The Triton

Engine Room

Diesel Digest: A primer on compression ignition engines

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Diesel Digest: by Capt. Jeff Werner

Rudolf Diesel envisioned a new type of engine, one that would replace the available steam and gasoline engines. In 1895, he perfected the breakthrough in technology he hoped for. Diesel’s invention, at first called a compression-ignition engine, eventually bore his name. Diesel engines now power just about every motoryacht and auxiliary engine on sailing vessels, as well as marine generators.

A yacht’s diesel engine works 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.

Intake stroke: During the intake stroke, as the piston moves down to the bottom of the cylinder, air enters the cylinder.

Compression stroke: As the piston moves back up to the top of the cylinder, the air in the cylinder gets squeezed together. Once the air becomes 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.

Power stroke: The explosion and expanding gases within the cylinders 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.

Exhaust stroke: As the exhaust valve is opened and the piston rises once again, the burned gases are pushed out of the cylinder. On a modern-day yacht, the exhaust gases exit via the exhaust manifold and are mixed with cooling seawater at the exhaust elbow and then discharged into the water. Many older motoryachts had a dry stack exhaust, which allowed the engine exhaust to exit via a smoke stack.

Throughout the last hundred years, engine designers have worked to enhance the efficiency and power output of diesel engines. Turbochargers and advanced fuel injection systems have been the primary methods of achieving these advances.

Turbocharging: Diesel’s original engine design called for air to be sucked into the cylinder during the intake stroke. This is known as a naturally aspirated engine. Engineers soon discovered that the power output of diesel engines is limited by the amount of air they take in. Therefore, if a greater amount of air could be forced into the engine during the intake stroke, the engine could do more work with better fuel economy. A turbocharger uses this idea of forced air induction by employing a compressor to raise the pressure, and therefore, the density of the air entering the engine. In short, that means a greater mass of oxygen enters each cylinder. Turbochargers use a small exhaust gas driven turbine to turn a small compressor to pressurize the intake air. Thus, the energy for compressing the intake air is taken from the exhaust, which is a novel bit of scavenging.

Common rail injection system A conventional diesel engine uses a gear-driven pump to send fuel directly to each injector through a series of metal tubes. When the fuel pressure is high enough, a valve opens and allows the fuel to enter the combustion chamber from the injector.

In a common rail engine, an electronically controlled pump feeds a single large diameter tube, or “common rail,” rather than an injector. This rail serves as a reservoir that accumulates the fuel at high pressure, and then distributes this fuel to each injector through a smaller tube.

The advantage of using the common rail is found in the extreme high pressure with which  the fuel is pushed through the injector. According to engine research by MTU: “The higher the injection pressure, the better the fuel atomizes during injection and mixes with the oxygen in the cylinder. This results in a virtually complete combustion of the fuel with high energy conversion during which only minimal amounts of particulates are formed.” In other words, there is a higher power output with fewer emissions per gallon of fuel.

However the performance of even the most advanced diesel design is only as good as the fuel being fed to it. With modern marine diesel engines, it is imperative that diesel fuel is kept scrupulously clean and within the specifications of the engine manufacturer. Periodic fuel sampling, testing, polishing and tank cleaning are the cornerstones of fuel preventive maintenance. A successful fuel maintenance program aboard a yacht is the simplest method to keep diesel engines running at peak efficiency. And that efficiency translates into more sea miles per gallon.

Capt. Jeff Werner is a 25-year veteran of the yachting industry as a captain on private and charter yachts, both sail and power, and a certified instructor for the RYA, MCA, USCG and US Sailing. He also owns Diesel Doctor (MyDieselDoctor.com). Comments are welcome below.

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