TWOSTROKE?

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T he beauty of the two-stroke engine is its simplicity.there are no camshafts, a camchain or even conventional valves. It is lightweight, compact and has an excellent power-to-weight ratio.that’s because it produces a power stroke during each revolution of the crank compared with the one power stroke for every two revolutions in a four-stroke engine (see below).

During the compression stroke (upstroke), the fuel-air-oil mixture (the charge) is sucked into the crankcase, normally through a reed valve (one-way valve), by the vacuum that forms as the piston moves up.then follows spark-initiated combustion forcing the piston down (power stroke).this compresses the fuel-air-oil mixture in the crankcase in the process.the piston passes the exhaust port rst, allowing the burnt exhaust gases to escape down the exhaust. As the piston moves further down, the inlet port is passed, allowing the charge under pressure to pass through the bypass port into the combustion chamber ready for the next compression stroke.the process then repeats.

The main issue with a two-stroke engine is the moving components in the crankcase need lubrication and this is achieved by mixing two-stroke oil with the petrol (manually or by an auto-lube system). Therefore, some of the oil reaches the combustion chamber resulting in the blue tint of smoke from the exhaust.

Another issue is the exhaust port is still uncovered during the intake stroke, with a portion of the fresh charge entering the combustion chamber exiting straight out the exhaust. Unburnt hydrocarbons now enter the atmosphere which is not environmentally friendly nor fuel-ef cient.

Exhaust tuning (with those expansion chambers) is used to create a pressure wave that re ects in the exhaust and runs back towards the engine. If the geometry is correct, the pulse will hit the exhaust port just as the fresh charge enters and keeps the charge from escaping down the exhaust.this effect is ampli ed at certain engine speeds and the term “powerband” is used to describe the behaviour when the engine starts to produce serious power. Now that the basic working of a two-stroke engine is explained, we will investigate some possible advancements in petrol units of the type.

SEPARATE CRANKCASE LUBRICATION

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