Common rail engine delivers power, less emissions at a price

Sep 28, 2015 by Capt. Jeff Werner

Diesel engines aboard yachts have featured common rail fuel injection since the late 1990s, but common rail engines are not new to marine applications. In Great Britain in 1916, Vickers built a submarine with a common rail diesel engine that used mechanical pumps controlled by engine rotation to deliver fuel at 3,000 pounds per square inch (psi), which is the same pressure power washers use today to do a bottom cleaning during haulouts.

Now, new common rail engines have an electronic control unit (ECU) that governs a high pressure pump that produces a fuel pressure of 36,000 psi, a 12-fold increase from 100 years ago.

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 to fuel to enter the combustion chamber from the injector.

In a common rail engine, an ECU-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. This design allows the fuel to be delivered to the injector at a constant pressure 100 percent of the time, regardless of the engine RPM.

The advantage of using the common rail is found in the extreme high pressure that the fuel is pushed through the injector with. According to MTU, which began offering common rail fuel injection as a standard feature in 1996, “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 less emissions per gallon of fuel.

It is this increase in efficiency and decrease in air pollution that has driven the rapid refinement of common rail engines in the past 20 years for marine applications. But it comes at a price.

“With the higher technical performance levels of the injection systems, the demands placed on the fuel in terms of purity and quality also rise,” says MTU. “Thus the fuel must comply with predefined values for viscosity and lubricity, as components of the high pressure pumps and injectors are lubricated by the fuel. It must also be free of any contamination that would lead to abrasive damage at the high pressures employed.”

Viscosity measures the fuel’s resistance to flow or fluid thickness, and is one of the most important physical properties of diesel fuel. The fuel’s viscosity index represents its change in viscosity with respect to changes in temperature. The viscosity index is determined by testing its flow rate at 40 degrees C and 100 degrees C.

The lubricity of diesel fuel can be problematic due to the regulatory requirements of removing sulfur from diesel during the refining process. This reduction of sulfur allows that fuel, when burned, meets emissions guidelines. However, since sulfur is a natural lubricant in petroleum, removing it takes away diesel’s innate ability to lubricate the high pressure engine parts. This loss of lubricity can be compensated for with fuel additives or special coatings on the injection system.

Particulate contamination, which leads to abrasion, can be tested with a portable particle counter and compared with standards set for cleanliness. Many engine manufacturers state that for high-pressure engines, a particle count rating of 18/16/13 should be considered the maximum allowable.

What this rating translates into is that in every one milliliter of fuel, there can be no more than 2,500 particles larger than four microns in size, no more than 640 particles larger than six microns and no more than 80 particles larger than 14 microns in diameter.

What does this all mean for an engineer aboard a yacht? That fuel must be tested on a regular basis as part of the yacht’s fuel preventive maintenance program to assure that the fuel feeding a common rail engine meets the engine manufacturer’s specifications.

And considering that most fuel filters mounted on diesel engines can only trap particles greater than 10 to 12 microns, it is imperative that a yacht polish its fuel through a multi-stage filtration system designed to perform diesel dialysis down to three microns, on a regularly scheduled basis.

The risk of not doing this is not worth the cost of installing a new set of high performance common rail injectors.

Capt. Jeff Werner has been in the yachting industry for almost 25 years. Contact him through MyDieselDoctor.com.

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