philipa_240sx wrote:From another post Re: NY Times Review:
I did some reading in the FSM tonight. The torque converter lockup clutch on the Nissan CVT engages at a very low 18kph. This is unlike some conventional automatics which engage at much higher speeds and gears (3rd -5th) so they can take advantage of the torque multiplication effect to improve acceleration and reduce shift shocks. When the torque converter does lock up, there is a significant improvement in efficiency.
Although I am no expert, I understand the main function of the torque converter in the Nissan CVT is to smooth operation when moving from a standstill. Since the CVT has a much wider range of ratios and no shift shock to deal with, the torque converter's other attributes (torque multiplication and vibration/shock isolation) aren't needed.
I really don't blame Nissan for the way thier CVT operates. They are pushing the technology to obtain the best balance performance and fuel economy... this one of the reasons they are using CVT's in the first place.
I for one am very satisfied.
The reason for the torque converter on conventional, planetary automatics is to prevent stalling when the car is fully stopped (but sitting in 1st or 2nd gear) or to change gears smoothly. The torque multiplier effect also happens to be a waste of energy (it is converted to heat), but it tends to be a necessary evil considering how automatic, planetary transmissions work. With the way the Xtronic CVT is designed, the torque converter serves only to prevent stall; because of the CVT's ability to maintain such a wide range of gear ratios and does not need to disengage to change gears like a planetary auto, the torque converter can be programmed to fully lock up at a very low speed (18kph in this case). Full lock means zero waste of power or energy to heat. An open torque converter works just like slipping a mechanical clutch, allowing a torque multiplier effect, but effectively wasting a certain percentage of energy in the process. Theoretically, a lot of energy (and thusly efficiency) is retained in part because of the low lock-up limit of the Xtronic CVT. The nature of the CVT, however, is such that it will exagerate the effect of driver input. If you're a leadfoot and constantly apply heavy throttle, accelerating hard, the Xtronic will be running shorter ratios more often and in general will use up more fuel than even a car with a conventional, planetary auto will, since the engine is effectively racing more of the time. On the same token, a driver who is very light on throttle with the CVT will conserve more fuel than a driver in the same car with a conventional auto, because there is less waste to open torque conversion, as well as effectively taller gearing more of the time. Essentially, there will be a wider variation in fuel consumption as a whole with CVT-equipped cars than cars with conventional, planetary automatic transmissions.
There is, however one caveat. That is that planetary autos do not use up any energy to maintain whatever gear it's set to. Once in gear, it's in gear. Belt-driven pulley-type CVTs always use a certain amount of energy to maintain belt tension/torque holding, no matter what gear it's in. Granted, JATCO has designed the CVT that Nissan tunes and markets as the Xtronic to be as efficient as possible and to minimize said hydraulic energy waste, so in general, the Xtronic-equipped cars do yield benefit over competing cars that don't have it. This explains why the Murano, Rogue and Altima all have virtually class-leading combinations of fuel efficiency and performance compared to its competitors. In a comparison of acceleration, Xtronic allows the best application of torque and power to get the quickest acceleration time. In a comparison of fuel efficiency, Xtronic allows the very tallest gear ratio to be effective at all times. Hydraulic losses are so low that it's comparable to energy lost to torque conversion on regular autos.
EDIT: Oh yes, and as to why you feel the shock more at torque converter lockup on the Xtronic, it's because it locks up at such a low speed and short gear ratio; the effect is amplified as a result. The taller the gear ratio, the less the effect of torque conversion lockup can be felt, because the taller the gear ratio, the less of a torque multiplier effect there is.