A question about the relationship between MPG and engine TORQUE

[karabiner98k]

Is it true that a car has it's best fuel consumption in max torque RPM?
For example, if a car produces it's max torque in 4500 RPM, does it use the least amount of fuel by driving constantly in 4500 RPM?

 

[rcgldr]

Is it true that a car has it's best fuel consumption in max torque RPM? For example, if a car produces it's max torque in 4500 RPM, does it use the least amount of fuel by driving constantly in 4500 RPM?

Generally an engine is most efficient in terms of power produced versus rate of fuel consumption, but this amount of power well exceeds the power involved for best fuel milage, which is distance covered versus rate of fuel consumption. Generally this occurs at relatively low rpms in a tall gear in a car, at a relatively low power output.

 

[Jobrag]

Just because the max torque is achieved at 4500 RPM it does not follow that if the engine is doing 4500 RPM it is running at max torque. Torque curves show the torque produced with your right foot buried in the Wilton, definitely not the most economical way to drive.

 

[Lsos]

Actually when your foot is buried in the Wilton it IS the most, or at least very "economical", or at least efficient, because the engine is unthrottled and the pumping losses are the least.

Of course, in most cars you'd also be accelerating in this situation, in which case I agree it would not be economical. If you had a very tall final gear though that would let you keep an even speed at max torque rpm with the throttle buried, you'd get some relatively good mileage.

 

[Jobrag]

No because resistance increases as a square of speed (ish) you'd get poor mileage, if you were using the engine to drive a stationary load you'd get good efficiency.

 

[Lsos]

...resistance increases as a square of speed (ish) you'd get poor mileage...

I agree, but I'm not sure how this is related to what I wrote.

 

[jack action]

The rpm for best fuel consumption depends on how much power you need and it changes depending on throttle position. You need a BSFC chart for that. This is the one for the toyota prius:

The red line represents the best fuel economy according to the power demand (= torque X rpm) from left to right.

Here is one from another engine, but with each BSFC «island» with a different color (the red one being the most economical spot, slightly below maximum torque at full throttle):

Best fuel economy is always at maximum torque, but «maximum torque» changes RPM with throttle position.

 

[Bob S]

The first plot in jack action's #7 post is precisely the right plot to use. It is called the BSFC (brake specific fuel consumption) map. These are different for every engine. This one is for a small displacement relatively high rpm (red line) engine. Manufacturers generally treat these maps as proprietary information. The "230" contour on the BSFC map represents 230 grams of gasoline per kilowatt-hour of brake output power on a dynamo. 100% efficiency is about 82 grams (more if the gasoline is diluted with ethanol).

The vertical axis is torque in Newton meters (N-m). Power is N-m times engine speed. As an example, you should be able to cruise at 35 mph at no more than 10 HP. The contour of constant 10 HP on the BSFC map is a hyperbola. To drive at maximum efficiency using 10 HP is the point where the 10 HP contour meets the red line.

I would probably draw the vertical red line for this engine at 1500 RPM. The intersection of the 10 HP contour with the red line at 1500 RPM is about 47 N-m. You should rarely exceed 1500 RPM under 35 MPH for best fuel efficiency using this engine.

added: Question for karabiner - If 10 Hp at 1500 RPM is 47 Nm, what is it at 3000 RPM? Using jack action's BSFC plot above, and the fuel consumption contours shown, estimate the % increase in fuel consumption (grams per kW-h) by shifting from 1500 RPM to 3000 RPM.

Bob S

 

[karabiner98k]

added: Question for karabiner - If 10 Hp at 1500 RPM is 47 Nm, what is it at 3000 RPM? Using jack action's BSFC plot above, and the fuel consumption contours shown, estimate the % increase in fuel consumption (grams per kW-h) by shifting from 1500 RPM to 3000 RPM.

10HP and 3000 RPM means about 24 Nm of torque.
I think with 3000 RPM and 24 Nm the BSFC map represents 350 grams of gasoline per kilowatt-hour

For 1500 RPM and 47 Nm it is about 270 grams of gasoline per kilowatt-hour

350 - 270 = 80

I think by shifting from 1500 RPM to 3000 RPM the fuel consumption increases by about 23%

Am i right?

 

[Bob S]

You are correct. If you are cruising around town in 3rd gear, shift up to 5th. Keep the RPM around 1500, downshift to 4th when needed. Of course, this depends on the BSFC map for your engine. Good luck in getting a copy.

Bob S