BSFC (brake specific fuel consumption) explained

[Focus-Ak]

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]

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]

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.


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]

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]

Originally Posted by Vekke

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]

Originally Posted by JRMichler

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]

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]

Originally Posted by Focus-Ak

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]

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)?



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

[user removed]

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