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How do Hybrids work? Newbie questions encouraged!



  • My Insight's regenerative braking slows at ~1 mph per second. If I'm about to slam into a pole at 60 miles an hour, I want faster slowing than 1 mph/second (a full minute to stop).


    I want the old-fashioned but highly effective friction brakes.


  • robertsmxrobertsmx Posts: 5,525
    I’m not sure how you could measure how regenerative braking is translating to deceleration (down to mph). I have my doubts on common understanding of what regenerative braking really is.


    Electric motors transform potential energy (electric charge) to kinetic energy and when they are run in reverse (i.e. as electric generator), they can capture kinetic energy to store potential energy. That’s all there is to the bottom line of regenerative braking.


    Vehicle is already moving, and when driver applies brakes, the controller knows to switch to generator mode and starts to capture whatever kinetic energy it can and before the vehicle comes to a complete stop (due to braking by the driver). Since some of the kinetic energy is being transformed into electric charge, there is lower loss to the atmosphere by way of transformation of the kinetic energy into heat energy.


    I’ve yet to understand why it is being assumed here that regenerative brakes is actually stopping the vehicle by itself (well it is but only in conjunction actual braking action by the driver, pretty much like engine braking except for the fact that there is less energy lost in translation).
  • stevewastevewa Posts: 203
    Consider the railroads which have used electric motors to brake trains for about 80 years. They still use friction brakes. The reason is simple. Regenerative (or dynamic braking in the case of Diesel/Electric locomotives) is only effective over a certain speed range. Once the vehicle is moving too fast or too slowly it doesn't work. In the case of the Prius regenerative braking stops working below about 6MPH. There just isn't enough kinetic energy left to resist the magnetism of the field. If you wanted to completely stop using just the motor, you'd have to apply so much field current that you'd defeat the purpose of rhe regenerative brakes (i.e. recharging the HV battery).

    There is another consideration which you need to consider w.r.t. use of regeneration during ABS conditions. There's a friction brake at each wheel, while there's only one electric motor/generator set. Since the regenerative brakes have to transmit force through the differential/transaxle, there's no way to modulate the braking force between the drive wheels (and no way at all to brake the rear wheels in a FWD system). One could remedy this by placing a traction motor at each wheel, but that would add weight and complexity to the system far more than having conventional brakes (which you'd still need because of the limitations mentioned above).


    Hope this helps to further clarify things.


    btw, the FEH is no different from the others w.r.t. overriding regen braking...
  • "I’m not sure how you could measure how regenerative braking is translating to deceleration [1 mph per second]"




    Because I have a brain? It's not that difficult to observe my Insight's deceleration with generator braking is about 1 mph per second.


  • robertsmxrobertsmx Posts: 5,525
    Are you sure it is not 0.8 or 1.2? ;-)


    What you experience is similar to engine braking in a conventionally powered vehicle except that in this case you're recapturing some of the energy to recharge the batteries. When you apply brakes, the electric generator continues to convert mechanical energy to electrical energy in the background (in a way, assisting your conventional braking effort), again something similar to engine braking used in conjunction with driver applying brakes.
  • You are still missing my point. Let me repeat it:

    - my insight's regenerative braking slows me at ~1 mph per second


    - my insight has slid off the road & is about to slam into a pole at 60 miles an hour


    - do I want to slow my car at only ~1 mph/second?


    - heck no! I want the old-fashioned friction braking to stop me. I'm perfectly happy with Honda's decision to disable regen & use only friction brakes for the emergency ABS stop.


  • robertsmxrobertsmx Posts: 5,525
    Don’t forget, you HAVE old fashioned brakes at your disposal. You’ve got issues with a system that is basically recouping some (otherwise) lost energy and works on the backside.


    Pike’s Peak Highway in Colorado is a good example to be brought up here. When you start your vehicle downhill from the 14K ft elevation, would you follow warning signs that ask you to use engine braking instead of applying brakes, or no? After all, engine braking isn’t going to “stop” your car.


    As far as ABS versus regen goes, the issue is largely with “speed” of hydraulics than anything else. Someday, we should see faster ABS systems, potentially integrated with regen braking. I have already read a few technical papers on the subject.
  • Someone complained that regen is disabled during ABS, and they don't understand why.


    MY point is that if I'm about to DIE, I don't give a darn about regen. I support Honda's decision to disable it during emergency ABS braking.


  • mistermemisterme Posts: 407
    I think ABS is currently applied where it needs to be....right at the wheel.


