Outboards - 2 Stroke or 4 Stroke?

Post by: TheYachtMarket News
26 February 2008

There are two types of outboards available, 2 stroke and 4 stroke. Each has its good and bad points, depending on what you want the engine to do. We've put together a handy guide to help you decide which is the best outboard motor for your 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. They are also more affordable and have a higher resale value. 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 outboards are not as environmentally friendly as 4 strokes as they produce far more pollution. 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.

Manufacturers of 2 stroke outboards include Evinrude, Yamaha, Nissan and Mercury.

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. You are more likely to use a 4 stroke engine if you are interested in a smoother and quieter ride.

4 stroke motors have more complex engines which could lead to more repairs. 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.

Manufacturers of 4 stroke outboards include Johnson, Honda, Mercury, Nissan, Suzuki, Tohatsu and Yamaha.

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 is sucked in.
  • Compression stroke: As the piston moves back up, it blocks of 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.