Toyota Prius: Best Engineered Car of 2001

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THS in detail
The basic configuration of the Prius' THS is the same as that introduced in Japan in 1997 for MY1998. It combines parallel- with series-hybrid systems to achieve a synergy of both. The gasoline engine drives the wheels and the generator, with the generated electrical power used to drive the motor directly; or the power is converted by an inverter into direct current (dc) and stored in a high-voltage battery pack. In the 2001 Prius, engine and motor output and efficiency are considerably improved to achieve better fuel consumption and a considerable reduction in exhaust emissions, according to Toyota engineer Shinichi Abe. The complete transmission - which consists of the power-split device, motor, generator, and reduction gear - was designed to be as compact as possible.

Improvements to the major powertrain components enhance standing and passing performance - which is now equivalent to a 1.8-L automatic-transmission (AT) vehicle, according to Abe. Gains were made in engine output by increasing maximum speed from 4000 to 4500 rpm, optimizing intake-valve-closing timing by using a VVT-i (variable valve timing-intelligent) system, and optimizing the compression ratio. Valve timing was advanced by increasing the VVT-i's operating angle, thus increasing intake-air volume. The compression ratio was lowered from 13.5: to 13.0:1 because the increased air volume made the engine susceptible to pre-ignition.

Because engine speed is limited to 4500 rpm, many internal parts experience lower-than-normal stress levels. Therefore, the crankshaft has a smaller diameter, piston rings have lower tension, and the valve spring load is less, compared to standard higher-revving engines. These advances result in reduced friction losses, which translate into increased efficiency. The cylinder bores also are offset from the crankshaft to help reduce friction.

The Prius' instruments are centered high on the IP at the base of the windshield, making it easy for the driver to refocus from the road to the dash. A display below the instrument cluster on the IP can monitor the condition and energy flow of the hybrid drivetrain and the audio system.

The basic construction of the electric motors is the same, but their mechanical and electromagnetic designs were revised. The improved motor is controlled using pulse-width-modulation (PWM) switching in the low-speed range and by a one-pulse method in the high-speed range; the previous motor was controlled solely by the PWM switching method. By adopting the one-pulse switching method, 27% higher basic-wave voltage can be applied to the motor in the high-speed range compared with the PWM switching method, and the output of the motor is increased.

Toyota engineers improved the efficiency of many components and reprogrammed the hybrid control logic to improve fuel economy. The improved efficiency is highlighted in Figure 6, which compares the new Prius, at 58 mpg (unadjusted), with other gasoline-fueled vehicles in the U.S. EPA combined mode. With the previous Prius, only two intake-valve-timing settings were used: one prioritized fuel consumption and the other acceleration. With the new car, intake-valve timing is continuously varied with engine speed and load conditions, to the benefit of torque and power as well as specific fuel consumption, the minimum being 225 g/kW•h (168 g/hp•h).

The basic construction of the transaxle, with an integrated electric motor, is unchanged. However, drivetrain losses were considerably reduced, and motor and generator efficiency improved. Mechanical efficiency gains were made mainly through the reduction of oil-pump and -mixing losses as well as reduced motor drag loss through the elimination of the oil seal. Electrical efficiency was improved via new motor windings and magnets as well as new motor control strategies.

In the previous Prius, the gear section was separated from the motor/generator chamber to prevent transmission oil from entering the chamber. However, this sealing structure created drag torque and efficiency issues. The resin sealing material was changed in the new motor to enhance the oil durability of the motor body; therefore, the oil seals are no longer required, and bearing friction - thus, mechanical loss - is reduced. By reducing losses from oil pumping and windage, mechanical loss of the overall transmission (including the torque-split device) is reduced by about 40%.

The use of one-pulse switching allowed a redesign of the electromagnetic circuit to enhance efficiency further. An increase in the number of turns led to a drop in the current required, reducing both inverter loss and copper loss. In general, an increase in the number of turns increases torque if the current level is the same, but in the high-speed range, it increases induced electromotive force and reduces output. Low-speed efficiency improvements were significant, with a noticeable benefit in fuel consumption. The efficiency of the drivetrain, including the electrical system, is about 19% greater at a constant 60 km/h (37 mph) and about 9% better during 120-km/h (75-mph) cruising.

In addition to the engine and drivetrain improvements, hybrid coordination control was refined. By raising the maximum speed of the generator, Toyota engineers were able to raise from 45 to 65 km/h (30 to 40 mph) the speed at which stop-and-go operation of the engine is permitted and expand the electric-mode driving "area".

Though the Prius powertrain was designed to be compact in 1997, the battery pack was mounted behind the rear seat, which restricted trunk room. A new rectangular battery pack allows an enlarged trunk and pushes the new Prius into the EPA compact class. The previous battery configuration comprised 7.2-V cylindrical modules, each consisting of six D-size 1.2-V cylindrical nickel/metal-hydride batteries connected in series. Forty of these modules were assembled into a body called a holder, connected in series, and placed in the vehicle. In contrast, the basic battery of the new car is a rectangular 7.2-V mono-block module. The module is thin but has a large surface area to dissipate heat. It is made of resin selected for its resistance to the alkalinic electrolyte, electrical insulation among cells, formability, and weight. When combined into a battery pack, the rectangular modules are tightly packed and require a minimum amount of space for cooling. Thirty-eight modules are stacked in series to form a single, compact pack.

To ensure electrical safety, the system's high-voltage parts are enclosed in the pack. These parts consist of a system main relay (SMR) with a built-in amperage sensor, a service plug with a built-in fuse, and a battery ECU. The battery-cooling blower was relocated outside of the battery pack to use vehicle dead space more efficiently.

The internal resistance of the battery was reduced and its performance improved vs. the cylindrical type. Although the number of modules in the battery pack was reduced from 40 to 38, equal or greater performance was achieved. Because the number of modules was reduced without reducing the output voltage, the change could be made without altering the specifications of the vehicle's electric components.

For North America, where vehicles need better hill-climbing performance, it was necessary to extend the maximum current available from the inverter. Compared with the previous inverter, cooling performance was enhanced by heat-sink fin optimization, a reduction in the cooling system's flow resistance, and improvements in water-pump performance.

The capacity of the condenser was reduced by a careful design of cooling construction and use of new materials for specific components - with inverter weight reduced by 40%. In addition to the plainer-type insulated gate bipolar transistors (IGBT), Toyota engineers developed a trench-type unit - both important parts of the interior permanent magnet (IPM). The adoption of trench-gate IGBT technology resulted in a large cell-area reduction and lower energy loss. The 2.1-V loss on the 13 x 13 mm (0.51 x 0.51 in) plainer unit was reduced to a 1.7-V loss on the smaller 9.3 x 12.3 mm (0.37 x 0.48 in) trench-type unit. Development of fine-process and trench-etching technology made it possible to achieve this performance. Basic cell unit length was reduced from 36 µm (1400 µin) for the plainer-type unit to 4 µm (157 µin) for the trench-type. Although the cell area was greatly reduced, avalanche energy remained the same as the plainer IGBT's.

Typical rotors use surface permanent magnet (SPM) technology with magnets attached to the surface of the cylindrical rotor core. With the Prius' reverse salient-pole IPM system, magnets are embedded inside the rotor; electromagnetic steel sheets are laminated to avoid the use of windings on the magnetic surface, reducing costs and providing higher torque and efficiency by adding reluctance torque to magnetic torque.

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