Unlike other transmission types that use solid gears in one form or another to effect fixed gear ratios, CVT transmissions do not use any gears, except for a set of planetary gears and a couple of clutch packs to facilitate a reverse function.
Instead of gears, CVT transmissions use two pulleys, the width of which can be adjusted in opposite, but equal amounts by actuators that are in their turn, controlled by microprocessors. As a practical matter, one pulley is attached to the engine (typically via a torque converter to allow for a neutral stance), and another that is attached to the driving wheels via a conventional differential. In some CVT transmissions, the two pulleys are connected by an immensely strong reinforced rubber belt, while on others, the pulleys are connected with a flexible steel chain to accommodate higher torque inputs.
In terms of operation, “gear ratios” are established by varying the effective diameters of the two pulleys in opposite but equal amounts to maintain the proper belt of chain tension. For instance, when the vehicle pulls away from a standing start, the drive pulley will be set so that its effective diameter is smaller than that of the driven pulley. While this reduces the rotational speed of the drive pulley at the point where the belt or chain runs within it, engine torque is multiplied proportionally by virtue of the difference in the drive and driven pulleys’ effective diameters.
As the vehicle gathers speed, the effective diameters of the pulleys are adjusted continuously and seamlessly, which produces seamless acceleration. However, the adjustments to the pulleys are made in a manner that ensures that the engine is always running at not only its most economical, but also its most efficient speed for any given transmission “setting”. In translation, this means that the engine may run at the same RPM’s over a wide range of vehicle road speeds, and that the effective diameters of the pulleys will only be changed when the engine is no longer running at its most economical and/or efficient speed.
The two biggest advantages of CVT transmissions over other transmissions types are that when driven appropriately, a CVT transmission can produce fuel cost savings, and that its mechanical simplicity translates into a lower weight than other transmissions.
CVT transmissions suffer from several distinct disadvantages compared to other types of transmissions, which include...
Many users find the fact that the engine runs at the same speed for long periods to be disconcerting, if not downright unpleasant and distracting. Although all manufacturers have by now updated CVT control software to simulate “gear changes”, the improvements are not always detectable or effective, and generally do not produce the correlation between increasing engine speed and increasing road speed.
Although CVT transmissions are fitted to many mid-range SUV’s, the design and operating principles of CVT transmissions mean that towing, load carrying capacity, and off-road capability is severely limited. Bear in mind that power is transmitted to the driving wheels through the small contact area between the belt/chain and the sides of the pulleys, meaning that heavy loads and aggressive driving can cause drive belts to slip, or steel chains to break.
Although most CVT transmissions are now comparable to manual transmissions in terms of reliability, the high servicing costs of CVT transmissions can make the true cost of ownership of most CVT-fitted vehicles up to significantly higher than that of vehicles with manual transmissions, when calculated over the useful life of the vehicle.
One major complaint about CVT transmissions is that they produce a variety of mechanical noises under different driving conditions. Typical noises include rattling, grinding, or whining sounds that typically appear during start-up, at low speeds, or during hard acceleration. The actual causes of most noises are not known for certain, but most technicians tend to agree that mechanical noises are the result of the normal operation of CVT transmissions, and that the noises can therefore not be eliminated.
Most common symptoms of defective, faulty, or malfunctioning CVT transmissions are much the same across all applications and can include the following...
This is typical of drive belts or chains that are slipping, and is most commonly caused by defects or malfunctions in the actuators (and their electronic circuitry) that control the adjustment of the pulleys. Other causes include worn or damaged drive belts or chains.
This is also common, and is mainly caused by worn or damaged drive belts or chains. However, in severe cases, the vehicle may fail to move at all; in some cases, the problem involves failures of the torque converter due to low transmission fluid levels, or a complete failure of the drive belt or chain.
As with any vehicle, regular maintenance will help identify problems early, potentially saving you time and money. Your local Repco Authorised Service centre is qualified to service your vehicle, no matter what make or model you drive. You can find your nearest service centre here.