Amazing Technology

The Prius is a very polarizing product.  There are many people who love it and many others who hate it.  This controversy has produced studies that say the Prius, though efficient in fuel usage, is not green if you take its entire life cycle into account.  One study even suggested that a Hummer H2 was actually more efficient than a Prius.  Contrary what these studies say, the Prius really is efficient and "green".   The negative studies generally make a number of flawed assumptions to arrive at the conclusion that they desire.  There is nothing about the Prius that should make its life cycle any more expensive or dirty than any other similarly sized car.  The battery is relatively small, has a very long lifetime, and uses recyclable metals.  With the exception of the battery, the Prius is made of metal, plastic, and electronics like any other mid-size car.

For years, I had not even considered purchasing a Prius, thinking of it as being a small, underpowered "gas mileage" car.  Then, on a business trip, I happened to rent one for three days.  I found it exceptionally roomy, being much larger inside than the outside dimensions would suggest, and fun to drive.  I drove a total of 411 miles and didn't have to put gas into it until I was returning it to the airport.  When I filled it up it only took 8.2 gallons.  I decided I needed to take a closer look.

Since this is not an electric car, how did the Toyota engineers manage to nearly double the efficiency of a gas vehicle?  All of the power in the Prius comes from gasoline so it has to make very efficient use of the available fuel.  It does so by excellent aerodynamics, using a more efficient engine (Atkinson Cycle), shutting the engine down when it is not needed, and recovering wasted energy from braking and rolling.   The engineers even went to the extreme of saving the heated engine coolant in a thermos-like device so that its heat energy could be recovered if the car is driven again within a short period of time.

The battery is relatively small.  It is a 1.2 kilowatt hour NiMH (nickel metal hydride) battery weighing about 100 pounds (not much more than a large lead-acid truck battery).  In addition, its condition is carefully managed, never charging above 80% or discharging below 40% of capacity in order to extend its life.  In California, the battery is warranted for 10 years and 150,000 miles.  The useable capacity of the battery, then, is only about 480 watt hours, or enough energy to travel about 2 miles on a level road.  Its function is not so much to provide power to propel the car, but rather to act as a reservoir to recover normally wasted energy.  It’s interesting that as the Prius line evolved  the battery was actually made smaller in each successive generation.

The engine is not a normal gasoline engine.  Usual 4-cycle gasoline engines use a process called the "Otto Cycle".  This process is a compromise between efficiency and performance.  Most 1.5 liter 4 cylinder engines in small cars produce around 110 to 125 horsepower, a lot of power for their size. The 1.5 liter "Atkinson Cycle" engine in the Prius produces only about 78 horsepower.  The difference between the two processes is that the Otto Cycle uses the same length of piston stroke to generate power as it does to compress the fuel-air mixture.  This wastes power because, among other things, there is much more pressure in the cylinder at the end of the power stroke than when the compression stroke started.  This excess pressure represents wasted energy.  The Atkinson Cycle engine has a shorter compression than power stroke so that more of the available energy is used for power generation and there is almost no excess pressure left in the cylinder.  The downside of the Atkinson cycle (invented in the late 1800’s) is that it generates less power and torque for its size than a similar Otto Cycle engine, making it unsuitable for normal cars.  The electric power system in the Prius, however, compensates for this deficiency.

The next clever bit of engineering is that the Prius has no transmission.  It is listed as a CVT (continuously variable transmission), but in reality it only has a differential-like device called a PSD (power splitting device).  It is a set of fixed gears that can continuously, under computer control, allocate engine power between a motor-generator and the car wheels.  All the PSD gear ratios are fixed.  There are no gear changes.  In fact, the Prius doesn't even have a reverse gear; it runs the large electric motor backwards in order to go in reverse.

So why do I harp so much on the technology?  For one, I am a geek and can't resist high tech.  But in a larger sense, the essence of what the Prius is comes from its technology.  Now that I own a Prius, all other cars that I drive seem "clunky".  You have to start them (the Prius only has an on/off switch), their engines run continuously even when not needed, and most use transmissions with fixed shift points so that their engines are rarely running at optimum speed.  Even some other hybrid systems, like Honda's, do not have the Prius' flexibility, requiring that the gasoline engine and electric motor always move in unison.

So how does all this make for a great car?  For one, the interior size of the Prius is nearly as large as the Camry.  Rear seat room is amazing for such a small car.  The hatchback trunk is roomy and flexible.  Headroom is excellent.  Seats are comfortable and visibility is very good.  Power is adequate and feels quite different from "normal" cars.  There is plenty of torque available at all speeds because the torque is supplied by the large electric motor (the Prius has a second small electric motor/generator used to start the gasoline engine and generate power to charge the battery).  Electric motors provide the most torque at low rpm so there is plenty of torque available to accelerate from a start.  The electric motor also supplements engine torque at freeway speeds and is used when extra acceleration is needed for passing.  Zero to sixty acceleration time is not overly impressive at about 10.5 seconds, but it's adequate.  Acceleration at highway speeds for passing feels better than what one would expect from the numbers because of the extra torque from the electric motor and because the gas engine can be run at whatever speed is necessary to generate the needed power independently of the speed at which the car is traveling.

Handling is pretty good on my car.  I chose the "Touring" edition because it has larger wheels, wider tires, and a stiffer suspension.  It does not handle as well as my other car, a 3 series BMW, but I have driven it on mountain roads and it's quite comfortable.  The downside of the stiffer suspension on the "Touring" version is that the ride is not as smooth, with more road feel being transmitted to the driver.  Maneuverability and turning radius are great.  It's easy to park.

Another feature of the Prius is that it is exceptionally quiet.  This is due to the aerodynamics reducing wind noise and the relatively low rpm at which the engine runs at normal driving speeds.  The only significant noise comes from roads that are not overly smooth and when heavy acceleration is needed and the engine needs to rev up to maximum power.  The speed perception, how fast the car seems to be going, is slower than most other small cars.  Overall, it's one of the most comfortable cars I have owned.

Although the Touring edition Prius is rated at 42 mpg highway and 46 mpg city, I find that it gets better mileage on the highway than in the city.  If you could drive on long city streets without frequent starts and stops, the EPA numbers would probably apply.  However this is rarely the case.  Stopping and starting, even in the Prius, reduces gas mileage.  Also, when first starting out in the morning, mileage is low (around 30 mpg) for the first couple of miles while the engine warms up.  Consequently, if I make frequent very short trips (less than 2 miles) the mileage suffers.  Driving in slow, stop and go traffic is a new experience in the Prius.  It will run for much of the time on electric only as long as the battery remains charged, creeping along silently with the gas mileage display indicating 99.9 mpg (the maximum).  Also, it no longer bothers me to use brakes in stop-and-go traffic and when going downhill.  Regenerative braking uses the motor generator to slow the car and recharge the battery when the brakes are applied so that the actual physical brakes are not used unless really heavy braking is required.  Overall, I average around 42 mpg.

Finally, being a long time BMW owner, it's surprising that I like the Prius so much.  In fact, the only time I now drive the BMW is when my wife is driving the Prius.  There are few conditions that I can imagine under which I would prefer to drive the BMW, even with its much superior power and handling.  For anyone considering buying a Prius, I would suggest renting or borrowing one for several days first.  The Prius is so radically different from normal cars that a short test drive does not reveal its true nature.  Once you find its true nature, however, you will either love it or hate it.  And you might surprise yourself by finding it much harder to hate than you thought.