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When the author was down in New Zealand trying to figure out why his little Seagull wasn’t working he asked a fellow cruiser how the internal combustion engine worked. His reply was Suck, Squeeze, Bang and Blow, then he went on to explain what he meant by that phrase.The internal four-cycle combustion engine requires that the piston travel up and down the cylinder four times in order to deliver one power stroke. One of the downstrokes sucks in a mixture of fuel and air (intake). On the following up stroke the piston squeezes this mixture (compression). When the piston is at the top of the cylinder the ignition delivers the bang that is provided by the spark plug and drives the piston down (power). On the next up and final fourth stroke the piston blows the used mixture of fuel and air out of the cylinder through an exhaust port (exhaust). A two-stroke motor combines two of these functions into one stroke by making use of both the combustion side and the crankcase side of the piston. As the piston is moving up, it sucks in on the crankcase side a mixture of fuel and air from the carburettor. The lubrication of the crankcase is provided by the oil that was previously mixed into the fuel and not by an oil sump. Most two-stroke engines have a reed valve, a one way valve, which stops some of the oil and exhaust from escaping from the crankcase side of the piston but allows the new mixture of fuel and air in. British Seagulls do not have this reed valve so they tend to produce a bluer exhaust. As the piston is still moving up, the combustion side of the piston squeezes the previous air and fuel intake. After the bang that was caused by the spark plug on the combustion side, the piston will move down delivering the power stroke. As the piston continues to move down, it will first open the exhaust port to blow the exhaust out. A little further down the piston will open the intake port allowing a new mixture of fuel and air into the combustion side of the piston. The unique design of the domed or slanted two-stroke piston allows this incoming mixture to be first deflected upwards and then downwards towards the exhaust port to help ‘sweep’ the cylinder clean of spent exhaust. In conclusion the two-stroke engine combines the Intake/Compression into the up stroke and the Power/Exhaust into the down stroke. The piston itself controls the flow of the air/fuel mix and the exhaust by the way of the intake and exhaust ports thus eliminating valves and cams. To this day when trying to explain or trouble shoot an engine these are four things I look for as each one deals with a certain part of the engine.

Function System Area Possible Remedy
Suck Intake Carburettor Check fuel and air supply
Squeeze Compression Piston and rings Compression can be measured
Bang Ignition Points, spark plug Timing, points gap
Blow Exhaust Height of engine

 

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