Something that's been on my mind for a while, but I never got around to asking: How efficiently can an EV or hybrid capture energy as it slows? Or put another way: if you braked to a standstill using regen, then used that energy to accelerate again, how much energy would you get back?

In the general ballpark of 30%, from what I've heard. Here are some of the many incremental ways in which power is lost: Unless you brake very gently, most hybrids don't use fully use regen most of the time, and dump a good chunk of the momentum into the brakes. At least in the case of Honda's IMA system, the motor/generator is driven indirectly through a CVT, which is going to cost you 10% or more. If I remember right, Toyota's and Ford's systems use MG2 for generation, which is a very direct connection and should be pretty efficient. There are generating losses in converting mechanical energy into electricity. There are charging losses in the battery charge controller and in charging the battery itself. There are storage losses in the battery. And this is more true of the NiMHs used in hybrids than of the other chemistries. There are substantial losses in the motor controllers. There are losses in the motor itself. There are losses in getting the power from the motor to the wheels: the CVT in Honda's IMA system is, again, at best 90% efficient. The planetary gear system (manipulated by less-than-100%-efficient generator and motor action in tandem) of the Toyota and Ford systems is worse on average (depending on speed), from what I've heard.

Rolling friction (tire) losses, too. At the mfr recommended pressures, coast down is significant. WriConsult can probably ignore this loss, though!

First, it depends on the generation of hybrid you are referring to. Each new generation offers an increase in captured energy via regenerative braking. This was marginally true for the 3 Prius generations and significantly so for the 2 Civic Hybrid gens. Then, you have to consider the aggressiveness of the regenerative braking as well. Aggressive regens usually lead to greater losses not only because the MGSet(s) is/are always able to produce current at higher rates than what the packs usually can take (discarded regen) but also because of the implicit downstream conversion losses (energy state conversion losses, thermal losses, friction, etc). So, the key to reducing these losses is to incurr the regen over the longest "coast" and the lowest amplitude possible so that little or no friction braking is ever needed except when close to the actual stop. Friction braking is always exclusively activated when the vehicle slows down below the minimum threshold (12-7 MPH, depending on the hybrid vehicle). So, you'll be lucky if you can use 40% of the recovered energy later on for propulsion purposes. And depending on the circumstances, it is more likely that there will be much less than 30% available since a great deal of the onboard systems run on electric (Climate Control+AC, Power steering, Hybrid specific control systems, legacy 12V subsystems, lighting, etc). Cheers; MSantos

This is something I often point out to people about using the battery power in a hybrid. It is, ironically, less efficient to rely on the battery too heavily than it is to conserve momentum so that braking can be avoided as much as possible. The Prius relies primarily on regen braking unless emergency braking is needed or at speeds under 7 mph. Even then it only captures about 30% of the kinetic energy.

Hi Larry: ___I was amazed to hear some very well placed engineers (you know the company) mention the same thing. The single lead scientist in the car questioned the engineers past statements after we had maybe 10 minutes of clinic time. And of course their FE numbers blew by anything any of them had put up in the past simply by moving to a more maintaining momentum "Glide" mentality than a recapture via Regen at every opportunity mentality. ___I can almost guarantee that this is going to be a key enabler in their next generation HEV/PHEV/BEV designs now that they know what conservation of momentum can do vs. the very lossy recapture via Regen. ___Manuel, as always, "d**n you are good" ___Good Luck ___Wayne

Hi Sean: ___You mean the BEV equivalent of a FAS switch? Yes, they know all about it now including a detailed history of the RAV4EV to go along with it ___Good Luck ___Wayne

The 30% number being bandied about is way too low. At certain speeds in my FEH regen storage efficiency is well over 70%. In absolute terms, it can be a reasonable if you look at the efficiency of non-regen coasting. If you look at the Prius, its characteristics are well known and we can actually crunch numbers with a reasonable amount of confidence. Published info for the Prius tells us that the force acting on it at STP and level ground is given by: F=98.33 + .4103v^2 with F in newtons and v in m/s. So in the limit of infinitesmally small v (Wayne driving?) the conversion of kinetic energy into work is given by KE=98.33d where KE is in Joules and d is the distance in meters. At a starting speed of 17.88m/s (40MPH), the coast-down equation tells us that the Prius will take 169 seconds to coast to a stop and cover 1.296km in doing so. Its kinetic energy starts out at 200.4kJ and ends at, of course, zero. Ideally it should therefore go 200.4/98.33=2.038km. So its efficiency is 1.296/2.039=63.6%. A coast from 20MPH to stop will be 86% efficient, 10MPH 95.4%. But what about P&G? If the Prius coasts from 40MPH to 30MPH with no regen, it will take 28.3 seconds to cover 443 meters which results in an efficiency of 49.7%. A 40-35 glide with no regen results in kinetic energy conversion into work of 46.8%. At higher speeds the efficiency is worse. For a 50-40 glide you will glide for 21.8 seconds, cover 439 meters for an efficiency of 37.6%. For the FEH the aerodynamic drag is much higher and non-regen gliding is not very good at speeds above 30MPH. For the FEH regen at high speeds switched to N coasting below 30MPH gives better results than either all regen or all coasting. Unfortunately, when using regen you still have the road and aerodynamic forces working against you which is why very low efficiency numbers are quoted for regen braking. Elevation change, wind, and other things greatly affect the results.

