On-road testing of new multi-speed transmission for EVs from Oerlikon Graziano/Vocis begins; potential for increased range or reduced pack size
05 September 2011
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| The red region shows the most efficient speed range for a traction motor. By keeping the motor operating in, or close to, this region, the new multi-speed transmission developed by Vocis could increase the range of electric vehicles with very little impact. Click to enlarge. |
Prototypes of a novel, multi-speed transmission for electric vehicles (EVs) have begun on-road testing with a European vehicle manufacturer. Designed and manufactured by Italian drivetrain specialist Oerlikon Graziano with UK control systems specialist Vocis (itself part-owned by Oerlikon Graziano), the concept is intended to increase vehicle range or to allow reduced battery pack size, as well as providing improved low-speed pull away and higher-speed cruising.
The wide operating range of a typical electric motor, which provides maximum torque from zero RPM, has led most EV producers to use a single transmission gear ratio. While reducing cost and packaging volume, the compromise is reduced efficiency and performance, according to the partners.
Electric motor efficiency drops off at low load levels and towards the extremes of speed. Multiple gear ratios with electronic control allow the motor to be kept in the region of greatest efficiency for a much higher proportion of the time, allowing significant range extension.
—Vocis technical director, Richard Taylor
The extra ratios also remove the compromise between top speed and hill-climbing ability.
An EV will typically have a transmission ratio that is higher than the ideal, simply to give it enough top speed. “With multiple ratios, we can provide much better laden pull-away as well as improved top speed without increasing the powertrain size.
—Vocis technical director, Richard Taylor
The heart of the new transmission is a novel gearshifting concept based on principles similar to those used in a DCT (dual clutch transmission), a transmission type in which Vocis has considerable expertise (earlier post). Unlike most EV powertrains, which use a single e-machine (motor/generator), the Vocis multi-speed transmission works with two small e-machines, each on an independently-controlled shaft.
The design allows the distribution of drive and recovered energy to be balanced between the two e-machines, with seamless changing provided by torque infill during each shift. The torque infill makes gearshifts so imperceptible that the first customer for the system asked to be shown that shifting was taking place because he couldn’t feel it when driving, according to Taylor.
Electronic control provides full driveline integration which, combined with multiple ratios, will also allow the implementation of alternative calibrations in order to tailor the feel and performance of the vehicle to the driver’s personal preferences or to match brand characteristics. It also provides the facility for strategies such as automatic ratio optimization to maximize range.
Because the technology is readily scalable, including the number of ratios, it is easily applied to a diverse range of vehicles.
The priority for a passenger car application is to increase the performance to that of a conventional car by improving acceleration and top speed. The priority for a delivery van would be to improve fully laden performance during pull away or hill climbing. And both will benefit from the significant improvement in range.
—Paul Taylor
Though more complex than a single-speed transmission, the cost and weight of the multi-speed system compares favorably to a conventional gearbox because it requires no clutch or synchronizers. This inherent simplicity makes the technology attractive to EV producers, the partners say.
Vocis will demonstrate this technology, along with its two-speed electric vehicle transmission (earlier post) at the upcoming LCV2011 (the Low Carbon Vehicle show) on 7-8 September at Rockingham Motor Speedway in the UK. The new twin-speed transmission will be demonstrated in a prototype electric minibus developed with powertrain supplier Zytek. (Earlier post.)
Our two-speed transmission demonstrated the benefits of EVs moving beyond a single speed. We are now extending those gains by using multiple ratios to make EVs more competitive in a variety of applications. This gives us single, twin and multi-speed EV transmissions, all of which are already in running vehicles. We believe this to be one of the most comprehensive ranges available anywhere.
—Vocis managing director Mike Everitt
Given that most of your driving is already done in the meat of the efficiency curve, I have to wonder how much exactly a 2 or 3 speed transmission will improve efficiency in real life.
Looking at the efficiency plot, even if one assumes that one could improve efficiency from a low of 70% to 95% (optimistic) one might improve range by 25%. More realistically we'd be looking at improvements of ~10% under typical situations.
I would have to think that the real benefits would come from improved performance - better acceleration and top speed while maintaining efficiency. Current mainstream EVs have 0-60s times in the ~10s range and a top speed of 90 mph. Gearing to allow 0-60s in 8s and a top speed of 100 mph would greatly improve driver acceptance.
I also have to wonder if reducing the peak torque requirements also might allow reduced currents through the motor and allow downsizing of the inverter and other related components.
It would be interesting to see some actual data with the product.
Posted by: Dave R | 05 September 2011 at 11:36 AM
I seem to remember the Tesla Roadster started out with a two-speed tranny before changing to a one-speed. Still has pretty good acceleration and top speed. For the mainstream non-enthusiast driver, 90 mph top speed would seem to work, as would 10-second 0-60 acceleration, for daily driving chores. If memory serves, the BMW Active E and the i3 also use a one-speed. There's always the Mercedes SLS E-Cell for people who use their cars for thrills.
