Tomorrow’s Technology Today ™
o Since we already know that the optimum torque is at 2,100 RPM, you could say that you’d get the most bang for the buck out of this engine at 2,050 RPM — a combination of best fuel efficiency and most speed. o As for the curve of specific fuel consumption, this is inconvenient in more ways than simply converting grams per horse- power hour to sensible units such as liters per kilowatt per hour or gallons per horsepower hour. This isn’t because the conversion is difficult (it isn’t) but because almost every engine’s specific fuel consumption curve seems to understate the real- world, in-the- boat fuel consumption. o In practice, I’ve found that almost all diesel engines consume approximately 0.274liters per kW/hr . o For the 420 CX Yanmar, propeller output power at 2,300 RPM is 250 HP, so fuel consumption at this RPM (assuming the propeller is properly matched) is 220gms/kW, while at maximum 2,700 RPM fuel consumption is 230gms/kW. (The shape of the specific fuel consumption curve and its point of maximum and minimum consumption are usually quite accurate in service, just not the absolute consumption numbers indicated.) Determining Optimum Cruising Speed o So far, we’ve seen what the engine performance curves mean and that maximum oomph and fuel efficiency occur for this engine at between 2,000 and 2,100 RPM. Is this the proper cruising speed? Well, an argument can be made for this in terms of sheer efficiency, but it’d be a bit slow. In- stead, we can look at the performance curves to determine where the best compromise between efficiency and speed falls. In this case, 2,300 RPM looks like a good bet. At 2,300, torque is still high, specific fuel consumption is still low, and we’re getting a reasonable 250 HP at the propeller. Low cruise, for quiet, efficiency, and maximum range would be around 2,000 RPM, and you’d only open her up to 2,700 from time to time to show off or outrun a storm. o OTE: If you don’t have any engine curves available, you can estimate that the propeller power at various RPMs as follows: o 90% of max RPM = about 68% of max rated engine power o 80% of max RPM = about 48%; 70% of max RPM = about 30%; 60% of max RPM = about 22%; 50% of max RPM = about 15%; 40% of max RPM = about 11% o To determine the impact of adding a 9 kW high-efficiency BMG to the fuel consumption of the Yanmar 420 CX engine at cruising speed (2,300 RPM), we can follow these steps: Diesel Engine Efficiency at Cruising Speed o At 50% of maximum RPM, the engine is typically operating at a point of high thermal efficiency, near its sweet spot in the fuel map. o Engines are often most fuel-efficient between 40–60% of their maximum RPM and load capacity. o The increased load of a high power BMG improves the engine's brake-specific fuel consumption (BSFC) slightly, as diesel engines are generally more efficient under higher load conditions within their optimal range. o BMG Efficiency: Running the BMG at full load typically results in its peak efficiency (85%), minimizing losses in converting mechanical power to electrical energy. o Engine Efficiency Impact: Adding the BMG load increases the engine's total load, potentially pushing it closer to its optimal operating point, which can slightly improve overall fuel efficiency.
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