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Oceanworks II
On the Toyota Prius
Some observations on the Toyota Prius
The Prius promises a future where automobile makers combine various energy storage and production technologies to yield more gas-efficient cars and a more sustainable future. As an early example of the new hybrid technology, the Prius raises issues that not all prospective buyers may appreciate.
First of all, its longevity is unknown. The first generation (through 2003) Prius' batteries are hoped to be good for about 7 years and will cost about $6,000 to replace. The current cars are warrantied for 10 years, so owners should pray for 9 year battery life, and not 11! There are other costly bits in the car, but they are less certain in terms of risk than the batteries.
Secondly, the Prius may not be getting those great mileage numbers from its hybrid technology. Consider a primary tenet of physics, that everywhere and always:
Force = Mass * AccelerationSince the "force" that accelerates cars (including the Prius) comes from burning gasoline, a car will get better mileage if it weighs less or accelerates more slowly. With the Prius accelerating from 0 to 60mph in a very leisurely 11 seconds, you can see that Toyota has increased the Prius' mileage rating in part by making it slow. You can improve the mileage you see in your car the same way: accelerate more slowly and reduce your top speed.
For cruising on the freeway, the story is a little different. The force your engine exerts is mostly pushing air out of the way of the car, and it increases exponentially as you increase your cruising speed. Specifically:
Force = Coefficient of Drag * Area * Density * (Speed2/2)"Coefficient of Drag" means "how draggy" something is: the higher the score, the more drag, the worse the fuel economy for a car. The Prius scores exceptionally well here. It has a slippery shape and is pretty small, both of which increase fuel economy.
What does all this mean?
Is the Prius a breakthrough in fuel economy? It may be helpful to compare it to Toyota's Echo, an economy car which looks a lot like the first generation Prius while costing a little more than half as much as the current Prius. The Echo is officially rated at 35 mpg city, 43 mpg highway: much better than any SUV, but noticeably less than the Prius' 60/51. However, when Consumer Reports tested the two cars, it got 38 mpg in the Echo and 44 mpg in the Prius--only a 15% improvement despite all that expensive extra technology in the Prius. This gibes with anecdotal reports from Prius owners that in order to get the great gas mileage, you have to drive in an extremely conservative fashion.
To sum up, most of the Prius' advertised mileage benefits over bigger cars probably come from its size and shape and the way people (supposedly) drive it. Its hybrid technology represents a modest improvement in fuel economy, not a revolution. And it comes at a large price premium over similarly-performing vehicles, with long-term reliability unknown.
Reviews and Commentary
- Corolland
- Car and Driver
- Edmunds.com
- Wired
- A Prius owner
- Engines turning themselves off at freeway speeds
Addendum October 2004
Toyota has anounced that they are on target to sell 30,000 Prius's this model year (2004). I do not have exact data, but people who have seen Prius batteries report that they are 12-15 lbs each, and it is known that there are 38 of them. So:
12 x 38 x 30,000 = 13,680,000That's 13.6 million pounds of new batteries, made to order this year, retired in about 10 years. And more each year. So this is our environmental saviour?