This is the most common engine design found in street motorcycles today. It refers to the number of times a piston moves up and down through each power cycle. 1) A downward stroke brings in the fuel/air mixture; 2) an upward stroke compresses the fuel/air mixture; 3) a downward stroke results when that mixture is ignited and expands, and finally; 4) an upward stroke expels the exhaust gases.
an internal-combustion engine in which an explosive mixture is drawn into the cylinder on the first stroke and is compressed and ignited on the second stroke; work is done on the third stroke and the products of combustion are exhausted on the fourth stroke
Internal-combustion engine operating on the Otto cycle principle of intake, compression, expansion and exhaust.