Nissan commits to CVT

CVTs are stepless since they use cones in a pulley system rather than gears. As there's no torque converter, they're theoretically mechanically more efficient and can be geared to higher and lower extremes of ratio than even the most numerically blessed epicyclic box. So there are good reasons to offer a CVT in lieu of a conventional epicyclic auto.

There are even good reasons to offer it in lieu of a manual box, since modern units provide artificially programmed steps for those drivers who want to feel like they're driving a manual when they're in a sporty frame of mind, but don't want the leg-straining clutch operation that goes with a manual in urban driving.

In the Dualis, the CVT offers six programmed steps for manual operation. Using the sequential shifter manually sets up the car to accelerate in a fixed ratio, so the engine speed climbs with the road speed.

You would think then that Nissan -- and other car companies with similar manual-shift CVTs – would have addressed the one shortcoming of the transmission, the inertial sensation. See, in auto mode the driver prods the accelerator and the CVT continuously adjusts the ratios upwards (hence the name) while the engine speed remains constant at a point that balances torque against fuel economy. The more throttle the driver is using, the closer to the torque peak the engine will rev -- or closer to the power peak if it's all-out acceleration you're after. If the driver is using less throttle, the engine will rev to a lower speed to save fuel.

The trouble is this, the CVT is so smooth in its operation, it feels chauffeur-like and slow, exacerbated by the engine operation, in which engine revs don't climb with the road speed.

CVT-equipped vehicles can essentially accelerate as fast or faster than a manual car, because there's no relaxation of torque as the driver lifts the foot to change gear. There's also no delay in acceleration due to the gear change itself. But there remains that tactile illusion that CVT cars are generally much slower than manuals.

In theory at least, the sequential manual-shifting facility should help overcome that perception, but in day-to-day auto operation, the public concern lingers: CVTs are a bit slow.

Nissan has tweaked the CVT in the Dualis to resolve this. Indeed, the Dualis CVT provides engine braking, a kickdown détente and a facility that allows engine revs to alter as the vehicle accelerates. We've tried it and it works.

Speaking to Peter Brown, Nissan's British engineering head for the P32L (Qashqai/Dualis) project, we learned more about the car's CVT shift operation.

"For the CVT, we spent a lot of effort trying to improve the driveability and controllability of the car. To give the driver better control, we added a kickdown détente to the accelerator so this allows the driver to get most of the engine torque without the engine revs rising excessively. We tuned the characteristics of the accelerator pedal to give a very linear response. Again, the driver can easily control the engine RPM," Brown explained.

In feel, the kickdown détente works like an overboost facility on a turbo-engined car or opening the second barrel of a car with a twin-barrel carburettor. It allows additional fuel to enter the engine and engine revs rise for a surge of torque when really needed, as in an emergency. It also allows the Dualis to walk a fine line between the soft shifting of typical CVT-equipped cars and more aggressive change modes that are a common characteristic of kicked-down epicyclic transmissions.

To illustrate the difference graphically, Nissan mapped the Dualis's throttle opening on a chart, against the acceleration G-force on the second axis. Two competitive vehicles were also graphed and Brown subsequently informed us that the two benchmarks were the Audi A4 Multitronic and a BMW with a six-speed epicyclic transmission. The Audi's curve was lower and smoother than the BMW's, with the Dualis riding a curve in between the two, striking a compromise between comfort and responsiveness.

"Cars like [the BMW] tend to have a very abrupt feeling, high revving, not so easy to control, whereas the [Audi] feels very slow and unresponsive, so we aimed for something in the middle to give the customer good control of the vehicle", Brown said.

The kickdown détente feels like a firmer spring under the accelerator pedal for the last few millimetres of travel, but is easily overpowered by those with a lust for torque.

Even more intriguing was the software-controlled means of varying engine revs with the CVT in auto mode. Either with or without the accelerator pedal pounded all the way to the floor -- past the kickdown détente -- the engine would rev to a certain point as usual for CVT-equipped cars. But the revs would continue to rise, adding perhaps an extra 500rpm as the system, depending on how much throttle and how long the driver held the throttle in that position, adjusted the CVT's input/output ratios and the engine management system's fuel delivery and ignition settings.

The only point where the system struggled was from middling speeds of 40-60km/h or higher, with the throttle kicked past the détente and held to the floor for an extended period as the vehicle accelerated hard up to a significantly higher speed.

In that scenario, the engine revs would climb up around 5000rpm or higher and then, without the driver lifting the foot off the accelerator, the system would be forced to reduce revs and adjust the CVT ratios to compensate. Thus, the engine revs would hunt around, climbing 500rpm, dropping back 250, climbing another 500rpm, dropping back 250, etc.

For the most part however, the system worked well and we see a bright future for the technology. That said, it's up against the established order of epicyclic transmissions, which do improve in leaps and bounds with every new generation, and twin-clutch transmissions, the current darling of the rodeo.

Will we see a re-run of the VHS/Beta debacle, in which one -- arguably better -- technology was ousted by the more freely available format? Probably not. There's promise in CVT and with companies such as Nissan and Audi promoting it, it won't fall by the wayside in a hurry.