Regenerative Braking Details?

When I let off my feet from gas padel, speed of Prius v lowering automatically. It seems something is slowly pulling back from the behind. Is it affected from the regenerative brakes?

 

When I let off my feet from gas padel, speed of Prius v lowering automatically. It seems something is slowly pulling back from the behind. Is it affected from the regenerative brakes?

 

When I let off my feet from gas padel, speed of Prius v lowering automatically. It seems something is slowly pulling back from the behind. Is it affected from the regenerative brakes?

 

When I let off my feet from gas padel, speed of Prius v lowering automatically. It seems something is slowly pulling back from the behind. Is it affected from the regenerative brakes?

 

Yes, the regenerative braking is designed to make the car feel a bit like a conventional car with an automatic transmission, so it regenerates when you let off the pedal.

 

The Prius strives to feel like a conventional automatic transmission, with engine drag slowing down the car when you let off the pedal, and creep forward when you are at a stop and let off the pedal. OTOH, Tesla has a "one-pedal" model. You speed up and slow down solely with the throttle pedal. If you let off the pedal completely, you will come to a quick complete stop. The only use for the brake pedal is emergency stopping. In fact, in the Tesla, using the brake pedal is 100% friction brakes; no regeneration. In the Prius, pressing the brake pedal uses either or both regeneration and the friction brakes, in whatever combination is more appropriate.

 

When you lift your foot off the accelerator or the drivetrain power requirement falls below zero, such as when decelerating or going downhill, the on-board electronics reverses the polarity of the connection between the motor–generator and the battery—this will cause the motor–generator act as a generator that charges the battery instead of a motor that is powered by the battery, which is known as regenerative braking. In the Prius Prime, regenerative braking is very efficient, and friction braking is only used at hard braking (shown on the MFD regenerative braking indicator as going beyond the maximum) or immediately before a full stop.

As for the transmission, the CVT in the Prius and Prius Prime employs the same planetary gear (sun, planet, and ring gears) system to change the gear ratio as in a conventional automatic transmission—the difference being that the gear ratio is tuned continuously by the varying speed of the motor–generator as opposed to discretely changing it by stopping the turning of specific gears by using brake bands on hydraulic clutches connected to these gears. There is also no torque converter but only a single hydraulic clutch to allow idling.

 

That is not how regeneration works at all. They are AC motor-generators so they reverse polarity many times per second all the time. If they were DC machines, reversing polarity would reverse direction of rotation, not direction of power-flow.

Those don't exist and that's not how the eCVT works.

 

You are correct that there is no Torque Converter, but there is also no "Hydraulic Clutch". Neutral is simulated by pinning the MG controlling the eCVT fast enough to cancel the rotation of the Internal Combustion Engine so that the drive shaft does not spin.

JeffD

 

Right, actually there is no reversing of polarity at all—it is done by controlling the voltage/current—if less power is being provided than the motor is applying, it becomes a generator.

Here is a summary:

(i) MG1 and MG2 Main Control

(1) MG1, which is driven by the engine, generates high-voltage electricity in order to operate MG2 and charge the HV battery. Also, it functions as a starter to start the engine.

(2) MG2, which is driven by electrical power from MG1 and the HV battery, generates motive force for the drive wheels.

(3) MG2 generates high-voltage electricity to charge the HV battery during braking (regenerative braking cooperative control), or when the accelerator pedal is not being depressed (energy regeneration).

(4) MG1 and MG2 are basically shut down when neutral (N) is selected. In order to stop providing motive force, it is necessary to stop driving MG1 and MG2, because MG1 and MG2 are mechanically joined to the drive wheels.

(5) The MG ECU controls the Insulated Gate Bipolar Transistors (IGBTs) in the Intelligent Power Module (IPM) based on the signals received from the hybrid vehicle control ECU. The IGBTs are used for switching for the U, V, and W phases of each motor generator. There are 6 IGBTs that switch on and off to control each individual motor generator in accordance with operation as either a motor or as a generator.

(6) The illustration below describes the basic control used when the motor generator functions as a motor. The IGBTs in the IPM switch on and off to supply 3-phase alternating current to the motor generator. In order to create the motive force required of the motor generator as calculated by the hybrid vehicle control ECU, the MG ECU switches the IGBTs on and off in order to control the speed of the motor generator.

They exist in a conventional ATM. I said that the functionality of the brake bands and clutches are replaced by the motor–generator (connected to the ring gear as far as I remember) in the eCVT, with the same principle of a planetary gear system otherwise.

