How can 1.8L be more efficient than 1.5?

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.

 

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

 

More engine efficiency, better aerodynamics, better designed tire all come to mind.

 

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.

 

G III/1.8L = Hybrid Magic

 

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.

 

And lower BSFC (below 220g/kwh)...

 

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").

 

This may help.