Hi,
I would recommend getting a copy of SAE 2009-01-1061, "Development of New 1.8-Liter Engine for Hybrid Vehicles" Kawmoto, Naiki, Kawai, Shikida, and Tomatsuri, Toyota Motor Corporation, and SAE 2009-01-0726, "Development of New Hybrid Transmission for Compact-Class Vehicles" Mizuno, Ibaraki, Kondo, Odaka, Watanabe, Mizutani, Kaneshige, and Kitada, Toyota Motor Corporation. The first fully covers the new 1.8L engine and the later, the transmission. However, these are not 'systems views' of the 2010 Prius and many times, a general discussion can help clear up some misconceptions.
The Prius engine 'cheats' the pumping losses by using an Atkinson cycle that keeps the input valves open during half of the compression stroke. Such simple words belie what is going on. It is an intake valve trick that significantly reduces pumping losses seen in an Otto engine but requires doubling the number of phases involved to understand:
Phase angle - Otto cycle - Atkinson cycle
0-45 - power stroke - power stroke
45-90 - power stroke - power stroke
90-135 - exhaust stroke - exhaust stroke
135-180 - exhaust stroke - exhaust stroke
180-225 - intake stroke (pumping loss) - intake stroke (pumping loss)
225-270 - intake stroke (pumping loss) - intake stroke from adjacent cylinder (no pumping loss)
270-315 - compression stroke - push charge to adjacent cylinder (no pumping loss)
315-360 - compression stroke - compression stroke, half charge
The Atkinson cycle trick occurs at angles 225-315 when the intake valves of two cylinders, one beginning compression and a second beginning intake, where the charge passes from the first cylinder to the second cylinder. There is no 'pumping' loss during this ~45 degree phase. But the variable intake valve also replaces part of the traditional throttle plate function.
Now if you'll re-read page 5 from SAE 2009-01-1061, the 1.8L engine paper, you'll notice the intake valve has a variable range of 41 degrees. This change in intake valve opening replaces some of the throttle plate by controlling how much fuel-air leaks from one cylinder to the next. In a traditional Otto engine throttle, all of air intake has to push through that narrow gap. In contrast, the Atkinson cycle is just passing part of the charge from one cylinder to the next without the throttle plate, narrow gap. But these are details of implementation whose real impact shows up in vehicle performance.
The bottom line, "the proof," is in the mileage numbers from an engineering lab, like the EPA requires, as found at
www.fueleconomy.gov:
City - Highway - vehicle engine
51 - 48 - 1.8L, 2010 Prius
29 - 40 - 2.0L, 2009 Jetta TDI
The outstanding Prius mileage is a product of the Atkinson cycle engine and efficient transmission/transaxle combined with a sophisticated set of control laws. One's understanding or lack of understanding of these Prius systems doesn't really matter when standing by the fuel pump and paying the bottom line.
I am reminded of the story of the aerodynamic analysis of the bumble bee that proved they could not fly. Yet there they are, buzzing around. It is possible that the problem was not the bumble bee but the limited, aerodynamic analysis. There comes a time when old concepts have to be replaced by improved understanding.
So let's assume that the starting motor power is the amount of energy needed to spin an engine at idle. This is a fair approximation of the minimum energy losses of each:
- 20-to-1 diesel - the high compression resistance requires a lot of torque, more power
- 8-to-1 Atkinson - the low compression resistance requires less torque, less power
Meanwhile, the overlapping, open valve of the intake and compression stroke further reduces the 'idle' fuel burn but the Prius has one more trick ... it turns off the engine. Unlike our diesel friends, the Prius turns the engine off and runs on the stored traction battery energy until more power is needed.
These are not easy concepts to master, especially after a life-time of dealing with traditional Otto and diesel cycle engines and traditional passive transmissions. I'm hoping our diesel friends might get a clue. If they do, good. If not, no matter, the EPA numbers are accurate.
Bob Wilson