Power Split Device

The Toyota Prius is a full hybrid electric vehicle. It is a mid-size vehicle developed and manufactured by the Toyota Motor Corporation. The XW20 series represented the second generation of the Toyota Prius, replacing its XW10 predecessor. According to the EPA standard City fuel economy for Toyota Prius is 48 mpg and HWY fuel economy is 45 mpg. The combined fuel economy comes out to be 46 mpg. Toyota Prius has Combined Series and Parallel power train transmission configuration. It uses a device called the Power Split Device to transmit power to the wheels. The Power Split Device is a planetary gear set that combines ICE, an Alternator and a Motor and transmits torque to the differential. It acts as a continuously variable transmission (CVT) but with a fixed gear ratio.

Toyota Prius uses efficient technology and blends torque using power split device which gives good efficiency and good driving performance, hence to design such model is challenging and a perfect opportunity for learning.

 Transmission 

A powertrain to match Combination of Series-Parallel HEV powertrain of Toyota Prius mainly depends on Planetary Split Device (PSD) as shown in Figure 1. PSD combines Generator, Motor, and ICE to transmit torque to wheels through differential gear system. ICE power is transmitted through the planet carrier of PSD, which passes power to outer-ring gear and the inner-sun gear. Rotating shaft of ring gear is linked to motor which transmits tractive force. 

Figure 1

Transmission of Power to the wheels can be explained using following modes:

1. Starting: When the car starts off, only the electric motors is used, powered by the battery, while the gas/petrol engine remains shut off. 
2. Low Speed Driving: A gas engine is not energy efficient in running a car in the low-speed range. On the other hand, electric motors are energy efficient in running a car in the low-speed range. Therefore, the electric energy stored in battery is used to run the car on the electric motors in low-speed range. (If the battery charge level is low, the engine is used to turn the generator to supply power to the electric motors.)
3. Cruising : The gas engine in the speed range in which it operates with good energy efficiency is used. The power produced by the gas engine is used to drive the wheels directly, and depending on the driving conditions, part of the power is distributed to the generator. Power produced by the generator is used to feed the electric motors, to supplement the  engine. By making use of the engine/motor dual powertrain, the energy produced by the gas engine is transferred to the road surface with minimal loss. (If the battery charge level is low, the power output from the gas/petrol engine is increased to increase the amount of electricity generated to recharge the battery)
4. Recharging: In its high efficiency range, the gas/petrol engine may produce more power than is necessary to drive the car. In this case, the surplus power is converted to electric energy by the generator to be stored in the battery.
5. Full Throttle: When strong acceleration is used, the power from the battery is supplied to the electric motors to supplement driving power. By combining the power from the gas/petrol engine and the electric motors, the vehicle gets power without any flaws.  
6. Deceleration : Under braking or when the accelerator is lifted,  the electric motors, functions as regenerators. Energy that is normally lost as friction heat under deceleration is converted into electrical energy, which is recovered in the battery to be reused later.
7. Stop: The gas engine, the electric motors and the generator are automatically shut down when the car comes to rest. No energy is wasted by idling.( If the battery charge level is low, the gas/petrol engine is kept running to recharge it. In some cases, the gas/petrol engine may be turned on in conjunction with the air-conditioner switch operation.)