BSFC maps and fuel economy

retiredphysicist · #1 08-24-2007, 02:06 PM

A BSFC (brake specific fuel consumption) map of a SI (spark ignited) ICE (internal combustion engine) describes the engine's fuel efficiency for every combination of engine torque (usually in Newton meters) and engine RPM. For every driving condition (engine horsepower requirement) there is an optimum combination of engine torque and RPM for best fuel economy. Usually the SI engine is most efficient at about 35% of redline RPM and 75% of maximum torque. For the 2.5L engine in my 2003 Subaru Forester with a 5-speed manual, the maximum efficiency is at about 120 Nm and 2200 RPM (27.6 kW or 37 HP). City driving usually requires less than about 10 HP (easily calculated for steady speeds). For power requirements below about 30 HP, the BSFC map shows that the optimum engine speed for best fuel economy is below about 2000 RPM. I usually get into 5th gear between 25 mph (1100 RPM) and 35 mph (1500 RPM), depending on driving conditions. I consistently get 38 to 39 mpg in the summer (34 to 35 mpg in winter) and drive over 500 miles between fuel stops. Do not be afraid of using torque up to 80% or 90% of WOT (wide open throttle). The engine computer will optimize the spark timing to prevent pinging and engine damage.

brucepick · #2 08-24-2007, 02:14 PM

Thanks - I find this helpful!
I drive "blind" in my '89 car that can't take a Scangauge.
I do have a tach.
With any luck I'll find the BSFC map for my engine.

Dan · #3 08-24-2007, 05:03 PM

Thanks for the low-down.

Quick search of Hobbit's Site yields what I'm after for my Prius

Hat's off to Hobbit for including it in his training slides.

11011011

xcel · #4 08-25-2007, 03:19 AM

Hi Guys:

___Love pretty BSFC maps but I never drive in an efficient area according to most I have seen. Even the Prius is supposed to allow garbage down in the 1.2- 1.4K range and I find just the opposite? Slower speeds of course but that is my own real world including pulses. The Accord allowed 54.5 into the drive tonight as. An Accord? Impossible! The BSFC maps do not allow anything near that level of FE under any regiment! The first and second laws of thermo would have to have been broken so it must be impossible! At least that is what some keep saying

___Good Luck

___Wayne

hobbit · #5 08-25-2007, 09:21 AM

Yeah, but Wayne, while you may not be able to break the laws
of physics either, you're the guy finding all the loopholes!
.
_H*

xcel · #6 08-25-2007, 12:23 PM

Hi Al:

___You are not exactly sitting on the fence allowing mother nature to control your Prius’ destiny either

___Good Luck

___Wayne

retiredphysicist · #7 08-25-2007, 07:35 PM

Reply to Wayne-
I think my engine's maximum thermal efficiency is at about 120 Nm and 2200 RPM, corresponding to about 37 HP. The thermal efficiency is about 31%, corresponding to 260 grams of gas per kilowatt-hour of brake (flywheel) power output (1 gram of gasoline contains about 44,000 joules of energy). However, if I could drive at a steady speed of 35 mph using only 8 HP (6000 watts) at 1500 RPM (5th gear), the thermal efficiency is probably closer to 24%, corresponding to 340 grams per kWh. This corresponds to 58 grams of gas per mile, or 48 miles per gallon (using 2800 grams per gallon).
Retired physicist.

Right Lane Cruiser · #8 08-26-2007, 01:21 AM

Ah -- see there is the rub. We do not drive in steady state -- and we choose not to because we can enhance fuel economy substantially by making this choice. Your calculations are correct but only cover steady state driving. They do not take into account the games we play with efficiency bands for accelerating and coasting for bringing the average consumption rate over a given distance down substantially from those steady state numbers.

That understanding (gained from discourse with Wayne here) was the "AHA!" moment that defined my departure from somewhat enhanced mileage numbers to "What the... ???" numbers.

ILAveo · #9 08-26-2007, 11:39 PM

Quote:

Originally Posted by Right Lane Cruiser

Ah -- see there is the rub. We do not drive in steady state -- and we choose not to because we can enhance fuel economy substantially by making this choice. Your calculations are correct but only cover steady state driving. They do not take into account the games we play with efficiency bands for accelerating and coasting for bringing the average consumption rate over a given distance down substantially from those steady state numbers.

That understanding (gained from discourse with Wayne here) was the "AHA!" moment that defined my departure from somewhat enhanced mileage numbers to "What the... ???" numbers.

So it sounds like what you're saying is that it needs to be studied as a dynamic rather than steady state optimization problem. Dynamic optimization with discontinuities (because the engine uses no fuel when it's off) would make you do all sorts of fun math to do a good job modelling what Wayne does. The BSFC map data might provide you with useful data for implementing a numerical solution.

Wayne's result that pulse and glide beats steady state is interesting and appears to have been informally reproduced by a number of people including me. Has anyone formally tested the proposition?

PaleMelanesian · #10 08-27-2007, 10:41 AM

I did a quick-n-dirty test a couple months ago. A 5.4 mile loop, with 4 right turns and rolling 50-foot ups and downs.
A - 45.6 mpg - cruise control 55mph
B - 49.0 mpg - no CC, steady 55mph
C - 63.5 mpg - P&G 60-45mph
A - 48.4 mpg - CC again

Hardly anything close to formal or scientific. Not enough to prove / disprove CC against foot steady-state. But I took the P&G number to heart, and now I P&G whenever I possibly can. My last tank was 56.3 mpg with that approach.