1.  **Toyota's goals for designing the Prius** hybrid car with a
    4‑cylinder, 1.5-liter, 76-hp internal combustion gasoline engine and
    battery‑powered electric motor/generators were to

    a.  maximize energy efficiency (which helps conserve fossil fuels
        and reduces costs),

    b.  minimize pollution (to help with global warming caused by the
        greenhouse effect),

    c.  use gasoline as the only fuel (no external electrical recharging
        is required),

    d.  have a range of at least 500 miles without refueling (with an
        11.9-gallon tank), and

    e.  have midsize (larger than compact) room to seat 5, with 4 doors
        plus a hatchback.

2.  Toyota engineers used the following **methods to achieve those
    goals** for the Prius:

    a.  **a generator** to charge the battery **and an electric motor**
        to provide 50 kilowatts (67 horsepower) at 1,200 to 1,540
        revolutions per minute, which significantly contributes to
        performance and economy;

    b.  a **32-bit microprocessor** which automatically controls the
        most efficient use of the electric motors alone or the gasoline
        engine alone or both together based on driving conditions,
        determines when to charge the battery, and controls a 50 kW
        inverter that efficiently converts between direct current for
        the battery and alternating current for the motor/generators ---
        a human driver could not control these things as efficiently;

    c.  car body shape with a **low coefficient of drag** at 0.26 (the
        second lowest in the industry, after the smaller Honda Insight
        at 0.25), which reduces the aerodynamic energy losses due to air
        resistance, especially at higher speeds;

    d.  stiff, high pressure **tires for low rolling resistance**,
        reducing road friction;

    e.  **regenerative braking**, which is a process for recovering
        kinetic energy that would otherwise be wasted when braking or
        traveling down a slope and storing it as electrical energy in
        the hybrid battery for later use while greatly reducing wear and
        tear on the brake pads except for panic stops;

    f.  a **nickel-metal-hydride (NiMH) battery** with 28 cells of 7.2
        volts each, providing a total of 201.6 volts with 6.5
        ampere‑hour capacity, which is less detrimental to the
        environment than most other rechargeable battery technologies
        such as lithium‑ion (Li‑ion), nickel‑cadmium (NiCad), and
        lead‑acid, although it takes about 1,000 pounds of batteries to
        store as much energy as 1 gallon (7 pounds) of gasoline;

    g.  a **continuously variable transmission (CVT)** --- the Prius
        does not use a typical CVT, but Toyota calls it the Power Split
        Device, which connects the gasoline engine and generator to the
        electric motor and wheels (through the differential) using a
        planetary set of gears which is always engaged, so there is no
        shifting and the gasoline engine can always run near its optimum
        speed (for its highest efficiency and to reduce engine wear)
        even while the car is changing speed;

    h.  a **flexible resin gasoline tank**, reducing the amount of
        hydrocarbon pollutant emissions in the form of escaped gasoline
        vapor;

    i.  a **vacuum flask coolant storage system** that stores hot engine
        coolant and then reuses it to reduce warm-up time;

    j.  **weight reduction** --- for example, the hatch and hood are
        made of aluminum instead of steel to improve fuel economy.

3.  **Ten hybrid cars** are available in the United States as of
    January, 2006. The Environmental Protection Agency (EPA) estimates
    of their **fuel efficiencies** are listed in the following table.

+---------------------------+--------------------+--------------------+
| **Hybrid model**          | **Miles/gallon     | **Miles/gallon     |
|                           | city**             | highway**          |
+---------------------------+--------------------+--------------------+
| > Chevrolet Silverado     | 18                 | 21                 |
| > Hybrid                  |                    |                    |
+---------------------------+--------------------+--------------------+
| > Ford Escape Hybrid      | 36                 | 31                 |
+---------------------------+--------------------+--------------------+
| > GMC Sierra Hybrid       | 18                 | 21                 |
+---------------------------+--------------------+--------------------+
| > Honda Accord Hybrid     | 30                 | 37                 |
+---------------------------+--------------------+--------------------+
| > Honda Civic Hybrid      | 49                 | 51                 |
+---------------------------+--------------------+--------------------+
| > **Honda Insight**       | **60**             | **66**             |
+---------------------------+--------------------+--------------------+
| > Mercury Mariner Hybrid  | 33                 | 29                 |
+---------------------------+--------------------+--------------------+
| > Lexus RX Hybrid         | 33                 | 28                 |
+---------------------------+--------------------+--------------------+
| > Toyota Highlander       | 33                 | 28                 |
| > Hybrid                  |                    |                    |
+---------------------------+--------------------+--------------------+
| > **Toyota Prius**        | **60**             | **51**             |
+---------------------------+--------------------+--------------------+

> Several of those vehicles are more fuel efficient on the highway than
> for city driving, especially because gasoline engines tend be
> inefficient at low driving speeds. Some hybrids like the Prius,
> however, are more fuel efficient for stop-and-go city driving than for
> constant high‑speed highway driving especially because the engine can
> be turned off while the car is briefly stopped (as at a traffic light)
> or when the electric motor provides sufficient power and because
> regenerative braking can reclaim energy that would otherwise be lost.

4.  How does the Prius' "Hybrid Synergy Drive" **balance electrical and
    gasoline power**?

    a.  Under **low loads**, the Prius can be powered solely by the
        electric motor with energy coming from the battery pack. This
        provides for silent driving for short periods of time. This is
        in contrast to other "mild hybrids" such as Honda Civic, Accord,
        and Insight which use the electric motor simply as a power
        assist to the gas engine, and can never run on the electric
        motor alone.

    b.  Under **heavy acceleration** the Prius integrates both the
        electric motor and the gas engine, again with additional energy
        coming from the battery.

    c.  At **highway speeds** the Prius uses the gas engine to move it
        along, generating additional electricity to recharge the battery
        if needed.

5.  European and Asian standard models of the Prius can operate in
    **electric vehicle (EV) mode** keeping the gasoline engine off for a
    few miles at speeds up to 34 miles/hour, but that so‑called "stealth
    mode" (because it is so quiet) is disabled on American models. One
    reason for disabling stealth mode in America is that otherwise
    Toyota would have to pay a fee for dual‑fueled vehicles if the user
    rather than the car can decide which fuel to use; another reason may
    be to avoid deep discharging (if the user could prevent the car from
    automatically turning on the engine for recharging) that would
    shorten the life of the battery covered by Toyota's 100,000‑mile
    warranty in America.

6.  **Web references** about the Toyota Prius and other hybrid cars:

    a.  http://en.wikipedia.org/wiki/Toyota_Prius

    b.  http://www.howstuffworks.com/hybrid-car.htm

    c.  http://hybridcars.about.com/od/hybridcarfaq/

    d.  http://www.fueleconomy.gov/feg/tax_hybrid.shtml