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Old 03-06-2009, 10:04 PM   #5 (permalink)
Engineering first
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Join Date: Mar 2009
Location: Huntsville, AL
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14 i3-REx - '14 BMW i3-REx
Last 3: 45.67 mpg (US)

17 Prime Plus - '17 Toyota Prius Prime Plus
90 day: 58.25 mpg (US)
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I was at the Detroit show and saw the new Prius:
Originally Posted by SVOboy View Post
I was just looking through some of the official Toyota videos on youtube when I happened to stumble across this explanation of some of the tech in the new Prius. I already knew that Toyota had made some changes to the engine to favor a more powerful, quicker Prius. . . .
The problem is internal engine drag is proportional to speed. So what happens is as the rpm increases, the Atkinson cycle engine, which is very efficient at modest rpm begins to suffer higher internal losses. This reduces the brake specific fuel consumption:

My NHW11 Prius has the 1.5L engine and you can see that the brake specific fuel consumption is falling off at higher rpm.

This chart shows another effect, which we suspect is fuel enrichment at high power settings above 3,7500 rpm:

The high power region is where cooled exhaust gas is added to cool the exhaust and allow higher power output with less fuel burned. At high power settings, the engine has to use a rich mixture to keep from burning out the catalytic converter.

So the larger, 1.8L engine and appropriately improved hybrid transaxle lets the engine avoid higher, energy wasting rpms. With cooled exhaust gas, the engine continues into higher power settings but without the extra heat. What we don't know is if a similar system fitted to the existing Prius would provide a similar mileage improvement not counting the friction losses.

BTW, the Otto cycle engines suffer pumping losses through the throttle that the Atkinson cycle avoids. What this means is what works with the Prius engine won't necessarily work with an Otto cycle engine.

Bob Wilson
2019 Std. Range Plus Model 3 - 134 MPG3 || 2014 BMW i3-REx - 117 MPGe, 39 MPG
JuiceBox 40 Pro (240 VAC, 40 A), KHONS portable (120-240 VAC, 12-32 A)
Retired engineer, Huntsville, AL (five times AutoPilot saved.)
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