More mass, more displacement, more gasoline burned. It seems a hyper miler would prefer the smaller engine. In test driving both 1.5 versus 1.8 at 45 miles/hour the smaller engine did do +1mpg better. Not a big difference but measurable.
Under the exactly same operating conditions, No! But using different gear ratios and under load and certain conditions, the 1.8 might win. Some small chars have better mileage than others with the same weight and size of engine. It all comes to efficiency in the use of fuel.
How "scientific" were your controls in that testing. +1 mpg difference is not enough for me to define it as more efficient. Too many other variables at any given moment could account for that small a difference. Upon release of the Gen 3, Toyota presented the larger 4 cylinder engine as a benefit in smoother, quieter, highway driving, less strain. Since Gen 3 overall is a slightly more efficient incarnation of Prius than the Gen 2, I'm just going to say the end results are "how" the larger 4 cylinder engine can be made to be more efficient working in concert with the whole HSD system.
The 1.8L engine, and the car it is placed in, are newer designs by five years, so not all else is equal. And yes, some hypermilers do prefer the smaller engine. But many other customers have differing wants and needs.
How can 1.8L be more efficient than 1.5? Easy. It's newer. You can put a 1.1 liter motor from a first-gen Honda CVCC (c. 1973) into a Prius and it might generate enough power to get out of its own way, but I'm guessing that both emissions and efficiency would suffer greatly. One of the things I like about the G3 Prius is the fact that it almost has as much power as a real car, but if you want it to it can knock down 50-mpg without all of the hypermiler hijinks. Nobody will ever confuse it with a sports car, but it has enough grunt to haul four American (over)sized people in something approaching comfort, at interstate speeds, efficiently, and cleanly. If you keep one long enough...it's about the cheapest car you can drive even with today's ($2.90) gas prices. Somebody in the head-shed figured out that you need 1.8l in a G3, and that 1.5l would suffice in the trial-sized Prius. Despite my earlier incorrect presumptions, the C-type can get better mileage than a G3. There's no replacement for displacement!
The answer most often given by the Toyota engineers is variable valve timing on both intake and exhaust. But newer more advanced technology will give better mileage. A larger displacement allowed the engineers to push the hybrid technology further. With more torque available they could go further into the "Atkinson cycle" area without danger of "stalling" the engine.
The 2ZR-FE engine used in the Prius (coded 2ZR-FXE) was introduced in NA in 2008 with the Corolla and Matrix redesign and is part of the current Toyota family of R coded engines. (and dates back to 2006 when the new Gen 10 Corolla was introduced in Japan). The 1NZ-FE (FXE in hybrid form) that the Gen 1 & 2 Prius used as well as the Prius c dates back to 1998 with the Vitz/Echo and is part of the last generation Z coded engines. Hence the 2ZR is 8 years newer. A variant of it (2ZR-FAE) includes Valvematic (Toyota's version of a continuously variable valve timing system) and that's being used in the 2014 Corolla LE Eco (140hp, 126lb-ft) and a slightly higher compression ratio. The 1.8 litre engine was chosen because Toyota wanted to increase the low end torque, allowing the car to get better mileage on the highway. The idea is that it doesn't have to rev as much when passing resulting in quieter operation (less engine noise), and less fuel being used. They also included a reduction gear in the Power Split Device to act as a torque multiplier. They wanted to better compete with the diesels in Europe at high speeds (130km/h) and as a bonus, can give Americans a better highway mpg number. That being said, the 1.5 litre 1NZ-FXE in the Prius c has been upgraded with the same stuff the bigger brother has (electric water pump for a beltless engine, cooled exhaust gas recovery system and exhaust heat recirculation system). Coupled that with a lighter vehicle and mpg rises (even with a smaller battery). The R engines from Toyota are much more efficient than their Z counterpart (see in part the 2AZ-FE engines in the Camry and RAV4 vs. the 2AR-FE engines they currently use. The fuel economy is much better, especially since they've also included a 6-speed automatic to replace the 5-speed). If you want to see how the old one fares under current testing regimes, just take a look at the Scion xB. It uses the same 4-speed auto and 2AZ-FE setup as our 2002 Camry and the city mileage is not comparable.
Agree. However, 2zr-fxe engine in Prius has only variable valve timing on intake. More here: Do we know why intake VVT only in 2ZR-FXE? | PriusChat
A school bus is more fuel efficient than a compact car for moving the same number of people over the same distance. It does in one trip that the compact car needs multiple trips for. Power does tend to mean more fuel consumed, but that little bit extra displacement could mean faster acceleration (less fuel over time to reach desired speed), easier time maintaining highway speed (less fuel), and so on.
I want to get the 1.8L plugin (used of course so it's around $15,000 in price). Of course for that price I could also get the sexier unlimited-range Volt EV (used).
So why are you wanting a PiP instead of a Volt? Or that $16k (new) Focus Hatchback that I cannot find that low?
Another possibility of how a 1.8L could be more efficient under some conditions (including the conditions of the EPA tests): Everybody knows that when you floor the gas pedal, you trade efficiency for power. Similarly, if you use a very light touch, you waste gas because it spends most of its effort just spinning the engine, and not accelerating the car. The Prius often uses electric power in both the low- and high-power cases - in the low-power, so that the engine can remain off; and in the high-power case, so that the engine can be smaller and perhaps not rev as high. In between the low and high-power cases, there will be a most efficient operating point. I'd bet that this point is different for the 1.5L and 1.8L engines. It may be the case that normal driving (or at least EPA driving) requires acceleration that is more efficient for the 1.8L than for the 1.5L. An extreme example of this is when Top Gear raced the 1.5L Prius against a 4L BMW, both at the Prius's top speed. Because the power demands were so high, the Prius was at pretty much its worst efficiency point; the BMW, meanwhile, was puttering along with ease, probably not too far from its most efficient power level. As a result, the Prius got 17 mpg, and the BMW (which under any normal circumstances would fare much worse than the Prius) got 19 mpg. So it's entirely possible that normal driving just requires a little too much power for the 1.5L Atkinson sometimes, and it's more efficient to use a larger engine at lower RPMs to generate the same amount of power.
Note also that the 'sweet spot' on the BSFC chart of the newer 1.8L engine covers a much wider RPM span than that of the older 1.5L engine.
My input: With our 2005 Prius (1.5 liter) - we would get max mpg 54-57 (in the Summer). With our 2010 Prius (1.8 liter) we get 57-60 mpg (Summer and pretty much the same "routes").
Here's the thread if anyone's interested in what telmo's talking about 2010 Prius 2ZR-FXE engine efficiency map | PriusChat You can see the lower BSFC area (so it's using less fuel) yet producing more torque. In addition, you can see the jump in torque after 3,650rpm that I spoke of earlier (Your ECO light will go out in the Hybrid System Indicator if the engine is in that mode at highway speed)
