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Efficiencies of a Genset

Apr 5, 2011

A right size genset can save fuel compared to using the main engine for electricity

Even the best-managed alternative energy battery-charging system will have a hard time keeping up with the power needs of a large sailing vessel. Pressurized hot and cold water, a high-volume watermaker, plus a bevy of navigation systems with color graphic displays, not to mention an entertainment system befitting a large yacht, all point to the necessity of an onboard generator in addition to the propulsion engine alternator, at least during peak usage.

Low-sulfur bio diesel and low-CO gasoline engines in some genset models will ease some of the guilt you may feel from not being a truly “green” voyager. In fact, your genset will be using less fuel to crank out more kilowatt hours per unit of fuel than your main engine easily qualifing the genset as a bona fide component of an alternative energy system, right along with the solar panels and wind generator.

Your first step in narrowing down the genset options is determining your requirement for power in kilowatts, the maximum genset size the vessel can accommodate and noise level. The industry gauges genset noise in decibels at seven meters with the sound cover fastened in place. To be sure, you will need to weigh a number of other factors as well to make the best choice.

Optimum power
“Sometimes folks come into my shop asking what size genset to buy,” said Don Peters of S&W Diesel, a Norpro genset distributor near the Port of Los Angeles. “I tell them, first do your homework. Assume you have everything on at once and see what it adds up to. That’s the minimum amount of power you need.”

Peters’ plainspoken advice will work in most situations. But what if the hole you have set aside for the genset is too small for the size and power rating of machine you have in mind? And what about load surges, especially from an old, worn-out motor in a refrigerator or watermaker, which can draw up to three times its rated power on start-up? You do not necessarily need to move up to a bigger, more powerful generator to cover such challenges.

There is also such a thing as too much power. Leif Johansen of Marine Diesel Engineering in Los Angeles pointed out the potential damage done to a genset if its output greatly exceeds a vessel’s amperage draw.

“When a genset has too light a load or no load,” Johansen explained, “it’s just like running your main engine out of gear. There is inadequate heat in the chambers to burn fuel properly. This causes carbon to build up and shorten engine life. So it’s important to select the generator you really need, not the biggest you can afford or squeeze into your boat.”

The conventional genset connects an engine and either a generator or an alternator through “closed coupling,” which is similar to the way an engine and transmission are coupled for propulsion. The main limitation of this system is that generator output remains constant, whether it’s powering a single light bulb or the total sum of electrical loads on the vessel.

A few manufacturers run belt coupling between the engine and alternator, making the whole genset smaller and permitting an optimum engine speed for a specified level of alternator output. By running an engine at its most efficient speed, you save fuel and extend the life of the engine. Adjustable amperage can also be found in some new genset models.

Today’s gensets use brushless generators and solid state voltage regulators to produce 120 volts of AC at 60 Hz. These include the popular models from Beta Marine, Westerbeke, Cummins Onan and Northern Lights. A few of these, to include those from Mastervolt and Fischer Panda, feature an asymmetrical, or “skewed,” stator to produce a pure symmetrical sine wave, well suited for refrigerator condenser motors, laptop computers and other sensitive equipment.

Some models, however, are engineered to provide 12 volts DC instead of the usual AC. Fischer Panda builds DC generators, in addition to its popular line of high-tech alternator gensets, for those who want to feed pure DC power directly to their onboard systems and battery banks.

The Polar Power 6250 “DC alternator” incorporates a DC generator with an inverter to turn out more amperage while conserving fuel. The 6250’s two-cylinder Lombardini diesel engine operates at speeds varying from 1,500 to 2,200 rpm to produce 2.5 to 5 kW of power. The whole unit weighs only 229 pounds.

Dimensions and layout
As a rule, the larger the genset, the more kilowatts it kicks out. Not always, though. Space-saving engine-generator couplings, high engine rpm and water-cooled generator windings can yield astounding levels of power in small packages.

The Fischer Panda 8 Mini DP, for example, weighs a modest 350 pounds and fits into a space of only 23.7 by 17.7 by 23.7 inches, yet it reportedly produces up to 62 amps at 120 volts, or roughly 7.4 kW. This compact, high-performance genset involves cutting-edge engineering and commands a pretty penny, but again, it is only one option in a wide and varied market.