    If you were to depend on IMA or HSD systems to pulse the wheels then you are going through differential, gears, bearings, belts and would work against the inertia of all the associated spinning parts and any wear and tear slop that may develop over time.


    The only way I can see for this to work is to also pulse right at the wheel, which would require traction motors at each axle. I think this could be a vastly better way but we don't have it yet.


    If you want regen at the same time as ABS, then regen might render ABS useless because the regen load on the wheel might keep the wheel sliding when it should be in its rolling cycle.
  • robertsmxrobertsmx Posts: 5,525
    Engineers may or may not have deliberately disabled regenerative braking during situations where other accessories like ABS or stability control take over. If they did attempt to keep them exclusive, reason could be reliability and effectiveness of the overall system (a situation I can imagine is lack of a good feedback system from the hydraulics based pumps that controls ABS could leave the controller undecided as it tries to switch between motor and generator modes).


    The other issue may be “automatic”. ABS triggers on loss of traction, an instance where wheel slips rather than rotates (regenerative braking requires wheel rotation). No rotation, no regen.
  • patpat Posts: 10,421
    Consider buying a hybrid? Reconsider after you learned about the long wait? Whether or not you chose to add your name to the waiting list, a reporter is hoping to hear your perspective. Please respond to with your daytime contact info by May 27, 2005.
    Jeannine Fallon
    Corporate Communications
  • I originally posted this in "Hybrid Tips: Optimizing mileage", but then found this forum and thought the post might be more appropriate here.

    I have read a lot of posts talking about how to drive to ensure that the electric motors only are on (referring to the Prius and other "full hybrids" that can drive on electric motors only), and I am a little confused about how that helps mileage. After all, the energy to power the batteries ultimately comes from the gasoline that is put into the car.

    From my understanding of how hybrids work, one of the main ways they get better fuel economy is by having smaller engines, and by running those engines more efficiently.

    However, whenever energy is converted from one form to another, I would assume that the conversion is not 100% efficient. If the gas engine was running at a similar efficiency when powering the car as when it charges the battery, wouldn't you get better mileage when the gas engine only is powering the car?

    I would think that when travelling at a constant speed, and with the CVT allowing the gas engine to operate at peak efficiency, you would get better mileage when using gas only. The electric motor helps mileage by providing extra power / acceleration when necessary while allowing the gas engine to operate at peak efficiency, and also by storing the kinetic energy from braking / coasting.

    I admit that I am not very knowledgeable about hybrids, so I was wondering if any of the experts could provide me more insight.
  • stevewastevewa Posts: 203
    You are correct. The reason people driving to maximize EV mode get better mileage is that they are using less torque as a result of their driving strategy. In theory the most efficient driving mode is when all the power from the gas engine is going to the road while at the same time the gas engine is working at its most efficient load. There are only a limited number of situations where this can occur, however. My Escape seems to do its best at about 45 MPH on level ground, this is just above the speed where the gas engine _has_ to be spinning to protect the electric motor from overspeeding...but it can operate with very little throttle.
  • Shadow11,

    Here is a simple answer.

    You are right conversion is not 100% efficient.

    In a normal car none (0%) of braking energy is recovered.

    In a hybrid with regenerative braking some of the enegry is recovered and used to charge the batteries.

    Later the batteries use this recovered energy to run electic motors to provide some of the car's power.

    Essentially this electric power is free.

    That reduces the power need from the gas engine and in some case also the size of the gas engine needed.

    This effectively increases the miles per gallon of hybrid cars.

    YMMV (Your mileage may vary),

  • Hi MidCow,

    Thanks for the response.

    I realize that the electric power generated by braking is one way that hybrids become more efficient. I believe the battery also charges when you let the engine do the breaking by coasting to a stop. However, I was under the impression that the majority of charging of the battery was performed by the ICE.

    Is this not the case?
  • Shadow11,

    Most of the charging comes from regenerative braking.

    In the Prius and other cars/vehicles that use the HSD system from Toyota, there is a small motor/generator ( MG1) which also charges the battery when you are driving because it is always spinning. Many critics say this is inefficinet in that it continues to take some power away from the ICE and try to charge fully charged batteries.

    The Hondas ( Accord Hybrid, Civic Hybrid, Insight) use an IMA (intergrated Motor Assist) and it primarily provides battery charging through regenerative braking.

    In both cases you will also obtain some charging when you let off the gas and coast in your vechicle. . However, in both cases this amounts to only a small amount of charging.

