The 2017 Prius Prime was revealed at the New York Auto Show recently and one of its major new features is the ability to use both of its electric motors together when accelerating with the gas engine off.
Several other plug-in hybrid models announced during the past year also have this kind of “dual motor” ability starting with the 2016 Chevrolet Volt, the 2017 Chrysler Pacifica minivan, and the 2017 Cadillac CT6.
Using both motors together seems like an obvious idea and some will wonder why it wasn’t being done by the first generation Prius Plug-in. The key to understanding dual motor use is, oddly enough, not really the motors. It’s about the battery.
The original 2012 Prius Plug-in Hybrid had a relatively small battery that could nominally store up to 4.4 kilowatt-hours of energy and put out up to 38 kW (51 horsepower) of instantaneous power. The bigger of the car’s two electric motors was designed to use up to 60 kW (80 horsepower) generated in part by the gas engine together with power taken from the battery during hybrid operation. So, 38 kW just from the battery’s output alone with the gas engine off was not a problem.
The Prius Prime, Toyota’s replacement for the original Prius Plug-in, has twice the nominal battery capacity at 8.8 kWh and almost twice the power output capability at 68 kW (91 horsepower). That’s more power than the big motor on the 2012 Prius Plug-in could have handled but the new Prius Prime actually has a somewhat less powerful motor.
The Prime shares most of its powertrain with the 2016 Prius. When Toyota redesigned the hybrid system for the new Prius its engineers realized they could obtain similar or better overall hybrid performance with improved gas mileage by optimizing the engine and parts of the transaxle and this included downsizing the bigger motor from 60 kW to 53 kW.
Thus, the big motor can no longer handle the full output of the new Prius Prime battery by itself. Toyota could have made a larger motor (and more powerful motor controls) just for the Prime. Instead, it chose to use the existing motors together in order to utilize the full 68 kW (91 horsepower) from the Prime’s larger battery.
The vehicles from GM and Toyota use a somewhat similar electrically variable power-split transmission design that ties these two motors, the gas engine, and the wheels together using one or more planetary gear sets. Chrysler is believed to use a similar design as well.
Due to the way the smaller motor/generator is geared to the gasoline engine, any attempt to use it during electric-only driving would cause the engine to spin backwards rather than drive the wheels of the car forward. This problem has now been fixed by adding a one-way clutch to prevent the gas engine from spinning backwards.
Every Prius has a flywheel and damper assembly which sits between the engine and the hybrid transaxle case which contains the electric motors, planetary gears, and front wheel differential. Normally the damper passes the engine rpm through unchanged but it can also give a little to dampen out vibrations and sudden torque changes.
As shown in the image above, the new one-way clutch in the Prius Prime appears to have been integrated into this assembly. GM locates it’s version of an engine clutch inside of the transaxle case in the Volt and CT6. The exact details of Chrysler’s design have not yet been revealed.
With its new battery and dual motors the Prius Prime now has stronger EV driving capability. As long as there is usable battery charge the gas engine will stay off even under full acceleration. However, the engine will start if the driver goes above 84 mph. The previous generation Prius Plug-in would start the engine under moderate acceleration or at speeds above 62. Being able to use either motor or both motors together to varying degrees may also allow for some efficiency optimizations.
Similarly, by using a new one-way clutch, GM was able to reduce their bigger motor in the first generation Volt from 111 kW (149 horsepower) to 87 kW (117 horsepower) in the new 2016 model because using both its small and big motors together allows it maintain the same 111 kW of drive power in the new model.
In these cars with a one-way engine clutch the larger motor is still used alone for slowing the car during regenerative braking. Some other car makers use a parallel type of plug-in hybrid powertrain that have just a single electric motor for driving the wheels or regenerative braking.