New Two-Speed Electric Vehicle Transmission For Improved Performance, Range and Battery Life
12 May 2009
| 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 two-speed transmission developed by Vocis could increase the range of electric vehicles by up to 10%, with little impact on cost. Click to enlarge. |
Transmission engineering and control specialist Vocis is developing a novel multi-speed transmission for electric vehicles. The company will preview the transmission at EVS, the Electric Vehicle Symposium (Norway, 13-16 May).
Simulations show that a two-speed configuration could reduce the battery energy consumed by 5-10% over the industry-standard New European Drive Cycle, without any significant increase in overall cost or packaging volume, as well as extending battery life and providing the potential to substantially downsize the powertrain and battery pack.
Electric motors have a very wide operating range, but that doesn’t mean that they are equally efficient at every speed. The torque curve of a typical traction motor is well suited to vehicle propulsion, having maximum torque from zero speed and a wide constant power region. However, there is a ‘sweet spot’, typically at medium speed and medium to high loads, where the delivery of power is most efficient. A choice of gear ratios allows the motor to be kept in this operating region during more of the drivecycle.
—Vocis EV transmissions specialist Andy Turner
Vocis says there are a number of benefits to this strategy. Because operating efficiency is increased, range can be substantially improved or the battery capacity can be decreased, reducing weight, cost and recycling issues. The availability of a low gear ratio for pull-away and for climbing gradients (especially important for laden commercial vehicles) allows the size, cost and weight of the motor to be reduced and this and the increased motor efficiency allows a smaller, lighter cooling system.
Battery life can also be improved because there will be less need for deep discharge to deliver the required range, so greatly improving the financial viability of electric vehicles.
The heart of the system is a novel gearshifting concept, based on proven twin-shaft principles, that allows the ratio to be changed with no break in torque delivery. New high-precision electromechanical actuators are being developed, working at traction battery voltage (typically 300V) to allow improved efficiency. 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.
This is not the first two-speed EV transmission concept, but it is the first that overcomes the issues that have so far prevented their successful introduction. Fundamental benefits of our technology are zero torque interruption during shifts, very low additional losses compared with a single speed transmission, scaleability to any practical number of ratios, and mechanical robustness through the use of technologies that have all been proven in other applications.
—Vocis managing director Mike Everitt
A development electric vehicle fitted with a Vocis two-speed transmission will be available for demonstrations before the end of 2009.
It's the old cost/benefit question; do you want to increase your efficiency from 85-80% to 90-85% for an insignificant increase in overall cost? Well, how much for a three speed transmission that increases your efficiency to 93-90? Or a four speed for 95-93? How about a six speed just to get 96-95?
Posted by: ai_vin | 12 May 2009 at 12:34 PM
I saw a motor power graph that showed efficiency versus load and RPM once. It showed that the efficiency was way down when running at zero RPM with a heavy load. This makes sense, because the torque is high and so is the current. Launching a 3000 pound car from a stop light could use a gear or two to help with the efficiency and range, not to mention battery life.
Posted by: SJC | 12 May 2009 at 03:30 PM
Its also a trade off with reliability. No transmission to fail is a selling point to me.
Posted by: paul in hampden | 12 May 2009 at 04:25 PM
"from one speed to 6 speed" are step approaches.
There is also something such as a continuous variable Transmission. delivered bij Bosch; using a push belt method.
This will allow the engine to operate continuously at its maximum efficiency level of 100%.
it is proven technology for 40 years and implemented in many cars.
Posted by: Wil | 13 May 2009 at 01:17 AM
Tesla tried to develop a two-speed transmission for their EV for a long time for exactly these reasons (improved efficiency and range).
In the end they abandoned it.
Posted by: clett | 13 May 2009 at 01:17 AM
Does the cost increase justify the efficency gain?
Posted by: kelly | 13 May 2009 at 06:40 AM
The new $2000 Tata Nano comes with optional (4-speed manual, 5-speed automatic or CVT) very low cost transmissions made by CVTech Transmissions in Drummondville, Quebec, Canada.
Those light transmissions were developed for recreation vehicles and can handle 125 to 150 HP in off-road rather rough usage.
It seems that Tata will offer the optional transmissions at about the same low price.
Wouldn't be surprised if Tata comes out with a small BEV equipped with a very light, low cost, 2-gear transmission, much sooner that we think.
That would be a smart way to keep the e-motors very small and extend the life of the battery pack at next to no extra cost.
Chinese e-car manufacturers are most likely taken notice of what Tata is planning and doing.
Posted by: HarveyD | 13 May 2009 at 10:33 AM
Harvey,
Tesla had to abandon two separate efforts to build a transmission for their electric. The torque is higher and seems to rip transmissions apart. Sure it can be built, but its heavier then you expect and now costs more.
Posted by: paul in hampden | 13 May 2009 at 05:27 PM
An obvious observation (one not cited in the press release) is that part of the gains to be had from a transmission will surely be lost to friction. IMO, money/effort is better invested in improving the electric motor design.
Posted by: GreenPlease | 13 May 2009 at 07:01 PM
It may depend on what kind of start you want. If you want the high G force take off, it will cost in power and energy. If you are willing to have a softer start from a stop light and accelerate from there, you can save range and battery life.
Posted by: SJC | 13 May 2009 at 07:13 PM
Paul:
It may be unfair to compare transmission requirement for two very different cars/vehicles.
The Tesla is a sporty high torque, high power vehicle requiring an optional very rugged transmission to handle all the power available.
The Tata Nano is a very light, low torque, low power vehicle. A very rugged transmission may not be necessary but could supply more take-off wheel power from smaller faster e-motors and possibly smaller batteries. Extending batteries life could be a worthwhile side benefit.
Posted by: HarveyD | 16 May 2009 at 05:43 AM
Yes; the problem is the Red light intersections.
Get rid of them and a major problem will be solved.
We can do it if the Governments Stop placing Old Time
Thinking as Road Blocks to The Future. BEE.
Posted by: bee | 18 May 2009 at 11:04 PM