Practical Classics (UK)

The two-stroke cycle explained

We get 2T fruity with the simplest of engines



The two-stroke (or 2T, from the German 2-Takt or stroke) engine converts chemical energy (hydrocarbo­n fuel) into kinetic (movement) energy in order to do useful work. Unlike the four-stroke engine, a basic, valveless, crankcasec­ompression spark ignition (SI) two-stroke is extremely simple and has few moving parts

(the twin-cylinder engines powering the Trabant and

Subaru 360 have five moving parts, and the three-cylinder engines in Saabs and DKWS have seven). The simple two-stroke engine not only uses the space above the piston, but employs the area below the piston, using crankcase-compressio­n, and can thus incorporat­e two events into one stroke, thereby providing a power stroke for every crankshaft revolution in comparison to a fourstroke’s power stroke for every two crankshaft revolution­s, providing excellent power-to-weight ratio.

The two-stroke engine is essentiall­y an air pump using combustion of a fuel

(usually petrol, diesel or crude oil) to expand gases to push down a piston in a cylinder.

Most simple twostrokes are lubricated by mixing oil with the fuel to create petroil (or via an automatic pump) on a total-loss basis (it eventually goes to atmosphere via the exhaust pipe). Larger diesel engines using a blower instead of the crankcase to introduce air for combustion, allowing a typical wetsump lubricatio­n set-up. Crankcasec­ompression SI engines are able to operate in any orientatio­n due to their lack of oil reservoir.

2 Pistons as valves

The crankcase-compressio­n twostroke engine uses the piston(s) as inlet and exhaust valves. As the top of the piston uncovers and covers the exhaust port, it’s like a four-stroke engine’s exhaust valve opening and closing, and as the bottom of the piston uncovers and covers the inlet port, it’s like the inlet valve opening and closing. The piston also opens and closes transfer ports.

1 Below ascending piston

As the piston A ascends, the volume in the crankcase B below it gets larger, so the pressure drops to below that of the atmospheri­c pressure outside the crankcase. When the ascending piston begins to uncover the inlet port C, atmospheri­c pressure fills the crankcase with a mixture of air, petrol and lubricatin­g oil from the carburetto­r.

8 Tuned exhaust

Preventing the inlet charge from exiting straight out of the exhaust by taking a short-cut across the piston crown can be combatted by the scavenging system used. Maximising the amount of exhaust gas exiting and stopping the inlet charge from following the exhaust gas out can be tuned by creating an exhaust system that creates negative pressure to encourage the exhaust gas exiting, then creating a timed reverse pressure wave to halt the exit of the inlet charge.

7 Above the descending piston

The ignited mixture begins to expand rapidly, pushing down the piston on its power stroke. As the piston crown begins to open the exhaust port F, the spent gases exit to atmosphere. A little afterwards, the transfer ports open and the fresh charge begins to enter, its volume helping to push out the exhaust gas.

3 Below descending piston

As the piston descends, the volume below it in the crankcase gets smaller, increasing the pressure to above that of atmospheri­c pressure and the pressure above the piston. When the piston crown uncovers the transfer ports D the air-fuel-oil mixture travels upwards via the port to the space above the piston.

4 Crankcase sealing

The key to the simple crankcase-compressio­n two-stroke engine is the ability to seal the crankcase against both atmospheri­c pressure (air getting into the crankcase) and compressio­n pressure (air/fuel/ oil getting out of the crankcase). For this reason, the crankcase has no holes, breathers or convention­al wet-sump lubricatio­n system. Multi-cylinder engines like the twin-cylinder Saab engine (above right) are sealed between cylinders.

5 Above the ascending piston

At the bottom of the cylinder, the piston has uncovered both the transfer ports to allow in a fresh fuel-air-oil charge, and the exhaust port. As the piston ascends, it covers both ports and the mixture is compressed into the combustion chamber at the top. Just before topdead-centre, the spark plug E begins to ignite the mixture. Scavenging is the process whereby the fresh inlet charge helps to purge the spent exhaust gases from the cylinder, and the exiting exhaust charge helps to draw in the fresh charge. When the piston is at the bottom of the bore, both the exhaust and transfer/inlet ports are open (akin to valve overlap in a four-stroke engine). Poor scavenging will result in low power and too much can result in more inlet charge being lost, unburnt, via the exhaust port.

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