Life with the Prius Hybrid
By Steve Rose
Wednesday, October 23, 2002
The Toyota Prius Hybrid
We bought our Prius in January of 2001 -- the first on Maui, and one of the first imported to the US market. We love it. Not only is it a great car, but we have a new game of who can get the best gas mileage. We are at a disadvantage, as we live on the side of a mountain (Haleakala) at about the 1,000 foot level, so that our milage is always poorer than if we lived in the valley ("infinite" mileage downhill averaged with 20 or so MPG uphill equals 40 MPG for that portion of our travel). In spite of that, we typically get about 42 to 44 MPG, with the AC running and always maintaining the speed limit. (I've checked the calibration of the speedometer with GPS, and it is remarkably accurate.)Life with The Prius The Prius is a remarkable car -- all the more so for being unremarkable to drive.The engineering masterpiece it represents is only obvious if you pay close attention, and know what to watch (and listen) for.As a result, it is a comfortable car for any driver.It behaves in a very predictable manner, just like one would expect from a car of its size.And yet what is really going on is completely different. For example, there is no conventional shifting transmission, or clutch mechanism.The internal combustion (IC) engine is coupled to the wheels via a planetary gearbox, which also couples to the generator (a planetary gearbox has three shafts: from the sun gear at the middle, the planetary gears which mesh with the sun gear at their center, and the ring gear which surrounds the planetary gears and meshes with their outside surface).The amount of energy transferred from the engine to the wheels is controlled by varying the load on the generator, so that no clutch or gear shifting is required.The electric motor is essentially concentric with the output shaft.To move the car in reverse, the electric motor is used by itself. (This image shows the "spider" that connects the three planet gears as stationary, even though there is nothing keeping it that way. In the Prius, the spider is connected to the third shaft. It is the rotation of the spider that represents the third shaft, not the rotation of the individual planet gears. All shafts are concentric.)
The electric motor and the generator are both actually "motor generators", which is to say that they can be used for either purpose.As a result, the generator is also used to start the engine, and the motor is also used for regenerative braking, which recaptures about 30% of the energy which would otherwise be wasted as heat.Both have permanent magnet rotors (so no brushes are needed), and electronic inverters to run the stator coils.The inverters work to control the energy going to or coming from the coils. The enclosure which holds the inverters is water cooled, as the generator puts out up to 15 Kilowatts and the motor consumes up to 33 Kilowatts (about 40 horsepower).The inverter is an electronically controlled variable frequency device.The frequency determines the speed of rotation, and there is a tight feedback loop with a rotation sensor which synchronizes the inverter. The primary battery of the system occupies a small portion of the trunk.It has a potential of 276 volts, and holds enough energy to run the car for about six miles (YMMV).However, this is more than enough for a hybrid car where all of the motive energy originates from the gasoline engine.When the battery gets low, the engine starts and runs the generator until the battery is suffiently charged -- even if the car is standing still. Stranger still, if the car is moving, but the battery is charged and energy from the engine is not needed at the wheels, the engine shuts off!The impact of this event is barely perceptible, except that things get even quieter.When the car is moving downhill, it essentially is coasting, except that energy that would otherwise be wasted as heat in the brakes or engine is recovered as electricity and used to charge the main battery.Unlike a standard vehicle, in which coasting is dangerous, in the Prius full control is maintained.There is even a B postiion on the "shift" lever, where engine braking is applied -- exept that the engine is off, and no fuel is being consumed.This also keeps the engine warm on long downhill runs when it might otherwise cool off. Strangest is coming to a stoplight.Unless the air conditioning is on full blast, the engine stops at about the same time as the car, and you sit there in total silence.Stepping on the accelerator launches the car using the electric motor first, and if sufficient pressure is applied, the engine starts.This is almost always the case.However, even after the engine starts, the majority of the starting torque comes from the electic motor unless the accelerator is pushed with considerable force. We have talked about the accelerator pedal and the shift lever.In reality, almost everything in the Prius is fly by wire.The accelerator is a pedal coupled to a variable resistor and a spring.It feels just like a regular accelerator pedal, but it's not.The "shift lever" is connected to a switch.Even the brake pedal, although there is a real mechanical / hydraulic connection to the brakes, has its action modified electronically so that the braking force perceived by the driver remains balanced and natural when the regenerative braking kicks in, in addition to standard braking. We have a cabin in the country, where we lived for several years before moving in with my Father in Law. The cabin is "off grid", in that we generate all of our own electrical power. It was clear from the time that we first considered the Prius that there was a special opportunity in applying its technology as a source of energy, and we made a proposal to Toyota. Unfortunately, the chose not to respond, so I've included the proposal here.
(Our thanks to TurboCAD and softdevspb.)