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2 Stroke v.s. 4 Stroke

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by

Josh Welch

on 8 January 2013

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Transcript of 2 Stroke v.s. 4 Stroke

2 Stroke v.s. 4 Stroke Josh vos-welch A "stroke" refers to the movment of the piston in the engine. In a two stroke engine the piston travels up and down once while the crank makes one revolution. In a four stroke engine the piston travels up and down twice while the crank makes two revolutions. It sound like the same thing just repeating the cycle but a two stroke ignition fires on every revolution of the crank and a four stroke ignition fires on every other revolution. Introduction to 2-stroke v.s. 4-stroke Wait... isnt it the same thing? The 4-stroke Engine The four stroke was invented by Nikolaus Otto in 1876, hence it is also known as the Otto cycle but the technically correct term is four stroke cycle The four strokes of the cycle are intake, compression, power and exhaust as the piston moves down wards the cam/lifter arrangement opens the intake vlave drawing in a vaporized fuel/air mixture into the cylinder. the valve closes, as the piston moves upwards it compresses the fuel/air mixture, when
the piston reaches the top of the compression stroke, the spark plug fires igniting the compressed fuel. As the fuel burns the gases expand driving the piston downwards creating power, at the bottom of the power stroke the exhaust valve is opened by the cam/lifter arrangement. the upward stroke of the piston drives the exhaust fumes out of the cylinder The 2-stroke Engine the first two stroke was invented by Sir Dugald Clark in 1881. His design has valves like a four stroke and required an air compressor to blow the fuel/air mixture. These Engines were used in big ships and trains but never hit the mass market. The crankcase compression two stroke more commonly known was invented by Joseph Day in 1889 Two stroke engines use both the crankcase and the cylinder to achieve all the elements of the four stroke in only two strokes of the piston. The fuel/air mixture is drawn into the crankcase by the vacuum that's created by the piston moving upwards. two stroke engines either have a spring valve or reed to regulate air flow, also as the pistons moving upwards its compressing the fuel/air mixture above the piston, as the piston reaches the top of the cylinder the spark plug fires driving it downwards compressing the fuel/air mixture in the crankcase. Toward the end of the power stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This also opens and pushes gases out of the exhaust port, unfortunately some of the fresh fuel is expelled as well. Two strokes usually have an expansion chamber. http://www.animatedengines.com/twostroke.html http://www.animatedengines.com/otto.html Expansion Chamber an expansion chamber exhaust has a chamber where gases allow the exhaust to set up a pulse(pressure wave) that returns towards the exhaust port. If the returning pulse arrives at just the right time, it will push the unburnt gases back into the cylinder. In Comparison Advantages of the 2 stroke engine -two stroke engines do not have valves simplifying their constuction

-two stroke are lighter and cost less to manufacture

-two stroke engines have the potential for about twice as many power strokes per revolution Disadvantages of the 2 stroke engine -two stoke engine dont have as long of a life as a four stroke due to the lack of a dedicated lubrication system.

-two stroke engines dont have good fuel efficiency, yielding fewer miles per gallon.

-two stroke engines produce more pollution due to
-the combustion of the oil in the gas, this is what makes the
"blue smoke"
-each time a fresh mix of fuel/air is loaded into the combustion
chamber part of it leaks out Advantages of the 4 stroke engine Disadvantages of the 4 stroke engine -four stroke engines produce more torque at idling speed

-four stroke engines have valves stopping fresh gasses from escaping

-four stroke engines have a dedicated lubrication system typically making them last longer

-four stroke engines put out less emissions because of the separate lubrication system(no oil is being burned) -four strokes have a lower power to weight ratio compared to a two stroke

-four stroke engines have more moving parts so routine maintenance and overhaul costs are more

- four stroke engines have less strokes per cycle gerenerally creating lower rpm and less power http://en.wikipedia.org/wiki/File:Arbeitsweise_Zweitakt.gif
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