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Old 02-27-2010, 02:59 PM   #11
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Originally Posted by i-DSi View Post
Impressive explanation!!
But seriously: what do yo mean with 'a CVT doesn't lock up' ?
A 'classic' AT's locking up is the moment where the torque convertor is 'bypassed' and the cranckshaft is locked with the AT's shaft.
A CVT... doesn't need a lock up I thought because there's no slip in a convertor.
A CVT let's you always drive the lowest possible rpm, also on the highway.
So I don't understand how a CVT can make the difference in mpg. There are clutches in the CVT, but there's no slip once you'e on the move.
I've never driven a car with a CVT, but from my understanding, if you're cruising at 1500rpm and you hit an incline, if you push on the throttle in order to maintain speed, the RPMs will rise instead of staying steady with a larger throttle opening. Aren't I correct in this assessment? When the torque converter is locked up or the clutch is released, one is capable of loading down the engine in this way and consequently can make more efficient usage of its power. Think of the engine in a car with a CVT like the batteries in a Prius, they won't be used below 20% and above 80%, but somewhere in between. I believe this is due to the inherent design of the transmission.

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Old 02-27-2010, 04:00 PM   #12
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I have two cars with CVTs, my Insight and the wife's Nissan Rogue.

The rogue will climb a grade without increasing RPM unless the grade gets fairly significant. It will accelerate very nicely at 1400 RPM up to just below 50 MPH, tachs 1500 at 50. That's tall gearing.

The Insight runs at higher revs, but that is because of the 1 liter engine. It has a clutch between the transmission and differential.

I don't think either has a torque converter to lock up. I know the Honda doesn't.

Last back roads trip in the rogue I managed 33.25 MPG in a 3200 pounds small SUV. Only mod was 40 PSI tires.

The Insight has averaged 64.8 MPG for over 20k miles. Not as good as a well driven manual and it does not have lean burn, earning a SULEV rating for Cal emissions, while the manual gets a ULEV rating.

I like CVTs. Nissan recently extended the warranty on theirs to 10-120 which helps when you consider the replacement cost.

Most of the losses in CVTs are belt friction and the high pressure actuators than are used to vary the ratio by forcing the pulley faces closer together to change the gear ratios.

We had a Murano that was rated to tow 3500 pounds (weighed 3800) with a 245 HP V6. That one also had the warranty extended to 10-120 by Nissan. I wish Honda would do the same with the Insight, but I doubt that will ever happen.


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Old 03-01-2010, 08:52 AM   #13
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The CVTs in the Toyota and Ford hybrids are not a traditional cone and band automatic. They are just a planetary gear at heart, and are about as mechanically complex as a single speed manual without a clutch. It is elegantly simple. There's no lock up because there is always a physical, direct connection between the drive motors and wheels.
It's annoying to see the old Insight get 70mpg on the highway while the Prius is like in the 50s and its weight isn't even all that important for highway cruising.
Take more than 2 people by Insight(s) and the combined milage drops. The Insight got what it got because Honda focused on a fuel efficient commuter. Toyota had other priorities; transporting a family and cleaner emissions. The 1st gen Prius beat the Insight in both areas. Plus, it was for sale in Japan since 1997. Japanese drivers spend a lot more time in city driving.

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