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05/03/2013
HAPPY BIRTHDAY, PRIUS!

FROM THE ENGINE TO THE WHEELS
The Toyota hybrid has just celebrated its 15th year but it seems to be in better shape than ever, thanks also to  its very special transmission system

Nicodemo Angì

IT WAS 1997 when Toyota launched in Japan the first version of its Prius, a “soft” hybrid car – the batteries were very small –  that although not very beautiful to look at, it was packed with technology. The world launch did not come until 2000 and by 2003, 160,000 examples of the first series had been sold, worldwide. Not bad for such a modern car that was ahead of its time. The second series arrived in 2004 and was very different. Its lines were more pleasing and it had become a slightly bigger hatchback, but it was more dynamic and engaging. The second series was also promoted from a Super Ultra Low Emission Vehicle (SULEV) to an Advanced Technology Partial Zero Emission Vehicle (AT-PZEV). This recognition – issued by the California Air Resources Board, which assesses, amongst other things linked to saving the environment, the ecological quality of cars – is even more significant considering that the second series was bigger and heavier than the first.
We had to wait for the third series until 2009, the year in which Prius acquired a more sporty look and was the same size but had more cabin space. But underneath, the changes were even more substantial because the internal combustion engine had been replaced by a 1.8-litre Atkinson cycle engine and considerable improvement of the components meant that emissions and consumption were further reduced, despite the bigger engine.

Plugged into the wall
Another generational leap came with the Plug-in version. The small Ni-MH, 1.3 kWh batteries – which did a handful of km in the all-electric mode – were replaced by much more performance-packing lithium-ion batteries: 4.4 kWh for a range of about twenty kilometres and zero emissions. The better energy/weight ratio with the lithium batteries meant that the weight could be limited to 80 kg: only 38 kg more than the nickel-metal hydride batteries in the first Prius, despite a capacity that had more than tripled. The energy/volume ratio was also more favourable and meant the loss of only two litres of boot space compared to the previous models and the creation of a 22-litre compartment under the floor.
It is called a Plug-in because the batteries can be recharged from a simple socket in about an hour and a half.  It would have been possible to charge the Ni-MH batteries from the mains, but energy-wise it was not very favourable because the addition of 1 kWh to these elements would have meant consuming about 40% more. Engine power also changed: the petrol engines started with 43 kW for the very first series, rose to 52 kW in 2000, 57 in 2003 and 73 kW with the 1.8-litre engine, but the Plug-in version was reduced to 60 kW.
Now let’s look at the mechanical parts and the decidedly interesting transmission, as mentioned at the start of this article. The transmission is composed of new fewer than three motors: one internal combustion and two electric; one of the latter is much less powerful than the other. The petrol engine, as we said, is an Atkinson cycle; it differs from the classic Otto cycle engine unit because the expansion ratio is greater than the compression ratio. This asymmetry can be achieved either with a special configuration of the crankshaft mechanism or, more simply, with an ad hoc distribution layout that delays the closing of the exhaust valve. In this way the effective compression ratio is reduced with respect to the theoretic geometrical ratio, which can therefore be raised to the advantage of output. In effect, the one in the Prius is 13 and is a compromise between the classic diesel and petrol ratio, but the danger of knocking is avoided because valve closure is delayed. We could say that the Prius’ 1,800 cc “exhale” or expand like a 2,000 cc or, more precisely, prolonged expansion means that better use can be made of the energy from the exhaust gas. Maximum power is less than that with the classic engine of comparable power, but the efficiency is much greater, especially at the constant revs of hybrid operation.
Prius’ other trump card is Hybrid Synergy Drive (HSD): it is a simple, but also sophisticated and creative way of distributing torque and power to the three motors. The two electric motors have very different powers and the main one (MG2) can generate 82 HP with fairly high maximum torque of 207 Nm compared to the 99 HP and 142 Nm of the combustion engine. The other electric motor (MG1) is a lot less powerful but capable of very high revs and also able to invert its rotation; it operates as a generator, reversing motor and starter motor.

Three passionate motors
The heart of the HSD system is another initialization, PSD or Power Split Device, a planetary gear set connected to the three motors which unites the functions of gear change (including reverse), clutch and the distribution of power and torque. To fully understand how the PSD works, we must first look at the construction diagrams. The electric motor MG1 is connected to the centre gear by a hollow shaft (green in the diagrams). The combustion engine is connected to the blue planetary gears by a second shaft that rotates inside the MG1 shaft.
The main electric motor MG2, the one with the red gears, is the only one directly connected to the drive wheels by a silent chain and the light-yellow gears. That’s it. But in this case simplicity is not synonymous with easy. We already met planetary gears when we talked about automatic transmissions with a torque converter: by braking one or the other of the shafts we could obtain various transmission ratios from a single group of gears. Toyota has substituted these brakes with motor MG1, which can be blocked on demand by the appropriate controller/inverter (shown in the figure).
To start the Internal Combustion Engine (ICE), for example, MG1 (green) turns the blue gears; when the ICE has started up it can turn freely without transmitting torque to the wheels. If MG2 moves the wheels, the ICE may or may not be activated by the large red external gear, depending on whether or not MG1 is blocked. If both main motors transfer torque to the wheels when accelerating, MG1 will be dragged and act like a generator to supply MG2 with power from the ICE and thereby save the batteries (famously long-lasting in the Prius).
When driving at a speed that is constant but not very fast, the ICE slows down considerably and therefore MG1 might rotate in the opposite direction to normal due to the change in gear speed. Reverse is also “interesting”, because MG1 rotates the 4 blue gears which, in their turn, drag the external (red) gear connected to the wheels. If rotation is fast enough (or if the battery level is too low) the ICE will start up, but reversing will continue: the small MG1 (which can have a very “high” rotation) only has to increase its revs to compensate for ICE rotation.
A refined technique that provides all the Continuously Variable Transmission (CVT) functions and a hybrid operation with a fairly simple mechanical unit. It is not by chance, therefore, that the Prius has acquired a reputation for being considerably reliable and with sales of the various versions – despite a price that is anything but low – totalling 2.8 million vehicles to date. A taxi driver in Rome, who undoubtedly had thorough experience of the Prius, also confirmed the car’s considerable reliability and economy: depending on driving style, consumption is never less than 21-22 km/litre in the city.

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