Quote:
Originally Posted by jeff88
I'm confused about the bsfc map.
Why doesn't highest load and lowest RPM produce the best BSFC (and thus MPG)?

It takes a while to understand these maps because there is so much happening.
BMEP is torque. More BMEP is more torque. Zero BMEP is zero torque. The maximum BMEP at any one RPM is the maximum torque at that RPM. There is a calculation involved if you want to get torque in foot pounds or other units.
Look at Vekke's BSFC plot with the power curves drawn on it. Follow the power curve labeled 11.2 kW. That's 15 hp. You can get 15 hp at 5000 RPM with only a little throttle, or at 1400 RPM at wide open throttle, or at any RPM in between at an in between throttle opening. I took some values from that BSFC plot for different ways to get 15 hp:
RPM BMEP BSFC
1400 10.5 260
1800 7.5 220
2500 5.5 230
4000 3.0 320
Since low BSFC is good (less fuel consumed for the power), the ideal RPM for this amount of power is 1800. 2500 RPM is almost as good, as is any RPM in between. Running WOT at 1400 RPM is much worse than part throttle at 1800 to 2500 RPM. And running around in low gear at 4000 RPM is the worst.
This shows why the gear ratios need to be matched to the vehicle. Too tall a final drive ratio (low RPM on highway) gives worse mileage than the right ratio.