Introduction to Prius Power Flow

[bwilson4web]

A frustrated automotive engineer once claimed that you had to have a doctorate to understand the Prius transaxle. Learning how it works can lead to a headache but it is understandable. Hopefully, the following charts and data will clear things up.

Power Split Device

The key to understanding is how the planetary gear, power-split device works. The ICE powers the planetary gear carrier and the planetary gears are in a computer controlled dance between MG1 driving the sun gear, and MG2 driving the ring gear.

(I edited in the ICE power shaft from images of these parts found at: http://privatenrg.com/01-03MGs%20&%20PSD.jpg)

Normal Power Mode

In this mode, MG1 provides the counter torque to the engine, always 28%. This counter torque generates power that electrically passes to MG2. The remaining 72% of torque passes to the ring gear and via the chain drive to the reduction gears and differential.

In order to make the power schematic easier to understand, I have shown MG2 connected to the chain drive pickup. In fact, it is connected to the ring gear in an very compact arrangement. However, this does not change the basic power flow.


Energy Re-Circulation ("heretical" or "overdrive")

Unfortunately, normal mode leads to higher engine power settings and speeds. To put the engine in a lower-speed, highly efficient region, the transaxle can use MG2 as a generator and pass power back to MG1. MG1 then combines this with the ICE power and it 're-circulates' via the chain back towards the wheels.

When some of the earliest Prius owners noticed this it was called "heretical" mode. Toyota engineers published a paper and called it "energy re-circulation" mode. Regardless, it provides a very efficient operating mode.



Transition Between Normal and Re-Circulate Mode

The following data was captured from a Graham miniscanner and shows the transition from 'normal' mode, used for higher power operations, to "energy re-circulate." Acceleration and hill climbing are high-power, normal modes of operation but once the target speed is achieved, the car transitions to "energy re-circulate mode."


'Kevin17' pointed out that in "energy re-circulate" mode, MG1 is working to keep the ICE from spinning faster. This additional load on the ICE keeps the ICE rpm low so it can generate maximum power for the fuel burned AT A LOWER POWER SETTING. It is similar to 'lugging the engine' only done so the ICE is never overstressed. Thanks 'Kevin17' for the insight. Also, Hobbit posted, ". . . the engine was running barely above idle -- 1100 RPM or so -- the throttle was significantly open, shaft torque was high, and RPM was being kept down by the "heretical mode" electrically-routed overdrive. . ."

Optimum System Efficiency

As the Toyota engineers pointed out, optimum system efficiency requires balance and tradeoffs. Sometimes one part has to operate in what appears to be a less efficient mode so another part can perform at peak efficiency. It is the total system performance that is important.



Example of Regeneration

One of the better ways to see regeneration in action is to descend a hill. In this example upon entry to the hill at 65 mph, the transaxle was put in "B". The battery was at a nominal 60% charge and quickly reached the 80% limit. Thereafter, no further changing could occur and the rest of the braking was from spinning the engine.




Climbing The Hill

To drain the battery from a nominal 60% down to 40%, drive up a hill at high speed. In this case, there was a small 'leveling' when the vehicle attempted to transit to "re-circulate" mode. Then the hill climb continued.



Hopefully, this makes things a little clearer.

About Efficiency

Dept. of Energy measurements indicate MG1 and MG2 typically have 95% efficiency as either a generator or motor. Switching power supplies like the motor drive circuits can achieve 90-95% efficiency. So though there are electrical losses, they are fairly modest, especially considering only a fraction of the power passes through this path.

As for mechanical losses, this remains an area of investigation. Some claim that each gear stage is only 95% efficient. This is not something I have direct knowledge of so I'll leave it unchallenged.

Bob Wilson

[usbseawolf2000]

Very nice Bob. I see you captured those graphs with your 03 Prius. I didn't know the battery power spiked during the transitional period between modes. During this time, ICE RPM dropped dramatically as well. I guess that is one way to tell.

How did you drive when you captured it? Did you keep your foot on the pedal firmly? The reason I am asking is because I've been letting go of the accelerator pedal and then slowly pushing back to maintain speed. This way, I could get about 10% better MPG on highway cruising. I now realize that I was probably putting my Prius into the "energy re-circulation" mode.

Also... How did you get the "Development of New-Generation of Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy" article? I would love to get a hold of it. Hungry for knowledge.

[bwilson4web]

<div class='quotetop'>QUOTE(usbseawolf2000 @ Mar 5 2007, 06:28 PM) [snapback]400609[/snapback]</div>

Very nice Bob. I see you captured those graphs with your 03 Prius. I didn't know the battery power spiked during the transitional period between modes. During this time, ICE RPM dropped dramatically as well. I guess that is one way to tell.

How did you drive when you captured it? Did you keep your foot on the pedal firmly?[/b]

The data came from an ordinary drive and wasn't 'scripted.' But the MG2_rpm is recorded which lets us plot the mph.

NOTE: I updated the chart to show the mph over the same interval. Just refresh the browser to get the new graph.

. . . Given the sharp cut-off, I probably put the car in "N" to turn off all energy flows and perform an energy neutral glide.

