BUYERS GUIDE - SOFT-ROADERS
The Nissan Murano uses an Xtronic continuously variable transmission in place of a conventional automatic transmission. A CVT has ‘stepless’ gearing within upper and lower limits.
The principle is quite simple. A flexible belt runs between two pulleys that have variable diameters. One pulley is connected to the engine and the other to the driveline. As one pulley becomes effectively larger in diameter the other becomes correspondingly smaller and this varies the gear ratio between the input and output pulleys.
Our first encounter with a continually variable transmission (CVT) was in a little DAF 600 sedan, back in 1975, when DAF made cars as well as trucks. The little DAF went much harder than it should have, because with a clog full of accelerator the engine went to the red line and stayed there, while the pulleys varied the gearing.
The main mechanical problem with early CVTs was the torque limit the belt could endure, so applications were restricted to small vehicles. In Nissan’s case the best known example in this market was the Micra mini-sedan.
Later CVT developments use a steel link-plate chain instead of a rubber band, increasing torque capacity up to over 300Nm and with hydraulic pulley diameter control to eradicate ‘slipping clutch syndrome’. This is the system used in the Nissan Murano.
Driving a CVT-equipped vehicle can be quite different from operating a conventional automatic, because at lift off the CVT pulleys adopt maximum gearing instantly, sending engine revs towards maximum power. As the vehicle accelerates the revs stay pretty much constant unless the driver backs off. The process feels similar to driving a manual transmission with a slipping clutch and many drivers don’t like the sensation.
The Xtronic CVT in the Murano doesn’t have the ‘slipping clutch’ feel of a conventional CVT, because in place of an automatic clutch or a centrifugal clutch to provide ‘neutral’ it has a torque converter between the power pulley and the engine.
The fluid coupling nature of the torque converter provides a neutral gear and also magnifies lift off torque, reducing the engine revs necessary to get the Murano mobile with a 1500kg caravan in tow. The torque converter provides additional gearing multiplication at stall, so the Murano can get away with a relatively high-geared 2.4:1 low ratio.
The overdrive ratio achieved when the power pulley is at its smallest and the output pulley is at its largest is an unprecedented 0.4:1, so the Murano has short-geared diffs with 5.2:1 reduction.
The Murano has a six-step manual mode, for those who just can’t leave automatic boxes alone. In this mode the transmission will hold onto a ‘ratio’, so it’s quite useful for situations that require some engine braking.
The future of CVTs looks bright and they might well have moved into racing circles as well, were it not for a decisive block by the FIA in 1994, following spectacular times clocked by David Coulthard in an experimental CVT-equipped Williams.