Prius c. Click to enlarge.

The Prius c (“c” for city) is targeted at young singles and couples, and is the gateway vehicle for the Prius Family, Carter said. The Prius c will offer a single grade with four equipment levels:

• Prius c One (MSRP $18,950) features: power windows and door locks, automatic climate control, Multi-Information Display (MID), Hill Start Assist Control (HAC), Remote Keyless Entry, projector-beam halogen headlamps, steering wheel-mounted audio controls, 4-speaker audio, Bluetooth hands-free telephone controls, Bluetooth and USB auxiliary outlets, rear window wiper.

• Prius c Two (MSRP $19,900) adds cruise control, tonneau cover, 6-way adjustable driver seat with vertical height adjustment, 60/40 split fold-down rear seat, engine immobilizer, tilt/telescopic steering wheel and 6-speaker audio system.

• Prius c Three (MSRP $21,635) adds Display Audio with Navigation and Entune, Touch Tracer Display, Smart Key System on front doors and liftgate with Push Button Start and more.

• Prius c Four (MSRP $23,230) adds SofTex-trimmed seats, heated front seats, alloy wheels, fog lamps.

The Prius Family will have a big hand in defining Toyota’s future. I can see combined Prius sales in the US leading Toyota by the end of the decade.

The Prius v is one of our quickest-turning vehicles at dealerships. We sold 8,399 in the first 10 weeks it was on the market; nearly two-thirds of Prius v buyers are first-time hybrid buyers. Now that we have two Prius Family members in showrooms [v and Liftback], it hasn’t dampened the strong popularity of the third-generation Prius Liftback. The early interest in the Prius v has drawn strong traffic to dealers, with some of those new Prius buyers liking the v but actually deciding that the Liftback fit their needs best.

The Prius c, like the v and the Plug-in, will add substantial incremental sales to Prius this year, with total family sales of about 220,000. Both the Prius c and the v will be about 15-20 percent of the mix, with the Plug-in around 5%.

—Bob Carter

The hybrid system in the Prius c comprises a SULEV (Tier 2 Bin 3)-rated 1.5-liter in-line, four-cylinder gasoline engine (INZ-FXE) that utilizes an Atkinson cycle to increase efficiency, a new hybrid transaxle (P510), an air-cooled, high-output 144V, 6.5 Ah NiMH battery and a variable-voltage system in which a boost converter boosts the operating voltage of the system to a maximum of 520 V DC. Maximum hybrid system output is 73.6 kW (98.7 hp)

Estimated EPA fuel economy ratings are 53/46/50 mpg US city/highway/combined (4.4/5.1/4.7 L/100 km).

Engine. The 1NZ-FXE is an in-line 4-cylinder, 1.5-liter, 16-valve DOHC engine. Although the engine number is the same as in earlier generations of the Prius, the engine for the Prius c is about 70% new, said Chief Engineer Satoshi Ogiso.

It features a high-expansion ratio Atkinson cycle, Variable Valve Timing-intelligent (VVT-i) system, Electric Throttle Control System-intelligent (ETCS-i) and Exhaust Gas Recirculation (EGR) system employing a newly designed, highly efficient EGR cooler from Denso (earlier post). The engine also uses Sequential Multiport Fuel Injection (SFI) and Electronic Spark Advance (ESA).

The adoption of a cooled-EGR system reduces cooling loss and pumping loss; with lower exhaust heat, heat efficiency was enhanced and, consequently, fuel efficiency was enhanced as well.

The VVT-i system controls the intake camshaft within a range of 47° of Crankshaft Angle to provide valve timing suited to the engine operating condition. This improves torque in all engine speed ranges as well as increasing fuel economy, and reducing exhaust emissions.

Toyota engineers eliminated the accessory drive belts, thereby reducing losses from friction. Too, the electrically driven water pump allows coolant flow rate to be controlled with greater precision based on vehicle conditions for better fuel efficiency.

Exhaust Heat Recirculation heats engine coolant using a newly designed heat exchanger. The more efficient fin-type heat exchanger provides improved cold-weather performance and reduces time for coolant to reach operating temperature. This shortens the time until the gasoline engine can stop and also improves heater performance, reducing time to warm air by 1 minute.

Maximum power is 54 kW (72 hp) @ 4800 rpm; maximum torque is 111 N·m (82 lb-ft) @ 3600 - 4400 rpm.

The new P510 transaxle in the Prius c is 21.0 mm shorter and 19.5 mm narrower than current Prius 3G transaxle (P410). Click to enlarge.

P510 Transaxle. Toyota has introduced several innovations with the Prius c’s transaxle, two of which represent a departure from the company’s previous hybrid transaxle architectures. The transaxle assembly contains Motor Generator No. 1 (MG1) which functions as the starter and a generator, and Motor Generator No. 2 (MG2) which also functions as a generator as well as the vehicle’s traction motor, along with a final drive and differential. As with all Toyota hybrid transaxles introduced from 2006 forward, it utilizes a planetary reduction gear for the traction motor in addition to the planetary gearset that is employed as a power-split device. The P510’s MG1 offers maximum output of 42.4 kW (56 hp); MG2 offers maximum output of 45 kW (60.3 hp) and 169 N·m (124.6 lb-ft) of torque.

