BSFC (brake specific fuel consumption) explained

[user removed]

The islands are not really islands, they are highlights to show the differences in fuel consumed versus power produced and at what speed-load you can achieve that range of consumption versus power. Between the "island points" the range changes gradually from the outer to the inner island.

regards
mech

[JRMichler]

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.

[jeff88]

Thanks for your help!

I think I'm starting to get it. So the "islands" represent the best BSFC in relation to the list you just gave (e.g. 1400 RPM = 260 BSFC). The red island represents the lowest fuel consumption for greatest power produced (as a ratio?). So in your list 1800 rpm would be the red island, 2500 RPM would be the yellow area, 1400 RPM would be the green area, and 4000 RPM would be way out in the blue or violet (if you were to compare those numbers to the AutoSpeed graph).

This kind of makes me want to incorporate a BSFC computer into the car which tracks RPM and torque and a little light(s) can change colors based on how close you are to optimal BSFC [say green for best BSFC (or in the "island"), yellow for second best, orange for OK, red for worst].

[Vekke]

Originally Posted by Old Mechanic

The islands are not really islands, they are highlights to show the differences in fuel consumed versus power produced and at what speed-load you can achieve that range of consumption versus power. Between the "island points" the range changes gradually from the outer to the inner island.

regards
mech

This is a good point because this means you can calculate a rough BSFC estimation anywhere on the map. So between 210 and 220 there are numbers 211,212,213,214,215,216,217,218,219. Just use "measuring tape" to get correct answer and diveide the gap to 10 sections... thats how I got those numbers that werent on the lines routes.

[some_other_dave]

You can also think of this as a 3D graph. The X and Y axes are, as stated, RPM and (effectively) torque. The Z axis, coming out of the screen, is fuel used per HP generated. The red is the lowest fuel per HP, yellow is next lowest, then green, and so on, up to the light-green which is the most fuel used per HP. The 3D shape is a bowl, with the highest edge of it at low load, and the bottom of the bowl being where the red zone is shown.



We'd love to stay in the high-load low-RPM regime, but that makes enough power to accelerate us. So we either have to stay in low-load low-RPM, or we get into the high-load low-RPM to accelerate, then coast. (That's "pulse and glide" or "pulse and coast", and can give very good results when done well.)

-soD

[pgfpro]

some_other_dave

That's pretty much what I was going post. Very well done!!!

I'm a visual person, not to smart with out my vision so I need to see everything in 3D to understand anything? lol

[Occasionally6]

Originally Posted by Hersbird

What I don't understand is why cars with cylinder deactivation end that deactivation almost as soon as you give it some throttle. I would like to be able to lock in the deactivation and increase throttle and load for a better BSFC. I wonder if it is just to hard on the working cylinders.

It may be an NVH thing. The two engine modes mean that the engine mounts etc. have to be designed to deal with two different "engines" in terms of vibration isolation and damping.

Originally Posted by jeff88

Also, what makes the red island the best? Why doesn't highest load and lowest RPM produce the best BSFC (and thus MPG)?

The BSFC has a peak wrt to engine speed because the engine is most efficient - due to valve timing and inertial effects in the intake and exhaust tracts - at trapping charge at a particular speed. Either side of that speed there is more wasted work done relative to the charge trapped.

Higher engine speeds always result in lower BSFC due to the effects of friction i.e. engine oil pumping, viscous effects in both bearing oil and the various air flows inside the engine.

An engine designer could optimise the engine for low speed operation, at the expense of maximum power output, or maybe spread out wrt rpm the way the engine pumps air/exhaust and flatten the BSFC plot.

[Occasionally6]

Originally Posted by jeff88

This kind of makes me want to incorporate a BSFC computer into the car which tracks RPM and torque and a little light(s) can change colors based on how close you are to optimal BSFC [say green for best BSFC (or in the "island"), yellow for second best, orange for OK, red for worst].

Like an LCD screen with the BSFC map on it with a real time marker or dot that shows you where you are on it and lines of constant power so you can see where you could be.

I suggested to you in the Guino thread that the most useful thing to do with the data you needed for that would not be to determine the ground slope. Know the acceleration of the car with its weight and you can calculate the power being used. Know also the fuel consumption, as per MPGuino, and you can calculate the BSFC.

[Vekke]

Yes you can also make calculatios after which speed the driving with load is more fuel efficient vs push and glide. That is related to push time vs coast time and then use the BSFC to calculate which is more fuel efficient. Usually after 100 km/h or 62 mph speed almost on any car its more fuel efficient to use driving with load.

[jeff88]

An LCD screen with a little BSFC graph and a pinpoint of where you are on it would be cool too. I was thinking simple, like a little LED that changes colors based on what island you happen to be running in.

I haven't forgotten about that thread! I think about it multiple times a week. Just working my way towards that project.