Toyota Prius: Hybrid innovation drives new Prius

Toyota's Prius has been at the forefront of series-parallel hybrid development for years. Hence the reason for Toyota naming the car so; 'Prius' is a Latin word translated as the English phrase 'to go before'.

With the introduction of the latest model Prius, now in its third generation, the hybrid-drive passenger car has advanced another step along the development path for this type of technology. The car retains the basic series-parallel hybrid setup of the previous model, but Toyota has wrought some significant changes -- starting with the petrol engine.

"We use the Atkinson Cycle engine because of its higher expansion ratio, compared with a conventional Otto Cycle engine," he told media during the local launch of the new Prius.

"It makes full use of combustion energy by effectively making the expansion stroke longer than the compression stroke. It therefore harnesses more of the energy into torque on the crankshaft. The new 1.8-litre engine produces more power and torque than the existing 1.5-litre engine.

"Against conventional wisdom, the larger engine actually helps improve fuel economy."

In the Prius, the Atkinson Cycle results from delayed closure of the intake valves during the early stage of the compression stroke. Some air drawn into the cylinder during the intake stroke is ejected back into the inlet manifold. According to Toyota, this back flow counteracts pumping losses due to manifold vacuum, under partial-load conditions.

Despite the shorter compression stroke, the engine still boasts a compression ratio of 13.0:1, but the Atkinson cycle enables the higher compression ratio without pre-ignition occurring.

An undersquare design (bore & stroke = 80.5x88.3mm), the 73kW petrol engine relies on variable valve timing and DOHC valvetrain and port fuel injection to provide a relatively flat torque curve -- although peak torque of 142Nm occurs at a reasonably high engine speed of 4000rpm.

Toyota has fettled the engine to comply with the Euro V emissions standard through such means as cooled Exhaust Gas Recirculation and a heat recovery system for faster warm-up. The latter pumps exhaust gases into a heat exchange unit to warm the engine coolant. A valve then redirects the gases through the exhaust system once the coolant has reached normal operating temperature.

To achieve new levels of fuel efficiency, the petrol engine is also 'beltless', meaning that ancillaries such as the steering and water pump are electrically-powered.

In both combined-cycle and city-cycle fuel consumption testing, the Prius now achieves 3.9L/100km, and 3.7L/100km for the extra-urban cycle. That's an 11.3 per cent improvement on the superseded car's combined-cycle figure -- also complementing the new car's 21 per cent power gain.

Planetary gear sets, which allow torque input on one side with output at differing rotational speeds, can be found in differentials and conventional epicyclic automatic transmissions.

In the case of the Prius, one planetary gear set provides a three-way power split (engine-motor-drive wheels). It permits the petrol engine, the electric drive and the wheels to turn at different speeds, according to what is delivering the bulk of the motive power at the time.

There are various scenarios in which the computer-controlled Hybrid Synergy Drive system can operate through the CVT.

Firstly, the petrol engine and the electric drive system can work cooperatively, both transmitting torque through the CVT to the drive wheels, propelling the car forward (or in reverse). The torque from the petrol engine is met by the torque from the electric system and both are diverted to the drive wheels.

At speeds up to 50km/h, the Prius will run on battery power (electric drive) alone, with the petrol engine not running. Above 50km/h or when the driver demands faster acceleration, the petrol engine kicks in, as described in the first scenario.

Still above 50km/h, but on the overrun, the petrol engine's torque will be diverted via the CVT to one of the Prius's two electric motor/generator units to recharge the nickel-metal hydride batteries that store recovered kinetic energy as electrical energy.

The same motor/generator unit can lock up, to brake one side of the CVT and entirely hand over the task of motive power to the petrol engine.

Should the vehicle travel 2km or more on electric power alone, the petrol engine can start automatically, acting as a range-extender to provide power generation for the batteries.

The electric drive system in the Prius comprises two motor/generator units. Ashley Edwards explains their respective roles: "Motor Generator 1 -- or what we call MG1 -- in the transaxle has more concentrated coil winding which contributes to reduced weight.

"It works as a generator when driven by the engine to charge the hybrid battery. It also functions as a starter motor for the petrol engine. And it acts as a controller for the power-split device in the transmission, balancing out the power flow between the electric motor and the petrol engine.

"Motor Generator 2 -- or MG2 -- drives the road wheels. The new 60kW motor is thinner and operates at increased revs -- the peak is now up to 13,500 rpm -- with torque output of 207Nm.

"A reduction gear boosts torque significantly, making for a much more efficient powertrain."

On that point, the reduction gear is the second planetary gear set, working on a ratio of 2.64:1. This effectively doubles the torque from MG2, which runs at 13,500rpm and develops 60kW of power. For the previous model Prius, the torque from the electric motor was around 400Nm, but the motor's top speed was 6400rpm. The new version of MG2 achieves much the same result -- via the torque multiplication from the reduction gear -- but with lower levels of power consumption and lighter weight. MG2 also diverts recovered energy from regenerative braking, back to the hybrid battery.

