Diesels for today and tomorrow

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The diesel’s safety, fuel efficiency and durability have made it the power of choice for marine applications ranging from the 0.318-liter, single-cylinder, 9-hp engine in a small sailboat to the 25,480-liter, 14-cylinder, 108,920-hp engine in the world’s largest container ship.

The diesel’s safety, fuel efficiency and durability have made it the power of choice for marine applications ranging from the 0.318-liter, single-cylinder, 9-hp engine in a small sailboat to the 25,480-liter, 14-cylinder, 108,920-hp engine in the world’s largest container ship.

While there are many differences between the engines we use in recreational and small commercial vessels and those that propel the world’s largest vessels, the essential aspects of diesel engine operation are common from the smallest to the monster noted above.

All diesels are “excess air,” compression ignition engines. Even under full load, the fuel-to-air mixture in the diesel is very lean, well below a stoichiometric mixture. The fuel to be burned in the diesel engine’s cylinder is injected only after the inducted air charge has been compressed, heating it to more than 700 F. The efficiency of the diesel combustion process contributes to the virtual absence of carbon monoxide in the exhaust; everything that can be burned has been burned.

The fuel injection system is the key element in the diesel engine. Unlike a fuel injected gasoline engine, where the atomized fuel spray is injected into a relatively unpressurized intake manifold at a pressure usually less than 6.8 bar (100 pounds per square inch), the diesel injector delivers the fuel directly into the combustion chamber and must deal with temperatures that can exceed 1,300 degrees and pressures measured in hundreds of bar (1 bar = normal atmospheric pressure = 14.50377 psi, 29.53 inches of mercury, 100 kilopascal).

The first practical mass production fuel injection systems were introduced by Bosch in 1927 and remained in essentially the same mechanical form until fairly recently (and continue in use today in many marine engines less than 50 hp). Most of these injection systems use a high-pressure fuel injection pump assembly that contains a variable-volume piston pump for each cylinder. Steel capillary lines carry the fuel from the pump to each injector. A pressure-operated valve in the injector is forced open to begin the injection process when the rapidly rising fuel pressure reaches the valve’s pressure setting. The tip of the injector is designed to produce the fuel aerosol required to achieve optimum combustion. The sudden delivery of a substantial amount of fuel at the beginning of the injection process causes a very rapid increase in the combustion gas pressure in the cylinder, creating the characteristic sharp metallic “rap” sound of an idling diesel engine.

Emission standards push technology

This elegantly simple, extremely reliable, easy to understand and maintain system might still be the gold standard for virtually all diesel engines today were it not for the worldwide imposition of increasingly stringent exhaust emission standards. The new standards could not be achieved using the traditional technology.

The first U.S. federal emissions standards (Tier 1) for new non-road diesel engines (which includes marine applications) were phased in for engines more than 37 kW (50 hp) from 1996 to 2000. The Tier 1 standards were progressively replaced by the more stringent requirements known as Tier 2 and Tier 3, with phase-in between 2000 and 2008. Tier 4 standards that require a further reduction in particulate and NOx emissions will come into effect between 2008 and 2015.

At the same time the allowable sulfur content of diesel fuel for marine use will be reduced to 500 parts per million as of this June, with a further reduction to not more than 15 ppm by June 2012. However, the diesel fuel dispensed at gas stations already is ULSD (less than 15 ppm sulfur), and the same fuel supply serves many marinas.

Therefore, many of our engines will have to deal with this new fuel. The sulfur content of diesel fuel can affect fuel injection equipment since the process of removing sulfur from the fuel also can reduce the lubricity of the fuel, although fuel producers are assuring users that additives are being used to restore lost lubricity to ensure safe operation.

The challenge of cleaning up the exhaust emissions of diesel engines necessarily falls most heavily on the fuel injection system. A number of parallel improvements were used to achieve the excellent results that have been attained, including the most visible: idle noise low enough to be mistaken for a well-mannered gasoline engine, unusually low and stable idle speed and excellent acceleration, and banishment of black smoke from the exhaust and with it the “smile” on the transom that used to identify diesel powered boats. The new fuel injection systems have to be computer controlled if they are to achieve the mandated emission limits. Now well-developed and highly reliable, the computers make it easy to continually record and store engine performance data and make troubleshooting more precise, effective and economical.

Benefits outweigh drawbacks

We gained a lot but did give up some things. While an “old” diesel would continue to run with the battery disconnected, the new electronically controlled engines must have a supply of electrons. A technician could service the old injection system with mechanical tools; the new systems want to be addressed with the help of a computer. That said, the benefits far outweigh the detriments.

