How does the CMAX transmission work?

[SPL Tech]

I was watching some videos on how the transaxle works on the Prius:

So basically it looks like there is the ICE, a flywheel and clutch combo, MG1 and 2, a planitary gear system, and then a final drive gearing set and that's pretty much all.

Then I was watching how CVTs work on standard automobiles:

 

So does the CMAX not have an actual steel-belted CVT like shown in the video above? Is it just the MG1/2 system and that's it with no real transmission?

Anyone know how the CMAX transaxle differs from the Prius' as shown in the video?

[jdbob]

The eCVT in the C-Max and Fusion Hybrids & Energi models uses a Ford designed HF35 transmission. It's very similar in operation to the first video and has nothing to do with the second video. Here's a diagram of Toyota's powersplit:

 

The HF35 uses a different layout and different gear ratios and of course more powerful motors. Some pictures of the HF35:

 

 

 

Unfortunately I haven't seen a video of anyone tearing apart a HF35.

[hybridbear]

Check out these threads:

http://fordfusionhybridforum.com/topic/6775-what-is-an-ecvt-how-does-it-work-here-is-the-answer/

 

http://fordfusionhybridforum.com/topic/9689-road-trip-observations-with-torque-pro/?do=findComment&comment=91919

 

http://fordfusionhybridforum.com/topic/9602-ffh-factoids-parts-quiz/page-2?do=findComment&comment=90956

 

http://fordfusionhybridforum.com/topic/8416-fusion-hybrid-powertrain-technical-analysis-with-torque-pro-a-scangauge/

[drdiesel1]

Toyota and Ford have traded numerous patents on these transmissions, but the HF35 is made by Ford in Warren MI.

 

 

Here's a Ford diagram of this trans. It's an eCVT and there is a difference.

[SPL Tech]

How does the ICE propel the vehicle? In the video above it shows the ICE is connected to MG1, but independent of the output shaft and MG2. I also recall he mentioning something along the lines that the ICE is not actually connected directly to the output shaft.

[plus 3 golfer]

ICE is connected to the planet carrier not the generator which is connected to the sun gear. If the generator is held stationary and ice spins, the planet carrier and planet gears spin which causes the ring gear to spin which propels the car. For charging when the car moving at a constant speed, ICE speeds up which will now cause the sun gear and the generator to spin while the ring gear maintains its speed.

[SPL Tech]

How do the gears stay lucubrated without affecting MG1 and MG2? I suspect having a 300V motor submerged in ATF could cause a problem, so I am curious how they get ATF on the gears but not the electric motors.

Edited January 12, 2015 by SPL Tech

[scottwood2]

Great Post.  Thx for sharing this.  I have seen some of the videos but not all of them.  I also did not know that these were made in the U.S. 

[SPL Tech]

Any idea how the varable gear ratio is created? I dident see any gears in there that look varable such as in the video I posted above.

[plus 3 golfer]

Any idea how the varable gear ratio is created? I dident see any gears in there that look varable such as in the video I posted above.

Yes.  Read this as it explains what happens in a planetary gear set when an element of the gear set is not spinning and when one or two elements are providing input like ICE and the traction motor at the same time. As an example, when ICE is running, ICE rpm can be directed to propel the car by holding the the generator stationary or can be split between propelling the car and running the generator to charge the HVB.  Also when needed, the traction motor can supply additional torque / rpm to the drive wheels and ICE rpm can be backed down or perhaps held constant in which case generator rpm would be increased. Thus, the overall gear ratio (ICE rpm/transmission output rpm) is variable.  

[jdbob]

This page http://eahart.com/prius/psd/ has an explanation for the Toyota implementation and a simulator so you can play around with different motor and engine speeds. The actual RPM's and speeds of course are different for the Energi but it gives you an idea of how it all works.