    Good Luck,


    P.S. -For what it is worth I think the Prius is probably the best hybrid car. The primary reason I didn't get one was because it doesn't ot have a manual shift transmission and I really like to shift :)
  • john1701ajohn1701a Posts: 1,897
    > Most of the charging comes from regenerative braking.

    That blanket statement doesn't apply to Prius and other HSD vehicles. They are hybrids with *PERSISTENT* electrical systems. Regenerative braking only contributes a small amount of electricity with respect to how much the generator creates... which is far from small. It is 10kW, which is the same size as the current Civic-Hybrid uses for its thrust motor. With the 2006 Civic-Hybrid, that increases to 15kW... which is still considerably smaller than the 50kW Prius currently uses.

    Civic-Hybrid and the other IMA vehicles are hybrids with *PASSIVE* electrical systems. With them, the regenerative braking is the primary source of electricity.

  • john1701ajohn1701a Posts: 1,897
    > try to charge fully charged batteries

    That is a misconception.

    In reality, the electricity is simply directed back to the thrust motor immediately, rather than using it to recharge the battery-pack.

    This is surprisingly efficient, due to the way the Planetary-CVT is designed. The reason for the misconception is that people don't realize how frequently this happens. 10 times per minute is quite common, which is fast enough to take advantage of an efficiency opportunity without allowing the penalty of charging to take place. In other words, their are inefficiencies with the gas engine that it prevents from occuring.

  • Actually you are wrong! why confuse people with incorrect answers electric motors provide high torque at low RPM to help intitial acceleration.

    This is one of the primary reasons the highway mileage on the Prius and other Toyota HSD deriviative hybrid systems is lower than the city mileage. EPA Prius 60/51 60=city mileage 51=highway mileage.
  • Actually you are wrong again! This has been a critcism of the Toyota HSD system all along. The small motor generator (MG1) always spins. It serves to top off the battery, but when the battery is full and when MG2 can no longer supply power and torque to the car becuase the car is going too fast, the energy is wasted.

    This has been discussed over and over. You might want to look at some of the early discussions in the Prius 2004+ thread

    This is one of the primary reasons the highway mileage on the Prius and other Toyota HSD deriviative hybrid systems is lower than the city mileage. EPA Prius 60/51 60=city mileage 51=highway mileage.
  • It is so weird how I get the opposite of what the EPA states. First off, the EPA testing is ancient and we all know that. If I drive in city traffic I get lower than I get on the highway. I am currently averaging 47.3 but I drive 75-80 all the time. To me, that's great.
  • john1701ajohn1701a Posts: 1,897
    What are you talking about?

    That most certainly is not how HSD works!

  • John Fargunat (John1701a) said:

    "What are you talking about?

    That most certainly is not how HSD works!


    The EPA on a Prius is 60/51 The first number means the expected miles per gallon in the City, 60 miles per gallon. The second number is the highway miles per gallon, 51 miles per gallon. The City miles per gallon is 9 miles higher than the highway. All cars are more efficient on the highway, but the Prius and other HSD based vehicles are not. One wonders why the little motor generator MG1 spins and spins needlessly converting ICE to electrical at the penalty of energy conversion loss when the traction battery is fully charged!

    Have a good day,

  • john1701ajohn1701a Posts: 1,897
    First, you grossly misspelled my name.

    Second, that reasoning is based on common assumptions... not how the system actually works.

    There are far more efficiency savings opportunities in city type driving than there are when cruising on an open highway. The HSD system takes full advantage of them by preventing the inefficiencies that would normally occur, hence the MPG observation.

    And where the heck did that "fully charged" claim come from? That isn't even close to correct. The Multi-Display very clearly shows 6 bars (68.57%) almost all of the time. I've personally only seen it up to 8 bars (80.00%) briefly 3 times over the past 40,600 miles. That's not full by any definition.

  • ctcmoctcmo Posts: 1
    Here's a newbie question for you:

    How does regenerative braking actually work?

    I've heard again and again that it converts kinetic energy, but I don't know by what means. Where on a hybrid vehicle is the actual mechanism that converts braking energy, and how does it do so?
  • toyolla2toyolla2 Posts: 158
    Hi midcow,
    on post # 88 you stated the following

    " One wonders why the little motor generator MG1 spins and spins needlessly converting ICE to electrical at the penalty of energy conversion loss when the traction battery is fully charged!

