2 stroke outboard Vrs. 4 stroke outboard

While the four-stroke has to make two  complete revolutions to produce a power  stroke the two-stroke engine only has to  rotate once this gives the two-stroke  engine and advantage in acceleration and  throttle response two-stroke engines are  much simpler than four strokes reed  valves are small and lightweight they  don't need any mechanical linkage to  make them open and close since the  pressure inside the crankcase does that  quite well on four-stroke engines.


The  valves are held closed by springs a  camshaft driven by the crankshaft opens  the valves at the correct time all of  these parts add weight and take up space  so the two-stroke enjoys a significant  advantage when you need a lot of power  in a small lightweight package however  the four-stroke theoretically has the  edge and fuel economy the two-stroke is  hampered because the intake and exhaust  ports are open at the same time.

This  process is called scavenging and we'll  come back to it in a moment the problem  is that some of the exhaust gases get  left behind in the cylinder while some  of the air fuel mix right through the  cylinder into the exhaust system without  ever being burned this wastes fuel and  increases hydrocarbon emissions on a  four-stroke the valves prevent the air  fuel mix from escaping out the exhaust  so the engine runs cleaner and uses less  fuel .

However there's a better solution  for two strokes Evinrude x' direct  injection these models don't have a  carburetor instead of an air fuel mix  they cycle only air through the  crankcase and fuel is injected directly  into the cylinder the injection is  precisely timed so none of the fuel  escapes out the exhaust port this  dramatically increases fuel economy and  reduces emissions direct injection  models enjoy all the performance  advantages of two-stroke engines  yet they are equal to or superior in  fuel economy and emissions when compared  to many for strokes