Two and Four Stroke Outboard Engines

!±8± Two and Four Stroke Outboard Engines

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. 2 stroke outboards have been in existence for about 80 years so spares are easily found, making repairs less troublesome. 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 repairs are needed they tend to be more expensive and spare parts can be harder to source.

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. Less pollution, reduced noise, better fuel economy and lower running costs are amongst the advantages.

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.

Two and Four Stroke Outboard Engines

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Outboard Engines - The Basics

!±8± Outboard Engines - The Basics

Basic Two-Stroke and Four-Stroke Cycles

Two-stroke outboards are still around, but more and more four-stroke outboard engines are appearing. Suzuki outboards are amongst the market leaders with a huge range of four stroke outboard motors. Each kind has its advantages and disadvantages but the four-stroke, with its leaner exhaust emissions, will set the standard for the future. In any case, to be an engine troubleshooter you need to know how both kinds of engine work.

Remember the four basic needs: Whether it's two-stroke or four-stroke outboard engines, they need a fuel/air mixture delivered to the cylinder; it needs to compress it; the mixture must somehow be ignited; and, when it's done burning, the exhaust gases must be removed. This process occurs repeatedly as the engine runs, and the order in which these things happen must be exact to within a fraction of a second.

Four-Stroke outboard motors

It may help to think of an internal combustion engine as a pump of sorts.

Let's take a look at how the four-stroke works:

Stroke 1-The piston travels down the cylinder, drawing the air-fuel mixture in behind it.

Stroke 2-On its way up again, the piston compresses this mixture to help make a more efficient explosion. (Incidentally, the piston rings stop the mixture from leaking out of the slight gap at the sides.)

Stroke 3-When it's nicely compressed this mixture is ignited and the resulting explosion forces the piston down the cylinder. The piston has a connecting rod attached to an offset on the crankshaft, and that forces the crankshaft to turn.

Stroke 4-So, finally, while the piston is being pushed back to its starting position by the revolving crankshaft, it's also pushing out the burned gases and preparing to start the cycle all over again. Thus, the engine has converted the reciprocating (back-and-forth) motion of the piston into something more useful for driving a propeller: rotary motion.

Two-Stroke outboard engines.

The significant difference here is the absence of conventional intake and exhaust valves. In fact, there are valves in a purely technical sense: The moving pistons alternately reveal and cover up ports (holes) in the cylinder wall, thus acting as valves. The ports are the means of gas entry and exit from the combustion chamber. The other big difference is that two-stroke outboard engines actually combine, the intake and compression strokes into one operation, and the power and exhaust strokes into another. As we've already seen, this enables it to complete all four necessary functions intake, compression, ignition, and exhaust, with just two strokes of the piston in its cylinder, and only one revolution of the crankshaft instead of two.

Two-stroke engines don't have crankcase oil. In addition, a four-stroke engine's valves will need adjustment now and then. A two-stroke gets along fine without valves-at the cost of reduced efficiency and increased pollution.

Suzuki Outboards

Suzuki have been so confident in their product that they have been offering 3 year warranties.

Outboard Engines - The Basics

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