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-   -   BSFC (brake specific fuel consumption) explained (https://ecomodder.com/forum/showthread.php/bsfc-brake-specific-fuel-consumption-explained-30791.html)

Focus-Ak 12-31-2014 11:07 PM

BSFC (brake specific fuel consumption) explained
 
There is a great explanation of BSFC on this webpage:

"Brake Specific Fuel Consumption: A really useful concept"
by Julian Edgar (2008)
AutoSpeed - Brake Specific Fuel Consumption

It is a much more complicated and engine-specific topic than I realized, even from the various discussions and BSFC mapping on EcoModder.

JRMichler 01-04-2015 10:07 PM

It is a good explanation of BSFC. The only thing lacking is some constant power curves overlaid on the BSFC diagram. Some constant power curves would show exactly why low RPM is usually good.

Vekke 01-05-2015 01:56 PM

Here is one constant power curve for 1.2 TDI with some reference points on my Audi A8 project with vaious gear and Cd ratios.
https://fbcdn-sphotos-d-a.akamaihd.n...ce915249c00f0c

You can clearly see that if you just lower your drag your engine efficiency will get worse. that is the reason why you should always match your gear ratios when you lower your drag figures.

JRMichler 01-05-2015 02:28 PM

To help interpret Vekke's graph:

If you need 7.5 hp (5.6 kW) at a particular speed, the best gear ratio is that which has the engine running about 1400 RPM at that speed.

If you need 15 hp (11.2 kW) at a particular speed, the best gear ratio is that which has the engine running about 2000 RPM at that speed.

If you need 60 hp (45 kW), you don't have a choice. You only get that much power at about 4600 RPM. But if you keep the gas pedal on the floor, and change gears to get 3300 RPM, the power drops slightly to about 56 hp and the specific fuel consumption improves from about 250 to 222 g/kWh. That's 13% better (more efficient) use of the fuel.

Focus-Ak 01-05-2015 02:54 PM

Quote:

Originally Posted by Vekke (Post 462427)
Here is one constant power curve for 1.2 TDI with some reference points on my Audi A8 project with vaious gear and Cd ratios. ... [chart] ... You can clearly see that if you just lower your drag your engine efficiency will get worse. that is the reason why you should always match your gear ratios when you lower your drag figures.

Or another way to look it is that the engine is now over-sized. So if there were more time and $ for the project, a re-engining may be productive both in improving engine efficiency and in reducing weight. (Which in turn means a slightly smaller engine is needed, which means less weight, which means ... which is why aircraft design, for example, is an iterative process).

Interesting curves and data points. Thanks for posting this.

Vekke 01-05-2015 03:21 PM

Quote:

Originally Posted by JRMichler (Post 462433)
To help interpret Vekke's graph:

If you need 7.5 hp (5.6 kW) at a particular speed, the best gear ratio is that which has the engine running about 1400 RPM at that speed.

If you need 15 hp (11.2 kW) at a particular speed, the best gear ratio is that which has the engine running about 2000 RPM at that speed.

If you need 60 hp (45 kW), you don't have a choice. You only get that much power at about 4600 RPM. But if you keep the gas pedal on the floor, and change gears to get 3300 RPM, the power drops slightly to about 56 hp and the specific fuel consumption improves from about 250 to 222 g/kWh. That's 13% better (more efficient) use of the fuel.

5,6 kW was correct in my opinion

For the 11,2kW I would say the optimal rpm is that 1800 RPM? but in general you have also take into account how many gear you will have at speeds you are using the most. I am trying to optimise my gears for highway use meaning 62-100 MPH cruising speeds. That means tallest gear ratios are always better.

For 45kW I have plenty of options in my car as there will be 89 kW+ available in the first stage. But with stock engine BSFC map that this is there is not much choices.

Hersbird 01-05-2015 03:23 PM

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.

Vekke 01-05-2015 03:24 PM

Quote:

Originally Posted by Focus-Ak (Post 462436)
Or another way to look it is that the engine is now over-sized. So if there were more time and $ for the project, a re-engining may be productive both in improving engine efficiency and in reducing weight. (Which in turn means a slightly smaller engine is needed, which means less weight, which means ... which is why aircraft design, for example, is an iterative process).

Interesting curves and data points. Thanks for posting this.

- Yes you can recalculate the gear ratios to match Cd drop
- Add more weigth do match Cd drop
- Downsize the engine to match Cd drop

From those options the gear ratios are easiest to play with. However I would say the 1.2 TDI is perfectly sized for this A8 project.

jeff88 01-10-2015 11:46 PM

I'm confused about the bsfc map. How do they get the "islands"? I understand the straight line, but don't get where the circles come from. Are those based on the change in throttle opening?