<div class='quotetop'>QUOTE(usbseawolf2000 @ Mar 5 2007, 06:28 PM) [snapback]400609[/snapback]</div>

. . . The reason I am asking is because I've been letting go of the accelerator pedal and then slowly pushing back to maintain speed. This way, I could get about 10% better MPG on highway cruising. I now realize that I was probably putting my Prius into the "energy re-circulation" mode.

Also... How did you get the "Development of New-Generation of Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy" article? I would love to get a hold of it. Hungry for knowledge.
[/b]

Google for "Development of New-Generation of Hybrid System" works for me. As for how to get into 're-circulate' mode, I have a four-line, Graham scanner configured for:

• MG2_Nm - positive sign for "normal" and negative for "re-circulate"
• MG1_Nm - positive sign to start ICE and negative for either "normal" or "re-circulate"
• Batt_I - indication of battery energy contribution or charging
• MG1_rpm - indirect indication of ICE rpm
• MG2_rpm - speed
• Batt_V - battery voltage for energy calculations

I can easily monitor the vehicle energy flows, in rough engineering units, while driving. So far, it looks like there are three acceleration modes to investigate: (1) normal, (2) MG2_Nm close to 0, and (3) MG2_Nm negative. Each is slower than the previous but I'll need to record injector timing to see how efficiently I can achieve a given kinetic energy . . . something for next month's posting.

Bob Wilson

[Ken Cooper]

Here's a planetary animation from inachan@mac.com that you might find helpful:

http://homepage.mac.com/inachan/prius/planet_e.html

For some of us, a little animation tied to the above bwilson4web posted PSD information helps in getting a clearer picture of how this little marvel works.

[Ken Cooper]

Here's another way of looking at it:

MG1 is tied to the sun gear. It's virtually always a generator except when used as a motor to start the engine.

As an aside, here's how MG1 acts as a starter motor: For engine start, the computer tells the battery to supply processed battery power to the generator (MG1) stators, thus speeding up the generator/sun gear. The ring gear with its low speed load can't follow the increased sun gear speed so the planet carrier (engine shaft) has to start turning, making up the difference between sun gear's and ring gear's speed. By the way, while MG1 has been used as a starter motor (turning the engine), the computer has also commanded both spark and fuel for the engine.

MG2, when NOT in regeneration mode, is always acting as a motor (at least at those times when the computer allows power to be provided to it). MG2 receives processed power from sometimes the battery, sometimes the generator (MG1), and sometimes both. MG2, by the way, is tied directly to the final drive through fixed reduction gearing. MG2 also ties directly to the ring gear of the planetary drive.

So:

. We step on the accelerator from stop, supplying processed power from the battery to the motor to the final drive and the car starts moving, then speeding up.

. When we reach that point where the benefits of the motor's low RPM torque aren't going to gain us that much more, the engine is started and it starts helping the motor, to the extent it can, drive that ring gear, thus speeding up the car.

. Although the engine (tied to the planet carrier), can increase RPM, the ring gear can't follow the engine RPM because the car's speed is still too low to follow without step down gearing (we don't have that). So, the engine's energy that doesn't transfer to the ring gear, transfers to the sun gear, and thus to the generator (MG1), increasing its speed, thus helping the battery to supply more power to the motor (MG2).

. With more power to the motor (MG2) the motor works harder, working with the engine to speed up the car (remember, both are driving the ring gear).

. With that additional help of the generator (MG1) to the motor (MG2), the further increased speed of the car allows higher ring gear RPM, thus helping to bring the ring gear's speed up to where ultimately its RPM will become a best possible match for the engine's high RPM maximum torque capability.

. And so it goes. The motor helps the engine which helps the motor which helps the engine (ad infinitum).

This cooperative effort between the power split device's sun gear, ring gear, planetary drive, MG1, MG2, and the gasoline engine makes up Prius' definition of a continuously variable transmission (CVT).

This post and the one before were lifted from heavy discussion on this forum from several years ago. Many of the Prius buyers then were somewhat technically oriented (technical entheusiasts love the Prius).

[MPG > HP]

Power Management Under Cruise Control?

It seems like a fully charged battery would want to help the ICE on uphill runs and use gravity to help regenerate on downhills. So, why does my 06 always generate a "green" battery on uphills and discharge on downhills when on CC? Is the CC power management algorithm as simple as, a) higher torque demand, use ICE, lower torque demand, discharge battery, c) approaching 40% discharge, use ICE?

Is there a chart or animation that simulates power management over trips with very moderate
(e.g., S 405 to N 605 freeway in LA) to high (e.g., Sepulveda Pass) elevation changes and various states of charge at the starting point of the measurement runs? E.g., what would a run from the SF Valley over the Sepulveda Pass and down into Marina Del Rey look like with the CC set at 60, 70 & 75 MPH with the battery at 40% and 60% charged at the starting point? How many "green cars" would be logged and what would be the different MPGs?

I'd be happy to hear any speculation. Thanks, Gary

[bgdrewsif]

wow... this makes no sense to me so i'm glad there are others here with far more mechanical aptitude than me!