Wayne, Larry, Manuel, you are all missing the real issue here. It is not the conservation of momentum vs "very lossy regen" that is the issue. For the Prius, you only get low regen efficency numbers if you are at low speed and low regen power. At speeds above 45 MPH, moderate regen to 30 MPH can recapture more kinetic energy than coasting even for the slippery Prius. In my opinion, the problem with both the FEH and Prius is the point stasis SoC management philosophy that they employ. In my FEH, when the ICE is running, charging up to 53% SoC occurs. For any SoC level above 53%, assist is used to drive the SoC back to 53%. The problem with this is that assist forces the ICE into a lower power, less efficient mode. Don't forget, going from 25% efficiency to 20% efficiency is NOT a 5% decrease, but a 20% decrease. I believe that overall better performance will occur over the broad spectrum of drivers if a band stasis method is used for SoC management. For the FEH, for example, I would stop charging @49% SoC and then not force assist untill SoC was above 56%. This would still keep the average SoC in the range desired for battery longevity, but allow for overall better FE. Thing is, the engineers should know all this and more. There must be additional reasons for not doing this, but I can't figure out why.......... The energy capacity of the FEH 1st gen battery is 6.62 MJ! Just 1% of this is 66.2kJ, which is the amount of kinetic energy it has when going about 18 MPH.

Hi Carl: ___Doubling the 08 EPA or higher is achieved by not using Regen almost exclusively. By using it, you will never achieve that target in the FFH, FEH, Prius, HCH-I or -II and Insight-I or -II. Not even close. ___At least that was the way the 06 FEH hit 75 + last May. With Regen, more like 60 if that. ___IIRC, the 05 FEH increased its FE by 18% because of regen on the pre-08 city/highway test cycles. Unfortunately, it can reach a lot more by smoothing the decels and gliding than any regen can add. ___Once those that designed the FFH and FEH moved beyond their regen at all costs - preconceived notions, their personal best FE numbers jumped by 25 to 35 + mpg. This was a huge jump for all of them! ___Good Luck ___Wayne

The way I see it, you double the EPA by P&Ging at speeds where no regen gliding is much more efficient than regen. Alas, I rarely have the time to drive at those speeds. In general, the laws of physics are very hard to violate; I know because I've tried. Maybe Ford's latest effort should be called the Cold Fusion if it does.

Hi Carl -- the way I look at it, momentum conservation avoids conversion losses. No losses always trumps conversion losses because (not violating the laws of physics, right?) of conservation of energy. You can't get more energy out than you put in.

Hi Carl: ___The Accord, MDX, Ranger P/U, Insight-I, BMW335d and Civic iCDTi can all double the EPA without resorting to P&G. I am sure there are quite few others out there as well? ___In the FEH and FFH, resorting to Regen and Reaccelerating is a form of P&G albeit electric only acceleration up to a point when its time to start force charging again. ___Good Luck ___Wayne

Hi Wayne Those EPA's of non Hybrid vehicles with non-Atkinson cycle are not rated with FAS and can easily double their ratings. Most are MT as can be seen in the Challenges here to compare and are using Key-Off FAS P&G to double EPA. The '10 FFH and my '09 FEH made a big change in software for regen and Carl is on a different page with earlier models. We could separate regen with "L" from braking pads totally but that has changed in the '09 FEH. The '09 FEH does a much higher efficient regen than my '05 FEH. I don't use regen in my '09 FEH like I do in my '05 FEH because of the design changes. As far as the 75mpg in the '06 FEH, I was at 90mpg plus during the MPG Challenge for a long time. It all depends on the conditions of the SOC and distance driving average. Your review of the FFH did discount my estimates of the FFH to 59mpg. Good Luck GaryG

Hi Gary: ___No, doubling of the EPA via AT's in both the Accord and MDX is as simple as DWL down the Interstate. ___Regen and reuse under EV or assist will never achieve what a no energy conversion glide will. ___Good Luck ___Wayne

