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