by Conrad & Judy Kreuter
Dear Boat Talk: During my annual visit to the local boat shows I noticed many engine manufacturers now have “4-stroke” outboard engines as well as fuel injected motors. What are these and how are they different from older engines? MB, Coram, NY.
Dear MB: The majority of outboard motors since their inception have employed “2-stroke” and carburetor technology. “2-stroke” engines get their name from the fact that power is produced on each revolution of the piston within the cylinder. On the upward stroke of the piston fresh gas, air, and lubricating oil enter the cylinder while exhaust gases are removed. As the piston continues upward the fuel/air charge is compressed and ignited at or near the top of the piston travel by the spark plug thereby producing an explosion of power. This pushes the piston downward within the cylinder. One upward and one downward stroke of the piston produces the term “2-stroke” engine.
One of the major disadvantages of the “2-stroke” engine is that the fresh gas and air mixture is entering the cylinder while the exhaust gases are being released. This allows a certain amount of the unburned gases to escape into the atmosphere. Unburned gases and other contaminates such as carbon monoxide, etc., (from all internal combustion engines) are a major cause of air pollution.
“4-stroke” engines, which are very similar to your automobile engine, use valves to allow the fresh fuel mixture to enter the combustion chamber and another set of valves to allow the exhaust gases to evacuate the cylinder. On the first upward stroke of the piston, air and fuel have entered the cylinder via the opened intake valve from the previous cycle while the exhaust valve is closed. The upward travel of the piston compresses the air/fuel mixture. At or near the top of the piston’s travel, the spark plug fires igniting an explosion which produces power pushing the piston downward. On its next trip upward the piston pushes the burned gases out the opened exhaust valve while the intake valve remains closed. On the next downward stroke of the piston the opened intake valve allows the fresh charge of gas and air to enter the cylinder. The cycle then repeats. It takes 4 strokes of the piston to produce a complete cycle of operation hence the term “4-stroke”.
In the outboard world, the 4-stroke engine tends to be slightly heavier than its 2-stroke counterpart. Additionally, the 4-stroke does not burn any oil. Oil is contained within an oil reservoir and used only for lubrication purposes. 4-strokes also offer much quieter operation than the 2-stroke.
Fuel injection refers to the method by which the air/fuel mixture enters the cylinders. The older designs rely upon carburetors which mix the air and gasoline together with the use of jets to meter either the air or gas.
The newest fuel injection systems use separate injectors which allow precise amounts of fuel to enter the cylinder along with air. These systems rely upon a computer controller to monitor the speed, temperature, and load on the engine to determine exact amount of fuel required. This generally results in a cleaner running engine since there is no excess fuel in the cylinder.