b. Comparison of TwoStroke Cycle Engines. It might appear that a two
stroke cycle engine could produce twice as much horsepower as a fourstroke
cycle engine of the same size, operating at the same speed. This is not the
case. To scavenge the burned gases at the end of the power stroke and during
the time both the intake and exhaust ports are open, the fresh airfuel mixture
rushes into and through a cylinder, a portion of the fresh airfuel mixture
mingles with the burned gases and is carried out the exhaust port. Also, due
to the much shorter period, the intake port is open (as compared to the period
the intake valve in a fourstroke cycle engine is open), a relatively smaller
amount of airfuel mixture is admitted. With less airfuel mixture, less power
per power stroke is produced as compared to the power produced in a fourstroke
cycle engine of like size operating at the same speed, and with other
conditions being the same. To increase the amount of airfuel mixture,
auxiliary devices are used with twostroke cycle engines to assure delivery of
greater amounts of airfuel mixture into the cylinder. Figure 111 shows one
device that uses compression in the crankcase. Other engines may use
superchargers or turbochargers or both to increase power. Obviously, the
greater the pressure developed, the more air carried into the cylinder.
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