Technology 12.4.2018 09:33 am

CVT gerabox is more economical and smoother

AUTOMATIC GEARBOX - LEATHER GEAR LEVER

AUTOMATIC GEARBOX - LEATHER GEAR LEVER

With a CVT, the optimal gear ratio to allow the car to travel at the required speed is selected and maintained automatically.

Recently I poked fun at a vehicle hailed as the range flagship, but it had a CVT gearbox.

Although we were having a tongue-in-cheek dig, it made us realise that it has been some time since we had discussed gearbox technology, which has moved along at a fair clip.

While I might despise CVTs, they are gaining in popularity among consumers.

The different types of gearboxes significantly affect how the car feels to drive and how economical it is, especially in stop/ start conditions like traffic jams.

As such, it’s critical to understand their characteristics to determine the right one for you.

In a manual transmission car, we change gears using a gearstick 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 by means of a torque converter.

The so-called dual clutch transmission is akin to two manual gearboxes – one for odd, one for even numbered gears – in one housing, which have been automated to allow for very rapid gear shifting, also without a traditional clutch.

Like many conventional automatics, dual clutch transmission can be fully or semi-automatic, that is using a gear stick but no clutch.

But there is a third type of automatic transmission with significant advantages. A continuously variable transmission (CVT) operates on a principle which in some ways is more like a bicycle gear system, except it does not have any gears.

As we speed up or slow down, our car changes gear.

But why does one need to change gear? Consider a single speed bicycle with one gear, which is fine if you live in a flat city and are happy to move at a relatively fixed speed.

But when you go uphill you need to apply more force to turn the pedals. On a bicycle with more gears, we can 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, so we need less force to turn the pedals to go uphill.

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 running on a flat road there is less of a requirement for power and torque to overcome gravity, so the engine does not need to work so hard, so a car can be put in a higher gear which has the effect of turning the wheels at a lower speed.

A gear ratio determines the number of revolutions from a power source required to create one revolution of the wheels.

A typical gear ratio for a bicycle is 2:1. So for every two revolutions of the pedals, there is one revolution of the rear wheel.

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 running 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 of these channels can be made wider or narrower, pushing the belt to the outer edge or closer to the centre of either pulley.

This creates a huge range of potential gear ratios. And the changes in gear ratio occur in a smooth, linear and seamless manner.

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 smoother and more economical driving and means the engine is nearly always in its optimal power band.

Perhaps the greatest technical data generated for Nissan’s CVT systems is in maintaining the correct temperature of  the fluid to cool the system.

The latest generation of CVTs has a greater capacity to ensure it remains within the specified temperature range for long periods.

 

 

 

 

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