 

There is a one-way clutch, but it's not hydraulic. Also, they have a transaxle damper instead of a torque converter. Interestingly, the parking lock actuator looks like a miniature torque converter.

OUTLINE

(a) A P610 hybrid vehicle transaxle is used.

(b) Containing the motor (MG2) for driving the vehicle and generator (MG1) for generating electrical power, this transaxle uses a continuously variable transmission mechanism with a compound gear unit that achieves smooth and quiet operation.

(c) This hybrid vehicle transaxle assembly consists primarily of a generator (MG1), motor (MG2), power split planetary gear unit, counter gear, final gear, differential gear unit and oil pump.

(d) By utilizing a pluriaxial configuration for the generator (MG1) and the motor (MG2), the overall length of the transaxle has been shortened. A compound gear that consists of the ring gear of the power split planetary gear, counter drive gear and parking lock gear is utilized to drastically reduce size and weight. By using high accuracy machining for the gear tooth surfaces, low-loss bearings and an oil sling type lubrication mechanism, driving losses have been reduced resulting in improved fuel economy and reduced noise.

(e) This transaxle has a 4-shaft configuration. The power split planetary gear unit, an oil pump and generator (MG1) are provided on the main shaft. The MG2 reduction gear and motor (MG2) are provided on the 2nd shaft. The counter driven gear and the final drive gear are provided on the 3rd shaft. The final driven gear and the differential gear unit are provided on the 4th shaft.

(f) A differential pretorque mechanism is used. Straight-line stability and acceleration performance during periods of low load and low differential rotation when the vehicle is being driven normally are ensured.

(g) Lubrication for each gear is performed via the trochoid oil pump of the main shaft and final driven gear slinging up ATF. Through the use of a lubrication structure (oil sling type lubrication method) in which the gears sling up ATF, reduction of oil pump drive loss and enhanced transmission efficiency of the powertrain system have been achieved. Also, a water-cooled type oil cooler which optimizes the flow of ATF is used to achieve high cooling performance, resulting in a high efficiency and high output powertrain.

(h) By using the dual motor drive system, both the generator (MG1) and motor (MG2) can be used as motors when the vehicle is driven by motor.

(i) By installing a one-way clutch between the engine and transaxle to prevent reverse operation of the engine, torque from the generator (MG1), which conventionally only generates electricity, can be transmitted to the output shaft and adds the torque of the generator (MG1) to that of the motor (MG2) improving drivability and the feeling of acceleration when the vehicle is driven by motor.

(j) The vehicle can be driven by motor even in areas of high acceleration, contributing to an increase in the frequency at which the vehicle is driven by motor.

(k) The oil pump with motor assembly is used to ensure cooling of the motor and lubrication of each gear while the vehicle is driven by motor.

 

It would be useful to say which motor-generator, as there are two of them.

The only clutch I can remember in a regular Prius is a spring loaded torque damper meant to limit shock loads. It has no hydraulics or any controls or actuator path to release it by outside command, it just slips if excessive torque arrives. During idling, it should be fully locked.

A Prime does have a sprag or one-way freewheel clutch, allowing the ICE to spin in only one direction. This allows both Motor-Generators to provide propulsive torque in EV mode, while the ICE is held stationary instead of spinning backwards. Absent this clutch in a regular Prius, only MG2 could provide meaningful EV-mode propulsive torque, while MG1 is stuck simply being actively spun at whatever speed is necessary to hold the ICE still. IF MG1 attempted to contribute to propulsion, its contribution would be limited by whatever drag torque the ICE provides when spun backwards unpowered.

 

MG1 drives the sun gear and the ICE drives the three planet gears mounted on a carrier (shown as two planet gears in the pictures below for simplicity). The speed of the ring gear, which is connected to the driveshaft through some other gears, is given by a combination of the speeds of the sun and planet gears. Therefore, by adjusting the speed of MG1, which drives the sun gear, the rpm of the ICE, which is connected to the planet gears, can be controlled, achieving a continuously variable shift ratio.

A conventional discrete-speed ATM on the other hand employs one, two, or more planetary gear units in cascade, depending on the number of gears, and shifting gears is achieved by stopping the rotation of some gears by applying brake bands on their hydraulic clutches. The construction and operation of the Prius Prime eCVT is a lot simpler thanks to the variable speed of the motor–generator connected to the sun gear, which eliminates hydraulic clutches, brake bands, and the need for cascading two or more planetary gear units.

