Brake Specific Fuel Consumption
December 22, 2015 2:33 PM   Subscribe

Dear MeFi mechanical engineers: In the back of my car's owners manual are the engine's HP/torque/BSFC curves (god bless those data-loving Germans). Will the overall fuel economy always be optimized at the lowest point on the BSFC curve? With the gearing and wheel diameter, this point works out to be 84mph. That can't be right, right?
posted by hwyengr to Travel & Transportation (9 answers total)
 
You're right, it isn't - that neglects air resistance.
posted by The Gaffer at 2:36 PM on December 22, 2015 [2 favorites]


Response by poster: Scan of curves
posted by hwyengr at 2:54 PM on December 22, 2015


Well it's not just about top gear, but bear in mind that 85mph is the default cruising speed for the majority of Europe, the UK and Ontario, Canada plus Florida/Texas/California and some other places in the US based on personal experience. So it does actually make sense.

Having max torque at 2750 is best for driveability and having a band of 60-85mph for max specific fuel economy in top gear would seem to be fine, wouldn't it? What does 1750rpm match up to on that car? 55 or 60?
posted by Brockles at 3:04 PM on December 22, 2015


since the gaffer has answered this, but hasn't been marked correct, perhaps the problem is that the answer isn't clear?

BSFC ignores air resistance. so that is the most efficient point for the car to run if it's sitting on some rollers (roughly).

in practice, you don't run your car on rollers. instead, it moves forwards through the air. and pushing the air aside (particularly at speed) takes a lot of energy. so the engine has to do a lot more work at 84mph than that plot indicates. so in practice fuel efficiency drops way off at higher speeds.

but that isn't shown in the plot because BSFC ignores air resistance.
posted by andrewcooke at 3:15 PM on December 22, 2015


Response by poster: You're right, it isn't - that neglects air resistance.

Well, it does and it doesn't. The units are in g/PSh, which I'm guessing is grams per PS-hr, with PS roughly being equal to HP. So number of grams goes up with the increasing horsepower to overcome the additional resistance, but decreases with speed because it's also based on time. Is this a case that the desired information isn't knowable without the hp/speed data available?

What does 1750rpm match up to on that car? 55 or 60?

58. But we got a fuel saver 2.20 rear end in the US vs the 2.64 ROW.
posted by hwyengr at 3:18 PM on December 22, 2015


Well, it does and it doesn't.

It does. Two clues: the word "brake", which refers to measurements taken with the engine shaft disconnected from everything downstream. Second is the units, g/PSh, grams of fuel per (Power * time) = grams of fuel required per unit energy extracted. This has everything to do with the way the engine is able to combust the fuel in the cylinder to extract work, and really nothing to do moving the car itself.
posted by kiltedtaco at 6:01 PM on December 22, 2015


I'd be careful about trusting the data. Most of these cars power bands are optimized to hit fuel economy standards at the requirements set for regulators while allowing the cars to drive well at speed. They do this by fiddling with the engine management systems. this is separate from the VW NOx issue.

Realistically the best fuel economy for you car is always going to come at below 60 MPH because that's the top speed the EPA tests at for fuel economy. This happens because those tests determine fleet CAFE adherence which is a huge issue for car manufacturers.

https://www.dieselnet.com/standards/cycles/hwfet.php
posted by JPD at 6:47 AM on December 23, 2015


Best answer: The low point on the BSFC curve provides you most energy per unit of fuel. It does not provide you with the most distance per unit of fuel. That is because the energy used per unit of distance increases at roughly the square of the velocity due to aerodynamic drag.

To find the optimal velocity that provides the most distance per unit of fuel, you need to solve simultaneously for the two factors, BSFC and aerodynamic drag. Drag says you want to go at the lowest possible velocity. BSFC says that you want to go at a much higher velocity. Given these two opposing factors, the velocity for most efficient fuel mileage will be somewhere in between, something like 45 to 55 MPH for most vehicles.
posted by JackFlash at 8:51 AM on December 23, 2015


Realistically the best fuel economy for you car is always going to come at below 60 MPH because that's the top speed the EPA tests at for fuel economy.

I can see how you have reasoned your way to this, but it entirely depends on the gearing in the car. Also, not everything is designed around the EPA guidelines because there is a bigger world than the US being designed for and manufacturers also design for customer use, not just one government entities restrictions. Also related:

Given these two opposing factors, the velocity for most efficient fuel mileage will be somewhere in between, something like 45 to 55 MPH for most vehicles.

This doesn't bear out in reality, though. I have driven quite a few recent and older cars that get better fuel economy above the EPA speeds because they have a very long top gear and so the engine is running inefficiently below 55mph. There is often a 'dead spot' of poor engine efficiency from gearing that hangs around those kinds of speeds. One was even a Corvette Z06 that had such a long over-driven top gear that below 60 it didn't sit very happily and you ended up at a higher rpm in 5th. That car got really very good (and similar) fuel economy at 60-70mph and better than it did at 50 (rpm was too low to be efficient in 6th, too high in 5th). Unexpected, especially from a car such as that.

So manufacturers have specified gearing to make sure that for the actual use of the car, the economy is within the range of expectation of the customer. It's no good saying "We get 50mpg EPA cruising" if your customer goes out and always drivers at 75mph and gets 10mpg. Unlike emissions compliance, fuel economy and expected use are designed for expected usage much more than just for the regulations.

That is because the energy used per unit of distance increases at roughly the square of the velocity due to aerodynamic drag.

This is kind of a misleading statement. The energy required only increases at the same rate of drag if aero drag is either the only or vastly greater of the resistance to motion of the car. Below 60mph it is generally not all aero, but rolling resistance of the tyres and mechanical friction etc play a bigger part than you're suggesting there. Above 55 or so the aero drag starts to be much more prominent, though. So the balance point is not completely based on specific economy versus aero. Aero loads don't become a major factor at low speeds, so while your balance argument is correct, it is skewed to a higher starting point than you're suggesting in your statement.
posted by Brockles at 9:27 AM on December 23, 2015 [1 favorite]


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