Is the one-speed transmission the last word?

[adric22]

The Chevy Volt uses its two electric motors together to keep the main drive motor in an RPM band that is more efficient. The Volt actually has 4 different drive configurations. 2 of those are for electric-only driving and the other 2 are for gasoline driving. The Volt is pretty darned efficient at higher speeds, compared to the Leaf. I don't have any raw numbers, I can just tell you from experience driving both cars that the Leaf suffers massive range loss when driving at highway speeds, where the Volt does not.

 

[DaveEV]

The Leaf's motor on the other hand, has a wide band of peak power starting at 0 RPM and ending somewhere around halfway towards max RPM of (IIRC; my google-fu fails me) around 11,000 RPM.

Correction, you have power/torque mixed up. Peak torque starts at 0 RPM and stops falling off around 4000 RPM or so (~30mph) at which point peak power is reached. Above those speeds torque tapers off but power remains at it's max output (~80kW).

 

[jlsoaz]

Apparently the problem was not one of actual durability of the transmission itself. Tesla evidently spec'ed out a two speed unit with no clutches. The design intent was to do clutch-less shifting and manage the torque output of the motor during the shifts. Unfortunately the rotational inertia of the motor made this plan unworkable as the torque output couldn't be changed fast enough. According to Siry there were never actually any mechanical failures of the X-Trac transmission, it was simply a matter of Tesla not being able to get their control strategy to work adequately with the hardware. Subsequently Tesla contracted Magna to develop a dual clutch two speed gearbox. Unfortunately, this is the design that had durability issues leading to Tesla's ultimate decision to revise the motor for more power and go with a single speed unit for production.

http://www.autoblog.com/2008/04/16/tables-turned-tesla-motors-sued-by-transmission-supplier-magna/" onclick="window.open(this.href);return false;

Thanks, good to know.

 

[jlsoaz]

The Chevy Volt uses its two electric motors together to keep the main drive motor in an RPM band that is more efficient. The Volt actually has 4 different drive configurations. 2 of those are for electric-only driving and the other 2 are for gasoline driving. The Volt is pretty darned efficient at higher speeds, compared to the Leaf. I don't have any raw numbers, I can just tell you from experience driving both cars that the Leaf suffers massive range loss when driving at highway speeds, where the Volt does not.

Thanks, good to have this anecdotal point. It seems to help a little bit buttress the hypothesis that there are some efficiency gains to be found with the Leaf at higher speeds.

 

[jlsoaz]

The cars are already grossly overweight compared to an oil burner, plus an oil burner gets lighter as it burns off energy.

The trade offs of weight plus cost (on a car that is already too heavy and too expensive) for some nebulous benefit is pretty easy for me to decide.

I think it's putting the cart well before the horse to focus over-much at first on the marginal utility to you or me of some benefit where we don't know the degree of benefit. To clear up some of the nebulousness of the matter, and for other reasons, I'd encourage the auto manufacturers and other industry participants such as transmission makers to investigate robustly whether they might have missed a key range-extending and time-saving improvement. From there, if improved range vehicles are created, whether by adding multi-speed transmissions or by light-weighting via simpler transmissions, then it will be possible for me and other buyers to make better decisions based on our own personal marginal utility calculations which (if I am using the concept correctly) I think will be different for each of us.

I say this as a long-time fan of the potential for EVs to eliminate the added weight, complexity and maintenance issues of traditional transmissions. Aside from in-wheel motor EVS (maybe they will render this point moot, if the unsprung weight issue and any other issues they have are fully solved?) there was/is also at least one Midwestern EV (AMP?) retrofit that somehow balanced a couple of motors to achieve no conventional mechanical gears (if I'm not mistaken) and I was/am always a fan of this sort of wonderful step-change light-weighting improvement in vehicles.

I can't speak to all the decisions made behind the scenes as to why the Leaf and other OEM EVS seem to have a single gear (is that the right terminology?) sort of setup, but, as mentioned, this thread has particularly got me thinking that there might be some potential for energy efficiency improvement, so I think it should be thoroughly investigated, or maybe Nissan already did look into this and proved to their satisfaction that the potential is not there or is for some reason not worth it in their estimation.

 

[BraveLittleToaster]

Hi bravelittletoaster,

The point is taken from your post and many others that many EV projects don't need or want any transmission, or more than one speed, but my question definitely stands as to what extent another gear (despite the weight and volume penalties, added maintenance and any other issues) might help add range for the Leaf and other EVs. As best I can tell, it seems a possibility that it would add some range (how much I don't know) even after taking into account energy drawbacks (primarily weight).

Actually, from what I've seen of conversion guys' range, especially those that left the original manual transmission in, it made very very little difference to range, if any at all. I've also seen controllers (like the Zilla 2000) that will do the job electronically by mucking around with power output so that at some point there's a switch from focusing on torque over horsepower to the other way around. That part is over my head, and I probably don't understand it properly anyway, but the point being, there's little to no need for a transmission with more than one gear, so why add the complexity? The main reason that anyone has ever bothered with multi-gear transmissions in the first place is due to the shortcomings of the ICE.

 

[jlsoaz]

Hi bravelittletoaster,

The point is taken from your post and many others that many EV projects don't need or want any transmission, or more than one speed, but my question definitely stands as to what extent another gear (despite the weight and volume penalties, added maintenance and any other issues) might help add range for the Leaf and other EVs. As best I can tell, it seems a possibility that it would add some range (how much I don't know) even after taking into account energy drawbacks (primarily weight).

