Prius retains quirky styling - its pretty much unmistakable on the road

Prius retains quirky styling - it's pretty much unmistakable on the road

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We recently reviewed the latest, fourth generation Toyota Prius. Now, Graeme Roberts looks at Toyota's hybrid sales success and some of the new technology in the first car built on the automaker's New Global Architecture platform.

Sales history

Even though I have been interested in hybrids pretty much since the off, the press release from Toyota Motor back in April still was cause for pause. Cumulative global sales reached 9.014m units on 30 April 2016, passing the 9m unit mark, and this latest million-unit milestone was achieved in just nine months. Quite an achievement since Toyota launched the Coaster Hybrid EV in August 1997 and the Prius in December of the same year and only let the latter out for export in 2000.

In 2015, it added a hybrid model of the popular Sienta minivan, began selling the fourth-generation Prius, introduced the Corolla and Levin hybrid models in China - with locally-produced hybrid systems - and added a hybrid RAV4 SUV to its ever-increasing hybrid line-up. As of the end of April this year, Toyota was selling 33 hybrid passenger car models and one plug-in hybrid (PHV) model in more than 90 countries and regions. And still they keep coming. Toyota is working to expand its lineup of hybrid vehicles and, having averaged roughly a half million a year since that 1997 domestic market start, targets annual sales of 1.5m units, and cumulative sales of 15m units, by 2020.

There'll be arguments any time you say the word 'hybrid', especially here in Europe, that diesels are just as fuel efficient, if not more so. But Toyota makes the environment argument and calculated that, as of 30 April 2016, the use of its hybrid vehicles in lieu of conventional vehicles of similar size and driving performance had resulted in approximately 67m fewer tonnes of CO2 emissions and savings of around 25m kilolitres of petrol compared to the amount used by gasoline-powered vehicles in the same class. And it says it has positioned hybrid technologies, which encompass all of the component technologies necessary for the development of environment-friendly cars, and which facilitate the use of different fuel combinations, as core environmental technologies for the 21st century. So there.

Real world zero emissions

That fourth generation Prius recently delivered exceptional results in an academic test of its ability to run on electric power and with zero exhaust emissions on busy commuter journeys. The results of research by a team from the University of Rome from more than 1,360 miles of driving on a typical Rome commuting journey reveal that the test cars spent an average 73.2% of the journey time (62.5% of the journey distance) producing no tailpipe emissions. These figures included periods when the car was stationary, with the engine automatically switched off. The results were even better in urban areas where the latest Prius achieved zero emissions for 79.4% of the journey time and 76.3% of the route distance.

The automaker said all this was possible thanks to the quality of its latest Hybrid Synergy Drive powertrain which has been improved continuously since introduction. In Rome, almost 45% of the car's drive power was sourced from the electric motor, with 34.3% of its electric energy produced by the regenerative braking system. Thus, more than one third of the electric power used to drive the wheels was produced simply by the act of braking or slowing down. For each journey the Eco drive mode was selected and the air conditioning was switched off; speed limits were observed.

Technological ambassador

This latest Prius is first Toyota to make use of the automaker's New Global Architecture-based platform which lowers the centre of gravity 2.5cm and it gets CO2 emissions down to a historic low of 70g/km. Load space is improved thanks to the use of a smaller, more energy-dense hybrid battery that is located entirely beneath the rear seats and a new double wishbone rear suspension system – features which do not intrude in boot capacity. The Prius remains Toyota's 'technological ambassador' so you'll see it here first, a showcase for new, relevant technologies that boost safety, convenience, performance and comfort. For example, the Safety Sense package is extended with radar-managed Adaptive Cruise Control with Full Speed Range Following Function and a Pedestrian Detection function for the Pre-Collision Safety system. The full hybrid powertrain was revised greatly to improve efficiency, reduce weight and sharpen performance. Detailed design changes to the engine have resulted in a 40% thermal efficiency – a world-best performance for a petrol unit. Other hybrid system components have been made lighter and smaller and have been repositioned for optimum packaging, further contributing to the car's lower centre of gravity. The new nickel-metal hydride hybrid battery is more compact than in the previous generation model, with even better durability and charging performance.

"Hybrid has become Toyota's key competitive advantage, the principal quality that differentiates it from other manufacturers and which gives it a specific strength in the marketplace," the automaker said. "This is reflected in the fact Toyota hybrids make up more than 50% of all the alternative powertrain vehicles sold in Europe – more than all the other hybrids, plug-in hybrids and electric vehicles combined." Last year, hybrids accounted for 63% of all Auris sales and 41% of Yaris sales. Hybrid  also brings high conquest sales from other brands – up to 63% for Yaris and 51% for Auris. "Toyota believes hybrid's market potential will increase further, with more people taking up the technology as the emissions performance of all vehicles comes under closer scrutiny and legislative control," it adds. "At the same time, more manufacturers are following Toyota by introducing their own hybrid models, increasing competition and giving customers a wider choice. Toyota will continue to enjoy the advantage of offering a full hybrid system, in which the electric motor can operate independently of the petrol engine. This allows the car to be driven in all-electric EV mode when possible, with zero fuel consumption and zero tailpipe emissions. These benefits are not available in "mild" hybrid systems, in which the electric motor can be used only to support the performance of a petrol or diesel engine. Toyota believes that continuous improvement of its full hybrid technology will enable it to strengthen its market position, ahead of the wider uptake of alternatives such as plug-in hybrids, electric vehicles and fuel cell vehicles."