[SPL Tech]

Yes.  Read this as it explains what happens in a planetary gear set when an element of the gear set is not spinning and when one or two elements are providing input like ICE and the traction motor at the same time. As an example, when ICE is running, ICE rpm can be directed to propel the car by holding the the generator stationary or can be split between propelling the car and running the generator to charge the HVB.  Also when needed, the traction motor can supply additional torque / rpm to the drive wheels and ICE rpm can be backed down or perhaps held constant in which case generator rpm would be increased. Thus, the overall gear ratio (ICE rpm/transmission output rpm) is variable.  

When the rotation speed of MG1 is modified to control the RPM of the ICE or MG2, does it also change the final gear ratio, or only the rotational speed of the motors? Does both MG1 and MG2 have the ability to propel the vehicle and charge the battery? In the Prius video it shows that MG2 propels the car and MG1 starts the ICE and acts as a regen, but I recall reading that both motors in the CMAX can act as either function, and when you are hard on your brakes both MG1 and MG2 max out the charge capacity.

[plus 3 golfer]

When the rotation speed of MG1 is modified to control the RPM of the ICE or MG2, does it also change the final gear ratio, or only the rotational speed of the motors? Does both MG1 and MG2 have the ability to propel the vehicle and charge the battery? In the Prius video it shows that MG2 propels the car and MG1 starts the ICE and acts as a regen, but I recall reading that both motors in the CMAX can act as either function, and when you are hard on your brakes both MG1 and MG2 max out the charge capacity.

Because the three components ICE, MG1, and MG2 are physically connected, changing the speed of one affects the speed of one or both the other two components.  Neglecting the internal gear ratios inside the transmission the speed relationship is:  Speed _MG1 + Speed _MG2 = Speed _ICE    MG2 rotates proportionately with the wheels.  So, if vehicle speed is held constant and MG1 speeds up to slow down ICE, the overall gear ratio will be numerically lower (ICE rpm / vehicle rpm).  This is the negative split mode we see from time to time (see quote below) where the HVB has a high SOC and the PCM: 1) stops the charging of the HVB, 2) in essence, runs MG1 as a motor using electric power, and 3) runs MG2 as a generator or motor to regulate vehicle speed.   So, MG1 speeds up, ICE rpm decreases and MG2 rpm is held virtually constant to maintain vehicle speed.

 

I don't know of any case where MG1 would be used solely as a motor to propel the car.  I've never heard of MG1 being used when the brakes are applied (but I'm not familiar with what other manufacturers might do).  My guess is that using MG1 and MG2 simultaneously for regenerative braking might provide too much electrical power for either the inverter or HVB to handle.  There may be a current limit that would be exceeded.  But, maybe not. Also, at some braking load if MG1 and MG2 were used for regenerative braking, the C-Max may become unstable and friction brakes may need applied to at least the rear wheels in addition to the front wheel regenerative braking.  I've also read research papers on regenerative braking and don't recall any that discuss both being used for regenerative braking.

 

Negative split mode: The vehicle is cruising and the battery state-of-charge is high. The battery provides power to both the motor (to provide mechanical power) and to the generator. The generator converts this to mechanical energy that it directs towards the engine shaft, slowing it down (although not altering its torque output). The purpose of this engine "lugging" is to increase the fuel economy of the vehicle.

 

Edited January 28, 2015 by Plus 3 Golfer

[plus 3 golfer]

I just checked some of my recorded data and computed the C-Max overall gear ratio when in negative split mode using tire revolutions of 803 rev. per mile, eco-cruise set at 55 mph and 65 mph, and SOC constant.  In both the 55 mph and 65 mph data, the overall gear ratio is about 2.2:1 - 2.25:1 (ICE rpm varies somewhat +-20 rpm or so).  For comparison, a 2010 VW Jetta DSG TDI has an overall ratio of 2.31:1 in 6th gear.  The manual transmission has an overall ratio of 1.99:1 in sixth gear.

 

So, assuming that the C-Max ratio in negative split mode is the numerically minimum gear ratio (best for FE when cruising) of the C-Max, the C-Max ratio compares favorably to the Jetta DSG TDI ratio.  

