Motors Get It In Gear

As gasoline prices escalate, existing hybrid vehicles have received even more attention from customers and, as a result, from carmakers. In many cases, original equipment manufacturers (OEMs) are looking to and, sometimes, counting on suppliers for design ideas and the products for these hybrids. One fresh approach comes from the U.K., where dealing with high petrol prices is not new. UK-based NexxtDrive's DualDrive transmission is a continuously variable transmission (CVT) with two built-in motors to improve the drive's efficiency.

The design concept for the drive originated over 30 or 40 years ago but the dual aspect was a combination of a hydraulic variator and mechanical gears. As recently as 10 years ago, power combining or power split drives were developed using hydraulics instead of an electrical portion. The hydraulic variator consists of a hydraulic pump and a hydraulic motor. More recently, companies such as Caterpillar developed this kind of system by combining electrical variators with mechanical gears.

"At Caterpillar, because of the very high range of speeds and torques required for the machines, we used to do them as multi-regime transmissions," says Frank Moeller, former Caterpillar engineer who's now director and chief engineer at NexxtDrive. After leaving Caterpillar, Moeller developed a single regime unit. A further development of that concept led to the DualDrive design.

CVT for MPG

Recently, the high interest in hybrid drives and the trend towards higher voltage systems for automotive applications has made these CVT systems very attractive. "This is something that really is of keen interest at the moment from tier one and OEM manufacturers because of the need to reduce CO₂ and the strong consumer and governmental pressure in different countries for hybrid drives," notes Rod Keech, chairman of NexxtDrive.

The spur gears used in the CVT are about 97 percent efficient compared to the process of generating electricity and then powering a motor, which is about 80 percent efficient. Reducing the engine speed to the minimum required to drive the vehicle always provides the best fuel consumption for the engine. The two motor-generators provide a stepless position of output speed controlled by electronics. The electronic control is not unlike the torque control commonly used in motors today. This portion of the system uses well-established feedback control techniques and a sensorless approach to provide torque and speed inputs for the control.

The two middle suns connected to the two motors are not visible. The green output sun connects all three planet gear sets

The system's motors can be any type of durable motor such as a brushless permanent magnet, induction, or even switched reluctance design. However, a brushless motor is most likely the candidate. NexxtDrives development partner will make the ultimate decision.

"The main advantage that we see of this drive is that it can be both a replacement for an automatic transmission, an ECVT, but at the same time that transmission can provide hybrid capability," says Keech. Unlike the systems that are on the road today, a different drive is not required for the hybrid and non-hybrid version of the same vehicle. This provides a strong manufacturing advantage to the design.

All that is required for a hybrid version is the addition of a battery. The operating voltage for the system can be chosen to provide the highest voltage to the motors and maximize their efficiency. The voltage will be similar to other hybrids and 288V is a good target.

The CVT in Action

The DualDrive CVT uses an epicyclic gear train to combine the torques from the vehicle engine and one of two motors. The combination of the electrical machines, engine, and output provide a four-branch design. The four-branch transmission allows the engine connection to either the carrier or the ring of the epicyclical gear structure. Likewise, the output can be connected to either the carrier or the ring. Once the location for one is selected, the other is on the opposite member. The two suns in the planetary gear set are each driven by an electrical machine.

NexxtDrive's transmission does not have a ring; instead a third gear set in the planetary cluster provides the output to another sun. The motors can run at any speed but if either of the motors is held stationary, no electricity flows to the electrical machines and the transmission acts like a mechanical transmission. This is desirable to obtain the highest efficiency from the transmission at particular operating points. Rotating on common shafts, the planet gears are keyed to the shafts so they rotate at the same speed. While the first and third planets mesh directly with their suns, the second set of planets uses a set of idler gears mounted on the planet carriers that reverse the direction of rotation from its sun.

Instead of the outer gear ring commonly found in epicyclic gear trains, the DualDrive has three sets of planet gears mounted on a single planet carrier. Each set of planet gears engages with its own central sun gear.

Since the size of the electric motor depends more on its torque, the design uses two concentric motors. The larger machine is for high torque and the smaller unit is for high speed.

The system comes together in a package that is comparable in size or even smaller (shorter) than today's automatic transmissions. In preliminary work for one OEM, NexxtDrive was able to package the transmission within a space for one of the smallest vehicles in Europe.

A System Comparison

Comparing the Toyota Prius transmission, which is a three-branch unit, to the four-branch DualDrive, for a 100 kW input, the DualDrive would only require 26.3 kW for each motor instead of the over 60 kW. The torque requirements for the CVT motors are also much smaller.

In an a electrical power comparison of something like a diesel electric locomotive, the electric output increases to 100 percent and stays at this high level. This limits the drive's efficiency. In a three-branch unit like the Prius, the unit has more than 60 percent power at its peak and 25 percent of the power on average. In the three-branch unit, there is an input from the engine and one of the electric motors directly connects to the output. In the four-branch system, two node points occur over the entire speed range where the electrical power goes to zero. At the node points one of the motors is at a standstill and the transmission operates at its highest efficiency. With four branches, the unit can have two node points whereas the three-branch can have only one. The straight line of the Prius only crosses through zero once. It is difficult to match and lock a three-branch transmission since it is only a single-design, whereas the DualDrive has the two speeds to lock the motors.

The less electrical power consumed translates into greater use of the more efficient mechanical portion of the transmission. Additional benefits of lower power requirements are smaller, less expensive motors and power electronics. Because of the electrical capability of the transmission, it can replace both the alternator and starter motor and it does not require a clutch. So, from a vehicle design standpoint, a considerable simplification can occur.

A Transmission for More Than Just Cars

The underlying technology of the DualDrive can solve the speed and torque control requirements of a number of mechanical power output applications. Since the drive technology is scalable over a wide range, it can address very small vehicles and even electric bicycles all the way up to diesel locomotives and wind turbines. Although the primary application is automotive automatic transmissions and hybrid powertrains, NexxtDrive expects variable speed superchargers, engine driven accessories, and light electric vehicles to take advantage of the design approach.