CVT idea (fixed "top gear" set?) - Fuel Economy, Hypermiling, EcoModding News and Forum

pete c · 08-13-2015, 12:05 AM

I have rented a few of them. The latest being a Sentra. Once you get used to odd sound/feel, they really aren't so bad. In fact, I kind of enjoyed the way I could "accelerate" if you want to call it that, with the tach sitting permanently at little more than idle. The sentra is a decent sized car and I still exceeded 40 mpg with ease.

But, I still don't like the mechanics of what is going on inside. It just seems as though tranny wear over time is inevitable. I was thinking there is a solution. How about a CVT which has as its top "gear" an actual direct gear drive. It could be 1:1 or even an overdrive.

A CVT is basically 2 variable pulleys on parrallel shafts. Why not add a pair of gears on this shaft that would engage when the engine got to its highest ratio.

In addition to lower wear, I believe this would increase efficiency as well. It seems like a very simple idea and I can't believe I am the first to come up with it. Maybe it already exists and I don't know about it.

MetroMPG · 08-13-2015, 02:25 PM

Interesting idea...

I wonder if one reason it's not done already (aside from cost/packaging) is because the latest CVT ratios are now so wide, the fixed gearset would have to be especially tall which would limit the amount of time it actually spent engaged.

It would probably be so limited that the impact on the EPA test would be minor, and there wouldn't be an incentive to manufacture.

...Even if, in the hands of an attentive driver, it would return a decent boost in MPG.

MetroMPG · 08-13-2015, 02:28 PM

PS: this kind of reminds me of one of VW's hybrid drivetrain ideas where they had EV mode up to X road speed, then the combustion engine came on with direct drive to one single gear.

pete c · 08-13-2015, 04:03 PM

Honda did that in the latest Accord Hybrid. It seems like a good idea, but, I haven't heard much more other than the Accord Hybrid was getting a re-do. So maybe it wasn't such a good idea.

Hersbird · 08-13-2015, 04:55 PM

They wouldn't need a separate gear, they could just beef of that end of the CVT. That is probably not where they are wearing out. More likely it is the lower end where the torque multiplication is the highest. The problem with trying to keep the transmission at a certain gear ratio is it sort of defeats the purpose of having a CVT that can vary the ratios while keeping the motor in the sweet spot.

niky · 08-14-2015, 11:59 AM

Sounds unnecessarily complicated. Many CVTs already "lock" in "top gear", giving no drivetrain losses when you're cruising.

The real killer for CVTs seems to be heat. When they're cruising in whatever final drive they've got, they don't build up much heat. When you're accelerating or hill climbing, they do.

A more important modification for a CVT would seem to be an auxiliary oil cooler. That's what Subaru added to the WRX for the CVT... and it's a necessity! CVTs on other turbo Subarus seem prone to heatsoak and reduced performance during spirited driving.

gone-ot · 08-14-2015, 12:38 PM

Heat soak problem is most in "V-belt/link-drive" CVT's, not in "geared differential" CVT's.

some_other_dave · 08-17-2015, 10:26 PM

There are geared-differential CVTs? Wow, have I got some reading to do!

-soD

niky · 08-17-2015, 11:48 PM

Quote:

Originally Posted by Old Tele man

Heat soak problem is most in "V-belt/link-drive" CVT's, not in "geared differential" CVT's.

Sadly, almost all consumer market CVTs nowadays are chain-drive.

Also, I've seen non-CVT transmissions overheat also... lots of new cars running these ultra-thin low-friction transmission fluids to meet economy standards... so the ECU throws a fit and pulls power (or even goes into limp mode) every time the temperature climbs from lukewarm to slightly hot.

gone-ot · 08-21-2015, 03:26 PM

Quote:

Originally Posted by niky

Sadly, almost all consumer market CVTs nowadays are chain-drive.

Also, I've seen non-CVT transmissions overheat also... lots of new cars running these ultra-thin low-friction transmission fluids to meet economy standards... so the ECU throws a fit and pulls power (or even goes into limp mode) every time the temperature climbs from lukewarm to slightly hot.

The eCVT in our 2014 Prius does not use chain drive...it has GEARS. The earlier GenII Prius, however, DID use a chain-drive drive-connection, but not chain-drive ratio-connection.