Can someone explain why you get better MPG in...

[burritos]

city driving? I understand that there is regenerative braking, but ultimately all energy is derived from the ICE, right? I don't understand how you can get more energy by diverting the combustive kinetic energy into a battery and then diverting back to kinetic driving energy and have it be more efficient than just getting the energy from combustion itself.

If one is in on the freeway, it is not intuitive that driving in a straight line with stopping and braking will be more efficient than just driving straight smoothly.

[jeneric]

Because, generally, it's more efficent to go slower. With non-hybrids, the stop and go cancels the benefit of going slower in the city. With the Prius, you get rid of some of that extra waste and the efficiency of going slower comes through.

[aaf709]

Plus, at city driving the ICE may not be on as the demand is lower.

[ScottY]

Originally posted by burritos@Feb 20 2006, 05:54 PM
city driving? I understand that there is regenerative braking, but ultimately all energy is derived from the ICE, right? I don't understand how you can get more energy by diverting the combustive kinetic energy into a battery and then diverting back to kinetic driving energy and have it be more efficient than just getting the energy from combustion itself.

If one is in on the freeway, it is not intuitive that driving in a straight line with stopping and braking will be more efficient than just driving straight smoothly.

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You are right! If you look at the method used by hypermilers (pulse and glide), they aviod using the battery and got more than 100 MPG. Check it out at http://www.toyota.com/html/hybridsynergyvi...l/marathon.html.

[kdmorse]

Two words - AIR RESISTANCE.

Air resistance increases exponentially with velocity (speed). The amount of energy required to keep a car moving at 65MPH is extreme - compared to the amount of energy required to keep the car moving at 20MPG. At 65MPH, much, much more energy is required to keep the car at 65MPH, and all this energy comes from only one source - your gas tank. The effect increases dramatically as speed increases, resulting in lower, and lower MPG the faster you go. (Someone posted a nice little chart somewhere).

In the city, you are correct that every cycle of speed -> regenerative breaking -> accelerate from battery -> speed looses energy. (Of course, that's still free energy compared to a car without regenerative breaking). However that loss of energy pales in comparison to the energy required to push air out of the way at highway speeds.

-Ken

[burritos]

Originally posted by kdmorse@Feb 20 2006, 06:48 PM
Two words - AIR RESISTANCE.

Air resistance increases exponentially with velocity (speed).  The amount of energy required to keep a car moving at 65MPH is extreme - compared to the amount of energy required to keep the car moving at 20MPG.  At 65MPH, much, much more energy is required to keep the car at 65MPH, and all this energy comes from only one source - your gas tank.  The effect increases dramatically as speed increases, resulting in lower, and lower MPG the faster you go.  (Someone posted a nice little chart somewhere).

In the city, you are correct that every cycle of speed -> regenerative breaking -> accelerate from battery -> speed looses energy.  (Of course, that's still free energy compared to a car without regenerative breaking).  However that loss of energy pales in comparison to the energy required to push air out of the way at highway speeds.

-Ken

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Thank you.

That's an explanation I can understand and accept. None of this, "Let's ignore the laws of thermodynamics and brake to give yourself more energy."

Let me ask you this then. If HWY speed limits were say 35-40 mph and were actually obeyed, then would the mpg be better than city driving?

I would love to see that chart. I'm assuming that idling at 10 mpg is not the optimal mpg speed considering you'd be running at a low gear. I'm also presuming that air resistance is nominal between say 1 mph vs 35 mph. Is that correct?

[p626808]

Originally posted by burritos@Feb 20 2006, 07:32 PM
Thank you.

That's an explanation I can understand and accept. None of this, "Let's ignore the laws of thermodynamics and brake to give yourself more energy."

Let me ask you this then. If HWY speed limits were say 35-40 mph and were actually obeyed, then would the mpg be better than city driving?

I would love to see that chart. I'm assuming that idling at 10 mpg is not the optimal mpg speed considering you'd be running at a low gear. I'm also presuming that air resistance is nominal between say 1 mph vs 35 mph. Is that correct?

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Yes... And I did it for 50 miles.. between 35 and 43.. got a little over 80 MPG. But I think it would be higher if the city speed limit was 25.. I think that was the average speed of the marathoners.. and the got 110 MPG.

Scott

[ken1784]

Originally posted by burritos@Feb 21 2006, 07:54 AM
city driving?

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42 mph barrier.
http://priuschat.com/index.php?showtopic=1...id=210092&st=26

Pulse and Glide.
http://hybridcars.about.com/od/owner...seandglide.htm

Ken@Japan

[EricGo]

In the city, less air resistance at lower speeds, but more energy wasted braking
On the higher speed highway, lets say no energy wasted braking, but more energy spent against the higher air resistance.

Which is better overall ?

Depends ..
How fast on the highway ?
How much braking ?

The Prius wastes less energy braking (70 - 80% of a regular car, because some of the kinetic energy is sent to the battery for later reuse), but these same general considerations hold, and account for most of the answer you are seeking. The remainder of your answer has to do with engine efficiency which plays a larger part in sorting this question out in regular cars, and less in the Pruis because of its design that minimizes poor engine performance efficiency wise in both the city and highway.

Pulse & Glide is a technique to run the engine at > 95% of practical maximum efficiency the entire trip, rather than 80 - 90% that the Prius achieves left to it's own devices.

This question *really* bugged me before I learned 'the way of the Prius'. Now it seems obvious