Edited January 29, 2015 by Plus 3 Golfer

[SPL Tech]

Because the three components ICE, MG1, and MG2 are physically connected, changing the speed of one affects the speed of one or both the other two components.  Neglecting the internal gear ratios inside the transmission the speed relationship is:  Speed _MG1 + Speed _MG2 = Speed _ICE    MG2 rotates proportionately with the wheels.  So, if vehicle speed is held constant and MG1 speeds up to slow down ICE, the overall gear ratio will be numerically lower (ICE rpm / vehicle rpm).  This is the negative split mode we see from time to time (see quote below) where the HVB has a high SOC and the PCM: 1) stops the charging of the HVB, 2) in essence, runs MG1 as a motor using electric power, and 3) runs MG2 as a generator or motor to regulate vehicle speed.   So, MG1 speeds up, ICE rpm decreases and MG2 rpm is held virtually constant to maintain vehicle speed.

 

I don't know of any case where MG1 would be used solely as a motor to propel the car.  I've never heard of MG1 being used when the brakes are applied (but I'm not familiar with what other manufacturers might do).  My guess is that using MG1 and MG2 simultaneously for regenerative braking might provide too much electrical power for either the inverter or HVB to handle.  There may be a current limit that would be exceeded.  But, maybe not. Also, at some braking load if MG1 and MG2 were used for regenerative braking, the C-Max may become unstable and friction brakes may need applied to at least the rear wheels in addition to the front wheel regenerative braking.  I've also read research papers on regenerative braking and don't recall any that discuss both being used for regenerative braking.

So is MG1's only purpose to start the ICE and modify the RPM of the ICE and MG2? Does changing the RPM of the ICE also change the overall gear ratio, or is the gear ratio fixed for all RMPs?

[plus 3 golfer]

So is MG1's only purpose to start the ICE and modify the RPM of the ICE and MG2? Does changing the RPM of the ICE also change the overall gear ratio, or is the gear ratio fixed for all RMPs?

MG1 is used to start ICE, as a generator charging the HVB, and to control the overall gear ratio of the hybrid transaxle. MG1 speed has to change for the overall gear ratio to change.  Changing ICE rpm alone does not affect the overall gear ratio.

 

I think what's confusing is there are two parts to the Hybrid transaxle - Cinematic chain (mechanical components - the plantetary gear set) and the Variator components (MG1, MG2, and power electronics).  I'm fairly sure I attached the paper below before as it explains these components and the relationship or speed (rpm) and torque in the Hybrid transaxle.  It's the electrical components (specifically MG1 via the power electronics) that varies the overall gear ratio of the C-Max not the mechanical components. 

 

Comparison of Hybrid Transmissions.pdf

[hybridbear]

In the Prius video it shows that MG2 propels the car and MG1 starts the ICE and acts as a regen, but I recall reading that both motors in the CMAX can act as either function, and when you are hard on your brakes both MG1 and MG2 max out the charge capacity.

Only MG2 does regen braking. MG1 is not connected to the wheels. MG2 also propels the car when in reverse. The ICE cannot propel the wheels in reverse.

 

So is MG1's only purpose to start the ICE and modify the RPM of the ICE and MG2? Does changing the RPM of the ICE also change the overall gear ratio, or is the gear ratio fixed for all RMPs?

MG1 sometimes acts as a generator, consuming mechanical energy from the ICE and turning it into electrical energy. Other times it consumes electrical energy to slow down the ICE. In the links I posted above you can find an explanation of the different modes of MG1 operation, particularly in the Road Trip thread.

Edited February 2, 2015 by hybridbear

[obob]

So this is my attempt to understand how the C-Max works from reading threads from this forum and fusion forum and watching the youtube videos. It is my best attempt and it has been a struggle to get this far. And additional insights or correction welcomed. It seems like a lot of the posters have more of a understanding of transmissions that me.  I am more interested in the strategy and why.

 

The engine powers the drive train and/or an electrical generator/motor (MG1) MG1 is responsible for starting the ICE, generating electricity with power from the ICE which may go into the HVB or directly to MG2, and perhaps moving the car in reverse.

