Before I get into the technical discussion I would like to say that this is NOT an anti Volt, pro Prius thread. It is simply a technical discussion delving into posed question. RPhooper posted a good thread today about how to improve the Volt for Gen2 from an MPG and cost point of view,and, in order to come up with some good suggestion we must first understand why the Volt does not get Prius MPG in CS Mode.
An excellent place to start is with a post from WOT as follows. I just went back to this post and did what he suggested. ie open 2 windows and pull up the interactive planetary animation and nomograph. Relabel the sun, planets and ring in the proper Volt nomenclature and start playing with it.
To more easily visualize the effect you can actually use this Prius nomograph animation
For it to make sense you will need to appropriately "rename" the "stick" shifters and RPM readouts:
Blue (MG1) becomes MG2
Brown (ICE) becomes OUTPUT (to final drive)
Yellow (MG2) becomes MG1 and/or ICE driven speed of the ring/internal/annulus gear (and dependant on clutching factors)
(you can even use a bit of masking tape on your monitor to label as such to keep it straight)
*also keep in mind the ratio of the Prius is 4.66:1 but the 4ET50 is geared a bit higher at 4.22
Starting with everything flat & level you can slowly raise the blue shifter slowly to 6500 rpm by using the brown OUTPUT lever (previously ICE) to simulate normal EV mode in CD.on the Volt this would equate to an output speed that delivers ~70mph after the final drive and wheel circumference is through with it.
To go any faster in order to mitigate the inefficiency of overspeeding MG2 (Blue) we can first uncouple the ring/internal/annulus gear from it's stationary position (locked to case by C1) then add a variable reactionary influence from either MG1 (when C2 only is applied in CD mode) or ICE (when both C2 and C3 are applied in CS) or BOTH.
To observe it's dramatic effect (again at 4.66:1 not 4.22) slowly drag the yellow lever upward to until it's RPM is 1508. While doing so, MG2 RPM is quickly decreased to 2653 rpm yet the output speed does not change! (again 2653 is not quite accurate, the Volt would be 7.6% faster due to the higher gear ratio but close enough) After that you would merely move the speeds of MG1/ICE (both assuming CS) and MG2 in unison up to 100mph (something this nomograph animation doesn't permit)
It's not perfect but you get the picture.It's NOT a 2-mode (OR a Prius) but a similar use of variably split ratios of planetary gearset
Of course a fringe benefit of this 3 way coupled "output split" mode is any regenerative opportunity in this mode could be potentially created and maximized by using either 1 or both MG units depending on the inverter controlled pathway back to the HV battery.
After playing games with engine speed, vehicle speed and loading I have concluded that the existing planetary gear setup DOES allow the ICE to operate with enough freedom to put it on it's best sfc point for a given load. It does this at the expense of driving one or both MG's to a slightly off design condition, however, the motor maps I've seen have a fairly wide efficiency band as long as one does not drive the motors to an overspeed condition. So, IF what I just said is true, then why can't the Volt achieve Prius efficiency. We essentially have eliminated a poor ICE match as the culprit. Sooooo, then why??
I don't know and I must say I am even more puzzled about this than before.
Ideally we would have the guy at GM that runs the off design computer model of the Volt explain it. Failing that I would first ask WOT, or anybody else on this forum to present a logical reason.
I suspect that it is not just one thing but a multitude of things. First on the list in my mind is still that the Volt is operating in pure series mode for much of the time and therefore is suffering from conversion loss penalties (although this should be mitigated with the output split mode). Second we know that the higher wt of the Volt has to have an effect as Tom's sensitivity coefficients have shown (even though the Volt has a superior regen system). Thirdly: Is the ICE cycle efficiency a culprit (as opposed to ICE match point on it's map), I guess it does NOT have direct injection but I would assume it is using VVT so it should be able to match the Prius ICE efficiency.