BSFC has SI units of g/kWh (grams / (kilowatt *hour)). So the BSFC value tells you what mass of fuel is required to burn to get a kWh of energy. The lower the BSFC value, the better as that means you have to burn less fuel to get the same amount of energy. Of course, BSFC is not constant across the operating range of the engine; hence, the goal of engine downsizing and downspeeding is to put the engine in its best operating range during as much as the engine’s duty cycle as possible (where it spends most of its time).
A few different variables affect BSFC including pumping losses, frictional losses, and other power sapping losses within the engine. Looking back at the S2000 data, the engine flows more air at 4k rpms compared to 2k rpms. We know this because the manifold pressure was very similar between the two engine speeds and we know the engine runs a constant 14.7 A/F ratio at light loads. Since the 4k rpm engine speed was spraying more fuel, there had to be more air too. Because the engine was flowing more air, there were greater pumping losses.
|Part of the power losses in the engine are from shearing the oil. This happens wherever the oil is between two surfaces; so between all the bearings, cams, pistons and all the other moving surfaces the oil touches. The shear stress is directly related to velocity. Increasing the speed the engine spins also increases the shear stress that sucks up more power.|
Engine friction is a huge power drain and engine friction increases exponentially with engine speed. More friction increases your BSFC and that hurts your fuel economy. I used my WOT datalogs (love the Hondata KPro!) in third gear to see how much fuel it sprayed at 4k rpms, compensated for the richer A/F ratio vs. light load, compared that to my cruising third gear data, stirred the numbers around in a hat and came up with roughly an additional 23hp lost due to friction in going from 2k rpms to 4k rpms (give or take a few). For another reference, I have some engine data from a 10.7L, 6-cylinder engine and it sucks up about 75hp to spin it 2400rpms. Yup, 75hp to just spin the engine and not actually making any power. The other ‘power sapping losses’ come from accessories such as the oil pump, water pump, alternator, etc.
|Remember these equations relating power, torque and rpm? As stated before, it should take the same power at the crankshaft with the car in third gear or sixth gear to keep the car going the same speed. Because sixth gear spins the engine at about half the speed of third gear, the engine needs to make about double the torque. Double the torque means double the cylinder pressure.|
|This is a BSFC map for a diesel, but I’ll use it to illustrate the concept of downspeeding. Downspeeding from 4k rpms to 2k rpms means doubling the pressure (and therefore torque). On this map, that goes from a BSFC of 450 g/kWh to 255 g/kWh for a reduction of 43% in fuel consumption. That value actually is very close to the 41% reduction in injector time (191 milliseconds to 113 milliseconds) from my S2000 testing….. go figure. For reference, gasoline engines typically have a best BSFC around 270 g/kWh and diesels around 200 g/kWh. Yes, diesels are more fuel efficient.|
So there you have it, turbos are awesome because they allow smaller engines to make more power and torque than larger displacement naturally aspirated engines while getting significantly improved fuel economy. The new BMW M5 and stable of Mercedes using new twin-turbo V8s demonstrate this. In the M5, the twin-turbo V8 is rated at 10% more peak power with truck loads more torque compared to the old naturally aspirated V10. The fuel economy jumps from 11/17 (city/hwy) to 17/22, or about 55%/30% improvements. The Mercedes goes from 12/19 to 15/23 for 25%/21% gains. Of course, AMS has demonstrated the untapped power potential of these turbo engines which is why we love them. Turbos allow the downsizing and downspeeding for improved fuel economy while still providing the power and torque we want. The government didn’t intend it this way, but we get to have our cake and eat it too.