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Old 03-07-2009, 07:06 PM   #8 (permalink)
roflwaffle
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Join Date: Dec 2007
Location: Southern California
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Camryaro - '92 Toyota Camry LE V6
90 day: 31.12 mpg (US)

Red - '00 Honda Insight

Prius - '05 Toyota Prius

3 - '18 Tesla Model 3
90 day: 152.47 mpg (US)
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Well I'll be! I've never though about starving the cat of oxygen but that certainly shows me!

Quote:
Originally Posted by bwilson4web View Post
It means:
  • 8-to-1 compression stroke - in an Otto engine, the throttle plate would cause a lot of pumping losses in low power regions whereas the Atkinson cycle makes a substantial reduction in throttle plate losses. This is especially useful in low power regions where low-drag vehicles cruse.
  • 13-to-1 expansion stroke - provides a high expansion ratio so a large percentage of energy is extracted. Only diesels have a higher expansion ratio but they also have a problem with NOx formation. Longer durations at higher temperatures favors NOx formation.
Scaled for torque output, the engine in the Prius exhibits the same difference in BSFC compared to load a V6 engine designed over a decade earlier exhibits. Specifically, a ~10% increase in fuel consumption when comparing half load and full load, and a doubling of fuel consumption at around 15-20% of load. The Prius is a bit better, around 3%, at lower loads, but I'm not sure if this is the result of the greater expansion ratio or fewer cylinders/offset crank. The wider ovals at max torque are from limiting torque output and the offset crank.

Clearly the 1NZ-FXE would have lower fuel consumption than a normal 1NZ-FE since it's effectively destroked, which improves fuel consumption through fewer throttling losses, but also caps torque output across the powerband, and while the higher expansion ratio may help it doesn't appear to help a whole lot when we scale the output compared to other engines. Capping the torque output by reducing the effective compression ratio and the offset crank are where most of the gains appear to come from based on comparing it to decade+ older/larger engines from Toyota. The only way to determine how much the higher expansion ratio helps would be to compared the 1NZ-FXE to a version destroked to have the same effective compression ratio.

In terms of low drag vehicles cruising in lower power regions, that isn't much of a problem where a hybrid cycles on and off and stores power to be used in a battery instead of operating at a steady/lighter load. This, along with the taller gearing, is what Toyota can increase the engine size in the new Prius and increase or keep mileage the same. It's the same idea behind P&G. It doesn't matter how poor low load engine efficiency is if the engine is almost never operated there. Granted, their is a loss from dumping energy into the battery pack and out the motor, but as long as that loss is less than the increase in BSFC, then there's no reason not to do it.

Diesels have problems with NOx formation because of CI, not necessarily the higher CR. SI engines can have the same CR as modern diesels, but low engine out NOx because the A:F mixture can be distributed relatively homogeneously, which minimizes hots spots during ignition. CI engines otoh see ignition start more or less when the bulk of fuel is injected. Modern versions tend to run a bit richer to cool ignition and prevent as much NOx from hot spots during ignition. Anyway... Variable valve timing almost certainly won't get a SI engine near a CI engine in terms of overall fuel consumption/load, that's something we need HCCI, or something similar, for. On the plus side, GDI gets closer at low load, but that's the best in use implementation I've seen so far.
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