 

Another electrical motor/generator (MG2) powers the drive train and is a generator of electricity when braking.

 

So when driving MG1 when generating makes noise. I wonder why? When I first got the car it didn't but seemed to start after a software update.

 

There must be some advantage for the ICE to generate electric and pass it to MG2 rather than just propel the wheels itself. I am not sure what the advantage is? Maybe it just keeps everything spinning a little slower and spreads out the work.

 

This is one of hybridbear's post from the fusion forum. Ashley relays info from Ford engineers.

 

“Additional technical info from Ashley:

The primary purpose for the ICE is to the charge the battery. However, it is also possible under heavy load conditions to have the planetary set direct torque to the final drive of the transmission assembly from the ICE. So the ICE can provide extra power for acceleration by turning the generator or directly providing torque through the planetary set. Most of the time it will only turn the generator.”

 

 

This may have something to do with why I get good mileage on the highway as long as regen happening. The ICE isn't directly doing much of the propulsion.  But the ICE with the Atkinson cycle is good at gentle stuff like the generator and perhaps some helping out with the pulling.   When things really get tough the ICE provides the torque but it gets mileage more like a conventional engine.

Edited July 15, 2017 by obob

[plus 3 golfer]

The transmission is a simple planetary gearset that can via software direct ICE rpm and torque to the generator (MG1) and the drive shaft.  ICE rpm and torque can be split between MG1 and the drive shaft independently.  The traction motor (MG2) is connected to the drive shaft and can be used as a motor or generator.  When used as a motor, MG2 propels the car.  When used as a generator, MG2 is simulating ICE braking (foot off the brake) or slowing the car down when pressing the brake pedal (regeneration).  MG1 can also be used as a motor or generator.  When used as a motor, MG1 can be used to reduce ICE rpm.  This allows ICE to operate more efficiently (high torque, lower rpm).

 

The software that controls the transmission has several modes of operation which has been discussed in previous threads and meets driver's requests (accelerating / braking / gear shifting and so forth).  The software has been updated several times.  The presumption is that the software updates either enhance efficiency or reliabiltiy.  I believe that increasing the electric mode operational speed up to 85 mph allows ICE to operate in positive split mode more frequently which should enhance higher speed efficiency since electric mode can now be used to deplete the HVB at higher speeds when efficient to do so and then to charge the HVB back up when efficient to do so. IMO, this change is beneficial to hypermilers and those using cruise control at higher speeds.  Of course, lower speeds is still more efficient with respect to FE because aero drag kills FE at high speed.   From the Manual:

 

Torque Determination and Energy Management

The TCM is responsible for torque determination and energy management functions. The TCM monitors gear selector position (PRNDL), brake pedal position (BPP) and accelerator pedal position (APP). The TCM then makes a torque command determination. Positive torque is perceived as vehicle acceleration and negative torque is perceived as braking. Based on the amount of torque requested by the driver, the TCM decides which power source has to deliver the torque to meet the driver demand while the powertrain system is running most efficiently. 

 

There are five fundamental operating modes in the hybrid electric system:

• series mode
• electric mode
• positive split mode
• negative split mode
• engine cranking mode

Series Mode

The system operates in this mode when the engine is running and the vehicle is not moving. This is the preferred mode whenever the high voltage traction battery is charging, passenger compartment temperature control, high voltage traction battery temperature control or catalyst warm up is necessary.

Electric Mode

The system operates in this mode when the vehicle is propelled by the electrical power stored in the high voltage traction battery. The torque is supplied to the output shafts by the traction motor. This is the preferred mode whenever the desired torque is low and can be produced more efficiently by the electrical system than the engine. The electric mode is also used in reverse because the engine can deliver torque only in a forward direction. 

Positive Split Mode

The system operates in this mode when the engine is running and powering the generator motor which produces the electricity. The power from the engine is split between the path through the generator motor and the path to the output shafts of the vehicle. The electricity produced by the generator motor charges the high voltage traction battery or powers the traction motor. In this mode the traction motor can operate as a motor or as a generator to make up the difference between engine torque and desired torque at the wheels. This mode is preferred whenever the traction battery needs to be charged or at moderate loads at low speeds.