New hybrid system

The new hybrid system comes in a more compact package that is lighter in weight and lower in cost and reflects significant advances in battery, electric motor and petrol engine technologies. The new hybrid battery offers higher energy density, is 10% smaller yet can now absorb 28% more energy in the same amount of time, which means that it is faster charging. The electric motors are smaller in size, yet also provide a better power-to-weight ratio, and the thermal efficiency of the petrol engine – at 38.5% already very high on the previous Prius – is increased to 40% – a claimed world's best for a petrol unit. A 1.8-litre VVT-i Atkinson cycle petrol engine continues, but completely re-engineered to deliver significantly better fuel economy. Gas flow, combustion, cooling and knock control have all been improved and much more effective use is made of exhaust gas recirculation. Toyota has developed a heat recovery system that uses spent exhaust gas to speed up the warming of engine coolant, among other measures, reducing engine use.

Further work was done to reduce energy losses, particularly those caused by friction. The air filter has been made smaller and reduced in height, which helped the designers bring down the line of the bonnet. The new intake system has a resonator that creates less noise at noticeable frequencies and the intake duct is made of a porous material that suppresses resonance. A fresh air inlet duct has been added to make sure ample air volume is obtained at motorway speeds and a clever air/fluid separating structure has been added to keep water and snow from mixing with the intake air. Should the fresh air duct fill with water or snow, a secondary inlet serves as the air intake. A thinner silencer secures optimum performance and noise reduction. The engine block has V-shaped drilled paths that reduce losses in water jacket pressure. There is also a new water jacket spacer which helps control cylinder wall temperatures in the combustion chamber, reducing friction and preventing engine knock, which in turn supports optimum ignition timing. Engine cooling has been improved with a new cooling module structure and attachment. There is a new grille shutter behind the radiator which automatically closes when full airflow isn't required, improving aerodynamic performance and saving fuel. The engine's maximum output of 97bhp/72kW is delivered at 5,200rpm, with peak torque of 142Nm at 3,600rpm.


The redesigned transaxle offers more efficient performance and packaging and reduced weight. It's 59mm shorter, allowing the auxiliary battery to be relocated to the engine compartment. There are two electric motor-generators (MG1 and MG2); a single planetary gear; and a reduction gear to the final drive. MG1 serves primarily as a generator and as the engine's starter motor. MG2 is the electric drive motor, which also acts as a generator in regenerative braking mode. It drives the car from start-up, at low speed and in EV (electric vehicle) mode and is the sole propulsion method when the vehicle is in reverse.

Software changes make the car draw more on its electric drivetrain, allowing it to accelerate in a low engine rev range. It has also permitted the speed range of the electric motor to be increased by 60% so less dependency on the petrol engine at higher speeds, improving fuel economy. Higher motor speed and new forced water-convection cooling in place of air cooling improve the efficiency of the electric-drive motor which delivers 53kW of power and 163Nm of torque.

Power control and battery

The power control unit (PCU) has been totally redesigned for a 33% reduction in size, a 6% weight saving and a 20% reduction in electrical losses. It inlcudes the inverter/voltage booster, a DC/DC converter for auxiliary power and the electronic control for the motor-generators. In place of a belt-driven alternator, the Prius uses a DC/DC converter to recharge the 12-volt battery, using energy from the HV battery.

The nickel-metal hydride (NiMH) battery is more compact so can be located entirely beneath the rear seats, avoiding any intrusion in the load space. Its cooling is more efficient and it has a greater energy regeneration range. The new cooling system features an air filter in the intake bezel, which is located in the trim beneath the right rear seat.

The petrol engine uses less fuel; the electric motor-generators are smaller, lighter and more efficient; mechanical losses in the transaxle have been cut by 20%; the power control unit is more efficient; and the nickel-metal hydride HV battery is lighter and more efficient. Further gains have been made by reducing the energy load of the air conditioning system (smaller, lighter and uses less power, benefiting from a new S-FLOW function to improve efficiency. It registers whether the front and rear passenger seats are occupied, using a sensor in the front seat, and detects when either of the rear doors is opened and closed, adjusting ventilation and heating performance accordingly, minimising air flow around any empty seats), improving the rolling resistance performance of the tyres and creating a more aerodynamically efficient vehicle design.

Styling, as always, remains controversial (see our review) and some bright silver/white interior trim may not to be everyone's taste but I've always liked the quirky Prius. As Toyota's hybrid technology leader, it's always worth a close look. And a plug-in's coming.

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