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Discussion Starter #1 (Edited)
I bit the bullet and downloaded the SAE paper on the Gen 2 Volt engine. Best BSFC island is 230 g/kWh. With the given fuel spec, that comes out to ~36.5% peak efficiency.

For reference:
Gen 1 Volt engine peaked at 240 g/kWh (~35%)
2010 Prius peaked at 220 g/kWh (~38%)
2016 Prius and Hyundai Ionic apparently hit 40% thermal efficiency, which is roughly 205 g/kWh
Honda EarthDreams peaks at 214 g/kWh (39%)
TDI engines peak out around 196-197 g/kWh for comparison (42.5%)

Having read the whitepaper, I think some sacrifices of efficiency were made to get higher specific output (on 87 too), as they pushed the new engine family as far as possible for displacement. If you notice, the Prius engine makes less power with a higher displacement engine, affording them the ability to make some more efficient choices. That's the benefit of having started with a larger gasoline engine.

I thought everyone would be interested.
 

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Not sure what kind of detailed data is available in there, but aside from peak, typical usage also plays a large role.

If setup A is 10% less efficient at peak, but spends 50% of it's time at that peak vs setup B that's the best efficiency, but only spends 25% of its time at that peak, A still wins.
You could eke out every last bit you can to be able to advertise a high efficiency number, but if you can only hit that number in real world once in a blue moon, is it really worth all the effort to get there?
 

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Very interesting, thanks for sharing :)
 

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Keep in mind, the Prius uses a full time Atkinson setup with a higher geometric compression ratio. That gives it both a smaller charge volume ("effective" displacement only about 80% of actual, depending on the Atkinson ratio they picked) and greater efficiency from the longer exhaust stroke.

GM folks have said that the engine does something somewhat similar using the variable valve timing, but since they didn't commit to full time Atkinson they can't build the really high compression ratios, so there's less opportunity to gain (but they can also run with fully filled cylinders and get more power from the engine.)

I think GM's decision makes sense, under the circumstances.
 

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Thanks for the info. It would be interesting to see the figures for the Malibu Hybrid, it gets about 12% better fuel economy than the Volt despite being a bigger and heavier vehicle with a larger ICE but using a very similar Voltec transmission. Obviously some compromises were made for the Volt that hurt fuel economy. Is there any information in the paper explaining why the Volt does so much worse than the other hybrids?
 

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Discussion Starter #9
The paper doesn't try to present any explanations for lack of performance. I'm guessing they don't see it that way. I'd also guess that in their eyes, it performs as well as anything could given the restrictions and goals they had. The engineering that went into it is very impressive... And I say that as a guy who's competed and won in SAE supermileage competitions as the engine builder.

I'd like to get their take on things. I might email the lead author of the paper and see if he'll indulge me. I'll also say that GM tends to have very accurate measurements of things. Not everyone does. There could be some element of measurement uncertainty here too.
 
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