MOTORING NEWS - Even those who are not mechanically minded have a reasonable idea of the role of an internal combustion engine in a car. It burns fuel to create energy which is transformed into motion by turning the wheels.
Many of us are perhaps a little hazier on the gearbox and precisely what role the gearbox plays in getting energy to the wheels.
In fact, there are many different variations on the basic principle of a gearbox, including the continuously variable transmission (CVT), which doesn't actually have any gears. Different types of gearboxes significantly affect how the car feels to drive and how economical it is, especially in stop-start motoring conditions, which we often find in cities around South Africa.
As such, it's important to understand their characteristics in order to determine the right choice for oneself.
• In a manual transmission car, we change gears using a gear stick working in tandem with a foot-operated clutch, which temporarily disengages the engine from the gearbox to enable gear selection to take place.
• A standard automatic gearbox changes the gears without the use of a foot-operated clutch by means of a torque converter.
• The so-called dual clutch transmission is akin to two manual gearboxes. One is for odd and one for even numbered gears in one housing. These have been automated to allow for very rapid gear shifting without a traditional clutch. Like many conventional automatic, dual clutch transmission can be operated as full or semi-automatic, using a gear stick but no clutch.
• There is a third type of automatic transmission, one which is quite different to the other two and offers some significant advantages. A continuously variable transmission (CVT) operates on a principle which in some ways is like a bicycle gear system.
HOW A GEARBOX WORKS
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 have you ever considered why we need to change gear?
One way to think about it is to consider a single speed bicycle. Such a machine 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 with one gear, when you need to go uphill you will need to apply more force to turn the pedals. Human legs have only so much strength and if the hill is steep and long enough we will eventually fail to generate enough force (or torque) to turn the pedals.
On a bicycle with gears, however, we are able to move the bicycle chain onto different sized cogs at the pedals and on the back wheel. Changing the relative size of the front and rear cogs changes the gear ratios, meaning 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.
Applying this analogy to a car, when setting off from rest to go uphill we need to be in a low gear, which will allow us to use higher rotations per minute (RPM), generating more torque to turn the wheels.
Once the car is up and running on a flat road, less power and torque is required to overcome gravity. The engine therefore does not need to work so hard, so a car can be placed in a higher gear, which has the effect of turning the driven wheels at a lower engine speed.
On a bicycle, while the length of the chain driving the rear wheel is fixed, the size of the cogs at the front and rear can be changed in relation to each other to provide different gear ratios. A gear ratio is what determines the number of revolutions from a power source, whether it is a car's engine or the turning of a bike's pedals, that are required to create one revolution of the driven wheel.
For example, a typical gear ratio for a bicycle is 2:1. That means, for every two revolutions of the pedals, there is one revolution of the rear wheel.
THE SENSE IN CVT
So how does a CVT compare to a bike's gear system? Instead of shifting a chain between different cogs at the front and rear to alter gear ratios, a CVT uses a specially designed belt which runs between two pulleys, one attached to the engine, the other driving the wheels. The belt runs between two V-shaped channels, one in each pulley.
Both these channels can be made wider or narrower, pushing the belt to the outer edge or bring it closer to the centre of either pulley. This creates a huge range of potential gear ratios, all without any traditional gear cogs. And because there are no separate gears, the changes in gear ratio occur in a smooth, linear and seamless manner, meaning the "pause and lurch" that you may get when a standard automatic car changes gear, is absent.
When driving up a hill, there is no sense of the transmission "hunting" up and down for the correct gear, which may in fact sit somewhere in between two gears.
With a CVT, the optimal gear ratio to allow the car to travel at the required speed is selected and maintained automatically. This makes for both a smoother and more economical driving experience and means the engine is nearly always sitting within its "sweet spot" or optimal power band.
MAKING CVT BETTER
The principle behind CVT is simple, but refinements are made with each successive generation based on real world experience. Perhaps the greatest amount of technical data generated in real world scenarios for Nissan's CVT systems has come in the area of maintaining the correct temperature of the fluid that cools the system.
The latest generation of CVTs has a greater capacity to ensure the fluid remains within the specified temperature range when operating for extended periods, even in extreme temperatures. Nissan is confident that once drivers understand how best to use their CVT, the system will function smoothly and reliably even in the harshest climatic conditions.
Embedded software also protects the transmission, should a driver attempt to operate it outside normal driver behaviour.
If you have never before driven a CVT vehicle, you will find it is a case of "driving is believing".
The new Nissan X-Trail has a CVT model specifically developed for South Africa's driving conditions. CVT technology is widely used in the local market and is increasing in popularity. This technology available in the Nissan X-Trail and Qashqai opens up a whole new driving experience that is smoother, more economical and more relaxing.
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