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Regenerated Energy and Battery Use

Discussion in 'Gen 3 Prius Technical Discussion' started by drysider, Aug 29, 2013.

  1. drysider

    drysider Active Member

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    There are a lot of posts concerning the use of the energy stored in the battery. Many of them contend that it is not cost/mileage effective to use it, usually stating that you have to first burn gasoline if you are going to regenerate energy into the battery. That is a true statement, but says nothing about the thermodynamics of the system.

    Consider: you accelerate from a stop to 35mph, then take your foot off of the accelerator. You use energy to get to 35mph. How much energy to you use to get to 0mph from there? You do not use any. There are many posts that want to assign some of the gasoline energy cost to the coasting down to zero. How do you do that? All of the gasoline energy was used to get to 35mph in the first place. There is nothing left. If you assign some costs to the coasting portion, do you decrease the costs for the acceleration? The 2nd law tells us that the burning of the gas is an irreversible, adiabatic process, and as such, the energy costs are sunk into moving the pistons. You don't get to put the worms back in the can. You can, however, reclaim some of the energy without upsetting the physics. The acceleration and coasting to a stop are separate events which have nothing to do with each other. How does the engine know when it is going 35mph whether you will be regenerating some energy or not? It doesn't....and further more it can't. Irreversible are, in fact, irreversible. The energy stored in the battery is not all regenerated, so the overall costs are more complicated. To the extent that it is regenerated, it is, by far, the cheapest energy in the car, and the regen-battery-motor efficiency is at least twice the efficiency on the gasoline engine.

    The Prius is a light, aerodynamic car with an efficient engine. So is the Yaris, but the Yaris is rated 37/30mpg. It is the Synergy drive firmware and the regeneration systems that make the Prius so fuel efficient. Fortunately, the Synergy drive is pretty canny about the thermodynamics and it all happens whether we understand it or not.
     
    #1 drysider, Aug 29, 2013
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  2. Former Member 68813

    Former Member 68813 Senior Member

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    So, what's your point, again?
     
    #2 Former Member 68813, Aug 29, 2013
  3. jhinsc

    jhinsc Senior Member

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    It is, but it isn't, but then again maybe not.
     
    #3 jhinsc, Aug 29, 2013
  4. Drdiesel

    Drdiesel Active Member

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    Anyone that says that, has no idea what they're talking about. Here's a trip data
    page from my C-Max Energi plug-in. You can clearly see that using the hybrid battery
    to augment the ICE use will benefit the vehicles ability to use less fuel. This is not
    from EV mode using the plug-in portion of the battery. This is using the hybrid battery.
    You can see the regen mileage gained back that would otherwise be 100% lost.
    It's captured in the battery and reused to reduce the use of the ICE and decrease MPG used.


    Untitled-1.jpg
     
    #4 Drdiesel, Aug 29, 2013
  5. chulbert

    chulbert Junior Member

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    It is more efficient to use the existing kinetic enegy of a moving vehicle by coasting than to convert it to electricity and back to motion at a later time.
     
    #5 chulbert, Aug 29, 2013
  6. Drdiesel

    Drdiesel Active Member

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    I live in CA and we have lots of hills. Failing to capture and reuse that energy
    would be a 100% waste. I can regen downhill and never loose the otherwise wasted energy
    and still make it to the stop sign. I think you guys need to recalculate your flatlanders formula
    data. It's not very accurate when it comes to the dynamics of a hybrid vehicle.
     
    #6 Drdiesel, Aug 29, 2013
  7. bisco

    bisco cookie crumbler

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    this is where pulse and glide comes in. it's more efficient than purposely trying to charge the battery and then drive electric. of course it's efficient to charge downhill, when momentum would take you over the speed limit. i think this argument is lost in semantics.
     
    #7 bisco, Aug 29, 2013
  8. Former Member 68813

    Former Member 68813 Senior Member

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    DUH! We do know that battery use in hybrids generates higher MPG. This why hybrids are hybrids. The car does it by its own automatically. That doesn't prove that users forcing EV get better MPG, LOL!
    Too many prius drivers think they are smarter than prius computer (equipped with lots of sensors). Sorry, not going to happen.

