In 1994 Toyota initiated its G21 project, with the aim of creating a ‘green and environmentally friendly’ car for the 21st century. It should have impeccable environmental credentials, but offer all the convenience and driving pleasure of a conventional vehicle.
Initial development goals focused on developing a powertrain that would be 1.5 times more efficient than that of conventional petrol or diesel cars. When hybrid drive systems became the preferred option, the target was raised, and the aim was now to double the efficiency of conventional cars.
With electronics now an integral part of an engine, the functions of all electrical and mechanical components in a mass-produced hybrid vehicle — in fact, in any vehicle — become interrelated and interdependent. Toyota decided to design, develop and produce every component in the new car’s hybrid drive system in-house, partly in order to gain specialist expertise, partly to avoid having to coordinate outside suppliers working with a rapidly developing technology.
The first generation Prius was launched in Japan in 1997. It was the world’s first mass-produced full hybrid vehicle. The car made its European début three years later. So far, Toyota has sold 1.8 million Prius hybrids, most outside Europe.
The early cars had the same broad layout as the current ones. A petrol engine — originally of 1.5 litres — was combined with an electric motor. Power to the wheels was provided either by the electric motor alone, or by the two power packs together in varying proportions. Decisions about how to power the car could be taken either by the vehicle’s control system or by the driver.
The power units drove through a continuously variable transmission. Maximum system power, with the petrol engine and the electric motor working in parallel, was 100PS. Overall fuel consumption was 55.4mpg with CO2 emissions of 120g/km. By comparison, a 2001 model year Golf TDI 100PS returned 53.3mpg overall and produced 143g of CO2 per kilometre.
Third generation Toyota Prius.
Predictably, the second generation Prius turned in better figures. Overall system power was up to 112PS, fuel economy tests returned 65.7mpg overall, and CO2 output dropped to 104g/km.
The current Prius is the third generation. Announcing the new car last Autumn, Toyota claimed to have redesigned 90 per cent. of the hybrid system’s components. With a new 1798cc petrol engine operating with an extended expansion ratio, the Prius can now manage a full system power output of 136PS, with overall fuel economy in the European test cycle of 72.4mpg or 3.9l/100km. The CO2 yield is 89g/km.
Toyota has developed its hybrid drivetrain — called Hybrid Synergy Drive — to be compatible with all-electric drive systems and for vehicles with hydrogen fuel-cells. Reducing the size, weight and cost of key hybrid system components, such as the electric motor, inverter and batteries, is seen as a priority. The Company sees its Hybrid Synergy Drive serving as a core technology, applicable to all future Toyota models.
The limited range and performance of the Prius under electrical power prompted Toyota to develop a plug-in version. The essential technology is the same, but there are substantial differences in detail — principally the adoption of a lithium-ion battery pack with a capacity of 5.2kWh, and the facility to recharge the battery from a public charging point or a domestic supply. The petrol engine and electric motor are unchanged, with flat-out performance affected only by the increase in weight of around 90kg. The maximum speed under electric power doubles from 30mph to 60mph.
The plug-in Prius’s superior electric range compared with its plugless sibling cuts test cycle fuel consumption and emissions substantially, though clearly this is by virtue of using power supplied externally. The means by which this external power is generated is obviously an unknown.
Toyota Prius Plug-in Hybrid Vehicle.
Roughly 600 plug-in Priuses will be introduced in Japan, the U.S. and Europe over the first half of 2010. Twenty will come to the U.K. The cars will be leased to government ministries, local governments, companies, universities and research agencies for use in a testing programme aimed at collecting real-world driving data. Toyota is also consciously trying to stimulate the development of battery-charging infrastructure.
Toyota will be analysing the feedback it receives with an aim to start public sales in around two years. Volumes are expected to be in the tens of thousands.
195/65 / 215/45*
Petrol electric hybrid
series & parallel
Engine power PS/rpm
Engine torque Nm/rpm
89g/km / 92g/km*
Third generation Toyota Prius hybrid powertrain. (Special thanks to Richard Seymour at Toyota GB.)
Toyota Prius NiMH battery pack cooling. Air is circulated around the casing by an electric pump. Battery life and performance depend on controlling heat in the casing, as charge and discharge rates are often high.