Diesel Engine

This burns a less refined fuel product and is fired by compression rather than a spark.

Transmission

This transmits the power from the engine into the drive line, in this case a propeller shaft.

How They Work

Diesels fire under compression. This is to say that the piston squeezes the fuel fast enough into a small enough space, into the combustion chamber, that the fuel ignites. An explosive detonation from the friction of fuel subjected to rapid compression.

Most diesels are four stroke engines. Each up strike or down stroke movement is a stroke. In a four stroke engine a firing cycle takes two up and down movements.

The firing cycle on a four stroke goes like this:

1. Down Stroke

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   Fuel is drawn into the combustion chamber. This is the intake stroke. The intake valves are open.

2. Up Stroke

   Fuel is compressed and heated (by friction), which causes detonation of the fuel. This is the compression stroke. All valves are closed.

3. Down Stroke

   The explosion of the fuel forces the piston down, this is the “power stroke”. All valves are closed.

4. Up Stroke

   The burned fuel (exhaust) is pressed out of the cylinder. This is the exhaust stroke. The exhaust valves are open.

Then it all starts again. A rarer diesel is a two stroke, however most small engines like your outboard, are two stroke engines. A two stroke draws in fuel on the power stroke and exhausts at the end of the compression stroke.

Keeping in mind how the strokes of the engine work, generally, the inner/outer workings of the engine are as follows:

• The pistons moving up and down in the cylinders turn the crank. The crank is along the center line of the engine, down low.
• The crank turns the flywheel, a chain and often a belt.
• The flywheel turns in line with the transmission.
• The transmission turns the propeller shaft.
• The propeller shaft turns the propeller.
• The chain (often called the timing chain) turns the offset camshaft.
• The camshaft pushes on the (in order) pushrod, rocker, lifter which moves the valves open and closed as the camshaft rotates.
• The valves allow in fuel and allow out exhaust as they move up and down.
• The belt turns the off line water pump.
• The water pump cools the engine.
• The belt turns the off line alternator.
• The alternator makes electricity.