How efficient is regen?

Discussion in 'General' started by smart-za, Apr 17, 2009.

  1. CarlD

    CarlD Well-Known Member

    Oh yes you can, Sean. As the numbers in my example showed, you only need a regen-retrieve efficiency of 37% in the scenario for this to be true. Now for your Insight, there is no scenario in which regen would be beneficial for FE if you didn't need it to slow down faster than coasting allows. That is because the aerodynamic drag is so incredibly low for that vehicle. If you can't grasp the fact that coasting is not lossless due to drag, I don't know how I can further explain this. And John, if we use the no-regen coasting as a reference for efficiency, I can easily show you regen cases that are >100% efficient which we know is not the case.

    But since I am apparently a glutton for punishment, I will take the numbers Dan recently posted in the warp-stealth article thread and use the equations in my earlier posts to look at the results. I will not use any regen because in all of these cases it would significantly reduce FE.

    Based on Dan's 1600 rpm and 14 ign pulse numbers, I use the BSFC chart and operating trajectory to get 7.3kW ICE power during pulse with a BSFC of 270 g/kW-hr. For pulsing, the force equation becomes: mdv/dt=6000/v-98.33-.4103v^2 where 6000 W represents the 7.3kW reflected through the drivetrain loss. The coast-down equation remains the same and is given by -mdv/dt=98.33+.4103v^2

    For the "competition" P&G we have a pulse from 12MPH to 34MPH which takes 30.3 seconds and covers 353 meters. 16.6 grams of fuel are consumed for an average pulse FE of 36.5MPG. The glide from 34MPH to 12MPH takes 87.3 seconds and covers 860.9 meters for a glide efficiency of a nice 66.7%. So the competition P&G segment yields a distance travelled of 353+860.9=1213.9m or .754miles. The 16.6 grams of gasoline amounts to about .006 gallons yielding a whopping 125.3MPG.

    For the light traffic P&G the pulse is at 19MPH and the glide remains 34MPH. This yields a pulse time of 25.3 seconds and distance of 316.8m. The glide from 34-19 takes 54.3 seconds and covers 632.7 meters for a glide efficiency of 62.4%. Fuel consumed is 13.9 grams for 117.1MPG.

    For the heavy traffic P&G the pulse entry is 28MPH and glide entry remains 34MPH. This results in a pulse of 13.3 seconds that covers 188.4 meters and consumes 7.3 grams of fuel. The glide from 34MPH to 28MPH takes 19 seconds and covers 266 meters for a glide efficiency of 56.1%. Fuel consumed is 7.3 grams for 106.7MPG.

    In all of the above cases the ICE efficiency is exactly the same. The improvements in MPG come from the improvements in glide efficiency.

    Wayne, I am in violent agreement with you that 75MPG can not be achieved in the FEH when averaging speeds at which regen is beneficial. And even at that, the regen-EV is only good until the battery temp reaches about 85F at which point the A/C compressor will run whenever the ICE runs. My personal best was a P&G from 15MPH-25MPH which yielded 78.9MPG on straight gas for 5 miles.
  2. msantos

    msantos Eco Accelerometrist

    Hi folks;

    I don't really know if we stayed on track or not particularly in light of the original post and question.

    I will state that the 30% value is not really a guess either. Perhaps, I misinterpreted the original question? Anyway, please let me know if this is the case.

    In any case, this is a basic breakdown of relative efficiencies for a typical hybrid electric topology:

    Electric Hybrid
    – Mechanical to electric (29%)
    – AC to DC (9%)
    – DC to chemical (10% + 0.003% per minute)
    – Overall round tip efficiency ~ 34%

    This is the breakdown we often use when comparing a typical hybrid electric system to a hybrid mechanical. Just for giggles, the hybrid mechanical (in this example: KERS) is as follows:

    Mechanical Hybrid
    – Gear transmission losses (1.5% per gear pair)
    – CVT transmission losses (8%)
    – Parasitic losses due to oil pumps (1 kW)
    – Flywheel storage losses (2% per minute)
    – Overall round trip efficiency ~ 70%

    Now, the reason why we often mention 30% is because it is a nice round number and academically encompasses a great deal of current HEV systems. Please do note, and as I tried to state in a previous post, the actual energy available for propulsion purposes may vary greatly to the point we may actually get MUCH less than 30% available for propulsion use. :(


    Last edited: May 4, 2009
  3. CarlD

    CarlD Well-Known Member

    I cited my reference.
  4. msantos

    msantos Eco Accelerometrist

    Hi Carl;

    Its OK. The numbers I posted were actually derived directly from the Prius NW20 baseline (if I am accurate). :)

    In fact, the very numbers I quoted come directly from a baseline established by the F1 racing commission while looking for a suitable hybrid system (back in 2006). Papers produced by several bidders reference that baseline. One of these bidders actually published their document online. :rolleyes:

    Torotrak Corp proposal for their Kers system. You'll find the quoted figures on page 11 (if I recall correctly). Obviously, I have more similar info but very little in terms of "safe public domain" stuff.


