What is an eCVT? How does it work? Here is the answer

[acdii]

The biggest difference I see between the two is Gen1 can cruise neutral, no charge/discharge on the HVB, but the Gen2 is Very Very difficult to get into that stage, I have achieved it on occasion and saw 50+ on the instant, but any little bump, anything that could get you to move a fraction on the pedal and it was gone. The other note on this is you just can't maintain a steady speed doing this, you will slow down. (unless going downhill, and then you speed up like crazy).

 

What this tells me is the ICE is putting out a lot more torque to the MG1 so it is always putting out power for either the traction motor or to the HVB or both. If you notice on the Gen1, the battery levels remain constant at anything over 45 MPH, which on mine was at 50% SOC on the dash. The only time this was not true was when it was in a conditioning mode, then it fully charged the HVB and the ICE ran 100% of the time. This new one doesn't do that, it charges similar to the Toyota HSD where it almost always went to full when above 42 MPH. When I go down certain roads that are horrible for any EV, I usually wind up with a full SOC, instant hovers around 40(higher in summer), and there is always a charge symbol on the battery. According to the PDF linked above, at this point the ICE should be spinning MG1, with MG2 providing the power to the wheels. Looking at the gauges provided, it does appear to be doing just that. SOC does not change, yet the ICE is always charging. When accelerating, or higher demand like climbing a grade, then ICE and MG2 are combined to propel the car. That is how I see it working.

[larryh]

Above are the Atkinson Engine Map and the Motor/Generator Map for a Prius. To maintain constant speed, the car needs to maintain constant power. It can do so by varying the torque and engine speed along any line of constant power. As an example using the Atkinson Engine, if 15 kW are needed, then the car could operate at about 4000 rpm and 35 Nm of torque, consuming about 290 g/kWh of fuel. It could also operate at about 2000 rpm and 90 Nm of torque using 220 g/kWh of fuel. Both operating points provide the same power (maintain the same speed), but the later uses far less fuel, only 220/290 = 76% as much as the first operating point. So you would want to design the car to operate at the second operating point when 15 kW of power is required.

 

You can think of engine torque and rpms analogous to the force applied to the pedals and cadence riding a bicycle. To maintain a constant speed on a bicycle, you can use a high gear, applying greater pressure to the pedals spinning at a slower speed; or you can use a low gear, applying less pressure spinning at a higher speed.

 

Suppose instead of operating at 2000 rpm and 90 Nm of torque, we operate at 2200 rpm and 90 Nm of torque. Then, from the chart, instead of producing 15 kW of power, we produce about 16.5 kW of power. The fuel consumption for 15 kW is 15 kW * 220 g/kWh = 3300 g/h of fuel. The fuel consumption for 16.5 kW is 16.5 kW * 200 g/kWh = 3300 g/h of fuel. Thus is it possible for the engine to produce more power using the same or less fuel by choosing a more efficient operating point.

 

The question is what to do with this excess power achieved by moving to a more efficient operating point. We can't just apply it to the wheels. That would make the car go faster. Instead, we could doing any of the following:

1. Use the generator to charge the battery and place a greater load on the engine to absorb the excess power.

2. If the generator produces more power than the battery can handle, we could divert some of the power from the battery to the electric motor. Going in series from the engine through the generator, then to the electric motor, and finally to the wheels is a less efficient route than having the engine directly power the wheels. That wastes some of the excess power that we need to get rid of, but allows the car to operate using less fuel.

3. If the battery is full, we could use energy stored in the battery by operating the generator in reverse and putting a load on the engine to absorb the excess power. This may seem wasteful at first, but it could place the engine at a more efficient operating point that uses less fuel.

4. It could be possible that the indirect route which requires more power from the engine (having the engine power the generator, which then powers the motor, and finally the wheels) actually allows the engine to operate at a more efficient operating point and use less fuel than using the direct route.

 

I'm not an expert on engines. But this is what I think is happening based on what I have read.

