How Does an Outboard Engine Work? 

An outboard motor works a lot like a car engine, but with design modifications to adapt to a marine environment and meet the unique challenges of vertical mounting.

The inner workings vary depending on what type of outboard you get, but the basics are more or less the same. Here's a guide to how an outboard engine works.

Two Stroke or Four Stroke?

A two stroke outboard differs from a four stroke in how it operates internally. Designers might choose a two-stroke configuration to save weight in a low-power outboard, but they’ll usually opt for a four-stroke design in a larger motor. Four strokes are more fuel efficient, produce less exhaust, and generally require less maintenance than two-stroke motors.

A two-stroke engine utilizes two strokes of the piston inside the cylinder to complete one cycle. When the piston is moving down, away from the top of the cylinder, that’s the intake and exhaust stroke. As the piston moves down, it exposes an exhaust port that allows exhaust gasses to escape. This creates a low pressure inside the cylinder that allows air and fuel to enter the cylinder through an intake port. A second stroke then moves the piston toward the top of the cylinder. At this point, the fuel and air are compressed, the piston closes off the intake and exhaust ports, and the fuel and air are ignited. That little explosion pushes the piston back down and the cycle starts all over again.

Intake, compression, combustion, and exhaust on a four-stroke happen during its own movement of the piston. There’s an intake stroke where the piston moves down while the intake valve is open, pulling in fuel and air. Then, as the piston moves up with the valves closed, it compresses or charges the fuel-air mix. Ignition happens at the top of that stroke, and the piston gets pushed down again. This is the power stroke. Finally, the piston is pushed up again with the exhaust valve open to let the exhaust out.

This process in an outboard doesn’t differ much from what you might find in any internal combustion engine. The pistons turn a crankshaft for the rotational movement that ultimately drives the propeller. But there’s a challenge: the motor is mounted vertically, and that rotational power needs to be turned 90 degrees so that it’s parallel to the water’s surface. This happens below the engine.

Lower Unit 

The midsection of the engine primarily offers an enclosure for the drive shaft that is headed toward the lower unit. But first, it will pass through the water pump at the top of the lower unit. 

The outboard pulls water from the lower unit and pushes it up into the engine to keep the engine cool. This water pump contains a rubber impeller that draws water in and pushes it up through the midsection to the engine itself. The top of the lower unit also features a shift shaft, which allows the operator of the boat to switch between neutral, forward, and reverse.

The shift shaft is connected to a clutch dog in the lower unit, and this moves back and forth to engage with the forward and reverse pinion gears. Six fingers line up with the forward gear and another six line up with the reverse gear. When they engage, it drives that gear, which is connected to the prop shaft, which in turn rotates the propeller and pushes the boat. 

There are countless variants of this basic design: more or less horsepower; two- or four-stroke; diesel or gasoline. Each has specific design characteristics that help it work best in specific conditions. 

Problems with outboard engines often happen in lower unit components. Watch the video above to learn more about how to troubleshoot the lower unit of an outboard motor.