  5. GaryG

    GaryG Well-Known Member

    Hey Carl

    I did a temporary trade of my '05 FEH with my daughter's '07 Honda Civic because she needed room for two baby's car sits and equipment. This means I can't conduct the test above any time soon. The '09 FEH does not regen in "L" like my '05 does so my test would not confirm anything in that vehicle.

    I'm not sure if there is another hybrid like the '05 - '08 FEH that can completely separate regen from the brake pads in "L". There is so much more weight over the Prius the momentum just pulls so much energy out of that large traction motor as a generator in just a short distance. This is how I explained how I hypermiled in that article here at CMPG. Pull some heavy regen in "L" once at 30mph accelerate back to speed and shift back to "L" for enough regen to go EV and then glide in "N" or hold your speed in "D". There is enough battery SoC to carry you further than the distance the ICE was running and on top of that you can glide in "N".

    With the changes in the '09 FEH and '10 FFH, heavy regen is off the table like in the Prius. What's good about the changes is I can generally get enough regen from the brake pedal coming to a stop from 25mph or higher to go EV before the stop.

    I see both sides of the debate here and I agree with both of them. The older FEH has the right package for using regen more efficient at higher speeds than the other Hybrids. It's off the table in my '09 FEH, but I'm getting much better mileage with the new 2.5L engine. I'm not changing how I drive my '05 FEH because regen is also a part of EV over 30mph where I cruise at most of the time.

  6. Right Lane Cruiser

    Right Lane Cruiser Penguin of Notagascar

    Carl, your suggested test appears to be an attempt to circumvent the air resistance and more severe frictional costs associated with higher speed. We can accomplish the same thing with low speed P&G (which gets around the inefficiencies of low throttle plate opening and engine load).
  7. smart-za

    smart-za Well-Known Member

    Well, I certainly didn't expect a sort of Spanish Inquisition. [looks around]

    Carl - I'm going to read your posts again very carefully. You've clearly put some serious thought into this question, and I don't want to miss out.

    In the mean time - how should I interpret the coasting efficiencies you calculated? The way I read it, if you coast to a stop on a flat road (without regen/KERS), no work would be done on the vehicle, because its vertical position would not have changed. Therefore, efficiency would be zero, which is obviously different from your results.
  8. CarlD

    CarlD Well-Known Member

    I actually enjoyed going through the math/physics again, even if it wasn't convincing for anybody. Work is defined as Force X Distance. For the Prius equation the static force of 98.33 Newtons represents the force needed to equal the rolling resistance. If you push with a force infinitesmally greater than that, you will convert all of your energy expended into work done moving the Prius down the road. As far as increasing the vertical position, that adds potential energy in the form of mass*gravity*height. With no aerodynamic losses, if you coasted from the top of a hill, you would convert the potential energy into kinetic energy such that mgh=1/2mv^2. For a mere 10 meter hill, lossless coasting down would convert the 124.17kJ potential energy into kinetic energy resulting in a velocity of 14.1 m/s (31.5MPH) at the bottom of the hill. Lossless coasting would then convert the 124.71kJ of kinetic energy into work done moving the vehicle a further distance of 124.71/98.33=1.27km. A real world coast down from 31.5MPH would only cover 922.5m, so the efficiency is about 73%.

    As far as interpreting it, about the only thing I could say is use it as a reminder of why speed is bad and aerodymanics are good. For the Prius, at 60MPH 75% of the energy spent is lost to aerodynamic drag. So when you enter a non-regen glide at 60MPH, initially you are throwing away 75% of your kinetic energy into the air. The 451kJ of kinetic energy that the Prius has @60MPH could be used to move it 4.6km at near zero speeds. That 451kJ is less than 10% of the battery pack's capacity, by the way.
    Last edited: May 8, 2009
  9. lightfoot

    lightfoot Reformed speeder

    I (and probably other Insighters here) am well aware of aero losses and do factor them into driving strategies. On the highway, I try to keep the speed up to 45-50mph (for practical safety reasons). But when the speed rises much above 50mph on a downhill I slow the car down with regen (via MIMA at 20-30A) to capture some of that energy into regen rather than just throwing it away. And to keep the battery pack above half full so that regen does not drag down the mpg on the next flat stretch. Whatever the efficiency of regen is, doing this is better than simply discarding kinetic energy, unless the battery pack is 100% full.

    It's always seemed ironic that the Insight has such a low Cd and here we are trying to drive at speeds where aero drag is minimized. I often wonder how the Insight would do FE-wise without all the aero tricks Honda so painstakingly incorporated....
  10. cephraim

    cephraim Noob

    I own the Nissan Altima Hybrid which has an analog EV gauge (in kwatts). During regen, the needle goes below 0 into a "blue range" to indicate regen in progress. See this link.

    So, my assumption has been that, the lighter you push the brake, the more likely that the braking will be entirely regenerative. Does anyone know for certain when the friction pads kick in in the NAH? If your needle remains "in the blue" and not pegged down at the low end, can we assume it's all regenerative?

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