Posted by: ChrisL | 05 September 2011 at 12:33 PM
Variable torque e-motors have high torque over a wide speed range, i.e. high low-end torque and high top-end speed and do not need mechanical transmission. They can be used in cars, trucks and buses. They were designed (six different patents) by Variable Torque Motors of Fort Wayne India USA.
Very high efficiency variable torque e-motors exist and will certainly be further improved and have higher power density with the use of ultra light weight materials in the near future.
There is no real need for multi-speed transmissions for electrified vehicles.
Posted by: HarveyD | 05 September 2011 at 01:35 PM
They don't need no stinking road test.
The speed/torque/efficiency plot shown can easily be drawn for any auto/battery/motor combination.
It is simply a matter of cost/benefit tradeoffs and competition.
Introduced in 1950, the 2 speed Powerglide was on more than half of all new Chevrolets by the mid 50s.
It had little competition.
Same for EVs; as the market evolves, only golf carts are likely to have single speeds.
Posted by: ToppaTom | 05 September 2011 at 05:10 PM
TT...you have more than 66% chance of being wrong this time. The two speed Chevy power-glide was about the worse automatic transmission ever produced. It almost doubled the fuel consumption over manual transmissions, specially in heavy city traffic.
Posted by: HarveyD | 05 September 2011 at 06:00 PM
Harvey,
1. The 2 speed Powerglide was on more than half of all new Chevrolets by the mid 50s. [All other Chevy transmissions were manual]
2. Some obviously believed, at the time, that; "There is no real need for more than 2-speed automatic transmissions".
3. You say; "There is no real need for multi-speed transmissions for electrified vehicles."
4. Single speed transmissions might be considered "about the worse BEV transmissions ever produced". For the same reasons.
Posted by: ToppaTom | 05 September 2011 at 07:11 PM
Unless the cost of the transmission is lower than the cost of an extra 10% or so increase in batteries there is no point, other than performance cars maybe.
Posted by: JRP3 | 05 September 2011 at 07:40 PM
Unless the cost of more gears in the Powerglide is lower than the cost of an extra 10% or so increase in fuel economy, is there no point, other than performance cars maybe?
And why are 2-speed automatics, with lockup torque converters so rare?
Posted by: ToppaTom | 05 September 2011 at 09:18 PM
Please inform me of any electric or diesel-electric lokomotives that operate with gear shifting. ..HG..
Posted by: Henry Gibson | 06 September 2011 at 02:02 AM
Locomotives should not be confused with EVs; locomotives are fed from the grid and EVs from batteries which is a completely different ball game.
Electric motors in EVs have a nominal and peak power rating. When accelerating an EV at peak power and peak torque, the motor takes a huge gulp of electrons which is certainly detrimental to a long batterie life.
A transmission not only enables various speed ratios, it also matches the torque correspondingly.
I.o.w. a transmission would deliver the necessary torque while accelerating and at the same time avoid the flow of huge currents; that is definitely a contribution to a longer batterie life.
Posted by: yoatmon | 06 September 2011 at 06:14 AM
We are missing the point here. Variable torque e-motors meet most of the performances obtained from multi-speed transmissions without the extra weight and mechanical complexity. Improving the performances of future e-motors, control system and batteries and reducing mass production cost should be the common goal, not adding more complex mechanical assemblies.
Race cars are another story. However, the Tesla (and other e-muscle cars) do very well without a multi-speed transmission. Those cars are available now at a reasonable cost.
Posted by: HarveyD | 06 September 2011 at 07:09 AM
The 2 speed Powerglide was on more than half of all new Chevrolets by the mid 50s.
The 50s??? During the 50s the price of gas was about $.20 a gallon. At that price the cost/benefit tradeoffs were a lot different.
Posted by: ai_vin | 06 September 2011 at 11:09 AM
The price of gas had little to do with the Powerglide.
It was cost effective (apparently) then.
A 2 speed transmission was acceptable for almost 10 years in the 50s when today it would be/is considered primitive.
A single speed BEV transmission is acceptable today, when by tomorrow’s standards it might be considered primitive, except for golf carts.
Posted by: ToppaTom | 06 September 2011 at 09:14 PM
I'll place my bet that TT is wrong and most of future EVs will not have multi-speed transmissions.
Posted by: HarveyD | 07 September 2011 at 09:02 AM
Motor in wheel, multi-speed transmissions, simple reduction gears - who knows, we will find out over the next 10 years.
Posted by: ToppaTom | 07 September 2011 at 09:18 PM