down as air is being sucked in by the downward motion against the piston. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage. Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft. Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve. History An early known example of rotary to reciprocating motion is the crank mechanism. The earlier hand-operated cranks appeared in China during the Han Dynasty (202 BC–220 AD). The Chinese used the crank-and-connecting rod for operating querns as far back as the Western Han dynasty (202 BC–9 AD). Eventually crank-and-connecting rods were used in the inter-conversion of rotary and reciprocating motion for other applications such as flour-sifting, silk-reeling machines, treadle spinning wheels, and furnace bellows driven either by horses or waterwheels. Several saw mills in Roman Asia and Byzantine Syria during the 3rd–6th centuries AD had a crank and connecting rod mechanism which converted the rotary motion of a water wheel into the linear movement of saw blades. In 1206, Arab engineer Al-Jazari invented a crankshaft. The reciprocating engine developed in Europe during the 18th century, first as the atmospheric engine then later as the steam engine. These were followed by the Stirling engine and internal combustion engine in the 19th century. Today the most common form of reciprocating engine is the internal combustion engine running on the combustion of petrol, diesel, liquefied petroleum gas (LPG) or compressed natural gas (CNG) and used to power motor vehicles and engine power plants. One notable reciprocating engine from the World War II era was the 28-cylinder, Pratt & Whitney R-4360