by Conrad & Judy Kreuter
Q: Dear Boat Talk: I am looking to re-power my outboard boat. I have heard a lot about four stroke outboards. Can you explain why I should consider installing one on my boat? E.D., Brooklyn NY.
A: Dear E.D.: Four stroke outboards are very similar to your automobile engine. A four-stroke engine has, as its name implies, four distinct operations to produce one power stroke. The first is an intake stroke where fuel and air are introduced into the combustion chamber or cylinder on the downward movement of the piston. The piston travels up compressing the fuel air charge. This is the second stroke. At the proper time when the piston nears the top of its travel, a spark ignites the compressed fuel air charge, which explodes producing a power stroke forcing the piston downward. This is the third stroke. The piston again travels upward expelling the burnt fuel air charge. This is the fourth stroke or exhaust stroke. This process continues until the engine is shut off.
In the two-stroke engine, the downward movement of the piston uncovers an intake port, which allows a fresh fuel air charge into the combustion chamber. The piston travels upward compressing the fuel air mixture. Near the top of its travel, a spark occurs producing a power stroke forcing the piston downward. As the piston travels downward an exhaust port is uncovered allowing the burnt fuel air mixture to exit while the intake port is allowing the fresh fuel air charge to enter.
The operation of the two-stroke engine allows unburned gas and air to escape into the environment because the intake and exhaust ports are opened simultaneously. The four-stroke engine has distinct intake and exhaust cycles, which does not allow unburned fuel to escape into the atmosphere. Additionally noise is reduced in the four-stroke engine because a power stroke is produced half as many times as a two stoke and the intake and exhaust ports are closed which reduces the level of noise the engine makes. This is why the four-stroke engine is a cleaner operating, environmentally and accquistically friendly engine.
Many of the new four-stroke engines come with computer controlled electronic fuel injection. Electronic sensors measure and monitor such functions as air temperature, barometric pressure, cylinder wall operating temperature, and RPM. This system allows precise fuel air measurement thereby increasing fuel economy and further reducing exhaust emissions and insures the engine runs at peak performance under all conditions.
The traditional carbureted systems cannot approach the level of performance of the fuel injection system. They use jets to meter the air fuel mixture and have no sensors to adjust for differing conditions.
Four-stroke engines develop more torque, which is a measure of work performed, lower in the RPM curve than an equivalent two-stroke engine. For the practical application on your boat, this means you can use a four-stroke engine of less horsepower and still achieve similar performance. This may translate to faster out of the hole and planing capability.
The 2002 model year will be a very exciting year for four-stroke technology. With the introduction of the 200 and 225 HP as well as midrange 140HP four-stroke models, the boat owner now has a full range of product to fit almost any application. These new motors also have new technologies, which make them comparable in size and weight to their two-stroke counterparts.