Prior to designing the P510 transaxle, Toyota had employed random-wound distributed windings, with the sole exception being the resin-encased, solenoidal MG1 winding used in the P410 transaxle (Gen 3 Prius Liftback, Prius v, and Lexus CT200h). While distributed windings often deliver excellent performance, they can be bulky as well as costly to build. For the P510 transaxle, however, Toyota designed compact, high-output solenoidal windings for both MG1 and MG2. The P510’s windings use much thicker wire than usual—some ten times thicker than that in the P410 generator winding—with a trapezoidal cross-section. The new winding design has made the P510 transaxle easier to mass produce, according to Chief Engineer Ogiso. In addition, solenoidal windings are typically less susceptible than distributed windings to turn-to-turn winding faults, which are themselves exacerbated by the fast rise times of pulse width modulated inverters.

The transaxle’s cooling system has also been simplified. Previous Toyota hybrid transaxles have supplemented a splash lubrication system with an engine-driven transaxle fluid pump for high-speed and/or sustained high torque operation. The transaxle fluid circuits have, in turn, been augmented by heat sinks which are cooled by a water-based coolant loop shared with the powertrain’s inverter. The P510 transaxle does away with the shared coolant loop, which is now exclusive to the inverter, and instead diverts some of the transaxle’s oil pump output, via an external pipe, to an internal manifold pipe mounted above the transaxle windings. Transaxle fluid then drips down onto the end turns of the windings, bypassing the transaxle’s rotors and keeping windage losses to a minimum.

The new transaxle is some 16% lighter than the P410 transaxle, and features a redesigned a new torsional vibration damper to help suppress engine noise as well as the transfer of vibration between the engine and transaxle. According to Chief Engineer Ogiso, Toyota will roll out the design changes to other hybrid platforms over time. A paper on the P510 transaxle, entitled “Development of New Hybrid Transaxle for Sub-Compact-Class Vehicles”, will be presented at SAE 2012 World Congress in April.

The battery pack in the Prius c is more compact than in the Liftback. Click to enlarge.

Battery pack. The 144V NiMH air-cooled battery pack in the Prius c has been made more compact and lightweight and is installed under the rear seat. (The NiMH pack in the Liftback is 201V.) The battery module includes the battery pack, battery ECU, system main relays and service plug. The 144V output from the battery is boosted to 520V DC maximum before conversion to alternating current.

The pack comprises 20 NiMH modules (120 cells), with each module 7.2V DC (1.2 volts x 6 cells). Pack peak output is 19.3 kW (25.9 hp). The smaller pack in the Prius c weighs about 68 lb (31 kg), compared to about 92 lb (42 kg) in the Liftback, with 168 cells.

The battery pack is installed beneath the rear seat. Click to enlarge.

The cool air intake is positioned beneath rear seat; cooling air velocity and volume are optimized for cooling efficiency. The battery is warranted for 10 years/150,000 miles. At end of life, every part of the battery, from the precious metals to the plastic, plates, steel case and the wiring, will be recycled or processed for disposal.

Power electronics. The inverter on the Prius c is smaller and lighter than that in the Prius 3G. Weight is reduced 10% (3 lb, 1.4 kg) and volume is reduced 12 percent (1.6 liters).

Newly designed thinner IGBT (Insulated Gate Bipolar Transistor) chips reduce energy loss, contributing to enhanced fuel efficiency.

Aerodynamic features. Toyota applied a number of aerodynamic features to the Prius c to achieve a good coefficient of drag of 0.28. (As vehicles get shorter, it makes it more challenging to deliver a good C_d; the Liftback, by comparison, has a C_d of 0.25.)

Among the features are aero stabilizing fins installed on the side face of the rear combination lamp and at the base of the outer mirror; underbody covers to improve airflow; and lightweight aluminum wheels and wheel covers designed with aerodynamic performance in mind.

Driving modes. In addition to its base hybrid driving mode, the Prius c offers an ECO mode and an EV mode which can be activated via buttons next to the parking brake handle.

• ECO mode maximizes fuel savings across all driving conditions. It modifies or smoothes out the electronic throttle control program to reduce throttle response, reducing throttle opening by up to a maximum of 11.6%. It also modifies air conditioning operation. It also can improve acceleration performance in low-traction conditions such as ice and snow as the reduced output helps to minimize wheel slippage.

  EPA label calculations do not include ECO Mode.

• EV mode helps keep the vehicle in electric-only mode longer at low speeds. In EV Mode, the ECU operates the vehicle using only MG2 if required conditions are satisfied.

  Under certain operating conditions, the vehicle can go approximately 25 mph (40 km/h) for up to approximately one-half mile (possibly longer depending on vehicle and battery conditions). The SOC (state of charge) level display on the Energy Monitor must be four bars or more to operate in EV mode.

Driving first impressions. GCC had the opportunity briefly to drive the Prius c under a variety of city and highway conditions during the preview. The little hybrid is quite likeable—it handles well and is comfortable; Toyota’s focus on the comfort of the seats (at least the front seats, we didn’t climb into the back) paid off, as did its focus on reducing NVH.

The most noticeable downside in delivered performance is on a steep incline, or when attempting to overtake on the highway. Under those conditions, the power is not there, and the car labors. However, as Toyota pointed out, that wasn’t part of the core design target. They opted for keeping overall cost down and delivering a very fuel-efficient vehicle optimized for less strenuous conditions. (Again, “c” is for city.)

That said, once the Prius c was at speed on the highway, it kept securely to the pavement.

The ongoing tension between cost and content also shows at points. As an example, the entry grades do not offer the push-button start familiar to Prius Liftback drivers—however the rather substantial opening (sealed) remains in the dash as a visual reminder.

Resources

• Development of New Hybrid Transaxle for Sub-Compact-Class Vehicles (SAE 2012-01-0623)