PC ACADEMY
Back to basics in the classroom
We get 2T fruity with the simplest of engines
OVERVIEW
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, crankcase-compression 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 four-stroke’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 two-strokes 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 wet-sump lubrication set-up. Crankcase-compression SI engines are able to operate in any orientation due to their lack of oil reservoir.
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.
2 Pistons as valves
The crankcase-compression two-stroke 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.
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 simp
