This unique design defines Toyota hybrids.
According to the Society of Automotive Engineers (SAE), a hybrid is defined as "a vehicle with two or more energy storage systems both of which must provide propulsion power, either together or independently." In today's hybrid designs, those two systems are the ICE and an electric propulsion system. The ICE takes care of mid- to high-speed operation (where it is most efficient) and it can be shut off at stops or under low speed conditions (where it's not). The electric motor handles low speeds, and it can also add its power to the ICE when full power is needed.
Hybrid Synergy Drive
Toyota's hybrid design is unique in the industry. It is the only design that can recharge its high voltage (HV) battery at the same time the electric drive is being used for propulsion. It's called the Toyota Hybrid System, or THS. Improvements were made to the first THS, and THS-II, now referred to as Hybrid Synergy Drive®, was introduced on the 2004 Prius, and later included on the 2006 Highlander and 2007 Camry.Instead of one electric motor, Toyota hybrids use two. In all hybrids, the electric motor can be used as an electric generator to produce the electricity needed to recharge the battery. By using two, and controlling how they interact, the Toyota system can provide electricity to power the electric drive at the same time. This ability is why Toyota refers to it as a series-parallel hybrid system. And the heart of the system lies in the hybrid transaxle.
Splitting the Power
Actually, it's not a transaxle in the sense you are used to. You won't find a torque converter, valve body or clutch packs.
What you will find are two electric motor/generators: MG1 and MG2. MG1 is the motor/generator driven by the ICE. It is used to recharge the HV battery and supply electrical current to MG2. It also serves as the engine's starter. MG2 is the drive motor, supplying power to the wheels with or without the ICE's help. It also can convert kinetic energy into electrical energy during regenerative braking to charge the HV battery.The design was improved in 2010 when the P410 transaxle was introduced. Compared to the previous Prius transaxles, it's about 33 percent lighter but provides more power. The PSD is incorporated into a new Compound Gear Unit.
In addition to the planetary gear set used to split power, there is another set called the motor speed reduction unit. This allows the 80 hp MG2 speed to be reduced by a 2.64:1 ratio and effectively boosts maximum torque to 403 foot-pounds from the motor's base of 153 foot-pounds. The chain drive used in the earlier versions is also gone, replaced by a direct gear drive assembly using helical cut gears for noise reduction.Modes of Operation
Start-Up: On start up, power from the HV battery is passed through the inverter, a device that converts DC voltage to AC, and vice versa. The AC voltage is applied to MG1 to spin the engine. The engine will run until it reaches operating temperature and the battery pack is fully charged.TIP: When servicing, keep in mind that the ICE can start anytime the ignition is on. Be sure to follow proper precautions before performing routine services. Last thing you want is a Prius engine starting while up on the lift with no oil in it!
Off Idle Acceleration: MG2 powers the wheels all by itself, feeding off of HV voltage supplied through the inverter. When drive torque requirements exceed the programmed limits, MG1 is used to start the ICE. The ICE and MG2 supply power as needed to continue acceleration.Full acceleration: When full power is needed, the ICE and MG2 both supply power to the drive train.
Deceleration: MG2 acts as a generator to take advantage of regenerative braking.Service Tips
To properly control MG1 and MG2, a speed sensor (also called a resolver) is used. It is made up of three coil windings on a stator and an oval-shaped rotor. Two coils are staggered 90 degrees and the air gap between them and the rotor varies with rotation. The MG control module detects the difference between these two output values and calculates rotational speed.
The Prius started using the P112 transaxle in 2004. It uses a special, low viscosity fluid called ATF-WS, and the oil pan was discontinued. MG1 is liquid cooled, so be sure you're removing the right drain plug for the service you are performing. The one closest to the engine is the coolant drain, and the one farthest from the ICE is the ATF drain.
The same applies to the P410 transaxle in use starting with the 2010 model year Prius. Its ATF drain is located almost dead underneath the CV axle, while the coolant drain is closer to center. On both, fluid change is recommended at 100,000 miles with 50,000-mile intervals specified after the initial service.
If servicing the inverter cooling system, be sure to use a new gasket and follow the service procedures to remove all the air when refilling. The procedure will require you to activate the dedicated electric pump with a scan tool.
Towing a hybrid vehicle with its drive wheels on the ground could cause MG2 to produce electricity. If that happens, the electrical insulation could leak and cause a fire. Always tow a hybrid with its drive wheels off of the ground or on a flat bed.
Toyota has always been friendly to the aftermarket, and its service information site (www.techinfo.toyota.com) shows it. It offers factory level diagnostics using your PC in a product called Techstream Lite. Considering the number of Toyota/Lexus/Scion products on the road and the increasing number of hybrids needing service, this could be a worthwhile investment for you and your shop to investigate.
Another worthwhile tool is the free membership in the AutoPro Workshop, the Motor Age community site at http://workshop.search-autoparts.com.