Negative Split Mode

The system operates in this mode when the engine is running but the generator motor is reducing the engine speed. This mode is never preferred but occurs if the engine is running, the vehicle speed is high, the high voltage traction battery is charged.

Engine Cranking Mode

The generator motor provides the engine cranking function to start or restart the internal combustion engine. When the PCM requests the engine cranking mode, the generator motor rapidly accelerates the engine speed up to about 950 RPM in about 0.3 seconds. When the engine speed reaches a calibrated speed the PCM commands the delivery of fuel and spark at the appropriate time. 

 

[nsteblay]

Wow, great info.  I have a 2013 and a 2016.  The 2016 has an Eco mode activated by the EcoSelect button. I drive in this mode most of the time and experience better mileage. The big difference I perceive when driving between Eco mode and normal mode is how the car de-accelerates when pulling off the gas pedal. I brake far less in rush hour traffic because of this.  The TCM must be doing something different in Eco mode to cause this to happen - not sure what.  

 

Ford provides a vague description in the manual ...

 

EcoSelect allows your vehicle to operate more efficiently. You will notice:

 

• Less aggressive heating and cooling.

• Softer acceleration.

• More regenerative braking.

• Changes in engine behavior.

• ECO cruise control activation.

 

FYI - My mileage in the 2016 for the year is 44.9 MPG and for this last week is 47.5 MPG.  I do not hypermile, so these numbers represent driving normal if not aggressively.

[stolenmoment]

I puzzled over this for a long time, and the simplest way I can lay it out is this:

 

The parts of a planetary gear system are ring (the outside), planets (the little whirly bits inside), and the sun (the central shaft).  These are always engaged with each other.  In our cars, the ICE is connected to the sun gear at a fixed ratio, the wheels and one motor are connected to the ring, also at a fixed ratio.  The axes of the planetaries are connected by a yoke to the second motor.

 

Under electric drive, the ICE and second motor are unenergized, the planetaries spin freely.  A load is placed on the either motor for regen.  The real trick is that the second motor controls the "gear ratio" between the ICE and the ring and thus the wheels.

 

All of those modes are ratios of thrust or drag among the ICE and the two motors.  For instance, to start the ICE, the motor on the planetaries spins against the ring to turn the ICE.  Propulsion by ICE has the planetaries held still (the motor can do that), so that the power from the ICE transfers through to the ring.  Varying the power fed to the planetaries can allow all three sources to contribute torque to the wheels.

 

Planetary gear systems are hard to think about, I think because thrust and drag can come from any of the components.

[joshg678]

FYI - My mileage in the 2016 for the year is 44.9 MPG and for this last week is 47.5 MPG. I do not hypermile, so these numbers represent driving normal if not aggressively.

I'm currently at 45.6 on my 2016. I never use ECO select as I can't stand the Prius Button.

 

I've had two large trips on the interstate with it (1,700 miles total) where we averaged about 38 on.

My daily driving my trips range 47-56MPG

Edited July 15, 2017 by joshg678

[obob]

I am still working on trying to understand and am making progress.  Thanks for the help.

 

This video helped me a lot.

 

[joshg678]

It's very similar to a differential on a car. I think every transmission should be like our cmax. It's just so cool imo

[djc]

For me a main attraction of the C-max was the high level of ingenious technology, coupled with a comfortable and useful body shape.  Regular drive trains just seem dumb.  The things I would like in my next vehicle are bigger battery, an electric cabin pre-heater operated off the same plug-in as the engine block heater, and, most important, AWD. 

 

Regarding Ford engine control software updates:  it seems likely that these would be aimed at reliability improvements, if possible, rather than mpg.  A slight mpg increase will hardly be noticed in the noise - and doesn't do anything for Ford.  A failed transmission will be noticed all around and can easily cost Ford quite a bit.