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April 21, 2004

Yamaha Outboard Engine

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Which Outboard Engine - 4 Stroke or 2 Stroke?

Outboard engines fall into two main categories; 2 stroke and 4 stroke outboards. Both types have their advantages and disadvantages and the type you pick will depend on the intended use. This guide will help you choose which outboard engine is best for your type of boat.

2 Stroke Outboards

2 stroke engines are lighter and faster so when speed and acceleration are required, this would be the engine of choice. 2 strokes are also generally cheaper and keep their value better when it comes to resale. As 2 stroke engines have been around for almost 80 years, parts are readily available, making repairs easy. Also, the design on these engines is less complex than a 4 stroke, meaning problems are less likely to arise.

There are some disadvantages to the 2 stroke engine. 2 stroke engines are more pollutive than 4 stroke engines so they are not so kind on the environment. Their engines are lubricated by the oil that is mixed into the gasoline, meaning the exhaust is smoky and contains unburned oil, which in turn pollutes the water.

With new regulations, outboard manufacturers are looking to find new ways of producing the same power with less pollution.

Some 2 stroke outboard engine manufacturers include Mercury, Nissan, Yamaha and Evinrude.

4 Stroke Outboards

4 stroke engines are quieter, smoother and more economical. Heavier with slower acceleration, they are normally chosen for lakes and rivers. With less pollution and smoke they are also kinder to the environment. If you're after a quieter, smoother ride, then the 4 stroke outboard is the better choice.

4 stroke outboards are more complicated with more parts so they often require repairs more often. When a repair is required it is usually more expensive and parts are limited.

4 stroke outboards are generally heavier than 2 stroke outboards, so if you own a smaller boat it may not be able to hold the weight of a 4 stroke engine.

Because 4 stroke engines are newer, they are continually being improved to make purchase and repair easier.

Some 4 stroke outboard engine manufacturers include Yamaha, Tohatsu, Suzuki, Nissan, Mercury, Honda and Johnson.

Environmentally Friendly Outboards

If you're looking for a new outboard, it's worthwhile considering one of the newer, environmentally friendly models. As well as producing lower emissions, there are a number of other advantages such as better fuel economy, quieter running and reduced operating costs.

The Mechanical Differences Between 2 and 4 Stroke Outboards

The terms "2 stroke" and "4 stroke" refer to the number of times the piston moves during one firing cycle of the engine. In the 4 stroke engine, the four movements, or strokes, are:

Intake stroke: With the intake valve open, the piston moves down allowing a mixture of air and fuel to enter the combustion chamber.

Compression stroke: The piston moves up the cylinder with the valves closed, so that the air-fuel mixture is compressed.

Combustion stroke: With the valves still closed, the fuel is ignited by the spark plug. The explosion pushes the piston down again.

Exhaust stroke: The exhaust valve opens and the piston moves up to push the exhaust gasses out of the chamber so the process can be repeated.

In a 2 stroke engine, there are only two movements of the piston for each combustion; one up and one down:

Combustion stroke: Combustion of fuel drives the piston downwards. As the piston moves down, it exposes an exhaust vent and fuel inlet in the wall of the cylinder allowing exhaust gasses out, and new air and fuel to be sucked in.

Compression stroke: As the piston moves back up, it blocks off the exhaust vent and fuel inlet and thus compresses the air/fuel mixture. The spark plug ignites the air and fuel causing the piston to move down again.

The fuel inlet and the exhaust vent in the wall of the cylinder eliminate the need for valves and the mechanism for opening and closing them which is why 2 stroke engines are much simpler in design.

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