2 stroke question

[kawikiller]

I understand how a 2 stroke the piston moves twice where as in the 4 stroke it moves 4 times. Question is why do you have to put oil in the gas?

 

[76GMC1500]

2-stroke engines don't have a stroke dedicated to drawing in an intake charge and pushing out an exhaust charge. Both processes have to be done simultaneously in a very short period of time in the 2-stroke. In order for air to flow in to the cylinder and push the exhaust out, the intake charge has to be at a higher pressure than the exhaust. When intake air charges are greater than athmospheric pressure, they are referred to as supercharged. All 2-stroke engines are supercharged. The method of supercharging is why you have to mix the oil with your gas. 2-stroke gas engines mostly use underpiston supercharging where the underside of the piston and crankcase work like an air compressor. Because the air and fuel move through the crankcase, they can not have an oiling system with a sump like a 4-stroke because all of the oil would be carried into the cylinder and consumed. The oil is mixed with the gas because the gas moves through the crankcase and the oil lubricates the bearings. Not all 2-stroke engines require oil to be mixed with their fuel. If you use an external supercharger like a roots blower, the engine can have a self-contained oil sump exactly like a 4-stroke. Detroit Diesel, EMD, etc... are all examples of 2-stroke engines that have external roots blowers and 4-stroke like oil sumps.

 

[kawikiller]

Thank you :)

 

[john stu]

the real answer is because two strokes are cool

 

[76GMC1500]

Depends on what part you look at. The pistons/cylinder run hot because the engine fires every revolution, but the exhaust is cool because the engine blows so much fuel and air out of the exhaust port.

 

[Grinning Reaper]

wow nice answer GMC. very detailed. thanks