TWO-STROKE ENGINE

This engine design manages to pack one power stroke into every two strokes of the piston.

1. Compression stroke: Fuel-air mix is compressed and ignited.

2. Power stroke: The piston is pushed downward by combustion and hot exhaust gases.

When the piston moves from bottom dead center (BDC) to top dead center (TDC) the fresh air and fuel mixture enters the crankcase chamber through the intake port. The mixture enters due to the pressure difference between the crank chamber and the outer atmosphere.

In a two stroke spark ignition engine, a cycle is completed in two strokes of a piston or one complete 360º revolution of the crankshaft. The two-stroke engine uses an intake and an exhaust port instead of intake and exhaust valves. Petrol is mixed with two-stroke oil for lubricating the piston and crankshaft or the carburetors ‘autolube’. As the piston proceeds downward another port is opened and the oil/fuel/air mixture enters the intake port. Air/fuel/oil mixtures come from the carburetor or fuel may be pre-mixed usually at a ratio between 20:1 and 30:1. When the piston moves further down and the cylinder doesn't have any more gases, fuel mixture starts to flow to the combustion chamber and the second process of fuel compression starts.

The piston acts as the combustion chamber with the cylinder and compresses the air/fuel mixture, receives back the liberated energy, and transfers it to the crankshaft. The piston motion creates a vacuum that sucks the fuel/air mixture from the carburetor and pushes it from the crankcase to the combustion chamber. The sides of the piston act like the valves, covering and uncovering the intake and exhaust ports cut out in the side of the cylinder sleeves wall.

With the help of a spark plug, ignition timing takes place at the top of the stroke or TDC. Due to the expansion of the gases the piston moves downwards covering the intake port and causes the fuel-air mixture inside the crank chamber to be compressed. When the piston is at bottom dead center the burnt gases escape from the exhaust port out into the exhaust pipes expansion chamber.

At the time the transfer port is uncovered the compressed charge from the crank chamber enters into the combustion chamber through the transfer port. The fresh charge is deflected upwards and removes the exhaust gases from the combustion chamber. Again the piston moves from bottom dead center to top dead center and the air/fuel/oil mixture is compressed when the both the exhaust port and transfer ports are covered.

Two-stroke engines have some advantages over a four-stroke engines. Two-strokes have no valves or camshaft mechanisms, simplifying its construction. Four-strokes execute one cycle per two crankshaft revolutions thus halving the process. It weighs less and is easier to manufacture and produces a high power to weight ratio.

Two stroke engines also come with disadvantages, Engines have a shorter life due to an inefficient lubrication system. They also do not consume fuel efficiently, thereby producing excessive pollution.

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