
| Design | A piston, a connecting rod, a crankshaft — and hardly anything else. The timing is done by windows in the cylinder wall and a self-acting reed valve. | Spring-loaded valves, one or more camshafts, a belt or chain drive — many more precision parts. |
| Weight | Light and compact, with a great power-to-weight ratio. With no oil sump it can run in any position — a chainsaw works even upside down. | Heavier for the same power: a cylinder head with valve gear, an oil sump and an oil pump. The oil has to drain back into the sump, so the engine can only run in certain positions. |
| Power per liter | High — a power stroke every revolution. In theory 2× more at the same rpm; in practice less, because some of the mixture escapes and the cylinder never fills completely. | Lower at the same rpm without forced induction; it makes up for it with higher rpm, turbocharging and better cylinder filling. |
| Lubrication | Oil mixed into the fuel (e.g. 1:50) or metered by a small pump. It lubricates the bearings and the cylinder bore, then burns along with the mixture. | A closed circuit: sump, pump, filter. The oil circulates through the engine and is changed periodically — in normal running hardly any of it burns. |
| Emissions | High: scavenging losses (unburned fuel in the exhaust) and burned oil — hence the bluish smoke. Modern direct-injection two-strokes are much cleaner. | Much lower; with a catalytic converter it is easier to meet strict emission standards. |
| Uses | Chainsaws, lawn mowers, leaf blowers, scooters, motocross bikes, outboard motors, in the past the Trabant and Polish WSK motorcycles — and also huge two-stroke marine diesels (with exhaust valves and uniflow scavenging). | Cars, most motorcycles, generators, light aircraft, locomotives and medium-sized ships. |
How does it work?
Four clever ideas that let a two-stroke do without valves.
The crankcase is a pump
As the piston rises, the volume of the sealed crankcase grows, the pressure drops and the reed petals let the mixture in. As the piston descends, it precompresses the mixture to about 1.3–1.4 bar so that it can rush into the cylinder.
Loop scavenging
The transfer ports aim the stream of fresh mixture upward, under the cylinder head, and then down toward the exhaust — in a loop that pushes the exhaust gas out. Some mixture always escapes straight into the exhaust, though: these are the scavenging losses.
An exhaust that plays music
An expansion chamber works like a musical instrument. Its diverging cone sends back a low-pressure wave that helps suck out the exhaust gas, and its converging cone sends a high-pressure wave that returns just before the port closes and pushes the escaping mixture back in.
Did you know…?
Three facts that surprise even mechanics.
- The largest piston engines in the world are two-strokes. The 14-cylinder version of the Wärtsilä-Sulzer RTA96-C marine diesel produces about 80 MW (over 100,000 hp) and weighs about 2,300 tonnes. Its piston is 96 cm across with a 2.5 m stroke, and the crankshaft turns only about 100 times a minute. Instead of crankcase scavenging, though, it has an exhaust valve in the head and turbochargers.
- A 1:50 mix means 20 mL of oil for every liter of gasoline. The oil settles on the bearings and the cylinder bore on its way through the crankcase, then burns along with the fuel — hence the bluish smoke and the typical two-stroke smell.
- The exhaust can push the mixture back into the cylinder. Thanks to Walter Kaaden's work on the expansion chamber, MZ 125 racing bikes made about 25 hp as early as 1961 — that is 200 hp per liter, unheard of at the time for a naturally aspirated engine.
Challenge
Check that you understand the rhythm of a two-stroke — the page detects by itself when you succeed.
- Stop the engine while the exhaust port and the transfer ports are open at the same time — that's scavenging. Use pause, the ±15° step or drag the flywheel.
- Catch the spark: stop the crankshaft at the moment of ignition (within 4°). Hint: look for the yellow star on the port timing diagram.
- Turn on the comparison with the four-stroke and wait until the two-stroke has made 10 power strokes. How many will the four-stroke make in that time? You can speed things up with the slider or the + key.
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From history
An engine almost 150 years old — and still running in every chainsaw.
- 1878–1881
Dugald Clerk
The Scottish engineer builds the first working two-stroke engine and patents it in 1881. To pump the mixture, however, he uses a separate pumping cylinder.
- 1889
Joseph Day
The Englishman hits on a brilliantly simple idea: the crankcase under the piston can itself be the pump. That's how the crankcase-scavenged engine is born — the one in the animation above.
- 1920s
Adolf Schnürle
The German engineer develops loop scavenging: the streams of mixture loop around under the cylinder head instead of flying straight into the exhaust.
- 1950s–60s
Walter Kaaden
An engineer at MZ in Zschopau, East Germany, turns the exhaust into a resonator. In 1961 the rider Ernst Degner fled to the West with his know-how — a year later Suzuki won its first world title with him.