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The Toyota Prius - Part 2

A technical background of Toyota Australia's hybrid vehicle - the Prius...

By Michael Knowling

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We recently gave you a road test of Toyota's fantastic Prius hybrid vehicle. Now we'll investigate the inner workings of this complex 'clean car'...

Toyota Hybrid System (THS)

The Prius is propelled by a parallel hybrid system, in which two drive sources are employed either separately or simultaneously - a petrol engine and an electric motor.

The electric motor - drawing from a large hybrid battery - serves as the sole power source in instances when the petrol engine is relatively inefficient; this includes driving conditions such as slow-moving traffic jams, sitting stationary, deceleration, light acceleration and low speed cruise. Under these conditions, the electric motor is relied upon for drive, and the petrol engine is automatically switched off - completely eliminating fuel consumption and tailpipe emissions.

Note, however, the control system will run the petrol engine more frequently when the all-important hybrid battery charge is low - in this instance, a portion of the engine power is directed to charging the battery.

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Whenever the driver needs more power than the electric drive system can generate, the petrol motor is automatically started. The petrol engine is typically run in driving situations such as medium-to-high speed cruise, moderate-to-high acceleration or when climbing hills. When the petrol engine is in service, its power output can be channelled solely to the drive wheels, or additionally to charge the hybrid battery.

When absolute maximum power is called upon, both the petrol engine and electric motor combine to provide power to the wheels. Electricity stored in the hybrid battery is channelled to the electric motor driving the front wheels, in addition to the power derived from the petrol engine. Maximum power from the combined sources is 74kW.

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An important feature of the Prius hybrid system is its regenerative braking system - this recovers the kinetic energy that is otherwise wasted during either engine or foot brake deceleration. This is achieved by one of the two electrical traction motors being used as a generator, with this energy converted into electrical charge for the hybrid battery.

There are two advantages to the regenerative braking system - reduced conventional brake wear and eliminated need for battery charging from an external source (such as a power socket).

Parallel vs Series Hybrid Systems

In a parallel hybrid system such as the Prius, both a petrol engine and electric motor are used to drive the wheels directly. The electric motor supplements the petrol motor and, in this particular vehicle, serves as a generator to recharge the battery while driving.

In a series hybrid system, the petrol engine is usually dedicated to running a generator. The electricity generated from this enables an electric motor to drive the wheels. Accordingly, a series hybrid system can be regarded as an electric car with an engine-driven generator.

The Prius Electric Drive System...

CVT with Two Electric Motors (Generators)

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The front-wheel-drive P111 transaxle fitted to the Prius has been purpose developed to suit the hybrid role. The driveline is equipped with two permanent-magnet AC electric motors (generators), which are dubbed MG1 and MG2 ("MG" = motor/generator)

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MG1 is used to recharge the hybrid vehicle battery and supply the electrical power to directly drive MG2. By regulating the amount of electrical power generated, MG1 also controls the continuously variable function of the Prius transmission.

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The MG2 motor's primary role is to drive the wheels. Its maximum power output is 33kW from between 1040 and 5600 rpm, while torque is 350Nm between 0 and 400 rpm. At maximum torque, the motor draws 351 amperes.

When the regenerative braking system is activated, MG2 is also used to convert the vehicle's kinetic energy into electrical energy for the HV battery.

A so-called power splitting device - incorporated inside the transaxle - serves to switch the petrol engine operation to MG2 assistance, HV battery charging and power generation for driving MG2. It's effectively the 'hub' of the whole petrol/electric operation.

HV (Hybrid Vehicle) Battery

The Prius's newly developed HV battery is an evolution of sealed Nickel-Metal Hydride (Ni-MH) technology. Jointly designed by Toyota and Panasonic, the battery is said to feature high power density, light weight and longevity.

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Stored in the boot, the battery unit comprises six 1.2 volt DC cells connected in series to form a single battery module. A total of thirty-eight modules are connected in series, giving a rated voltage of 273.6 volts DC. The current Australian-delivered Prius has a HV battery 40 percent smaller than that fitted to the first generation Prius released in Japan.

The temperature of the HV battery is very important; an electric fan drawing air from inside the cabin provides essential cooling. The fan itself uses a 60W DC motor and flows up to 150 cubic metres of air per hour.

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As mentioned, the Prius battery doesn't need charging from an external power source. Instead, the MG1 and MG2 electric generators are used to maintain battery charge. Controlling the state of charge is a dedicated battery ECU, which takes inputs from two HV battery temperature sensors, amperage sensor, air-conditioning signal and a voltage from each battery module. Outputs from the battery ECU are to the air-conditioning ECU, HV battery cooling fan, Data Link Connector and separate HV ECU.

In the event of a malfunction, the battery ECU will either partially restrict or eliminate charging and discharging. A warning light will also be displayed and a code is stored in the ECU memory.

