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Tuesday, February 03, 2009
Update On Diesel/Electric: Hybrids
This coming from the Seattle Boat Show, where Nigel Calder gave an update on his evaluation of diesel/electric technology. His preliminary analysis indicates that "pure" diesel electric systems, where the propulsion is only driven by an electric motor, are no more efficient than straight diesel propulsion systems. In fact, they are more likely to be inefficient. Reason: efficiency of the diesel generators under low loads. As an example, when the propulsion battery bank (if you have one) or house bank is topped up, and the generator is only driving the propulsion motor, the traditionally sized generator would be burning more fuel than the straight diesel at low speeds. An advancement in generator technology would be needed to see this inefficiency eliminated. Not likely to be very soon. Expect to see Nigel's articles in future issues of Professional Boatbuilder magazine in the coming year. Enter Hybrids The Hybrid concept is something relatively newer in the marine market over pure D/E. Hybrids are a system where the propeller can be driven by a diesel engine, or an electric motor, or both. These systems show more promise to be more efficient over pure D/E as they are more likely to overcome the efficiency problem described above. A type of hybrid is called the "parallel hybrid" where the prop can be driven by an electric motor, in addition to the diesel engine ('parallel'). One approach to this is to fit an electric motor between the engine flywheel and the transmission, operating on the same output shaft. Using a clutch mechanism, the electric motor can provide all the propulsion, or, with the engine running and clutch engaged, the electric motor can work in tandem with the diesel engine. This setup provides the opportunity to "tune" the system to achieve the best efficiency throughout the range of boat speed. At low boat speeds, the electric motor is doing most of the work, and, as boat speed increases, with the diesel engine running, the electric motor is "backed off" to favor the diesel engine, whose efficiency improves as it approaches nominal RPM. A sort of, "best of both worlds". Other advantages: when relying solely on the diesel engine, the electric motor can be turned into a generator, driven by the diesel, to charge battery banks. And for sail boats under sail, the system can "free wheel" by having the propeller drive the electric motor and charge batteries as well. Examples of Parallel Hybrids Austrian motor manufacturer Steyr Motors (pronounced "Schtire") has recently entered the marine market (though they have been around for years as part of Daimler-Puch). They are now offering a parallel hybrid "Add-on" to all motors in their lineup, including their new 55hp naturally aspirated four cylinder. A motor like this would be perfect for the Westsail, and, provides an easy way to repower an existing engine setup to D/E hybrid. UK engine maker BETA Marine will also be offering a "parallel" option. However, it is believed that with this setup you can run diesel only or electric only, but not both.