Actually, from what I've seen of conversion guys' range, especially those that left the original manual transmission in, it made very very little difference to range, if any at all. I've also seen controllers (like the Zilla 2000) that will do the job electronically by mucking around with power output so that at some point there's a switch from focusing on torque over horsepower to the other way around. That part is over my head, and I probably don't understand it properly anyway, but the point being, there's little to no need for a transmission with more than one gear, so why add the complexity? The main reason that anyone has ever bothered with multi-gear transmissions in the first place is due to the shortcomings of the ICE.

It is possible that the conversion guys' input that you got can be extended to all or most EVs, as you seem to be doing, but I am going on the working theory that it may be more a case-by-case matter, particularly when we are talking about a significantly different (in some ways) matter of a major auto OEM dedicated EV design. We had a useful-seeming anecdotal input from a Leaf and Volt driver today as to raising the question of possible higher-speed efficiency differences. In particular, I am going on the theory that the 55 to 75 mph efficiency of the Leaf might possibly be improved by a different approach to gearing. Of course, I might simply be wrong (maybe the efficiencies at those speeds are already close to optimal) or even if they are not, a change in gearing might not help. Even if it did help, then this would involve compromises elsewhere (as most or all engineering does). Even if the alternative gearing setup seemed competitive, it is possible (or even probable) that a completely different competing technology such as in-wheel motors or some-such could moot the question of a traditional mechanical transmission of any sort (though I'm not stumping necessarily for a traditional transmission, I'm stumping for more than one gear, some confirming answers as to the degree that it could help efficiency, and regardless of the technology, however it can be done in a high-quality durable way).

I'm wondering if we'll be able to get some empirical feedback from one of the auto OEMs in terms of a competing vehicle, or maybe some extended robust data from one of their labs showing how they have looked into this matter in a way that revealed a lot of useful answers. It will also be helpful if we see several competing vehicles on the road with different approaches to this matter, and this would not only help the marketplace sort things out (as someone remarked) from a demand-for-features sort of view, but with basic efficiency figures on record with EPA et. al. it would allow us to try to make some comparisons.

 

[Sublime]

The difference in efficiency across the rev range of an induction motor are going to be in the single digit %, probably below 5%. The addition of extra gears to a transmission means extra rotating mass, friction loss, etc. I wouldn't be surprised to hear that it hurt overall efficiency.

Its more likely that there are marginal gains going the other direction. Removing the single gear that is there, plus the differential and going with 2 higher torque motors (1 per driven wheel).

 

[RegGuheert]

The difference in efficiency across the rev range of an induction motor are going to be in the single digit %, probably below 5%.

The LEAF does not use an induction machine, but rather it uses a synchronous machine.

But the point still stands, the old motor curves the efficiency did not go down significantly after its peak. That said, Tony's range test with a 2013 LEAF makes me think Nissan may have lower the speed at which peak efficiency occurs to try to get better EPA numbers. Such a change will certainly cause a bigger drop in efficiency at highway speeds.

 

[gbarry42]

...an elegant form of something like a multi-ratio planetary drive - has some advantages for the future, both for performance and efficiency.

This naturally makes me think about Toyota's Hybrid Synergy drive. Here, a second motor is placed where the clutches usually go. Hold that second motor still, and you get the natural gear ratio of the planetary drive. Drive the motor forward, and you get a high gear effect of the output turning faster than the motors. For a low-gear effect, you do the opposite, forcing the second motor backward. That turns it into a generator, and you route the output back to the first motor. The end result should be a variable ratio transmission, with the obvious advantage of no clutches, no lurching, and no stresses from suddenly meshing overspeeding gears.

I think this may allow higher top speeds (though I can't fathom the use for that) and more torque at higher speeds (without blowing up the motors--we have to assume the battery and inverter can supply it). I do not get the impression it would help efficiency. If you put 80kW into a drive system and it's already 90% efficient, you can guess the limit that any improvement could possibly make.

I guess I had been assuming that the seeming rapid drop-off in efficiency from the 40 to 50 to 65 to 75 mph progression was attributable largely or entirely to aerodynamics or something else, but what if some decent amount of it is attributable to gearing? Probably not, but the conversation has me wondering.

Aerodynamics is real, and there's nothing you can do inside the vehicle that's going to change the amount of power dissipated while pushing through the air at 70 MPH. So, no, you didn't imagine that part. In the end, that's a lot of power wasted, especially in comparison to gains you get from tinkering with gear ratios, load currents, or the type of oil in the gearbox.

In the glory days of the automakers, where they could build such a thing just to see what happens. I still think they should try it.

 

[LKK]

Don't know why my previous post was delete, I didn't think it contained anything inflammatory. Anyway, what I said is it's all about a trade off between efficiency and complexity. The Volt is the complex one with its fancy planetary gear set transmission and 10 million lines of code. The Leaf is pure simplicity with just a simple reduction gear. The Volt's complexity works as its electricity usage is almost the same as the Leaf (98 vs 99 combined MPGe) despite lugging around a complete ICE system making it weigh over 500 Pounds more than the Leaf, but is it worth the extra complexity?