Quote:
Originally Posted by renderfarmer
Theoretically yes, yet empirically I've now driven my 60 mile commute (highway) at about ever conceivable speed and i consistently get over 27mpg when i can maintain a steady speed of 65-70mph.
I think that modern engines and fuel management systems are too pro-active in maximizing performance to reduce fuel economy to a two term equation.
I just drove to and from Montreal (336 miles each way) and with 4 people on board and some light luggage I got 23mpg with an average speed of 70mph. The temp was -12C (10F) which didn't help matters.
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I'm not sure your point? Try driving the same trip at 55MPH. You'll get better than 27mpg. Drive it at 40MPH. You'll get close to your best fuel economy there.
Quote:
Originally Posted by RhoXS
IMO the theory is indeed reality. The losses that determine the actual fuel economy vary primarily with speed and driving practices.
Some losses are just a function of speed such as wind resistance. Since wind resistance is the really big hitter at constant cruise controlled higher "highway" speeds, the energy needed to propel the vehicle is indeed a function of the cube of the speed. At "highway" speeds there is essentially no braking or acceleration so windage is the only consequential energy loss.
At lower speeds" windage is negligible. The big hit energy losses are due to breaking where all the vehicles kinetic energy is converted to heat. This energy is recoverable (regenerative breaking) and that is why hybrids do so well in traffic but no better than non-hybrids at higher speeds where brakes are not used very much.
Of course, the weight of the drivers foot is also a factor as the vehicle's engine must abide by Newton's second law. That is, the force needed to accelerate a car (mass) is directly proportional to how quickly you want to accelerate it. Higher acceleration means greater force which means more gasoline. At a constant cruise controlled speed the ONLY energy needed is that necessary to overcome losses as the vehicles inertia (Newton's first law) will keep the car going forever if it was not for the losses.
The actual biggest energy loss is due to the inefficiency of the thermodynamic cycle that governs the engine. Greater than 2/3 the energy extracted from the gasoline is peed away out of the radiator and engine block. However, there is some point that the engine is most efficient so, when considering the rpm for optimum engine efficiency, braking, acceleration, and windage, that is why there is some speed at which the curve peaks. This point is invariably well below "highway" speeds. Bearing losses exist but are negligible so are typically ignored.
In other words, you can't F%$$ with mother nature when it comes to mpg. The faster you make the car go, the lower your mpg (unless, of course, you choose to drive in a vacuum). At highway speeds the aerodynamic slipperiness of the car is the dominant factor. At lower speeds the energy losses due to windage are essentially zip and braking/acceleration losses/inefficiencies dominate. A hybrid recaptures the low speed breaking losses but can't do squat for windage. Acceleration in a hybrid, is the result of a much more efficient energy conversion process (battery and electric motor vs an internal combustion engine).
BTW, temperature is a double edged sword. At lower temperatures the air is more dense. This means higher windage but more oxygen per unit volume of air sucked into the engine. I have no clue how significant these factors are or which dominates. I do remember from many years ago a friends GTO seemed to accelerate better on cold days.
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I'd like to add a few comments to this.
At low speed losses are (in approximate order of significance)
- Heat lost in the engine
- Braking / Accelerating
- Vehicle accessories (A/C, steering pump, transmission pump, radio, lights, ect)
- Tire rolling resistance
- Aerodynamics
At high speeds losses are (in approximate order of significance)
- Heat lost in the engine
- Aerodynamics
- Tire rolling resistance
- Vehicle accessories (A/C, steering pump, transmission pump, radio, lights, ect)
Here is an interesting article Tesla Motors put together for their electric vehicles range versus speed. Relevant, but not perfect since it does not have a combustion engine!
http://www.teslamotors.com/blog/road...ency-and-range