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Impact on Fuel Consumption with a high output BMG on Marine Engines Key Insights Using Performance Curves of Yanmar 420CX as Example o The BMG load at maximum output has a minimal adverse impact on overall system efficiency at cruising speeds.
o In some cases, adding the BMG load improves the fuel efficiency of the engine by moving it closer to its optimal operating range. o Adding a 9 kW BMG load to the Yanmar 420 CX engine at cruising speed increases fuel consumption by approximately 2.74 liters/hour or about 5.4%.
o This is a relatively small impact, demonstrating the efficiency of using the BMG within the engine's cruising efficiency range. o There is no impact on boat speed because of the white space to propellor load. The Torque Curve o Torque is the twisting force on your prop shaft. You could have all the power in the world, but if the shaft didn’t spin you’d have no torque and no go. Torque is actually defined with the formula: Torque, KGM = (975.175 x kW)÷ RPM o The torque curve shows the torque generated by this engine at various RPMs. The interesting o thing is that maximum torque on normal internal- combustion engines doesn’t occur at maximum engine rated power and RPM. In fact, the torque curve of the 420 CX Yanmar is pretty typical. The maximum torque occurs at about 77 percent of maximum RPM, or 2,100 RPM. Indeed, on most engines maximum torque falls somewhere be- tween about 55 percent and 80 percent of max RPM. (Light gas engines tend to have peak torque at lower engine RPMs and heavy diesels at higher RPMs.) The units for the torque curve are in kgm (kilogram meters) and Nm (Newton meters). This is the metric equivalent of pound feet. We’re not particularly concerned with the absolute numbers; however, we’re simply interested in where torque is highest. o It simple terms, the 420 CX Yanmar is delivering the most oomph per gallon of fuel consumed from 2,100 RPM. This wouldn’t be a bad low-cruising speed, but you don’t unnecessarily want to limit operating speed this much. Let’s see how fuel consumption fits into the picture. Specific Fuel Consumption o The specific fuel consumption curve reads—as is often the case—in rather inconvenient units. In this instance, in grams per kilowatt per hour (g/kW). For the moment, though, what we’re really interested in is the shape of the curve and where fuel consumption is lowest for the output power and torque. In other words—just the opposite of the power and torque curves— the best spot on the specific fuel consumption curve is where it’s lowest. For the 420 CX Yanmar, this is at 2,000 RPM.
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