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

http://us1.webpublications.com.au/st...112611_9lo.jpg

Hopefully, this isn't painfully obvious where I'm just overlooking something. I'm probably just slow... :turtle:

user removed 01-11-2015 11:19 AM

The "island" is a range of speed (horizontal) in RPMs and load (vertical) in percentage of full (100%) load.

1500-2500 at no more that 80% of full load is a good average for gasoline engines, avoiding full load enrichment. In Diesels the load is closer to 100%.

regards
mech

user removed 01-11-2015 11:26 AM

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 01-11-2015 03:29 PM

Quote:

Originally Posted by jeff88 (Post 463155)
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 01-11-2015 04:10 PM

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 01-11-2015 04:31 PM

Quote:

Originally Posted by Old Mechanic (Post 463186)
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 01-11-2015 09:16 PM

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.

http://us1.webpublications.com.au/st...112611_9lo.jpg

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 01-11-2015 09:33 PM

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 01-13-2015 06:10 AM

Quote:

Originally Posted by Hersbird (Post 462443)
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.

Quote:

Originally Posted by jeff88 (Post 463155)
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 01-13-2015 06:18 AM

Quote:

Originally Posted by jeff88 (Post 463202)
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 01-13-2015 06:00 PM

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 01-13-2015 08:46 PM

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. :)

jeff88 01-13-2015 08:59 PM

Hypothetical situation time...

Lets assume somebody had an alternator disconnect that automatically connected and disconnected based on various parameters. Let's use the AutoSpeed BSFC graph as a reference.

So if somebody had 60% engine load and 2,000 RPM (green island) and they "turned on" the alternator which increased the load say to 90% load (but same RPM), then the BSFC would be in the red island.

Would the car get better gas mileage with the alternator on rather than off, since the BSFC is in a better range?

Focus-Ak 01-13-2015 09:31 PM

Quote:

Originally Posted by jeff88 (Post 463506)
Hypothetical situation time... Lets assume somebody had an alternator disconnect that automatically connected and disconnected based on various parameters. Let's use the AutoSpeed BSFC graph as a reference.

Jeff, that's brilliant.

Also, it occurs to me that although a BSFC chart is at the foundation of the indicator lights you conceptualized a few messages ago, what we really need is an indicator for ASFC -- Acceleration Specific Fuel Consumption (new term?). The nut of the driving problem for us is how to get from one speed (zero or whatever you are moving at) to a higher speed with the least investment of consumed fuel. A logic board with inputs from an accelerometer and the OBD-II's gal/hour computation could feed an LCD screen could give the result of dividing those two instantaneous quantities. During acceleration we then use that instead of the %-Load or vacuum readout on an UltraGauge or ScanGauge. (At constant speed or coasting it doesn't matter, just during acceleration.) That would give a direct reading of what is really of interest to us.

Focus-Ak 01-14-2015 05:23 AM

I just started a new topic, "On Coasting (Two interesting papers)," and those papers have some really interesting stuff on the integration of certain hypermiling techniques with BSFC factors. See here: http://ecomodder.com/forum/showthrea...tml#post463536

some_other_dave 01-14-2015 06:45 PM

Quote:

Originally Posted by jeff88 (Post 463506)
Would the car get better gas mileage with the alternator on rather than off, since the BSFC is in a better range?

No. You're making more power with the alternator on, so the total consumption goes up. The consumption per horsepower generated goes down, but the overall consumption goes up. (There may be exceptions, but I'm thinking they are very rare.)

-soD

Focus-Ak 01-14-2015 07:11 PM

Quote:

Originally Posted by some_other_dave (Post 463603)
No. You're making more power with the alternator on, so the total consumption goes up. ... -soD

Dave, what you say is true. However, what Jeff proposes is that if the alternator has to be run some of the time but not all of the time, what it is the most advantageous circumstance for having the alternator on? One answer to that is at times when using the alternator can contribute to raising the engine's efficiency to a higher level, i.e. using the BSFC plot to its best advantage. Doing so has a double benefit -- not only does it make the electrical generation more fuel efficient, it also makes the driving power delivered by the engine more fuel efficient. So, yes, turning the alternator on for that instant does raise the fuel consumption at that moment, but it is the best time to perform this necessary function. It is not an additional fuel requirement in the context of the length of a trip; it is a minimization of that necessary fuel requirement.

some_other_dave 01-15-2015 01:06 PM

Ah, that is a different question than the one I thought I was answering.

