First step in good maintenance? Know how diesel engines work

May 20, 2016 by Capt. Jeff Werner

Diesel engines are the mainstay for both propulsion and generating electricity aboard yachts. Understanding how they work is the first step to knowing how to maintain diesel engines. Comparing the similarities and differences between the operation of a diesel engine aboard a yacht and the gasoline engine in an automobile is an easy way to grasp the fundamental concepts.

Gasoline and diesel engines are internal combustion engines. Engine combustion is just a controlled burning of gas or diesel within the engine cylinder. In order to have combustion, oxygen, fuel and heat must be present. For both types of engines, oxygen comes from the air, which is drawn through the engine air filter.

The fuel supplied from the tank is either gasoline or diesel oil, both of which are refined from petroleum. In a gasoline engine, the heat is supplied from the spark plug. It is the spark that ignites the air-gasoline mixture. In a diesel engine, the heat is generated by compressing the air in the engine cylinder. Compressing air generates heat as the molecules are pushed closer  together. It’s the same reason that dive tanks are bathed in cool water when an air compressor fills them. The heat of air compression warms up the tanks. A boat’s diesel engines and a car’s gas engine work on a four-stroke principle.

The four-stroke cycle is broken down into the intake stroke, the compression stroke, the power stroke and the exhaust stroke. These different strokes correlate to the movement up and down of the piston within the engine’s cylinder and the opening and closing of intake and exhaust valves. During the intake stroke, the piston moves down to the bottom.

In a gasoline engine, a precise mixture of air and gas enters through the open intake valves. In a diesel engine, only air enters the cylinder in this stroke. As the piston moves back up to the top, the air/fuel mixture or the air in the cylinder gets squeezed together. In a gasoline engine, at the time when the air/fuel mixture is fully compressed, the spark plug sparks and causes ignition.

In a diesel engine, when the air alone is fully compressed it reaches a temperature in excess of 1,000 degrees Fahrenheit. At that point, diesel fuel is sprayed into the cylinder by an injector and the fuel ignites spontaneously. With both types of engines, the explosion and expanding gases push the piston back down.

This converts the chemical energy of the fuel into the mechanical energy that provides the power to turn the crankshaft.

As the exhaust valve is opened and the piston rises once again, the burned gases are pushed out of the cylinder. In an automobile, those exhaust gases vent out through the tail pipe.

On a yacht, the exhaust gases exit via the exhaust manifold and are usually mixed with cooling seawater at the exhaust elbow and then discharged into the water. Gasoline engines use just enough air to burn all the fuel, while diesel engines always run lean which means they have more air than needed to burn the fuel. This allows diesel engines to be more efficient than gasoline engines in converting the energy of fuel into the work produced.

On the average, diesel engines are over 50 percent efficient, while gasoline engines are between 30 and 33 percent efficient. That’s why diesel engines have better fuel economy. In order to deliver that better fuel economy, diesel fuel must be kept scrupulously clean. At the end of the compression stroke, when the fuel is injected into the cylinder particulate matter in the diesel fuel can clog or damage the injector tip, which causes a poor spray pattern.

If the injector spray pattern does not meet the manufacturer’s specifications then the efficiency will decrease. Periodic fuel sampling, testing, polishing and cleaning are the cornerstones of fuel preventive maintenance. A successful program is the simplest method to keep engines running at peak efficiency. And as the cost of fuel begins to rise once again, that efficiency translates into more sea miles per gallon.


Capt. Jeff Werner has been in the yachting industry for 25 years, and is the owner of Diesel Doctor ( All Triton readers receive a 10 percent discount on online orders. Contact him at