Cleantech Market Intelligence
Better Fuel Economy Through Better Transmissions
Arguably, the most complicated piece of machinery in the automotive world is a transmission. Transmissions transmit the power from the motor to the wheels in a way that prevents the engine from damaging itself while maximizing the power of the engine. Some battery electric vehicles may be able to skip the transmission all together using just two or three simplified set of gears. However, in internal combustion engines (ICE), the transmissions provide the gears to best capitalize on the power curve of the engine (as well as provide the reverse function).
In the quest to snatch every last joule of energy from each drop of fuel, ICE vehicles have been undergoing a lot of interesting developments recently, including turbo charging, lighter weight materials, and revising pistons and fuel injectors. All of these contribute to better fuel economy, but most likely do not have the same impact as an improved transmission.
Transmissions have evolved over the years. When I first started to drive, manual transmissions were considered much more fuel efficient than automatic transmissions. That difference has diminished significantly and now we are seeing automatics that beat or match manual transmissions for fuel economy. In the mid 2000’s, the continuously variable transmission (CVT) emerged as the leader for fuel economy. These transmissions do not have the mechanical gears of a traditional transmission, instead using a chain on two v-shaped pulleys that move in and out making the chain rotate on a larger or smaller “gear.” This allows the transmission to operate at an infinite number of gear ratios, which keeps the engine running at its optimum speed.
However, in the last couple of years, in North America, the CVT transmission has been slipping in popularity. Why? One reason is because they are usually more expensive than comparable, traditionally geared transmissions. Another is that they lose the shifting feel that consumers are used to experiencing in vehicles. Some may view this as a positive, but many apparently do not. In fact, auto companies like Nissan are putting a shifting feel back into their CVTs with software. Finally, there tends to be a torque limit with CVT transmissions which lowers vehicle performance in comparison to geared transmissions making them better suited to small engines.
Meanwhile, transmission manufacturers like ZF Friedrichshafen have been adding additional gears to their transmissions while reducing the lag between shifting. This summer, ZF launched the first nine-speed automatic transmission for passenger cars with transverse mounted engines (typically used in front and all wheel drive cars). ZF’s nine-speed transmissions have very fast shifts with only two open clutch events, all but eliminating the fuel wasting time between shifts. The new transmission is expected to improve fuel economy 15% or more compared to a six-speed transmission and do so with fewer parts.
Dual clutch transmissions (popular in Europe) typically improve fuel economy 4% to 12% over comparable traditional automatics with torque converters according to transmission-builder, Getrag Corporate Group. Getrag explains dual clutch transmissions on their website as “one gear is engaged, the system has already preselected the next. Once the relevant rpm has been reached, one clutch is opened while the second is closed simultaneously, precluding any interruption in tractive force.” In addition, new torque converters and transmissions are being optimized to provide start/stop capabilities to automatic transmissions, all in the name of increased fuel economy.
Thanks to the relationship with Fiat, Chrysler is now going to be licensing ZF technology to manufacture eight-speed transmissions for its vehicles, replacing its current six-speed transmissions. While Ford’s 2012 Focus launched with Ford’s dual clutch six-speed transmission, in June the company announced two new transmissions, an eight-speed automatic aimed for Lincolns, initially, and a new CVT for its next generation of hybrid vehicles. This CVT allows the vehicle to share traction from two separate power plants, an electric motor and gas engine. At the same time, Hyundai has introduced a new CVT for its small cars and a dual clutch transmission that in essence automates manual transmissions.
Does all of this mean that to improve fuel economy we just need to add more gears or another clutch? Is there a 10-or 12-speed automatic transmission in your future? It seems the consensus at the moment is more gears probably won’t help in passenger cars, but perhaps in large engine light duty trucks in years to come. CVTs will continue to appear in various forms, and may have another day in the sun coming as fuel economy restrictions get tougher and the appeal of smaller, low-torque cars grows.