Edited January 18, 2014 by larryh

[hybridbear]

The biggest difference I see between the two is Gen1 can cruise neutral, no charge/discharge on the HVB, but the Gen2 is Very Very difficult to get into that stage, I have achieved it on occasion and saw 50+ on the instant, but any little bump, anything that could get you to move a fraction on the pedal and it was gone. The other note on this is you just can't maintain a steady speed doing this, you will slow down. (unless going downhill, and then you speed up like crazy).

You can't really do this with our cars. Even when the dash doesn't show the up/down arrows for HVB charging/discharging, there is still power flowing in/out of the HVB to "other" on the Power Flow screen. The Prius will actually isolate the HVB where the ScanGauge shows 0 amps of current flow in/out of the HVB. What we can see occasionally is where the Power Flow screen only shows the HVB sending power to "other" and not to/from the wheels, but this is quite rare.

[acdii]

It goes back to what was discussed months ago. When the up down arrows disappear, it most likely means that the output of the ICE on the generator is matching all requirements of the electrical system. Its producing just enough power to drive the car and the internal electrical. I would like to see the current draw on MG2 and output of MG1 during different states, Acceleration, cruise, grades, etc.

[lolder]

In the Gen 1, my understanding of the arrows is that they indicate HVB charge/discharge. If there are no arrows visible, the HVB is not much involved at that point. The arrows are very vague so there may be a little participation by the HVB. That source of EV inefficiency is then lowest. There still may be EV transfer going on between MG1 and MG2. If you keep a steady throttle position in P & G acceleration with no charge arrow, there is probably not much transfer of EV as the ICE will run at the steady power level you are calling for and this system doesn't use much EV in that situation. In any case, my P & G tests show about a 30% improvement in efficiency.

I could keep the arrow from appearing on acceleration at a steady pedal pressure. I could also keep it off while coasting ( ICE off ) by a touch of pedal pressure to eliminate the built in regen.

This is not worth doing in the real world but it shows how the drivers got the 81 mpg in the 2009 1400 mile run. They did it by avoiding EV mode. That's the only way.

The hypermilers that did the test can get the same numbers from conventional cars by shutting off the ICE during coasting but they lose power steering, etc and it's much harder than with a hybrid. Hybrids do most of the same thing automatically.

Edited January 19, 2014 by lolder

[OneSpeed]

I can imagine the Tech's who are members of this forum, just itching to answer the questions, at the same time they like the guessing and secret keeping aspects of the trade.

[lolder]

These eCVT hybrids of Toyota and Ford have been around since the 1997 Prius in Japan and the 2004 Ford Escape. The basics of electric motors, batteries, Atkinson cycle ICEs and planetary gears have been around 100 years. Toyota took the plunge and merged them with modern electronics, battery technology and software control. "The Prius that Shook the World" is a fascinating read here: http://r.search.yahoo.com/_ylt=A0LEV7swA4lUokQAimwPxQt.;_ylu=X3oDMTBybnV2cXQwBHNlYwNzcgRwb3MDMgRjb2xvA2JmMQR2dGlkAw--/RV=2/RE=1418294193/RO=10/RU=http%3a%2f%2fwww.vfaq.net%2fdocs%2fPrius_that_shook_world.pdf/RK=0/RS=j7A4tQjmQZjPEGHMx1liCcoZQMo-

 

We don't like secrets and we do have to guess a little about some things because the manufacturers don't like to reveal too much, particularly Ford. We know pretty clearly how these cars function but the software is very complex and proprietary. The hardware is simple and bulletproof in theory.

 

I think the word that best describes the design of these cars is "elegant".

[lolder]

Most non-hybrids operate with the throttle plate significantly less that wide open at cruising speeds. When you want to accelerate, opening the throttle plate instantly allows more fuel and air into the engine for more power in a small fraction of a second. These hybrids operate with the throttle plate almost wide open most of the time for peak efficiency so there is little more power available for acceleration. That's where the EV assist comes in. A pulse of addition torque is instantly added by the traction motor and the computer starts to increase the RPM of the ICE to yield the new power level desired. This might take a full second or more. As the ICE reaches the new power level, the EV assist pulse dies down.