    It is first necessary to to have a clear idea of what the Toyota HSD is. It is a power split device and takes the form of a planetary gearbox except that unlike conventional planetary gears where the the outer ring gear is fastened to the gearbox casing and is therefore stationary, in the HSD this gear is allowed to rotate (upto 6000rpm @100mph) and is connected to the drive wheels via a 5:1 ratio.
    MG1 is connected to the central sun gear and though it may appear to be spinning needlessly to midcow and others, it is in fact splitting off some power which it must do (about which more later )
    The engine is connected to the planetary carrier and of course provides the power which gets split. The three planet gears are distributed and held symmetrically around the carrier and make contact with the internal teeth of the ring gear. Clearly (I'm hoping it's clear!) if the sun gear was removed there would be nothing for these three gears to 'bite' on and the engine would be rotating easily while those three planet gears would be spun around by the internal teeth of the ring gear as they passed by. So almost no torque would be imparted to the ring gear and the car would sit still. If now the sun gear and MG1 assembly is mounted there would still be no torque if the sun gear with MG1 were allowed to start flying off in the opposite direction. At this point the engine cannot put any useful power into the system so there is no power to split and MG1 would - in midcow's words - spin needlessly.
    But supposing the electronics to MG1 are turned on. And the electronics says "I am going to continue to allow MG1 to be pushed backwards at 6000rpm but I will not allow it to be pushed backwards at 6001rpm." Of course with a little more engine throttle you know this is bound to happen. But the electronics are probably measuring the shaft speed about 100 times/sec with great accuracy at this time and when this tries to happen will change the electrical conditions to make MG1 start to regenerate probably as much 22kw. This will have the effect of applying a restraining force of up to 20lb-ft on the sun gear. The planet gears will pass this force on from the sun gear to the ring gear which due to the 2.6 ratio between sun and ring will exert a torque of 50lb-ft. This example pertains close to full throttle. It shows how quickly torque can be generated and it attempts to explain the power splitting process and how a small torque at the sun gear can control a much larger torque from the HSD output. It should be mentioned that the engine is effectively supplying both these torques . At lower throttle openings the same strategy applies with MG1 always extracting power from the sun gear in order for the HSD output (the ring gear) to be able send power to the wheels.
    So MG1 never spins needlessly.

    BTW where does the generated power go?, it goes to the electronic inverter which powers MG2. MG2 is made to devour all the electrical energy brought in by MG1 unless the battery happens to need charging in which case it cuts back to allow the battery to absorb the excess. MG2 happens to be conveniently connected to the ring gear where it drives the wheels by the same 5:1 reduction as previously mentioned.

    Why is MG2 twice as big as MG1 ? The answer is because during acceleration MG2 not only has to absorb all the power from MG1 but also absorb up to 20kw from the battery as well.

    But what's the point of all this ? A very good question, I'm glad I asked !

    The Prius is capable of a lot more things than has been mentioned here. To get through everything would take half a dozen posts at least. And some involve speculation because not many people play with synchronous motors and inverters. And, I might add, those that do utilise industrial servos do so without needing to understand the why. Getting two servos with the same inverter bus when either can be called on to motor or generate under a wide range of speeds particularly when mechanically coupled with a planetary gear and with the addition of a prime mover in the mix is something that needs a write up from the designers themselves. And this is what we are not getting.

    The real point of a spinning MG1 is to form a CVT function. The so called Continuously Variable gear ratio Transmission. Toyota chooses to use an electro-mechanical implementation while Honda have chosen to use a wholly mechanical implementation as found in the 2006 HCH. I have to say that now that Honda have dispensed with the manual transmission option on the Civic hybrid it is to be hoped they can concentrate their resources to really nail this device and improve its reliability. Something that industrial CVTs were lacking when they were discarded years ago.

    The reason for the CVT was originally for it to be an alternative to automatic three speed transmissions which were no more efficient than now particularly on subcompact cars. Today the reason is to allow a much smaller engine perform as if it were a larger engine when needed but to have that fuel consumption of the smaller engine most of the time.
    When using a manual transmission, the power profile of a constant torque engine is triangular, power increases with rpm. The area under the curve represents the energy delivered over time.
    The power profile of the output after an engine goes through a CVT, however is closely rectangular - at low revs the torque is much higher than in the previous case dropping slowly until at high revs the torque will become the same. The area under the CVT's rectangular profile will be almost double that area found under the manual transmission therefore the CVT will have delivered twice the energy in the same time.
    This can be played two ways by doubling the road performance of an existing engine or accepting the same performance from a weaker engine.
    The Toyota Prius and HCH obviously go for the latter and the fuel economy it brings. I would say that CVT's can provide fuel economy approaching double that of the manual transmission car and then that's it folks ! There will have to be another shift in thinking.
  • mistermemisterme Posts: 407
    "Civic-Hybrid and the other IMA vehicles are hybrids with *PASSIVE* electrical systems. With them, the regenerative braking is the primary source of electricity"

    Regen accounts for 5-10% recharge, the other 90% comes from the gasoline engine.

