Hi,
I would like to add a few details. The BSFC charts come from an excellent SAE paper, "Development of New 1.8-Liter Engine for Hybrid Vehicles", Kawamoto, Naiki, Kawai, Shikida, Tomatsuri, SAE 2009-01-1061. It costs $15 to download a copy and I would recommend it ... a great read.
Now I have an NHW11 (2003 Prius) with a Graham scanner. This allowed me to plot the operational BSFC data, what happens in the real world, and compare it to the 'full throttle' BSFC chart:
After rescaling the rpm X-axis to align the rpm scale, I drew vertical lines bounding the peak BSFC region and passing vertical. You can quickly see that the field measured, peak BSFC region, the small red dots, occur at lower rpms than the 'maximum throttle' region. This also corresponds to ad hoc reports of the target rpms for P&G drivers who consistently reported peak or target rpms less than the paper's BSFC chart.
My hypothesis is the 1.8L engine is using a more aggressive intake valve cycle (i.e., later closing) to further off-load some of the partial throttle operation. By further delaying the intake closing, a smaller charge is in the cylinder for low power operation without the traditional pumping losses of partial throttle operation. The smaller charge takes less energy to compress but the 13-to-1 expansion remains. But this is not evident in the 'full throttle' BSFC chart. In contrast, the existing 1.5L drivers are using or stretching the longest intake valve closing to achieve a peak BSFC that is better than the SAE paper would suggest.
Seriously, I would recommend reading the SAE paper and the companion, "Development of New Hybrid Transmission for Compact-Class Vehicles", Mizuno, Ibaraki, Kondo, Odaka, Watanabe, Mizutani, Kaneshige and Kitada, SAE 2009-01-0726. This paper discusses the very efficient transaxle that has significantly reduced rolling drag. I have been very pleasantly surprised that when I shift into "N" you can feel the ZVH30 (2010 Prius) 'release' in forward roll. Compared to my NHW11 (2001-03 Prius,) it is brilliant. For example,
reduction in lubrication stirring losses.
If you'd like to see the logical conclusion of advanced valve timing, the paper "Newly Developed Inline 4 AR Series SI Engine", Tsuchiya, Hosoi, Hoshi, Shimamura, Hagiwara, Ito and Ari, SAE 2009-01-1048. Ultimately, the throttle plate goes away.
Bob Wilson