 

At slow speeds, the power required is too low for the ICE to deliver it efficiently so EV is used. As the HVB charge gets low, the ICE is started and used to propel the car AND recharge the HVB. That puts the ICE in a higher power and efficiency operating range. When the HVB is recharged, the ICE stops and EV resumes. This repeats.

 

When braking, the HVB is recharged.

 

That in a nutshell is how these hybrids work. Hondas, Kias, and others are significantly different. The whole intent is to recover braking energy and to insure the ICE only runs when it can be run most efficiently. The EV assist makes up for the open throttle Atkinson ICE's slow acceleration response. The Atkinson cycle ICE's are close to the efficiency of diesels per pound of fuel.

 

If you figure fuel by weight and fuel cost the FFH efficiency is about the same as the VW TDIs at highway speeds

 

Currently in SW FL the price of diesel is 22% higher per gallon and 7% higher per energy ( BTU, KWh, etc. ) than gas.

Edited December 11, 2014 by lolder

[hybridbear]

"The Prius that Shook the World" is a fascinating read here: http://r.search.yahoo.com/_ylt=A0LEV7swA4lUokQAimwPxQt.;_ylu=X3oDMTBybnV2cXQwBHNlYwNzcgRwb3MDMgRjb2xvA2JmMQR2dGlkAw--/RV=2/RE=1418294193/RO=10/RU=http%3a%2f%2fwww.vfaq.net%2fdocs%2fPrius_that_shook_world.pdf/RK=0/RS=j7A4tQjmQZjPEGHMx1liCcoZQMo-

The link doesn't work for me. I get an error from Yahoo saying "403 forbidden".

[corncobs]

Talking about TDI on the highway since I had one while over there I took some pics after a 300 miles Autobahn trip.

 

 

While on the way I took another pic at 100 MPH which yielded 34 MPG instant.

 

[lolder]

It worked OK when I posted it and I can get to the PDF file by clicking on the "here" link in that Yahoo page.

[GrySql]

It worked OK when I posted it and I can get to the PDF file by clicking on the "here" link in that Yahoo page.

http://www.vfaq.net/docs/Prius_that_shook_world.pdf

[aschulz261]

After watching and rewatching the videos, I am pretty sure I understand the basics of how the system works. The part I don't get is how the ICE supplies torque to the final drive. It would seem that the only way to do that would be to apply a torque to the planetary gears. I can see that happening if the generator is charging the HVB. But what happens if the HVB is charged? Can anyone explain how ICE torque is applied across the planetary gearset?

[acdii]

Some energy from the HVB is used to encourage the power to flow from ICE to final drive. Simplified, When max torque is requested, the MG on the ICE will spin at the same RPM as the flywheel into the planetary gearset to transmit the full torque to the drive MG, and when in generator mode the ice spins it faster (or slower), putting less torque on the PG. In a conventional transmission PG are controlled via clutch packs to lock drums at various speeds. In the eCVT this is all controlled electronically.

 

In a belt driven CVT its done with sheeves, controlled by hydraulics, as power demands are requested the CPU sends more or less pressure to the drive sheeve to change the belt position. When it is slow the final sheeve has more belt than the drive sheeve, and at high speeds the drive sheeve has more belt. Think of the chain on a 10 speed bike. In eCVT this is all done with the MG and planetary gearset. at high torque demands, the ICE side MG is spinning faster to transmit the power to the final drive, and at cruise it is turning slower.

 

I would love to disassemble one to see how they are interconnected between the ICE and output shaft to the diff.

[aschulz261]

so you are saying that a torque is applied to MG1 to lock it to the ICE rpm? does that put MG1 into generator mode or motor mode? It sounds like the software has to be extremely complicated.