    Either you don't know the IMA system or are just spreading misinformation about it.
    Hybrid vehicles don't need more misinformation spread about than already is.
  • Wow that was pretty interesting explanation but it still didn't explain why the MG1 keeps on providing a charge even when it is not needed. It you could put a centrifical electric operated cluth on MG1 then it would have to dump excess power when the MG2 didn't need it the PSP didn't need it and the battery was at it optimum charge ( 80% according to JOHN).

    It seems to me that the wider the CVT range the better. With the palnetray CVT concept is is practical to be able to get a very low equivalaent first gear and a very high evivalent high gear. Typically a apread of 5-6 is considered very wide. This afford the best acceleration and the best no load mileage. However, with the torque of the electric motor the lwo end is already sufficient.

    Actually you are really trying to simplify it too much to call the torque cure triangular and the power outpiut profile retangular.While it might approxiamte to that on an occiliscope it is actually closer to a standard bell curve and inverted "U" . Actually an ideal engine would always run at the most efficient output maxiumm torque , WOT. The with CVT you just adjust the effective gear ratio to the desires speed. But once you have charege the battery and are at the desired speed what do you do with the excess power. You could back off the engine speed but then you reduce the eficiency of the engine, which an ICE has low effciency to start with or you could pulse the engine, but there is a startup lag time.

    I am sure you realize the torque curve of an electric brushless motor , It is almost maximu torque from 0 to approximately 1000 rpm after that is significantly falls off. The HSD system effectively extends this by mutiplying the efective rpm through gearing.

    If you need more explanation please let me know, but you have pretty well written most of the information written in the Prius technical manual. Let me guess, you are an engineering student just getting ready to graduate. Good luck with your futre job search. Again thanks for your efforts to explain the Prius CVT system.


  • toyolla2toyolla2 Posts: 158
    Hi midcow,
    too bad you couldn't follow my explanation. I tried hard to put up the best post I could. I even used spell check, something that you could use also if you don't mind my saying.

    Perhaps this is the bit that you, or the person reading for you, may have missed.

    MG1 is always extracting power from the sun gear in order for the HSD output (the ring gear) to be able send power to the wheels.
    So MG1 never spins needlessly.

    BTW where does the generated power go?, it goes to the electronic inverter which powers MG2. MG2 is made to devour all the electrical energy brought in by MG1 unless the battery happens to need charging in which case it cuts back to allow the battery to absorb the excess. MG2 happens to be conveniently connected to the ring gear where it drives the wheels (through the 3.95 : 1 reduction)

    Of course when the cruise speed is reached, accelerating torque will no longer be needed and power will drop considerably. MG1 must still act as a generator albeit at a much lower level in order to maintain control of the sun gear speed. Remember the car still needs torque transfer from the engine's planetary to the ring gear and it can only do that with an (dare I use this word with you midcow) "obstinate" sun gear. If a sun gear were to offer no resistance you may as well not have it there at that time. Now in that circumstance can you tell me where the torque would be coming from to continue to propel the vehicle. ?

    BTW If you going to reply spare me the remarks that appeared in the last paragraph of your previous post please and thankyou.
  • stevewastevewa Posts: 203
    Just because MG1 is spinning doesn't mean it's generating power.

    By the way, this is NOT an electric brushless (DC) motor. It's a permanent-magnet, 3-phase AC motor. Same for MG2. IMA uses a brushless DC motor. Easier to wire but more complicated mechanically and electrically.

    The rate of rotation of MG1 is controlled by the computer to deliver the ideal "effective gear ratio" for the ICE. Depending on conditions that could mean that MG1 is acting as a generator, acting as a motor, or being forced back-EMF to provide a braking effect against it's input shaft. When acting as a generator, the power from MG1 can go to either MG2, the battery, or both.

    Oh, and internal combustion engines have their maximum POWER at or near redline, not their maximum TORQUE.

    I would not dismiss John out of hand, he's probably more knowledgable about THS and HSD than most Toyota dealership staffs.
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