What you describe is more in the way of pulse-and-glide for the alternator. But that can easily be done by simply pulse-and-engine-off-coast. That way you not only avoid having to power the alternator, but the whole engine as well!

If you're keeping the engine on, then yes--turning the alt on while pulsing is the best. Same for A/C in the hotter months.

-soD

teoman 09-27-2018 04:16 PM

What would be needed to obtain this bsfc chart from an existing car assuming you had all of the car's canbus data?

redpoint5 09-27-2018 05:14 PM

Quote:

Originally Posted by teoman (Post 580190)
What would be needed to obtain this bsfc chart from an existing car assuming you had all of the car's canbus data?

You'd need all the data contained within those efficiency maps. Torque, RPM, and fuel consumption (HP can be derived from RPM and torque, or torque can be derived from RPM and HP).

What you would be missing is a dyno; a way to measure horsepower at a given RPM.

I'm not sure how precisely fuel consumption can be measured from the canbus either. If I understand correctly, fuel consumption is calculated based on fuel pressure, injector flow rate, and "open" time. If that's the case, you're assumptions about flow rate for a given open time would have to be fairly accurate.

Even with a dyno, you'd need to run many scenerios, such as low torque and high RPM, high torque and high RPM, and everything inbetween. And then the same for low RPM...

There has to be a way to mathematically estimate the map based on a few measurements.

teoman 09-28-2018 05:42 AM

The torque app uses the accelerometer (if i am not mistaken) to calculate the acceleration of the car and provided the weight of the vehicle that information is used to calculate the power output of the engine.

I do not need something that is accurate, but I do need something that is very precise. Some value that I can minimize or maximize.

http://cdn.antarcticglaciers.org/wp-...n_accuracy.png

teoman 09-29-2018 07:51 AM

What if i build a multi dimensional map with the data. Log injection ms, rpm, speed (acceleration derived from that), boost pressure, afr?

Then for 2 points in the map if the injection ms is lower then I have lower fuel consumption for that point.

Anyone see problems? It would not take the hills in to consideration so maybe an accelerometer can take care of that. Am I missing something?

teoman 10-13-2018 07:46 PM

A friend did a quick demo with his vagcom. There was a torque reading there.

Torque vs rpm vs fuel in g/s would provide a good map?

Not sure how the torque is calculated or if it is measured.

teoman 10-30-2018 04:22 PM

I have some data now if anyone is interested in collaborating.

hat_man 05-06-2019 01:28 PM

OK, for the not so smart guy (me) in the room.

Someone suggested an indicator showing different colors representing the BSFC "islands" as a visual reference to help keep the driver in a better FE range (speed/rpm). Would an old school vacuum gauge do the same thing? Or is my confusion showing again?

teoman 05-07-2019 01:23 PM

That is one indicator.

redpoint5 05-07-2019 01:43 PM

Quote:

Originally Posted by hat_man (Post 597510)
Someone suggested an indicator showing different colors representing the BSFC "islands" as a visual reference to help keep the driver in a better FE range (speed/rpm). Would an old school vacuum gauge do the same thing? Or is my confusion showing again?

I'm not so sure such a visual would help. A driver needs to travel a particular speed, and the only variable to maintain that speed is which gear they are in. Generally, you want to be in the tallest gear that will allow you to maintain speed.

Engines produce a certain amount of power at a certain torque and RPM. In fact, horsepower is torque * RPM / 5252

The reason that the BSFC map isn't that useful is that it doesn't factor in the other variables such as increasing wind resistance due to traveling faster. It simply shows the most efficient RPM and throttle position (torque) to produce a given amount of power.

MPG or liters per 100 km is a better measure of efficiency because it factors in all of the variables.

So, BSFC is a good tool for engineers, but MPG is the right tool for drivers.

teoman 05-07-2019 02:05 PM

True, but having the chart and where you are at would enable you to make the proper adjustments without trial and error.

redpoint5 05-07-2019 02:52 PM

Absolutely, which falls more under the category of engineering than driving. The more extreme ecomodders here would use BSFC to inform their designs, but the casual eco drivers here would not have use for the info.

sendler 05-08-2019 07:02 AM

An instantaneous fuel economy vs acceleration gauge would be helpful for those of us that use PulseNGlide to use the best throttle opening position during the pulse. And help us choose the best gear for slower speeds while traveling through slower city streets.


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