Sean, your post is EXACTLY why I posted in the first place. You seem to think that no regen coasting is lossless coasting. This is absolutely not the case. This is why I felt it was important to put some context for the 30% regen numbers that were being tossed around. The reference efficiency is not 100%, as the Prius coasting efficiency numbers I posted clearly show. I did not make these numbers up; they are the result of solving the coast-down equation given the forces acting on the vehicle. The parameters are taken from the excellent text "Propulsion Systems for Hybrid Automobiles," by Dr. John Miller, a noted expert in the field. Yes, for the Prius a coast down from 40MPH really is only 63.6% efficient at converting the kinetic energy given up into work done in moving the vehicle. 30MPH is 74.7%, 20MPH is 86%, 10MPH is 95.4%. No debating this. A "no energy conversion glide" is not possible. By definition you are giving up energy in exchange for work done moving the vehicle. Only when absolutely NO work is done moving air would it be a "lossless" glide. Aerodynamic forces are PARASITIC and energy spent pushing the air is never recovered. That is why speed kills FE and why Basjoo's vehicle looks like it does. I will do one last example for the Prius in what is probably an exercise in futility, but so be it. In this example, I will compare 50MPH to 30MPH glides in a Prius. Case #1 is no regen, Case #2 is a glide with 2kW of regen from 50MPH to 40MPH, followed by a no regen glide from 40MPH to 30MPH. I will then calculate what round-trip regen-store-retrieve efficiency would be required to achieve exactly the same results with either method. So, the force equation for the Prius becomes F=98.33 + Pregen/v+.4103v^2 and we know that this equals -m*dv/dt. For case #1 Pregen is zero and we have the following results: Time of coast=49.5 seconds, distance coasted=868.7m, coasting efficiency=42.6%. For case #2 we have time of coast=15.3 seconds, distance of coast=311.2m for the regen part, then a time of coast=28.3 seconds and distance of coast=442.9m for the 40-30 non-regen coast. For case #2 we have a total time of coast=43.6 seconds and a total distance coasted of 754.1m. So the efficiency is 37%, but we have 2000W*15.3s=30.6kJ of energy that needs to be stored and retrieved and added to the mix. So for the two cases to be equal, we need to have a round-trip conversion efficiency of (868.7-754.1)*98.33/30600=36.8%! Is this achievable? Close, it requires each way to be 60.6% efficient. Any higher than that and the regen case becomes more efficient! The math and physics do not lie. Typical Prius efficiencies are: .83 driveline, .9 MG2, .91 inverter, .94 battery for a combined efficiency of 63.9%, so that is definitely in the ballpark. However, the numbers Dr. Miller shows for 30MPH EV are 59.2%. Still, very close. If you were to bump the speeds up, what do you think the result would be? But I do not drive a Prius, I drive an FEH. It is a brick aerodynamically compared to the Prius. In fact, the .4103 coefficient for the Prius becomes .9879 for the FEH! So when the following experiment is performed on a flat road and no wind, the results can be quite surprising: Case #1, with SoC=53%, at 43MPH shift into L, lift the pedal, and when the tach bounces shove into N and record speed and SoC. Coast to a stop and record the distance coasted and SoC. Case #2, with SoC=53%, at 43MPH shift into L, and when the tach bounces record speed and SoC, but leave the shifter in L. Then at 30MPH, shift into N and coast to a stop. Look at SoC, and then shift into D and let the FEH creep until the extra SoC is burned off to equal the ending SoC for Case #1. If you're impatient, lightly press the go pedal to speed up the burn-off. Compare the distances. I have my results. What will you find? Gary? Debbie? Anyone?

Maybe there is a terminology problem here: no-regen coasting is "no-conversion-loss coasting", not "no-loss-of-any-kind coasting". If one considers no-regen ICE-off coasting as 100% "efficient" in terms of distance traveled (i.e., maximizing distance traveled), not in terms of conservation of energy, then coasting with regen will be a less "efficient" way to cover distance due to the losses incurred in converting the kinetic energy to electricity, storing it chemically, generating electricity from the battery, and converting that to kinetic energy. But as you point out how large that penalty is depends on many other factors of each individual situation, which is why 30% is only a guess. There may be very valid reasons for accepting this tradeoff (e.g., needing SOC to keep the gas engine from running to charge the battery and dragging down FE later). Also worth keeping in mind that SOC is almost certainly not a linear function.

Thanks, John. You've highlighted the reason I stated that. My goal is to get down the road with as little energy as possible. I can't somehow get further on the same energy by converting to electricity and then back to kinetic energy.