I travel between the bay area and lake tahoe all the time. When climbing long steep hills, I see the engine reving quite high, approaching 5K rpm for short periods even though I'm not accelerating. Most of the time the revs are 3-4K. I would think that the CVT could do a better job of holding a constant rpm in situations like this. I certainly could if I were using a non-cvt.

 

If I understand correctly, it's not really a CVT in that there are no changing ratio elements, it's basically in 5th gear all the time, and in order to generate the torque at lower speeds it uses the big electric motor, powered by spinning the ICE faster to generate power with the small electric motor. If it used the battery going up a hill it would not last long, so revving the ICE is generally a sign that it's trying to generate power for the electric motor, on top of the work it's doing moving the vehicle directly, in some kind of constant torque split like 70/30. Your engine revs will vary based on required power, rather than speed of vehicle, as with a fixed ratio gearbox.

This explains the workings of a Prius I http://home.earthlink.net/~graham1/M...OnAsIDrive.htm
FEH uses a similar system, but obviously different specifications, as with the Prius II and Toyota hybrid SUVs.

 

I haven't seen any graphical items out there showing how the Ford hybrid system is configured. My only frame of reference is Toyota's HSD system. The Ford design and the Toyota design are very similar in terms of concept, however Ford has done something different because of the integration of a fluid coupling 4wd system.

Both systems use a power split device, which is a planetary gear pack which connects the gas engine to the two motor/generators. The manipulations of RPM outputs from the MGs and the ICE by the vehicle's computers determines the final drive RPM and that results in a ground speed. See the above link at Graham's site. Here is another link to a site which shows how the Prius I operates and how the PSD works when running. http://www.wind.sannet.ne.jp/m_matsu/prius/ThsSimu/index_i18n.html?Language=en?Country=US

I hope you have a current version of a Java VM or else this site will not work all that well.

There is no "cone and belt" cvt as you might be expecting. The power split device is the CVT and works as such because the computers are always evaluating device RPM in relation to ground speed and economy maximization. If the ICE revs up while pulling a hill, it is usually because the computers are determining that extra torque is needed and the best provider of torque is the larger of the electric motors. The engine then spins at a higher RPM, through the PSD, the smaller of the motor/generators and direct electrical power is created to assist in moving the vehicle.

In short, engine RPM is not related to ground speed at all. I'm confused as to why Ford even provided a tach on the Escape considering how incoherent RPM can be related to ground speed in a full hybrid of this configuration?

 

I'll be the first to admit that I don't completely understand HSD but I don't think your explaination is quite correct. The SPD does in fact change the "gear" ratios between the engine and the drive wheels and does not need to switch gears or change pulleys to get that job done. To prove the point think about the OP's question. The engine revs up but the speed stays the same. If the engine were locked in 5th gear it could not rev up unless the car went faster. The way I understand the PSD is that the spinning of the motor in one direction has the effect of gearing the engine down for more torque, PLUS (as you said) the motor adds it's own torque. As you speed up the electric motor turns slower and slower and at some point the motor actually spins backward (into heretic mode) which has the effect of raising the gear ratio into an overdrive range.

 

It's a little more complicated than that. Strictly speaking, there are no gear ratio changes occurring. However, the effective gear ratio does change thanks to the interaction of the engine and the two electric motors.

The computer is constantly balancing the torque of these three inputs to seek optimal efficiency at the desired torque output level.. That means if it can run the engine at a higher RPM but at a lower load, it often will.

Basically when you're looking at the FEH and the HSD system, you have to throw out a lot of what you know about how automotive drivetrains work...

BTW, the 4wd coupling is not fluid drive but a clutched mechanism, if I read the service manual correctly.

 

I'm wondering what we can do to all get on same page. It seems some of you object to the term gear ratios because no gears are actually clunking in and out of place changing between fixed ratios. I object to the "locked in 5th gear" way of describing how it works. Perhaps I just don't understand what you all are trying to say but that sounds flat wrong compared to my understanding of how the SPD works.

Ratio changes are indeed happening even if are not traditional "gear ratio" changes. We all agree that the engine turns faster which is proof that ratio has indeed changed. Saying that the gears don't change or that the PSD is not a CVT is like saying that my Civic does not have power steering because no hydraulics go to the rack and pinion unit. My Civic does indeed have power steering and the power comes from an electric motor. Same goes for the Prius. It does absolutely have a CVT, just not the same kind as Subaru or Honda use.

 

I think the confusion comes from the fact that the ICE is not connected to the wheels through a solid connection "in 5th gear" and that's quite true. What's "in 5th gear" is the output of the PSD and the large electric motor, the ICE is always splitting it's torque approximately 70/30 with the output and the sun gear (small motor/generator) but the power is continuously variable between the sun gear and the ring gear (output) so that if the sun gear spins faster, more power goes to it, and when the ring gear is spinning faster than the sun gear more power is flowing through it (and to the wheels). What varies is the flow of power, rather than actual gear ratios, because there is no variable multiplication of torque, like in a real CVT where the ICE can power the vehicle from a start... I guess the term 'Continuously Variable Transmission' still fully applies, because there is something varying continuously, although it's not the gear ratio, so it's a bit confusing to the technically inclined.

 

Ok this is where the problem is. I believe that there is in fact a variable multiplication of torque. I might be wrong but I've done a lot of reading trying to understand the PSD and as best as I can tell it is a CVT and it does increase torque that the engine puts to the wheels mechanically when you first take off. As you speed up the final drive ratio gets taller so that the car goes faster for each engine revolution.

 

Hmmm... Well, I'm no expert, just trying to understand as best as I can too, but from what I understand it seems like the ICE has to deliver all it's power to the small motor/generator, then the current generated there passes to the large motor/generator, thus applying large torque to the wheels. To me it seems like the conversion of power to torque happens all within the electrical system rather than the usual mechanical arrangements. If there were mechanical means to multiply up the torque of the ICE, then it would be able to take off under ICE power alone rather than needing the large motor to push the vehicle at first. Perhaps this eCVT name makes more sense, since it really is using the electronics to do the power to torque conversion.

It may help to think about the situation in terms of series and parallel hybrids where in a series hybrid the ICE must generate electricity for the motors to use, but the ICE cannot ever power the vehicle directly. That's the situation when the vehicle is starting off, but as it speeds up the ICE can provide more and more power to the wheels (and consequently less power to the small motor/generator) and eventually it's operating in fully parallel mode where most (or all) the ICE power is applied to the wheels, and the electric motors are out of the way, unless a burst of acceleration (high torque) is called for, and in that case both ICE and electric motor combine their output.

 

You might be correct about the electrical power path. I've been going over Grahm's pages trying to get a handle on what is really going on. I now see where you are getting the top gear thing but I still think it is misleading. As best as I can understand only 20 some percent of the power path is electrical so I'm still thinking that the mechanical power path is "geared down" electrically but you might be right about the torque coming from the motors. You all might be able to convince me I'm wrong and if you do I'll retract and apologize for adding confusion. Actually I'm sorry anyway for opening up my mouth without being sure I actually knew what I was talking about.

Thank you for being a good sport about this. I would really love to be able to understand exactly what is going on in those fancy cars of yours.