Wikipedia explains how a planetary gear system works and gives the formulas for the effective gear ratios etc. in terms of the number of teeth and rotational speeds of the sun (MG1 in Prius Prime), planet, and ring (drive output in Prius Prime) gears and rotational speed of the planet carrier (ICE in Prius Prime).

Planetary gear system on Wikipedia

 

The outside material you have copied here today is almost entirely a rehash of subjects posted by numerous others over the years, but hopefully the different style and presentation has helped you and various others understand how the system works.

What is the maximum electric power that the Prime can pump into the battery during regeneration? I don't remember the specific number, but it seemed not all that much higher than on a regular Prius, which is about 27kW for my Gen3.

And in the regular Prius, this maximum battery charge rate means that regeneration can provides only light to moderate braking power, depending on vehicle speed. So friction braking must be engaged well before one gets up to hard braking levels. Hard (friction) braking on a loaded Prius at highway speed can exceed 500 kW.

 

It is not outside material. The two pics I posted are a small excerpt from my 1234-page Prius Prime new-car features manual. The text is my own explanation. I don't know if anyone fully explained the operation of the planetary gear system and eCVT here before. It is also not clear to many people how the engine rpm is controlled in an eCVT, let alone in a conventional ATM, and what the similarities and differences for the two are.

 

My new-car features manual lists 23 kW (31 hp) and 53 kW (72 hp) for MG1 and MG2, respectively. During deceleration MG2 does all the regenerative braking. The HV battery is listed as 351.5 V and 25 Ah (8.8 kWh) at C/3 discharge (3-hour discharge). MG specs are probably the same as those for the non-plug-in Prius, but the larger battery should allow more regenerative braking. However, Toyota may have chosen not to allow more regenerative braking in the Prius Prime.

 

I like this post, I learned a lot here. It seems that the American education system is great.

 

Every post that reminds readers the New Car Features manual exists and can tell them stuff is a useful post.

There have been plenty of explanations over the years. The model Niels made in Blender is pretty darned good.

The thing about Prius motor control is that you can talk about it a whole range of different ways, from "we flip the polarity" at the very simple end, up through a ten minute video from Mathworks, up through an hourlong video from TI, and every one of those could be called a fair description for a certain level of detail and audience interest.

The TI video is great if you've got the time, but the ten-minute Mathworks one is pretty good. And there's a companion video that picks up where it leaves off and shows how you can do that with some IGBTs that only switch on and off.

It doesn't explicitly show you how to do regen braking, but it's not a big stretch to see that if you shift the q vector so that it is lagging the rotor field rather than leading it, you have a torque that pulls against the MG rotation rather than with it, and the math tells you there's power coming out then instead of going in. And shifting that vector from 90° leading to 90° lagging amounts to nothing but putting a minus sign in front.

It's kind of neat that you can start out all oversimplified with "all we do is flip the polarity", and then you go into all kinds of three-phase AC messiness, and then you get the transforms that let you look at all that in the reference frame that moves with the rotor, and you end up back where you can say "see? all we did was flip the polarity!"

 

Regenerative braking power is limited to the traction battery's maximum charge rate, which is missing from this list of ratings. In a regular Prius, it is significantly less than MG2's power rating.

 

For visual clues to regenerative braking in the Prime, DrPrius can give clues to how much and how long higher levels of charge are produced. It's not often that I've seen the apps graphs switch color from green and / or yellow to RED, but it does happen. I've seen DrPrius report 80Amp during deceleration once and 100Amp during acceleration once, although very briefly!! , so it's hard to catch unless you are anticipating, and there is time to SAFELY watch for it.
Again as described above, how much charge actually transfers from the generator to the traction pack, I would surmise (because I don't have proof of concept) that the electronics, or if you will, the data present in the various ECUs, determines the current sent to the traction pack at any one time.
Just my take on a subject that can be hard to fully understand.

 

Correct. More precisely, it is limited by the maximum battery power, which is equal to the battery energy capacity in kWh multiplied by the maximum C rate in 1/hours (charge/discharge rate in 1/hours), the latter of which only depends on the precise battery type and model, which should be the same for the Prius and Prius Prime. So, it is directly proportional to the battery energy capacity in this set containing the Prius and Prius Prime. However, as I said, I suspect that the Prius and Prius Prime have the same maximum regenerative braking power, as Toyota is very conservative in its designs, despite the Prius Prime battery being about twelve times the size of the Prius battery. Moreover, the electronics is also a limiting factor, as it has absolute maximum current and voltage ratings. There are also cooling issues.