This, the largest of my topic areas, is all about how the Toyota Prius works. Or, to be more accurate, how I believe it works based on information from a wide variety of sources and a little hard thinking of my own. I must begin by stating clearly that you can choose, purchase, own, look after and, above all, enjoy a Prius without knowing anything of what I present here. Everything you need to know is in the Owner's Manual, supplemented, if you like, by other topic areas of my Web site, other Web sites and the Yahoo! discussion groups. However, after "lurking" for a few weeks in the technical discussion group, I began to appreciate the sheer brilliance of the engineering work that has gone into this car (not all original to Toyota) and I was hooked by the desire to learn more about how it works. If you feel the same way, I've written these topics for you. If you don't - just drive it! - you're in the majority.
Before delving into my individual topics specific to the Prius, you may want to investigate hybrid vehicles in general. An article in Scientific American is a good place to start [but the link has moved, can I find it again?]. You will learn about the two basic types of hybrid. A parallel hybrid, such as the Honda Insight, has an electric motor which can power the car alongside the engine. This allows a smaller engine to be used because the motor can help out when a lot of power is needed. The motor can also act as a generator when the car is slowed down and recover energy to partially recharge the battery. Series hybrids have fallen into disfavor and there are no examples in commercial, light-duty vehicles. A small engine runs a generator to charge the battery and a separate motor uses power from the generator and, when necessary, the battery to power the car. Because the engine cannot directly power the car, this arrangement suffers from greater electrical losses and needs a bigger electric motor. The Prius uses a combination of both parallel and series hybrid techniques to gain advantages from each. Some of the engine power goes directly (mechanically) to the wheels and some goes through a generator and a motor, where more power can be added from the battery. The direct path is mechanically most efficient, but the electrical path allows the engine speed to be selected for the best efficiency in getting power from the fuel burned.
If you should be highly technical and have access to MATLAB/Simulink, you might like to look at the ADVISOR Hybrid Vehicle Simulation, which can be downloaded free. This allows you to specify a hypothetical hybrid vehicle and evaluate its performance. There are built-in models of the Japanese Toyota Prius and the Honda Insight. A model of the US Toyota Prius is being added.
The USA is one of only three countries in the world in which business is not routinely conducted using the International System of Units (SI), also known as the MKS system (metres-kilograms-seconds) or sometimes just "the metric system". (The other two countries not fully embracing SI are Liberia and Burma.) I was educated in a country that does use SI in science and engineering (although it was also responsible for the awful system the USA seems to be stuck with). So, in my explanations I will be talking mostly about kilograms, megajoules and newton metres instead of pounds, British thermal units and foot pounds force. I'm afraid I can't go back to all those weird conversion factors. If you need an explanation of what the units I use mean, please refer to the index of units at the University of North Carolina. You can also read a description of the SI in general and the history of the metric system in the United States.
If you haven't used your high school physics in a while, a quick refresher course in energy physics might be a good idea. This will remind you about basic concepts such as potential energy, kinetic energy, mechanical work, etc. and the simple mathematical relationships between them (or, is "simple mathematical" an oxymoron?).