The BSFC chart thread (post 'em if you got 'em)

[TEiN]

That makes sense.

What do the dashed red lines indicate?

[pasadena_commut]

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.

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.

[dcb]

Ya lost me. The BSFC chart is useful for acceleration. So you know how much throttle to give it and when to shift to keep it in the "sweet spot". It is your job to conserve the energy that went into that last acceleration by not over accelerating into a heavy breaking situation. So get off the gas in time to coast to the next traffic obstruction, or feather the throttle as soon as you are up to speed, or p&g the whole way.

For cruising you can use it to determine the optimum gear ratio for an anticipated load at an anticipated velocity, but that obviously isn't for everyone.

[jonr]

Exactly.

It would be nice if the Saturn BSFC chart had one more island - there might be a 225 around 2500 rpm/125 Nm.

[pasadena_commut]

Originally Posted by dcb

For cruising you can use it to determine the optimum gear ratio for an anticipated load at an anticipated velocity, but that obviously isn't for everyone.

Ok, let's say you can open the gearbox and replace the gears with any ratios you want. How do you choose those gears for optimal efficiency?

Say you want to optimize for efficient cruising at 65 mph. So for the top gear you look at the BSFC and pick the RPM centered on the lowest fuel use, then 65 mph and that RPM (and tire size) determine the gear ratio. That does not determine the load though, so there is nothing much you can do to pick the location on the BSFC chart along that vertical line for your cruising speed. Is that optimal? Probably not, since a higher gear will increase the load on the motor but move the RPM to the left of the center of the most efficient point, and could lead to better cruising efficiency. Now you want to add a gear for accelerating at that same optimally efficient RPM. How do you choose that acceleration gear? Seems like an even more difficult problem since for a fixed gear, and a given RPM, there will be a fixed speed. It would help if we knew what the path of RPM vs Torque looks like as a function of RPM for a fixed gear ratio.

[dcb]

Well, you don't actually get to choose the vehicle speed, unless you can choose the engine size too Most vehicles are way overpowered in terms of optimal efficiency. I suppose it is possible to detune the engine too, hadn't really thought about it.

But assuming you have complete control of the gear ratio for cruising and can hold the target rpm, the load (of which aerodynamic load is no small part) and the velocity are dependent. If you make aerodynamic improvements you will be going even faster for a given engine load after the appropriate gear ratio adjustment. And thus could also downsize the engine to keep the previous cruising speed.


I don't know what the best acceleration gears are except that you can have too few and to many. The shifts shouldn't be so far apart that you are forced far from the "sweet spot" rpm wise, but not so numerous that you spend more time shifting than accelerating.

[Daox]

Its really not that difficult to pick an optimal gear ratio if you have a BSFC chart for your engine.

First off, you have to figure out how much horsepower your car demands at 65 mph. Sorry, I don't have the equation handy, but its findable (you'll need Cd, frontal area, and speed). I think I got it off autospeed at one time. For example, my Matrix takes roughly 14 horsepower to stay at 65 mph.

Second, you'll want to plot a bunch of points on the BSFC chart to see where that lands you. To do this you need to convert your horsepower to torque at a specific rpm.

torque = horsepower * 5252 / rpm

So lets plot a few points on the Saturn BSFC chart since I don't have a chart for the Matrix. This is obviously going to skew things, but it should get the idea across.

RPM....Torque @ 65 mph
1000...73
1500...49
2000...37
2500...29
3000...25
3500...21
4000...18






So, you could actually gear it all the way down to cruise at 1000 rpm and that would give you the best fuel economy. However, you would be still only be at 50% load and you would have next to no power avaliable should you need to pass or keep speed going up a hill. That may not bother some of you (it wouldn't bother me much), but it would bug the majority of buyers out there which is one reason why auto companies have not done this except on their high mileage cars (Metro xfi, Insight, Civic VX/HX, etc.). Even then they have been conservative so as to provide you with something that is easy to drive vs incredibly efficient. This is also the reason why you see such huge gains in FE from P&G.

[pasadena_commut]

Originally Posted by Daox

For example, my Matrix takes roughly 14 horsepower to stay at 65 mph.

First, thanks for the good explanation.

Does this HP number take into account the 15% or so (for a manual, more for an auto) drive train loss? That is, is that crankshaft HP or drive wheels HP?

[Daox]

I'm not sure, but it probably doesn't account for anything other than aero (and maybe rolling) resistance. Keep in mind that these are only rough guesses and thats really good enough for our calculations. In the real world you'll be fighting wind, getting helped by wind, going up and down hills, being passed by cars and tons of other things. So, the hp variable is really going to alter from second to second while driving. The basic point is to see that lower rpms almost always leads to increased fuel economy, but at the cost of readily avalible power without shifting.

[MazdaMatt]

Yeah, thanks for that explanation... now... just to play devil's advocate here... would you not be even better off if you actually reduced your gearing so that you had to be at 1500rpm for that cruise speed, THEN mess up your aerodynamics to the point where you needed 124.8 torque so that you are on the centre island? Then you are at highway speed with the highest gear maintaining speed at the throttle position that results in the least possible fuel consumption... crazy? Adjust gearing and aero for optimal rpm and tp for most efficient load...