    My commuting with favorable weather and traffic nets me mid 60's MPG. It's all city, P&G if possible. If I used EV aggressively, it would be much lower. Been there, done it.
     
    #8 Former Member 68813, Aug 29, 2013
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  9. Drdiesel

    Drdiesel Active Member

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    The OP made it sound like someone is claiming otherwise and that's why, I posted
    some data. It's total B/S to say it's not a benefit :ROFLMAO:
     
    #9 Drdiesel, Aug 30, 2013
  10. chulbert

    chulbert Junior Member

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    In the situation you describe, you clearly had more energy than you needed to reach your stopping point (the stop sign). If that doesn't go into the batteries then it turns into heat and is lost. You're absolutely correct and this is the exact purpose of the electric system.

    However, that doesn't change the fact that gliding is more efficient than braking. Energy conversion incurs loss so the optimal driving strategy always minimizes energy conversion.
     
    #10 chulbert, Aug 30, 2013
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  11. chulbert

    chulbert Junior Member

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    It seems to me you are saying drivers cannot outsmart the computer, then you say that's exactly what you do. Do I misunderstand you?
     
    #11 chulbert, Aug 30, 2013
  12. Drdiesel

    Drdiesel Active Member

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    I agree the Glide is the most efficient, but you'll need to stop at some point and you
    can't coast to a stop without making other drivers want to run you over/off the road.
     
    #12 Drdiesel, Aug 30, 2013
  13. Former Member 68813

    Former Member 68813 Senior Member

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    Let me help you understand. If you are pressing EV and run down battery to low levels (to improve short term MPG numbers) that forces ICE to run or run longer/harder to bring it back to optimal mid levels hurting MPG in the long term. This is how you don't want to outsmart the battery management that is very well optimized.

    On the other hand, the car has no eyes (yet) and can't see the red light approaching, but you can take advantage of that and glide to the stop. NASA does it too, short rocket pulse and long glide to the destination. Very energy efficient. Now, on earth, our glides are not so long of course.

    Does it make sense now?
     
    #13 Former Member 68813, Aug 30, 2013
  14. Drdiesel

    Drdiesel Active Member

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    Unfortunately we have gravity and traffic controls and you'll always need to stop somewhere along
    your way. Take gliding out of the picture for a moment. Now it's easy to see how battery regeneration
    is the capture of otherwise wasted energy. It would otherwise be lost to heat from the friction brakes.
    You're cutting this part of the picture away from the canvas to make your point look better. Yes, gliding
    is the most fuel efficient form of motion, but being able to take advantage of regen braking is the second best.
    Can you drive in traffic without using the regen capabilities of the system ? I doubt it :eek: I know if it was
    a space shuttle, we could :ROFLMAO:
     
    #14 Drdiesel, Aug 30, 2013
  15. drysider

    drysider Active Member

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    In the city, the engine isn't the primary recharge source for the battery, the regen systems are. That is the point of the post. In city driving, you can be pretty aggressive with using the battery because it will be recharging due to the natural flow of traffic. The energy has little or no cost and will improve your city mileage. As stated above, I also get 60+ mpg in the city (using pulse and glide, but extending the glide with 10 amps or so of battery), with some longer segments into the 80+mpg range. 90+ is not that unusual. On the freeway, as I stated, it is a lot harder to get the higher mpgs. If you are willing (and it is safe), you can go 65 instead of 75 and it will make a big difference. The mpg difference between a 60mph speed limit and a 75mph limit is dramatic. You can also bite the bullet and become a Prius nerd and follow a truck in the slow lane. I have always been able to find one going 65mph (with a 70mph limit).

    The battery will also, of course, charge during the pulse segment, but it is the deceleration and braking that provide the majority of the charge. I do not know how to glide to a stop and be a safe driver. I use the brake and the Synergy system takes care of the rest.
     
    #15 drysider, Aug 30, 2013
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