Staying in the sweet spot with Nissan’s upgraded CVT

Most of us – even those who are not mechanically minded – have a reasonable idea about the role of an internal combustion engine in a car. It burns fuel to create energy that is transformed into motion by turning the wheels of our vehicle. Many of us, however, are perhaps a little less clear on just how that energy is transferred to the wheels and the role of a gearbox in that process.

In fact, there are many different variations on the basic principle of a gearbox. One of them is the continuously variable transmission (CVT), which doesn’t actually house any gears.

Drive experience and fuel economy are heavily affected by the choice of transmission, especially in stop-start situations. Because of that, it’s best to understand the characteristics of different gearboxes and transmissions in order to determine the right one for you.

In a manual transmission car, we change gears using a gearstick in tandem with a foot-operated clutch that temporarily disengages the engine from the gearbox, allowing the gear shift to occur. A standard automatic gearbox changes the gears without the use of a foot-operated clutch by means of a torque converter. A second type of automatic gearbox, the Dual Clutch Transmission, is akin to two manual gearboxes in one housing – one for odd, one for even numbered gears.

But there is a third type of automatic transmission – quite different from A/T and DCT – that offers some significant advantages. CVT operates on a principle which, in some ways, is more similar to a bicycle gear system.

Let’s assume that we all understand that as we speed up or slow down our car changes gear, whether we do it ourselves using a clutch and manual gearbox car or simply select “Drive” on an automatic and let the car make the gear changes for us. But why do we need to change gear? Take the single-speed bicycle as an example. It has just one gear, which is fine if you live in a flat city and are happy to move at a relatively fixed speed, but going uphill you will need to apply more force to turn the pedals. At some point, as humans, we would fail to generate the necessary force – or torque.

On a bicycle with gears, however, we are able to move the bicycle chain onto different sized cogs, both at the pedals and on the back wheel. Changing the relative size of the front and rear cogs changes the gear ratios and means that we can use less force to turn the pedals when going uphill, although it means the rear wheel will not rotate as much for one pedal stroke as it will in a higher gear. We accept this trade-off because it is preferable to get up the hill using more pedal strokes at less effort per stroke than to run out of energy using fewer strokes that

require much more effort. For another comparison, imagine trying to climb the Pyramids in Egypt on foot. If there are fewer steps, but they are larger and higher, we will run out of energy faster than if they are shorter but more frequent.

Applying these analogies to cars, we need to be in a low gear setting off from rest to go uphill. This will generate more torque to turn the wheels and the engine will run at a higher rpm. Once the car is running on flat ground, there is less need for increased torque to overcome gravity, meaning that it can run in a higher gear and at a lower rpm.

On a bicycle, the length of the chain driving the rear wheel is fixed, while the size of the cogs at the front and rear can be changed to alter the gear ratio. A gear ration determines the number of revolutions from a power source required to create one revolution of the wheel (or wheels). A typical gear ratio for a bicycle is 2:1, meaning that for every two revolutions of the pedals, there is one revolution of the rear wheel.

In comparison, a continuously variable transmission uses a belt that runs between two pulleys – one attached to the engine, the other driving the wheels. The belts run between two V-shaped channels – one in each pulley – that can be made wider or narrower to adjust the belts proximity to the pulley. This process allows for a much greater range of potential gear ratios, all without the need for traditional gear cogs. It also results in much more smooth and seamless gear changes, meaning that there is no pause or lurch as with a standard automatic car.

When going uphill, regular automatic transmission vehicles will often ‘hunt’ for the correct gear, which may in fact lie somewhere between two of the limited gear ratio options. This is removed with CVT, which automatically selects and maintains the optimal gear ratio to allow the car to travel at the desired speed. Not only does this allow for a smoother driving experience, it also improves fuel economy, as the engine is nearly always operating at the ideal power band.

Like many of the world’s great inventions, the principle behind CVT is a magnificently simple one. However, as with any new technology, CVT has required successive refinements upon introduction to the mass market.

For Nissan’s CVT systems, the greatest amount of technical data generated in real world scenarios concerns temperature control and the special cooling fluid. In the Gulf market, the latest generation of CVT cars are more capable of keeping the cooling fluid within the necessary temperature range over extended periods. Embedded software also protects the transmission, should a driver attempt to operate it in a manner which falls outside normal driver behaviour.

 

Naturally, no system can withstand an infinite amount of misuse, but Nissan is fully confident that once drivers understand how best to use their CVT, the system will function smoothly and reliably even in the harshest climatic conditions.

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