Today I would like to discuss the difference between a two stroke outboard verse a four stroke outboard. While the four stroke has to make two complete revolutions to produce a power stroke, the two stoke engine only has to rotate once. This gives the two stroke engine an advantage in acceleration and throttle response. Two stroke engines are much simpler than a four stroke engine. Reed valves are small and lightweight. They do not 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 on your boat. However, the four stroke theoretically has the edge on fuel economy. The two stroke is hampered because the intake and exhaust ports open at the same time. This process is called savaging. The problem is that some of the exhaust gases are left behind in the cylinder, while some of the air fuel mix gets blown right through the cylinder into the exhaust system without ever being burned. This wastes boat fuel, and increase hydrocarbon admissions.
On a four stroke, the valves prevent the air fuel mix from escaping out the exhaust so the boat engine runs cleaner and uses less fuel. However, there is a better solution for two stokes -- evinrude direct injection. These models do not have a carburetor. Instead of an air fuel mix they recycle only air through the crankcase, and fuel is injected directly into the cylinder. The injection is precisely times so none of the fuel escapes outside of the exhaust port. This dramatically increases fuel economy, and reduces admissions.
This concludes my boating tips for the week. Check http://http://bit.ly/1RdM5AN/ next week for more tips on boating.
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