Gasoline powered outboard engines all depend on a sequential combustion cycle to operate. There are two possible combustion cycles used with outboards, four-cycle and two-cycle. The terms cycle and stroke are synonymous, two-cycle and two-stroke describes the same process. Up until only a few years ago, two-stroke outboard engines were the only option available to boaters. While gelcoat repairs on the boat you always focus on something hence, you strive for bringing perfection in the repairing task. Improvements in technology have brought four-stroke engines into the boating realm and two-strokes have seen substantial improvements as well.
Conventional Carbureted Two-Stroke Marine Engines
Conventional two-stroke outboard engines use a carburetor to deliver a mixture of gasoline, air and lubricating oil into the motor’s cylinder. Two-stroke engines depend on the oil added to the fuel to lubricate internal moving parts. In a conventional two-stroke engine, a complete combustion cycle takes one revolution of the crankshaft. The rotating crankshaft is what eventually drives the propeller.
The first half of the two-stroke cycle sees the piston moving down and drawing the fuel/oil/air mixture in the cylinder. The second combustion cycle compresses and burns the mixture introduced earlier into the cylinder. There are two ports in the cylinder of a two-stroke engine that allow for the intake of fuel mixture and exhaust of burned gasses.
Conventional two-stroke engines waste fuel because the exhaust port remains partially open as the piston begins its down stroke and the next fuel-air charge is drawn into the engine. A least 30% of the fuel consumed by one of these outboards motor is wasted and pumped into the marine environment.
Low Pollution 4-Stroke Marine Engines
Four-stroke outboards use the same combustion process used in automotive engines that burn gasoline and are the cleanest outboards to operate. Unlike their two-stroke cousins, four-strokes never have an exhaust and intake valve open at the same time, which prevents unburned fuel from being ejected from the engine. These engines run on straight gasoline and don’t require additional lubricating oil to operate.
The first four-stroke engines available to boaters were small and it has taken few years for the technology to develop to the point were these outboards could be produced at higher horsepower levels and not be too heavy for use on a boat. Four-stroke engines are available in ratings of up to 225 horsepower.
Low Pollution Direct Fuel Injection Two-Stroke Marine Engines
A new alternative has been developed for the carbureted two-stroke outboard engine. The technology is known as direct fuel injection (DFI). The required fuel mixture is injected directly into the cylinder after the piston passes the exhaust port, preventing unburned fuel from being prematurely forced out of the engine.
Use of DFI takes advantage of a two-stroke engines efficient power cycle and lighter weight, while greatly lowering pollution levels generated by these outboards.
It is good for the boating vocation to see improvements and alternatives being developed for carbureted two-stroke outboards. There are hundreds of thousands of these engines currently being used by boaters worldwide. They will be in use for many more years. One additional way to improve this situation is to use a biodegradable two-stroke oil like the product marketed as Pure Oceans by West Marine