When matching a genset to the space you have designated, make sure you have unobstructed access to everything on the unit that may require your attention. You should find all the main points of operator contact located on one side, but check just to make sure. Imagine where each checkpoint on the engine and generator will be located once the unit, along with its electrical harness, fuel lines and wet exhaust, is permanently installed.

That means having access to such vital points as the controller, circuit breaker, fuses, raw water pump impeller, heat exchanger zinc, oil dipstick and hopefully the hose clamps on the wet exhaust. Can you imagine having to extract a 400-pound genset just to get a wrench around a faulty injector?

Engine manufacturer
The majority of marine genset engines have been designed for some sort of propulsion, not for operating at one fixed speed to power a high-output alternator. When we hear names like Kubota, Isuzu, Westerbeke, Yanmar and Volvo Penta, we think of propellers, not microwave ovens, yet you can find all these names cranking out power in gensets as well.

Kohler manufactures both the power plant and the generator side of its gensets. Designing a genset as an integrated apparatus straight from the drawing board has allowed Kohler to claim a level of efficiency that reduces engine CO emissions by 90 percent.

In short, whatever you have learned about the quirks of different engine manufacturers will hold the same for genset power plants. Feel free to apply what you have gleaned from experience and dockside advice about noise, vibration, cooling, bearing life, and so on while musing over various brands and models of gensets.

The ideal choice of engine will offer the longest life with the least amount of hassle from oil leaks or prematurely burnt-out main bearings due to perhaps an inadequately lubricated crankcase. At least with this engine you won’t have to worry about prop shaft alignment, packing gland or cutlass bearing.

Engine performance
Few genset manufacturers advertise the horsepower of the engines in their gensets. However, virtually all the major brands list the number of cylinders, operating speed and fuel usage per hour. Compare two or three gensets with the same engine and check for significant discrepancies in power output and fuel consumption. Is fuel usage more or less the same across the different units at equal engine speed?

The same make and model of an engine may run at 1,800 rpm in one unit and at 3,600 rpm in another to generate two different levels of amperage at the same 60 Hz for the North American market. If so, which engine can we expect to last longer?

Speed consistency is the most critical demand we place on a genset engine. Manufacturers use two main strategies to achieve stable rpm. The traditional method is to employ an extra-heavy flywheel and governor springs specially designed for constant speed.

The other solution for regulating speed is a microprocessor-controlled servo motor, which reduces the need for a heavy flywheel and, arguably, ensures more precision than governor springs can deliver. On the other hand, as Johansen pointed out, one big splash of seawater can render the microprocessor, its related wiring and the remainder of the high-tech wonder useless until you can order new parts.

Life is not always easy for a genset aboard an offshore cruising yacht. You never know when a plastic bag will lodge itself in the raw water intake hose, or when a heat exchanger tube will spring a leak, or when a voltage regulator will fizzle.

When you see the phrase “automatic monitoring and shutdown” advertised, you can expect the late-model genset to offer comprehensive monitoring of battery charge, engine speed, voltage regulation, fuel consumption, emissions, oil temperature, and other diagnostic and protective capabilities.

The unit should offer automatic shutdown capability for, at the very least, high coolant temperature and low oil pressure. The unit may also feature auto shutoff for high exhaust temperature, over-speed, lack of raw water flow and starter motor over-crank.

Load surge protection
In calculating power demands, we may overlook start-up surges from heavy-duty loads like a windlass or even a watermaker or refrigerator motor. Rather than installing a genset rated for the highest possible combination of surges, why not install an inverter that distributes the momentary surge to an array of other charging sources?

Victron Energy’s microprocessor-controlled DC-to-AC inverter system picks up the slack when the genset cannot keep up with sudden extreme draws of wattage. This may allow you to select a smaller genset, saving you money in initial cost and fuel consumption, and cutting down on extra weight and space dedicated to the unit.

Theoretically, coupling a smaller genset with a surge inverter can also add to genset engine life by ensuring a better-matched load.

The following five examples should give you a rough idea of the main technologies available.

Traditional genset: Westerbeke
Founded in 1937, Westerbeke’s gensets range from 4 kW for small pleasure craft to 95 kW for commercial ships. Westerbeke’s 5.5-kW genset weighs 356 pounds, making it slightly heavy for its class, but its durability and dependability make up for the added pounds. Based on traditional 1:1 ratio solid coupling, this unit has kept pace with the digital age by incorporating a full text display control panel and digital controls. You may also opt to install this genset within an NMEA 2000 communications network.

