• Adharit

What is Otto cycle?

The Otto cycle is the ideal cycle for spark-ignition reciprocating engines. It

is named after Nikolaus A. Otto, who built a successful four-stroke engine

in 1876 in Germany using the cycle proposed by Frenchman Beau de

Rochas in 1862. In most spark-ignition engines, the piston executes four

complete strokes (two mechanical cycles) within the cylinder, and the

crankshaft completes two revolutions for each thermodynamic cycle. These

engines are called four-stroke internal combustion engines.

For 4 stroke IC Engines

Initially, both the intake and the exhaust valves are closed, and the piston is

at its lowest position i.e. bottom dead centre. During the compression stroke, the piston moves upward, compressing the air–fuel mixture. Shortly before the piston reaches

its highest position top dead centre, the spark plug fires and the mixture ignites,

increasing the pressure and temperature of the system. The high-pressure

gases force the piston down, which in turn forces the crankshaft to rotate,

producing a useful work output during the expansion or power stroke. At the

end of this stroke, the piston is at its lowest position (the completion of the

first mechanical cycle), and the cylinder is filled with combustion products.

Now the piston moves upward one more time, purging the exhaust gases

through the exhaust valve (the exhaust stroke), and down a second time,

drawing in fresh air–fuel mixture through the intake valve (the intake stroke). Notice that the pressure in the cylinder is slightly above the atmospheric value during the exhaust stroke and slightly below during the intake stroke.

For 2 stroke IC Engines

In two-stroke engines, all four functions described above are executed in

just two strokes: the power stroke and the compression stroke. In these

engines, the crankcase is sealed, and the outward motion of the piston is

used to slightly pressurize the air–fuel mixture in the crankcase Also, the intake and exhaust valves are replaced by openings in the lower portion of the cylinder wall. During the latter part of the power stroke, the piston uncovers first the exhaust port, allowing the exhaust gases to be partially expelled, and then the intake port, allowing the fresh air–fuel

mixture to rush in and drive most of the remaining exhaust gases out of the cylinder. This mixture is then compressed as the piston moves upward during the compression stroke and is subsequently ignited by a spark plug.

The two-stroke engines are generally less efficient than their four-stroke

counterparts because of the incomplete expulsion of the exhaust gases and

the partial expulsion of the fresh air–fuel mixture with the exhaust gases.

However, they are relatively simple and inexpensive, and they have high

power-to-weight and power-to-volume ratios, which make them suitable for

applications requiring small size and weight such as for motorcycles, chain

saws, and lawn mowers.

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