MG1 develops 30kW of power, so when it comes to starting the petrol engine, the Prius isn't lacking. Most conventional starters produce perhaps 1.5kW. For the Prius, which frequently demands immediate start-up of the petrol engine, MG2 has to crank the engine rapidly and quietly.

Computer control of the CVT and the two different drive systems has been revised for the latest Prius. Toyota has developed a smaller unit that is better cooled than its predecessor in the second-generation Prius. Its boost converter provides 650 Volts in lieu of the 500 Volts for the previous model. The power control unit also incorporates an inverter and ancillary power converter.

It's this power control unit that allows the Prius driver the option of four operating modes: Power, Normal, Eco and EV. As the names indicate, each mode can be selected for its specific driving style; Power for better performance, Eco for better fuel efficiency and EV for electric motor-only operation. The Normal mode is the default if the driver doesn't select one of the other modes from the three buttons on the dash.

The alloys themselves are a five-spoke design, developed for a combination of light weight and strength -- but separate plastic covers have been designed to enhance aerodynamic efficiency and looks. Since the covers don't contribute to the structural strength of the wheels, they could be constructed from plastic in a finish that matches that of the wheels. These wheels are shod with 195/65R15 91H low-rolling-resistance tyres.

Weight-saving and drag-reduction measures can be found elsewhere on the new Prius. The new car now boasts a drag coefficient of 0.25Cd (bettering the superseded model's figure of 0.26Cd). This has been achieved with careful tuning of the car's styling and proportions. There's a slight increase in overall length and the roof has been extended rearwards, providing additional rear-seat headroom in the process, but much of the aerodynamic improvement has come from such changes as: smoother frontal styling to reduce turbulence, revised rear-spoiler height, narrower panel gaps, restyling of mirrors and a wind-cheating undertray.

To reduce weight, Toyota has opted for aluminium bonnet and tailgate, with the lightweight material also being adopted for the front anti-roll bar and brake caliper. Super high-tensile strength steel is used in the construction of the rocker panel inners, the B pillars and roof reinforcement.

The Prius i-Tech is equipped with LED low-beam headlights, further contributing to the car's frugality. According to Toyota, these headlights use 40 per cent less power for the same level of illumination.

It principally displays road speed, but can also display energy consumption or route instructions from the satellite navigation. Toyota has ensured that the outer layer of the windscreen will reflect the HUD information back in alignment with the reflection from the inner layer, so there's no ghost image.

Other features of note in the Prius (the high-grade i-Tech version, specifically), include Intelligent Parking Assist (IPA), Dynamic Radar Cruise Control, Pre-Crash safety system, 'Touch Tracer' and a sunroof with solar-power cells for the collection of energy to power air-circulation for the cabin on hot days.

First things first, the IPA system is a facility seen before in German prestige cars and the Volkswagen Tiguan. In the Prius, it operates in much the same fashion, but allows the driver the added flexibility of selecting how fine the angle the car will take, turning into a parking spot.

To enable the system, the driver presses the button to the left of the steering wheel and cruises past the desired parking spot at around 5km/h. The car will prompt the driver to select the spot on the touch screen in the centre fascia. Once the driver has selected reverse, the only human intervention necessary is a moderating foot on the brake pedal. The system will immediately cancel once pressure is applied to the accelerator.

Dynamic Radar Cruise Control modulates the car's road speed in accordance with its proximity to the vehicle in front. If the vehicle ahead brakes, the Cruise Control will brake the Prius at the appropriate rate of deceleration. Once the vehicle in front accelerates back up to speed, the Prius will do likewise, maintaining a set distance between the two vehicles. The system allows the driver three options for braking distance, but is not to designed to bring the vehicle to a full stop, since it would thus be confused by parked cars.

Pre-Crash draws on information from the forward-looking radar, stability control and the braking system to anticipate a collision. It alerts the driver with an audible alarm; prepares the braking system for optimum braking effort; pre-tensions the front seatbelts and -- if the crash is unavoidable -- actuates full braking without waiting for the driver to do so.

'Touch Tracer' uses two stage dimples in the remote controls on the steering wheel (for 'multi-information' display, trip computer readout and audio system) to indicate which button the driver is depressing, without the driver needing to take eyes off the road. By pressing the dimpled button lightly, the driver can see the respective button highlighted in a display within the centrally-located instrument binnacle. Pressing harder will select the option for that button.

Finally, the Prius i-Tech features an air-conditioning enable button on the key fob, to cool the interior up to three minutes before the driver enters the car. This system, and combined with the solar cells integrated in the sunroof to power fans when the vehicle is parked, can cool the interior of the cabin from 80 degrees to 45 degrees -- a reduction of 40 per cent.

The upshot of all this technology is that Toyota has made the Prius more appealing than ever for its blend of frugality, comfort and safety.

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