Quote:
Originally Posted by MetroMPG
If that line does show torque at WOT, that may make it incredibly useful: could you not essentially use it to extrapolate what throttle/pedal position to use to get the engine closest to the sweet spot at a given RPM?
EG. in the Saturn chart, 7/9 throttle @ 2000 RPM looks like it would put you squarely in the 250 g/kWh island.
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To make one of these they mount the engine on a test stand and attach a brake to the output shaft. It looks like they measure four dependent variables (fuel consumption rate, heat generated at the brake, RPM, and torque) while varying two independent variables (throttle position and brake setting). These charts show us a map of those 4 dependent variables distilled down to contour lines for a constant ratio of fuel consumption to heat generation plotted over RPM and engine toque. What they do NOT show us is the paths across this map that one would get for a given constant brake setting (load) as the throttle varies.
So, unless I'm missing something, I don't see how a BSFC map is going to help anyone to drive in the "sweet spot". For instance, the Saturn map shows the best efficiency at 2500 RPM and 124.8 Nm. One can vary the RPM with the accelerator pedal but the only way to vary the load on the engine on a flat road is to choose the appropriate gear. So to be really useful, what one would need in addition to the BSFC chart, is an overlay showing RPM vs. Torque for each gear as a function of throttle position AND another independent chart showing speed versus RPM for each gear. That latter one can usually be found in a car's service manual, but the former one, well, I have never seen one. Have any of you?
The data that would be really useful is a handful of traces on a single 2D plot showing MPG (on Y) versus speed (on X), with one trace per gear. Somebody with a ScanGauge, some time, and an empty road, should be able to make a complete one of these in a couple of hours. This plot would tell a driver at a glance which gear to pick for optimal efficiency at a given constant speed, and also in an allowed range of speeds, which speed is best. With an automatic, which in a sense has one gear for each constant speed, this is just a single trace. Here is one somebody did (look at the instantaneous curve)
http://www.randomuseless.info/318ti/mphmpg.png
My guess is that the curve for most cars will be a lot like this one.