The Westerbeke 5.5 runs at a conservative 1,800 rpm fixed speed and consumes an average of 2.3 liters of fuel per hour.

Belt drive: Next Generation
Who said gensets are only for the largest of boats? Even my Cal 30 Saltaire can find room for Next Generation Power Engineering’s UCM1-3.5, which achieves its diminutive size and weight through a parallel-mount coupling system for engine and alternator. This 3.5-kW unit, powered by a 7-hp single-cylinder Kubota diesel, sips 0.8 to 1.6 liters of fuel per hour, weighs only 160 pounds and fits in a space measuring just 28 by 15 by 15 inches. The optional enclosure expands those dimensions to a mere 30 by 18 by 16.5 inches.

The two secrets behind this unit are the rigid attachment plate and belt-drive system joining the two halves of the genset. The two pulleys are sized to keep the engine running at 2,800 rpm, and an elastic, helical drive belt saves you the trouble of checking and adjusting belt tension.

Gino Kennedy, the company’s president, explained, “Keeping the engine at 2,800 rpm gives the best torque, the best fuel efficiency and maximum lifespan. We don’t use any fancy electronics that you would have to look for if they were to break down in the middle of the ocean. The Kubota engine has dependable mechanical speed control that lasts for years and years.”

Asynchronous: Fischer Panda
Fischer Panda of Oakland Park, Fla., boasts a line of freshwater-cooled, asynchronous marine diesel gensets, which they claim are “smaller, lighter, quieter and more fuel-efficient than the conventional synchronous generators.”

This new wave of genset technology uses no windings, brushes or diodes, requiring less maintenance of the generator assembly. Freshwater coolant flows through a water jacket in the alternator casing, minimizing the amount of space required for airflow around the whole genset unit.

Fischer Panda’s AC and DC gensets range from 4 kW to 20 kW.

Fuel-injected gasoline: Kohler
Ranging from 4 hp to 64 hp, Kohler’s list of Aegis, Kohler Diesel, Command PRO, Courage PRO, Magnum and Triad engines power mini tractors, lawn mowers, pumps, and other small applications. The Kohler, Wis.-based firm manufactures low-CO electronic fuel-injected gasoline marine generators in 5 kW, 7.5 kW and 10 kW models, covering a wide range of vessel sizes.

Hybrid genset: Mastervolt
The Mastervolt Hybrid Generator goes a step further in tailoring output specifically to fluctuations in electrical loads. The GPX-6 produces up to 10 kW of start-up power and then supplies a continuous 2 kW from Mastervolt batteries.

Whenever a continuous load exceeds 2 kW, the generator turns on automatically to fill the power void. Mastervolt claims fuel savings of 30 to 40 percent, a message that certainly resonates with the cost-conscious sailor in this era of spiraling fuel costs.

At 176 pounds with the sound cover and roughly the size of the Fischer Panda 4200, the GPX-6 is one of the lightest permanent-mount marine gensets on the market. A two-cylinder Steyr diesel engine powers a brushless permanent magnet alternator at 3,000 rpm to produce a “perfect sine wave” in a package that looks more like a sleek, new photocopier than a diesel genset.

Got juice?
Genset prices fluctuate, and they vary even more when we factor in such options as electrical harness, sound cover, frame mounts, repair kit, NMEA 2000 capability and with one manufacturer, even the grade of paint your prefer.

As we breeze past rows of numbers denoting genset output levels, it is easy to become desensitized to the sizzling power behind those numbers. At 20 kW, 120 volts AC equates to 167 amperes, a mind-boggling amount of current for a cruising yacht. One-fifth of that, only 4 kW, yields 33 amps, slightly higher than the common rating for house wiring. To put it in perspective, a 1,200-watt microwave consumes only 10 amps.

Whether you choose AC, DC, gas, diesel, direct drive, belt drive, synchronous or asynchronous, an efficient, fuel-saving genset can be a fuel-saving, ecologically correct upgrade to your cruising lifestyle.

Circumnavigator Bill Morris, a frequent contributor to Ocean Navigator, is the author of The Windvane Self-Steering Handbook, published by International Marine.

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