[bwilson4web]

Hi Gary,
<div class='quotetop'>QUOTE(MPG > HP @ Mar 5 2007, 11:02 PM) [snapback]400728[/snapback]</div>

Power Management Under Cruise Control?

It seems like a fully charged battery would want to help the ICE on uphill runs and use gravity to help regenerate on downhills.[/b]

With my older NHW11 and the Graham scanner, I have access to over 50, decoded OBD signals:

http://ecrostech.com/Products/MiniScanner/Intro.htm

It has been a terrific road-map to how the Prius systems work and their modular construction. One thing it has done, along with the Toyota Maintenance Manuals and Toyota SAE papers is show the modular construction and integration of the Prius. So think of the cruise control as just a device that tries to maintain a given speed and sends its inputs to the hybrid vehicle controller, the master mind. The hybrid vehicle controller manages everything going on.

<div class='quotetop'>QUOTE(MPG > HP @ Mar 5 2007, 11:02 PM) [snapback]400728[/snapback]</div>

So, why does my 06 always generate a "green" battery on uphills and discharge on downhills when on CC? Is the CC power management algorithm as simple as, a) higher torque demand, use ICE, lower torque demand, discharge battery, c) approaching 40% discharge, use ICE?[/b]

There is not enough information to answer this question. It is a complex function dependent upon speed, grade and vehicle maintenance and one of the reasons why having a scanner is so important for getting quantitative data. At least with the NHW11, Toyota SAE papers and insights gained from this and other Prius technical forum, we're gaining a clue. Graham Davies, Ken, Hobbit and other early adopters have done seminal work and I've been lucky enough to build upon their efforts.

<div class='quotetop'>QUOTE(MPG > HP @ Mar 5 2007, 11:02 PM) [snapback]400728[/snapback]</div>

Is there a chart or animation that simulates power management over trips with very moderate (e.g., S 405 to N 605 freeway in LA) to high (e.g., Sepulveda Pass) elevation changes and various states of charge at the starting point of the measurement runs? E.g., what would a run from the SF Valley over the Sepulveda Pass and down into Marina Del Rey look like with the CC set at 60, 70 & 75 MPH with the battery at 40% and 60% charged at the starting point? How many "green cars" would be logged and what would be the different MPGs?[/b]

The closest I know of is Wayne Brown's fairly extensive Prius model:

http://privatenrg.com/

Some of my work has been building upon Wayne's earlier work with my own data. But if you have an interest in this subject, I'd also recommend reading the Miller and Miller paper, "Comparative Assessment of Hybrid Vehicle Power Split Transmissions" (Google for sources.) Ultimately, it comes down to using your Prius as an experimental device and doing the hard work of measuring what happens in well defined test environments. which has been the greatest teacher of us all.

BTW, the ICE was already running at the stop because it had yet to reach a temperature where the normal hybrid vehicle control laws could auto-stop the ICE. But the data had cleanly captured the transition from "normal" to "energy re-circulate" (aka., heretical) mode. Being visually oriented, I thought folks might enjoy seeing what happens.

Bob Wilson

ps. BTW, I had created the PSD graphic just a day or so before Florian Steiper's excellent transaxle disassembly photos. Also, Ken had asked about one of the earlier power flow schematics and that sent me back to the Toyota SAE papers. So together, we've advanced our understanding.

[bwilson4web]

<div class='quotetop'>QUOTE(bgdrewsif @ Mar 6 2007, 04:11 AM) [snapback]400779[/snapback]</div>

wow... this makes no sense to me so i'm glad there are others here with far more mechanical aptitude than me!
[/b]

Don't feel bad, it is not easy. It took seeing 'torque' and other data before I could get this far. So how far do you get before it makes no sense?

Understanding the Power Split Device, the planetary gears is the foundation. Do you see how the parts assemble and with the exception of the planetary gear carrier, freely rotate around the ICE power shaft? The planetary gears on the carrier freely rotate but on the inside is the sun gear operated by MG1 and the outside has MG2, the ring gear, that transfers power to the reduction gears, differential and wheels. So to get ICE power to the wheels via MG2, MG1 has to provide 'counter torque' to the planetary gears. This is how the 'split' part of the power split device works.

The largest part of the ICE torque passes through the ring gear and MG2 towards the wheels. The rest of the ICE torque passes to MG1, which works as a generator. The power of MG1 can then be routed to MG2 to recombine and pass on to the wheels. But in the meanwhile, the relative speed of MG1 and MG2 keeps the ICE operating in 'the sweet spot.'

Bob Wilson

[john1701a]

<div class='quotetop'>QUOTE(bwilson4web @ Mar 6 2007, 05:06 AM) [snapback]400782[/snapback]</div>

Understanding the Power Split Device, the planetary gears is the foundation.[/b]

That's why this document was created...



With a good understanding from the 10 pages of pictures & descriptions, you can take that next step (Bob's info). Understanding the relation of RPM, vehicle speed, and electricity available stimulates a new level of interest. The system is quite clever... and remarkably simple, despite what that first impression leaves you thinking.