Upon tearing apart a Toyota Prius—spanking new from the dealer—what strikes an engineer's eye is that this first parallel drive train gas/electric hybrid car is more like other modern vehicles than not. Ignore the large nickel-metal hydride battery tucked out of sight behind the rear seat; filter out the associated starter-motor-like heavy-gage high-voltage orange cables running along the frame to carry current to and from the electric traction motor for power and regenerative braking. Now you have a system that seems "conventional," despite the optimized hybrid-power-train architecture.
The Prius overall appears to follow the Toyota build philosophy: standard modules, bits and pieces--from electronics to doors and other components--that readily fit in place, enabling the same vehicle model to be built to the same quality standard anywhere in the world.
Like any modern vehicle, the Prius packs numerous electronic subsystems. Six, however, stood out: the inverter/converter, the user-interface/dash module, the engine control module, the navigation/display system, the airbag control module and the anti-skid system. The functionality, placement, electronic content and packaging of those subsystems represent the cutting edge not only in terms of technology but also in the context of reliability and power management.
The Prius uses a parallel gasoline/electric power train, which means the car can run on electric power alone at low speeds (and shut off the gas engine at stops), accounting for its high city mileage. The architecture incorporates two motor generators mounted within the transaxle. One MG starts the car and recharges the high-voltage nickel metal hydride battery. The other MG boosts the internal-combustion engine's output and, conversely, regeneratively brakes the car to reclaim energy back into the battery and optimize brake effectiveness. Under heavy load, output from the first motor generator is sent to the second, adding to traction power.
When running, the Prius drives similarly to a conventional car but with better low-speed acceleration from the MGs' torque.
Obviously, the module of most interest is the one critical in making a hybrid car unique: the inverter/converter. This unit—placed atop the transaxle—is the electrical-system "switching yard," providing voltage control and switching dc to dc, dc to ac (for the motor drive) and ac to dc (to charge the battery with the gasoline engine and during regenerative braking).
The system voltage values that the inverter/converter must handle include 201 V from the NiMH battery and 500 Vac system max for the two motor generators in the transaxle. The circuit board is pressed into a substantial aluminum casting that acts as a heat sink, and the assembly is closed by a stamped steel cover. Within the module are robust relays and bus bars.
Engine control module
The ECM looks and functions much like those in conventional cars when providing gasoline engine control (such as modulating the timing and width of fuel injector pulses) and monitoring engine and emissions sensors (including cam, crank and O2 sensors). But in a hybrid, it must also interface with other modules, such as the inverter/converter, to allow running on all-electric power at low speeds and to use the gas engine to recharge the battery pack.
Placed under the instrument panel cover, behind the glove box (and attached on a common bracket with the climate control module), the engine control module is difficult to access. Such positioning, however, gets the unit out of the harsh under-hood environment, with its higher temperatures, potential moisture and higher electromagnetic interference. (In addition, since the module is difficult to reach, mechanics may be less inclined to replace this costly component first when troubleshooting problems.)
The touchscreen at the top of the center stack is one of two key driver interfaces, along with the interface/dash module. In addition to providing navigation information and input to navigation functions, the display allows control of the audio system and climate control, as well as updating the driver on the status of the hybrid system and in which mode it is running. The latter feature fascinates new hybrid operators—sometimes to the point of distraction—as they try to squeeze out maximum mileage and track which mode they are in by watching the current flows on the schematic.
The navigation system is connected to a DVD player under the driver's seat. The data storage device holds nav system map data that is read in conjunction with the real-time GPS satellite location to yield the guidance information displayed on the screen. The location of the GPS antenna is not obvious; it may be located behind the touchscreen module to enable a good sky view through the windshield.
The radio is integral to the center stack as well, with control via the touchscreen. The lighting implementation for the radio, however, was not obvious.
All in all, the stack comprises basic, easy-to-assemble technology with few fasteners. That facilitates assembly, which in turn minimizes costs as well as the opportunities for problems (and associated warranty work).
User interface/dash module
Just below the center windshield is a display module that gives the driver speed and odometer readings; provides gear, turn and high-beam indicators; and contains system status and warning lights. Its location serves as a quasi-heads-up display that comfortably allows drivers to focus on the road.
The upper portion of the display, comprising a digital electroluminescent speedometer, odometer, fuel gauge and gear position indicator, is separate from the warning and status indicators positioned below. The latter are LED-backlit behind a black polycarbonate laser-etched panel.
On other cars, such a display would usually be one large assembly. Here, the large size of each of the displays (23 x 10 cm for the speedometer cluster and 20 cm wide for the warning lights) probably dictated the dual-display configuration. The two displays are individually prewired and then electrically connected to the vehicle main wiring harness via inline connectors pressed together during the module assembly. Flex circuits serve as the interconnects between the circuit boards and the displays.
Airbag control module
The airbag module governs selective deployment of the various airbags (front, side, side-curtain and belt pretensioners) in a crash. Inspection of this unit reveals that the front airbags are dual-stage devices with two inflation rates, depending on crash severity.
The module is centrally hard-mounted on a spring steel bracket to the body directly below the instrument panel center stack. Located at the centerline position is a piezoelectric sensor, one of many on the vehicle (such as at the front corners and side pillars) for crash detection. Because this location is right below a set of cup holders, a clear, vinyl-like cover is glued to the module's die-cast aluminum housing to prevent liquid intrusion.
Closer inspection of the airbag control uncovers standard yellow connectors, denoting safety system circuits. All airbag squib-firing circuits have shorting bars to short the connectors when they are disconnected for service so that an electrostatic discharge cannot fire off a bag.
Stability control module
By selectively braking individual wheels, the stability control module mitigates skids and understeer during severe maneuvering. The module is directly above the throttle (gas) pedal. But one of its key inputs is a yaw sensor placed about a foot toward the rear of the car from the airbag module position. This spot is roughly at the center of gravity of the car and determines rotation (yaw) about the vertical axis.
Other inputs to the module include steering wheel angle (driver input), speed, brake inputs and wheel speed (from the antilock-braking system sensors, via the ABS controller).
The stability control module has an unusual two-piece case. One half is a die-cast housing whose chief feature is the many metal "posts" on its surface; they appear to be related to heat rejection. The other half of the case is sheet metal and includes the unit's mounting brackets. But the circuit board inside this case is only attached at the ends to bosses that are placed well away from post-studded areas of the case. Perhaps the housing was adapted from another unit that had more-critical heat issues. The stability control module circuit board has "typical" ICs and passives, with no sensors on the board.n
Al Steier is an associate at engineering analysis firm Munro & Associates Inc. (www.leandesign.com). Rick DeMeis is editor of TechOnline's Automotive DesignLine (www.automotivedesignline.com).
Video: Tear down that Prius!
Inside the Toyota Prius: Part 1 - The airbag control module
Inside the Toyota Prius: Part 2 - The dashboard display system
Inside the Toyota Prius: Part 3 - Skid-control module plays it safe
Inside the Toyota Prius: Part 4 - Controllers keep dual motors humming
Inside the Toyota Prius: Part 5 - Inverter/converter is Prius' power broker
Inside the Toyota Prius: Part 6 - Navigation unit bridges automotive, CE
Blog: Hybrids on the road