Antonov's 3-speed transmission for electric vehicles boosts efficiency by 15 percent
July 3, 2011
Electric vehicles have been a reality for more than 100 years, but it's only in the last decade or so that the world has truly woken up to their potential as a viable, cleaner urban transport alternative to their combustion engined cousins. During this EV renaissance much of the focus has been on developing improved power sources like batteries and fuel cells in order to deliver the range and performance consumers have become accustomed to during the age of oil. Transmissions on the other hand, despite being so important in the ICE space, hardly rate a mention because the wide torque curve of electric motors makes them largely irrelevant. It could be time to rethink that approach according to U.K. based engineering firm Antonov. The company has produced a 3-speed transmission designed specifically for electric vehicles that promises to bring significant efficiency gains and a better driver experience. The company's Business Development Manager Dave Paul outlined these benefits in a presentation at the IDTechEX Electric vehicles conference this week in Stuttgart.
Why use a 3-speed transmission for electric vehicles?
Internal combustion engines typically reach maximum torque at between 3000 and 5000 rpm, so a gearbox is needed to keep the engine in this operating range as the speed of the vehicle increases. Electric motors on the other hand have full torque at 0 rpm and a much wider operating range. That's why most EVs have a single speed - or in some cases dual-speed - transmission. Despite this, the efficiency of electric motors still varies at different speeds - they operate at a peak efficiency of around 90 percent but this can fall to 60-70 percent, particularly at low speed. The question is whether or not it's worth adding a multi-speed transmission to the EV drive-train to optimize efficiency at all speeds. According to Antonov, the answer is definitely yes.
Benefits of the Antonov multi-speed transmission
The Antonov system grew out of the Jaguar Limo-Green project in 2009. Antonov was initially asked to provide a fixed ratio transmission for this range extended series hybrid, but it was decided to look at additional gears with the aim of improving off-the-line performance and top speed. It worked. In testing, the 3-speed transmission improved efficiency by 14.7 percent while delivering the same performance of the base line vehicle, which translates to greater range or alternatively, a smaller battery pack. On the flip-side, it can deliver better performance at the same level of efficiency. The unit was also compact enough to drop into the existing transmission tunnel.
Almost 15 percent. That's a significant figure, but Paul says that it can also bring a raft of other benefits to EV architecture. The addition of a multi-speed transmission means a lower torque motor can be used which results in cost benefits and an overall reduction in the weight of the powertrain. It can offer better launch acceleration, lower noise and a higher cruising speed, while the hill climbing ability of utility vehicles can be improved.
Paul also argues that integrating the Antonov transmission can result in simpler, lighter and cheaper power electronics, and because the motor can be kept within its optimum speed range there's less wear. There are also gear overlap benefits compared to a 2-speed transmission, meaning that not as many gear changes are required at mid-range speeds.
One of the most interesting benefits is the potential to link the system to telematics so that the optimum gear can be selected when, for example you are approaching an uphill stretch of road.
Similarly, a "limp-home" facility could be incorporated to maximize efficiency over performance as the telematics system recognizes that you are likely to run out of juice before you reach your destination.
Another benefit to the overall efficiency of the vehicle is the ability to use first gear to maximize the regenerative braking.
The transmission has a "fairly conventional" twin shaft layout and uses two lubrication pumps - one mechanical, one electric - and a dual clutch for seamless gear changing. It could also be modified to become a 4-speed.
This week the company also took out the IDTechEx Electric Vehicles Land Sea Air "Technology Award‟ for the most significant EV technical development over the past two years.
So why haven't we seen three (or more) speed transmissions in electric vehicles before? The idea goes against conventional thinking regarding electric drive systems and Paul believes that engineers engaged in EV development have thus far kept a narrow focus. Perhaps no-one realized the significant difference a multi-speed transmission can make ... we'd love to hear your thoughts in the comments section.
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