I’m looking to buy a fuel efficient car. While comparing the 2009 Prius to the 2010 Prius (the one with the new Chick adds) on the Toyota web site, I noticed that the newer model claims better milage, with a larger (1.8L vs 1.5L) engine and more torque (105 ft-lbs vs 82 ft lbs)!! However the newer engine has less horsepower (82 hp vs 105 hp). I was surprised to see this; I thought greater displacement and torque would have yielded worse milage. Is there some some principle, or better yet, some equation that relates these variables? Does it have to do with some mysterious component, like carburetion?
Thanks.
Neither has a carburetor. It has to do with Toyota refining the drive system.
According to Wikipedia the old Prius engine is rated at 76 HP. The 105 HP was net power train HP, which I guess is both the electric, and gas motors combined?
The mysterious component, is a Atkinson Cycle engine. These engines are mostly responsible for the high gas mileage (highway). The only problem is they don’t put out a lot of horsepower, and that is where the electric motor comes in as a power booster, that supplements the gas engine and saves fuel when accelerating, especially in stop a go traffic by using free electric from regenerative braking.
According to Wikipedia the New Prius 1.8L VVT Atkinson cycle engine puts out 98HP on its own. IMHO plenty enough power to be able power a small car without the need for a hybrid system, and achieve great gas mileage.
An undersized engine will also cause fuel mileage to suffer since it has to work harder to move the car around. For example if you stuck a 2 Liter 4 cylinder engine from a Focus into an F-150 it would likely get poorer mileage than the standard 4.6L V8. The principle is the same in the case of the new Prius. The slightly larger/more powerful engine in the new model means that the gas engine doesn’t have to work as hard to power the car and fuel mileage is a little better because of that.
FoDaddy, I will have to respectively disagree. A gas engine is at its least efficiency when at light throttle do to engine vacuum power loss. And Gains efficiency under more throttle. I suspect the F-150 would get better gas mileage with the 2 Liter engine, but a dog it would be.
If you look at the the fuel economy estimates for some cars, my original thesis is supported. The 2001 Dodge Intrepid with the base 2.7L V6 gets the same mileage as the same car with the 3.2L V6, same tranmission. The 1987 Taurus 2.5L I4 gets marginaly worse mileage overall than the 3.0L. The 1988 5.7L Trans-Am gets slightly better than the 5.0L model, and perharps most telling of all. The 1988 Jeep CJ-7 4.2L I6 gets a little better fuel mileage than the 2.5L 4 banger.
Granted these examples are much more the exception than the rule. And 9 times out of 10 a smaller engine will get you better fuel mileage than a bigger engine in the same vehicle. But that’s a rule that’s set in stone.
Your theory is correct once the cars have reached cruising speed. The power to maintain the speed would give the mpg advantage to the smaller motor. However the smaller motor would have to work much harder for a longer period of time to achieve the cruising speed and therefore would get worse mileage overall. At one time Ford put a 4 cylinder motor in a mid size car, the Ford LTD I think this was in the early '80’s. The idea was to get better gas mileage. The car was god awful slow and still got worse real life mpg than a V6 motor in the same car. The V6 was slow too, just not god awful slow.
You’re kinda right, all things being equal, but they aren’t. There are many changes and improvements to the new Prius that give it better economy. The Toyota site has the 2010 at 98 hp, not 82.
The second generation Prius is a great car. The third generation is slightly better in some ways, but I don’t think you’ll see a big difference in real world mpg. Toyota is claiming 50mpg for the new Prius. I’ve done a lot of Prius driving and get 45-55 mpg. The improvement in mpg is marginal. See if you can get a deal on a second gen, used even, as people want to buy the third gen. You won’t be sorry.