The best bit about using an Atkinson-cycle engine in a car is the fact that it's much more efficient, therefore it uses much less fuel than a regular Otto-cycle. Of course, what many people don't know is that the Toyota Prius, along with pretty much all the other hybrid cars out there, is not using a true Atkinson-cycle IC engine, but one that is more closely connected to a Miller-cycle like in some old-school Mazdas, without being supercharged or turbocharged though.
What makes us say this is the fact that the 1.8-liter four-cylinder under the Prius' hood is not using different power and compression strokes, like in the original Atkinson engine, but becomes more efficient by adding a so-called "fifth stroke". More specific, in the "Prius Atkinson-cycle", the intake valve is held open longer than normal to allow a reverse flow of intake air into the intake manifold. This translates into a compression ratio that is smaller than the expansion ratio, therefore the engine uses less fuel but also makes less power.
In total, there are three engines and a pretty compact (compared to its predecessors) stack of NiMh batteries in the Prius. The aforementioned Atkinson-cycle 1.8-liter is used to power the driving wheels and/or recharge the batteries via one of the electric engines. One of the electric engines is used to either power the driving wheels when the batteries have enough juice or to recharge the batteries by using electricity from the regenerative braking system or from the ICE.
The second electric motor is actually the most interesting bit about this whole ensemble. Believe it or not, the Prius - just like all Toyotas that use the Hybrid Synergy Drive system – doesn't have a transmission, or at least not in the regular sense you all know. The second electric motor is actually the "transmission", acting pretty much in the same way as a CVT (Continuous Variable Transmission). That is also probably the reason why Toyota brochures always mention its hybrids to have an "eCVT".
In other words, there is no direct mechanical connection between either the internal combustion engine or the second electric motor and the wheels. Both engines transmit their power via another computer-controlled electric motor, which varies the input/output ratio just like an average Continuous Variable Transmission. All in all, the Prius takes so many automotive breakthroughs for granted it's not even funny.Continue reading