- Author: Mike Riley, Transmission Digest Technical Editor
- Subject: Hybrid-vehicle transmissions
- Units: 2ML70, 4L60-E PHT; A6LF2, A6MF2, A6GF2, A6MF2H
- Vehicle Applications: GM, Hyundai
- Essential Reading: Rebuilder, Diagnostician
In an effort to improve fuel economy, drivability, durability, and cost, many changes have occurred to automatic transmissions over time. In the last decade however, the proliferation of new models has been unending. Step-type transmissions have ratcheted up the gearing from old-time three- and four-speeds to five- through seven-speeds.
Thanks to transmission companies like Aisin and ZF, production eight-speeds are a reality and a ZF nine-speed is close at hand. Even Hyundai has come of age with an eight-speed and is supposedly going to launch a 10-speed transmission shortly.
The battle of the gears continues!
One departure from step-type transmissions has been CVTs. Regardless whether the CVT uses a belt or chain, has or doesn’t have a torque converter or uses a start clutch, they all function basically the same. There is a constant speed (ratio) change that occurs with a CVT, unless the computer overrides the base programming and forces a fixed ratio, such as a manual shift.
The first successful production CVT in the U.S. was in the 1996 Honda HX. Nissan began to offer vehicles with CVTs in the new millennium and has continued to expand the models using CVTs. There are now several variations of CVTs to contend with, but the main manufacturer of CVTs today is Jatco.
Because of the unrelenting fuel issues, many OEMs started to change their focus to an electrical solution – not the electronics that have controlled various transmission functions for decades, but rather using electricity to actually propel the vehicle.
There have been several approaches by the OEMs that have integrated electrical propulsion into the vehicle. To date, the most successful vehicle to use this electric approach is the Toyota Prius. It was first released in Japan in 1997, then in the U.S. in the early 2000s, and there are well over 2 million vehicles worldwide.
The Prius transmission basically has primary and secondary electric motors and not much else. The MG1 (motor/generator 1) starts the ICE (internal-combustion engine) and charges the power pack. The MG2 mainly propels the vehicle as well as charging the power pack through regenerative braking. The Prius can run using the ICE, the MG2 or both.
The Ford Escape Hybrid also uses a similar transmission to the Prius. Both are produced by Aisin. Although Toyota and Ford refer to these transmissions as CVTs, they are not. There isn’t a pulley or drive belt to be found. They are both EVTs (electric variable transmission).
More recently, GM released another variation, the 2ML70, which is a two-mode transmission. It is basically a four-speed automatic with two large electric motors inside. Unlike the Prius transmission, the 2ML70 can function like a traditional four-speed transmission driven by the engine or it can switch over to an electric vehicle for fuel economy. The 2ML70 is built to handle heavy loads and high-torque engines. Both EVTs and two-mode transmissions have all the key components integral within the main case halves to provide a total drive system.
Another electric-motor design is IMA (Integrated Motor Assist). By comparison, the IMA approach is more of an “add-on” type of system. An IMA setup does not propel the vehicle exclusively but rather helps the engine to turn under certain conditions. The electric motor will also start the ICE and charge the battery.
Honda chose this path with the 1999 Insight and later on with the 2003 Civic Hybrid. By doing that, Honda was able to achieve the desired fuel-economy goals. The electric motors are inside the transmission case on the Toyota Prius, whereas the Honda IMA electric motor is situated between the engine and transmission where a torque converter would normally be (Figure 1). The transmission used is a CVT.
Although an IMA vehicle could be considered an econo-box by certain standards, it is a way to reach specific fuel-economy goals without completely reinventing the mousetrap. GM took a stab at a “mild” hybrid, but for a different reason. The 2005 4L60-E PHT (partial hybrid transmission) also had an electric motor of sorts. The purpose of the GM PHT was primarily to provide the start-stop feature and charge the battery. The 4L60-E PHT (M33) was used only from 2005 to 2007. Any transmission could be outfitted with an electrical motor and function as an IMA.
Hyundai released a family of six-speed automatics in 2009 designated A6LF2, A6MF2 and A6GF2. The units are different only in size and torque capacity. In 2011, Hyundai released a hybrid six-speed, the A6MF2H. The back end of the transmission is comparable to the A6MF2. The front end is where the difference lies. The bellhousing is relatively flat to make room for the motor housing. The IMA electric coil bolts into the housing. The outside diameter of the coil is about a foot with the I.D. about 8 inches (Figure 2).
The armature (Figure 3), which is driven by the coil, is a powerful magnet that must be handled with caution. Fingers can be pinched if they get between the armature and a steel bench.
The A6MF2H does not use a torque converter, but rather a start clutch like Honda uses. The clutches are single sided and operate like a normal clutch pack (Figure 4).
The clutch hub is also the input shaft, and when the clutch applies the engine will drive the start-clutch housing, armature and ultimately the rest of the transmission (figures 5a and 5b).
Another word of caution has to do with voltage. Hybrids use a ton. As shown in Figure 6, the electric-cable connectors are large to accommodate the juice. Make sure to follow procedures while working on any hybrid, regardless of type.
Time will tell as to which electrical propulsion system will dominate the market; however, it is a reality today. Tomorrow, who knows; it may come down to an electric motor on each wheel.