The main difference between the second generation Prius and the present one (third generation), is shown in the picture above. Toyota managed to eliminate the chain that led motion from planetary gear, positioned between the two electric motors, to the differential one. Japanese engineers have adopted a single pair of helical gears, considerably reducing transmission losses.
The results of these changes are mostly evident in middle and high speeds, where fuel consumption of Prius II was not comparable to modern Diesel cars.
This technical step forward has not been easy, since Toyota had to redesign both gear and engine. They designed a new gasoline engine of 1800cc, called 2ZR-FXE, with a new characteristic: rotated back closest possible to the differential axle, so they could avoid the chain that connects gear to the differential. Despite a downsizing approach, Toyota used a bigger engine, compared to the previous 1.5 used by the Prius II, increasing capacity to operate at lower rpm and a high p.m.e.
Novelties are not limited to these; Japanese engineers installed a system that could heat the coolant liquid during warm-up. As I wrote in various articles, the most critical time for consumption and pollution are the first minutes after start when oil is still tacky and catalytic system does not work. So they designed a heat exchanger in the drain line that carries the coolant liquid close to the exhaust line for heating.
In addition, the need to gain operating temperature as soon as possible for catalyst system to convert NOx, CO and HC is obtained; this result is reached placing in the combustion chamber a greater amount of fuel so to heat more exhausted gases. This component, in fact, starts to convert the pollution into harmless substances from 300 °C (its operating point is at about 500 °C). With rear engine’s valves, the exhaust line is most protected from air that circulates inside the engine compartment. This option becomes more and more important both for hybrid and start&stop cars for continuous shutdowns and restarts of combustion engine.
Based on the actual version Toyota’s engineers had been working on plug-in version which should ensure an electrical autonomy of twenty kilometers. So plug-in version would not need to turn on the internal combustion engine in short urban use in which engine and catalyst don not reach operating temperature increasing pollution and consumption, especially for the Euro IV and Euro V version.
What, however, is still missing (and I have been emphasizing this for a long time) is the integration of the navigation system with the hybrid, in order to have a strategy to minimize fuel consumption. To achieve this aim, you have to manage not only route data and the estimated average speed of traffic, but also topography. Prius or RX450h hybrid system does not recognize if you are going up the ramp of Pordoi or you are driving in your garage at home; in the first case the hybrid could use all battery capability because the system knows that there will be a long descent to recharge the battery. This information is immediately important, but will become essential for the plug-in hybrid.