Is the Volt More Efficient Than a Standard Serial Hybrid?

[larry4pyro]

Remember all the cries of foul awhile back when GM revealed the Voltec transmission? At that time most folks believed that the Volt when operated in the range extender CS mode is a simple serial hybrid, i.e., ICE powers a generator, which makes electricity to turn the traction motor. As it turned out, the Voltec transmission had 4 modes, the last of which caused all the fuss. In this mode the ICE, M/G and ring gear were all connected together. The problem for the critics was GM called the Volt an EV, and now there was a direct connection between the ICE and the wheels, making it more Prius like, and thus the cries of "GM lied".

GM said this was done for efficiency. This paper models the Voltec against a serial hybrid and determines the relative efficiency of each approach.

http://www.autonomie.net/docs/5%20-%...12-01-1027.pdf

I won't pretend to say I totally understand the contents as it tends to be quite technical, but the conclusion is clear; "It was determined that the Voltec powertrain achieved lower levels of fuel consumption during all driving condition modes, as compared with a pure series configuration."

[scottf200]

Thanks for that cool document.

New folks here is the sticky thread on the 4 modes:
Voltec-drive-unit-has-four-basic-modes-of-operation
http://gm-volt.com/forum/showthread....s-of-operation

[focher]

That document makes me feel dumb. Until I got to the conclusion, and saw that my purchase of a Volt was smart.

[DonC]

Nice paper. Thanks for the cite.

My understanding is the computer modeling indicated that the Volt would have been more efficient had it been more of a pure serial -- running the ICE at a higher RPM once it came on rather than at one of several sweetspots and using the battery less -- rather than running as what has been described as a "strong serial" -- using the battery much more -- offered a much more refined driving experience.

"When we modeled the Volt's engine, theoretically we could have gotten to high 40s or low 50s mpg in gasoline mode. But we would have had to run it continuously at 3500-3800 rpm and just switch it on and off, and the noise and pleasability wouldn't work. We had to drop it down, which got us to 37-38 mpg. But I think a lot of fuel economy still can be gained without major expenditures in tooling or engineering."

http://green.autoblog.com/2011/11/03...-go-from-here/

[marlow]

May be some of the refinementioned are in the 2013 Volt accounting for its better numbers?

[Neromanceres]

Making the Volt run as a standard serial hybrid would have been easier for GM. GM took the hard road for a reason.

[QB 45]

I think, were it my choice for my car, I would have sacrificed a little "noise and pleaseability" for a little more fuel economy. I know it would subtract from the "luxury" of the overall experience, but an approximate 20%-25% decrease in fuel efficiency is quite a bit. Noise is one thing, but I don't mind a little vibration; I lean toward toward the practical side of things.

[MTN Ranger]

I think, were it my choice for my car, I would have sacrificed a little "noise and pleaseability" for a little more fuel economy. I know it would subtract from the "luxury" of the overall experience, but an approximate 20%-25% decrease in fuel efficiency is quite a bit. Noise is one thing, but I don't mind a little vibration; I lean toward toward the practical side of things.

Or even better: have a "Super ECO" CS drive mode and have the choice of more efficiency. The difference in running at a higher RPM is just a software change, right? I don't see how this would be a difficult to add in a future model (2014 hint, hint). This should seriously be considered.

While it doesn't make a huge difference to me, imagine the feather in GM's cap with a Prius-level MPG.

[bobchr]

Or even better: have a "Super ECO" CS drive mode and have the choice of more efficiency. The difference in running at a higher RPM is just a software change, right? I don't see how this would be a difficult to add in a future model (2014 hint, hint). This should seriously be considered.

While it doesn't make a huge difference to me, imagine the feather in GM's cap with a Prius-level MPG.

it may not be as simple as that. We don't know what the emissions profile would be running at those higher RPM's . The EPA emissions test requires the engine to run continuously and does not take into account cycling in CSS mode. This to me seems silly but that can be the only explanation as the why all current Volts didn't get full CARB certification from the get go. IMO the EPA test on the Volt has no basis in reality. After all 5 year old Priui are CARB certified with the same size engine as in a Volt, the mystery may be in the weight differential but the fuel consumption patten should mitigate that.

[saghost]

The current engine is definitely not more efficient at that 3500 rpm range they mentioned in its current form. If you look at the timing on the quote, I believe it referred to the 1 liter 3 cylinder turbo engine that they thought the Volt would use during the first year.

Given that some reviewers already find the engine harsh and jarring in town, I wouldn't be quick to dismiss the drawbacks in NVH or assume the higher economy would be worth it to the buying public.

For a quick development of a revolutionary car, GM did an awesome job. With more time to reflect, and budget for a unique engine, they can likely do more - a ~1.8l DI Atkinson engine with about a 16:1 geometric compression could probably put the Volt into or past the Prius numbers in the second generation without negative NVH impacts.

For the really exotic (seriously doubt it'll happen,) a ~.8 liter two stroke diesel three cylinder with electric turbo can probably produce better mileage from a lighter smoother quieter engine on a more stable fuel - if the emissions could be managed (and if a production electric turbo comes along...)