Cleaning up a diesel’s emissions, specifically reducing the amount of particulates and oxides of nitrogen in the exhaust, was addressed by improving the way fuel is burned in the cylinder, beginning with an idling engine and extending to full-power operation. One key to achieving the emission goal is to optimize the way in which the fuel charge is burned. The new systems use injection pressures far above the few thousand pounds per square inch common in traditional injection systems. High-pressure injection in the range of 20,000 psi was experimented with in the early 1980s in an effort to produce the finest possible droplet size in the fuel aerosol. These high-injection pressure experiments have led to today’s fuel injection systems that operate at pressures in excess of 1,600 bar (24,000 psi). Some companies are planning injection systems that operate at 2,000 bar.

The increased injection pressure aids in achieving the smallest possible fuel droplet size; however, changes in the fuel injector were required to achieve the mandated emission limits. For example, vaporization of even a minute amount of fuel remaining at the tip of a fuel injector at the end of the injection cycle can increase the hydrocarbon emission level, therefore the amount of residual fuel must be kept to a minimum.

A great deal of effort has been devoted to perfecting the nozzle designs needed to match different engine applications. Although the terms may be abstract without a substantial amount of additional information, two of the designs are “blind-hole” nozzles and “Sac-less” nozzles. The hole referred to in the former type is microscopic in diameter and runs from the interior of the injector, just below the sealing valve, to the exterior face of the injector’s nozzle. The amount of fuel that can be contained in this volume is truly minute. However, for some engines even this amount is unacceptably large. In the Sac-less injector design the fuel delivery hole from the valve seat to the exterior of the nozzle is much shorter and has a much smaller volume, thereby limiting the amount of fuel that can remain after the injector’s valve closes.

In addition to optimizing the atomization of fuel as it is injected into the cylinder, engineers had to develop technology that would ensure that the fuel burned evenly and completely during each combustion cycle. The designers also were interested in reducing the noise created by the very rapid pressure rise and high peak cylinder pressures that are typical of engines using conventional injectors. A mechanical, two-stage injector was developed that initially delivers a small amount of fuel, enough to “get the fire going,” followed a bit later by the remainder of the fuel intended for that power stroke. These injectors are now common in small diesel engines, including the high-speed engines used in vehicles that are often marinized for small-boat use.

Precise fuel control

The introduction of electromagnetic control of fuel injection marked a major advance in diesel engine emission control. Controlled by the engine’s computer, the injector cycle begins with the introduction of a very small amount of fuel into the cylinder, starting the combustion process without creating the very rapid-pressure increase that occurs with a conventional injection system. The remainder of the fuel required to achieve the desired power output is injected shortly thereafter. The result is a smooth increase in cylinder pressure accompanied by very complete combustion of the fuel charge. A “third generation” injector now entering the market replaces the electromagnetic fuel control valve in some systems with a faster-acting (approximately twice as fast) piezoelectric element, allowing even more precise control of the injection process.

Most of the new generation engines use a single high-pressure fuel pump to feed a pipe-like reservoir, the “common rail,” from which fuel is delivered to the individual injectors. The fuel pressure in the common rail is carefully controlled and monitored to ensure that the high-pressure pump is immediately disabled on detection of a pressure drop indicative of a fuel leak. A spray of pressurized diesel fuel can be highly combustible; a spray at the 20,000-plus psi pressure common to these engines also can cause serious damage to flesh. (Similar pressures are used for cutting materials in industrial processes.) For these reasons the injection system components in common rail engines usually are protected from accidental damage by safety covers.

Distributing highly pressurized fuel to the injectors in large engines presents mechanical challenges. Unit injectors — devices that contain individual camshaft-operated high-pressure pumps — eliminate the need for long high-pressure lines. The fuel supply plumbing need deal only with low pressures. Electromagnetically controlled unit injectors and unit injectors using the more advanced piezoelectric technology are being used on new very-high injection pressure computer controlled engines, including the Volkswagen V10 TDI, the most powerful automobile diesel.

Some larger engines, such as the Caterpillar 3412E, use hydraulic-electric unit injectors (HEUI). As with other unit injector systems, the HEUI injectors do not require a central high-pressure diesel fuel pump and common rail. Each injector creates the required fuel injection pressure by applying moderate pressure (800 to 3,300 psi) lubricating oil to a hydraulic pressure booster device housed within the injector body. The difference in the areas of the input and output sides of the booster’s piston creates the 20,000-plus bar pressure needed at the injector nozzle. The injector is electrically controlled similarly to the injector in a common rail system. The HEUI injector’s most significant advantage is its ability to separate the rate at which fuel is injected from the amount of fuel injected.

The effort to “clean up” diesel engines is far from complete. The emissions standards already on the books define the work that must be done between now and 2015, and there is no reason to think that these standards will not undergo further tightening.

Chuck Husick, an electronics engineer who runs his own consultancy in the marine and aviation fields, teaches a diesel engine seminar for boaters. A former chairman and president of Chris-Craft, he holds a Coast Guard 100 ton license, sails a 46-foot Irwin ketch and is a commercial pilot/flight/instrument instructor.