[lolder]

MG1 can be a motor or a generator and turn in either direction. The software is very sophisticated but Toyota has been doing it since 1997 and Ford since 2003. When the HVB is full or the system doesn't want to charge it, MG1 generator load is created by driving MG2. This is all done by software that controls the three phase, synchronous, , permanent magnet and brushless MG1 and MG2. The torque can be controlled very finely and instantaneously. There are no moving parts except the MG rotors, the planetary gear and the gearing to MG2 and the differential.

The transformation back and forth from the high voltage DC of the HVB to the variable 3 phase AC voltage and frequencies of the MGs is done by "Insulated Gate Bi-polar Transistors" ( IGBT, http://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor). These were developed in the early '80's for industrial electric motor control such as on high speed trains and were built by electronics companies. Toyota realized they would need millions of them when thousands were then being produced so their own automotive electic and electronics company improved them. Toyota improved the reliability enormously because when one fails in a hybrid, the car is DOR ( dead on the road ). There are at least two in every eCVT hybrid and more in hybrids like some Fords and others that boost HVB voltage. The variable speed AC compressor may use one also.

There have been very few IGBT failures most of which were in 2006-10 Toyota Highlander and 2006-08 Lexus RX 400H SUVs which were recalled in 2013 for replacement of the power modules which contain the IGBTs. It sounds like a manufacturing defect cropped up.

Edited April 21, 2015 by lolder

[aschulz261]

That would explain it. I couldn't get my head wrapped around where the load on MG1 was coming from if the HVB was charged.

[hybridbear]

When the car is on the highway it will go into negative split mode (described in the power flow screen thread). In this mode the generator motor consumes electrical energy and turns it into mechanical energy to control the ICE RPM and to route the ICE torque through the planetary gearset to the wheels. The traction motor then consumes some of this mechanical energy and converts it to electrical energy which supplies the generator motor. This is why highway driving in our car is inherently less efficient than the Accord Hybrid which uses a clutch to circumvent the electric motors and their conversion losses when driving at highway speeds. This adds additional mechanical complications to the system but improves MPGs. The new Chevy Volt will behave similarly.

 

That would explain it. I couldn't get my head wrapped around where the load on MG1 was coming from if the HVB was charged.

[jdbob]

This adds additional mechanical complications to the system but improves MPGs. The new Chevy Volt will behave similarly.

 

Chevy has really doubled down on the complexity in the Volt. The original had the two motors and one planetary gear set like ours but with two electrically operated clutches. The new one adds another planetary gear set and a mechanical clutch to keep the engine from turning backwards. Where does it stop?

[lolder]

When the agree to pay the patent license fee to Toyota or Ford.

[Raymond J Ramirez]

According to the Escape Hybrid development history, Ford engineers did their own CVT design and later discovered that their design was similar to Toyota's. So Ford negociated licensing agreements to Toyota before production. The "jab" that Ford copied Toyota is false. This is another case where two different designs came up with the same results.

 

The Gen 1 design that GM used in the Chevy Volt is also different than Ford's or Toyota's and no licesing agreements were needed.. You can find how the Volt works online. And the Gen 2 Volt is more of a parallel hybrid than Gen 1, whcih will begin sales in October.

 

But GM's design allows the Volt to run 100% on electricity at 99MPH.

Edited September 28, 2015 by Raymondjram

[lolder]

There are three parts to the planetary gear sets that these cars use; the "Sun" ( center ), the "Planet Carrier" and the "Ring" ( outer ) gears. The Volt uses them differently than the Toyota/Ford method and it is inferior and needs clutches. As they modify it to be closer to T/F, it's performance improves. It never was an electric car as GM claimed, it was a series-parallel hybrid like the T/Fs and not as good. It has a niche, however that the plug in hybrids are inching towards with bigger batteries. The problem is the design goals are different for hybrids than plug in hybrids and all the ones we see now are so - so compromises. If the Volt had gone with the eCVT T/F transmission, it would have been a better and cheaper car but they didn't want to pay the royalties IMHO.

Edited September 28, 2015 by lolder