Inverter

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An electric invertor - located above the transaxle - is used to convert DC and AC current between MG1, MG2 and the HV battery. It converts the voltage travelling from the battery to MG2, and from MG1 and MG2 back to the battery - the battery runs on DC, while the motors are AC designs.

To help keep the invertor unit cool - together with MG1 and MG2 - a dedicated radiator is fitted. A pump is used to circulate coolant whenever the ignition is switched on.

Converter

On the underside of the inverter is the Prius' DC converter. The role of the converter is to transform the HV battery output of 273.6 volts DC to 12 volts DC. This 12-volt source is used to recharge the auxiliary battery, which resides in the boot. The Prius' electrical equipment - the lights, audio system, ECUs etc - are all based on a 12 volt system.

The Petrol Engine Drive System...

The 'other half' of the Prius' drive system is its purpose-developed 1NZ-FXE petrol engine.

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This 1.5-litre four-cylinder engine uses a DOHC design with VVT-i, 16 valves and features the adoption of the high expansion ratio Atkinson cycle principle. The Atkinson cycle sees the exhaust valves closed until the end of the expansion stroke - this extends the expansion stroke, enabling more combustion energy to be applied to the piston.

The engine block is made from alloy and comes with bore and stroke measurements of 75.0 and 84.7mm respectively. Its pentroof type combustion chambers deliver a high 13.0:1 compression ratio, yet - partially thanks to a knock sensor - the Prius is rated for normal (91 RON) unleaded fuel.

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The alloy DOHC 16-valve cylinder head has its intake and exhaust valves set at a relatively narrow 33.5-degree separation to help achieve compactness. Toyota's VVT-i ("Variable Valve Timing - intelligent") varies the opening and closing time of the intake valves to improve torque in the medium to low speed zones, improved economy, purification of exhaust gasses and reduced vibration on start-up. VVT-i varies the Prius' intake cam timing over a 35-degree range.

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Interestingly, Toyota says, "It is not necessary to improve the intake air efficiency through inertial [ie tuned length] intake due to the adoption of the Atkinson cycle. The length of the intake pipe of the intake manifold has been shortened and, furthermore, the intake pipes for cylinder 1 and 2 as well as for 3 and 4 have been integrated midstream to achieve a large-scale weight reduction."

The intake to the engine is throttled by an ETCS-i system ("Electronic Throttle Control System - intelligent"). A single throttle butterfly is rotated by a DC electric motor, which is controlled by the engine ECU. A return spring ensures the throttle closes when necessary, and a warm coolant passage is provided below the throttle body to prevent it from freezing in cold temperatures.

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The engine management system fires four 12-hole injectors into each cylinder and a non-return fuel system is used - Toyota has integrated the pressure regulator and filter with the fuel pump assembly inside the tank. The resulting reduction in temperature rise inside the fuel tank reduces evaporative emissions.

Ignition control is via a DIS (Direct Ignition System). A coil is fitted for each cylinder, giving improved timing accuracy and control and - by eliminating the distributor - enhances reliability.

Note - the conventional alternator has been discarded as MG1 and MG2 are integrated into the charging system. The conventional starter motor has also been discarded because MG1 is used to start the petrol engine.

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The Prius' petrol engine generates 53kW at 4500 rpm and 115Nm at 4200 rpm - note the low peak power rpm. Toyota claims the maximum engine speed is set at 4500 rpm in order to reduce frictional losses. Further reductions of friction are achieved by using low-tension valve springs and piston rings, lightweight reciprocating components and an offset crank with 12mm deviation from the centre axis of the cylinder bore (to reduce piston resistance).

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Despite the frequent stopping and starting of the petrol engine, the use of twin oxygen sensors and a super thin ceramic wall high-density cat converter maintain precise emission control (the super thin ceramic cat converter wall helps reduce the amount of time necessary before it is activated).

Power Steering Assistance?

Rather than use a conventional power steering pump driven off the petrol engine, the Prius uses EMPS (Electric Motor-assisted Power Steering). An electric motor provides rack and pinion steering assistance (based on road speed) so the steering doesn't load up when the petrol engine is switched off by the control system. Further advantages of this arrangement include improved energy conservation, less weight and an eliminated need to fill the power steering fluid.

The Major Benefits of the THS

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The Toyota Prius is listed with an AS2877 city cycle fuel consumption figure of 4.6 litres per 100 kilometres and a highway figure of 4.2 litres per 100 kilometres.

Its evaporative emissions total 8 percent of the ADR permitted level, while tailpipe hydrocarbons, carbon monoxide and oxides of nitrogen are respectively 5, 8 and 2 percent of ADR maximum levels! The Prius is rated as an U-LEV (Ultra Low Emission Vehicle) under the California Air Research Board criteria and meets Euro IV emission requirements - reputedly around a decade in advance of Australia certification.

Next week in Part 3 we look beyond the hybrid system...

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