By: Graham Eagle
The need to meet ever-more stringent fuel consumption and emission regulations has become an expensive and complex challenge for motor manufacturers. Reduction of friction in the drivetrain, lessening of aerodynamic drag, deletion of vehicle weight and elimination of rolling resistance are all important focus areas, but the continued evolution of the vehicle’s internal combustion engine remains the number one factor. A general move to engine downsizing has been countered by the widespread adoption of turbocharging, direct injection, variable-camshaft timing, sophisticated electronic engine-management systems and several other technologies, all intended to boost engine efficiency and meet these targets.
However, without an appropriate transmission – carefully matched to the characteristics of the engine and thus allowing it to operate at optimum speeds and engine loads – the improved efficiencies promised cannot be realised. As a result, the most popular transmission types – the torque converter automatic with 37% share worldwide and the conventional manual transmission with approximately 23% share – have seen significant improvements to better match the characteristics of improved engines. The most obvious change has been the trend to more gears with eight, nine and even 10 speeds becoming common. Generally speaking, the more gears in a transmission, the easier it is to maintain the optimum engine speed in different driving conditions, thus improving efficiency and fuel economy.
The growing popularity of CVT
More significant has been the increased popularity of the continuously variable transmission or CVT, now found in 17,5% of the world’s car park and outranking the dual-clutch transmission, which holds approximately 14% share. CVTs enjoy a market share of more than 50% in Japan and are also popular in other Asian countries, as well as the US. They are recognised as a more cost-effective, compact and efficient alternative to traditional torque converter automatics and their characteristics are considered particularly well matched to small- and medium-sized vehicles with petrol engines developing up to but no more than 350-400 N.m. More than 70% of the CVTs currently produced hail from Japan’s leading manufacturers; unsurprisingly, it is these manufacturers that offer CVTs on many of their models in SA.
How does CVT differ from manual or automatic?
Both manual and automatic transmissions use sets of gears to provide several fixed gear ratios. These ratios provide “steps” between low- and high-speed operation and are selected either manually by the driver or automatically according to the required acceleration, vehicle speed, gradient and other operating conditions.
In a CVT, each of the driving (input) and driven (output) shafts have a pulley splined onto it. Each pulley is split down its centreline and comprises two conical surfaces facing each other, forming a V-shaped groove between them with an included angle of approximately 20 degrees. A metallic belt runs in the V-groove between the two conical surfaces of each pulley, transmitting drive from the input to the output shaft. One side of each pulley is fixed and the other side is movable, actuated by a hydraulic cylinder. Movement of the cylinder increases or decreases the amount of space between the two sides of the pulley, forcing the belt to ride lower or higher along the pulley walls, changing its pitch radius and, in turn, the transmission “gear” ratio. The variable distance between the pulley surfaces thus continuously varies the ratio between engine and vehicle speed according to driving conditions.
The metal belt connecting the pulleys is often referred to as the push belt as it works in compression, not tension. It is made of hundreds of high-strength, bow-tie-shaped metal elements strung together between two rings, typically made of between nine and 12 thin bands of high-alloy steel. Drive power is transmitted at the contact between the metal elements of the belt and the pulley surfaces and, with the use of specially developed transmission oils, these belts do not slip, are quiet in operation and can reliably transmit torque of up to 350-400 N.m.
The benefits of a CVT
CVTs offer several benefits over conventional automatic transmissions, both in terms of fuel economy and manufacturing costs. The stepless nature of their operation provides a major advantage as the infinite number of smooth transitions from low to high gearing keeps the engine in its optimum power range, improving efficiency and reducing fuel consumption. On average, this is claimed to improve fuel consumption by about 6% between CVT models and their manual transmission equivalents.
In addition to these efficiencies, the CVT also has a major benefit in terms of simplicity. Compared to a traditional automatic transmission, which can contain hundreds of moving parts, its major components include a high-strength belt, a hydraulically operated driving pulley, a mechanical torque-sensing pulley and several microprocessors and sensors. The lack of multiple gearsets also reduces weight and transmission length, improving packaging, particularly in transverse-engine applications.
Despite all these plus points, customer acceptance of CVT has been mixed, the primary complaint being the well-known “droning” or “mooing” of the engine running at a constant speed while the vehicle is accelerating. Many find the noise disconcerting and, to the uninitiated, may conclude a clutch must be slipping as the engine note is not unlike that experienced. The reality is that nothing is slipping at all; the CVT has sensed the acceleration required by the driver through the throttle position and other sensors and has allowed the engine revs to increase before gradually adjusting the gearing to maintain the optimum acceleration. Other complaints are that acceleration feels slow as it is smooth and continuous rather than the series of interrupted surges so familiar in a conventional automatic. The reality is that from the moment the engine revs reach and hold peak power, acceleration will be superior to any transmission with fixed-gear ratios, as peak power always delivers maximum acceleration.
Manufacturers have acknowledged this customer feedback and continue to make changes to improve the driving experience. Sophisticated electronic controls utilising a larger number of sensors have reduced many of these characteristics while incorporating override features to hold gear ratios through corners and downhill gradients. Some have created steps in the range of variable gearing, to mimic conventional gearboxes, even referring to this as 7-, 8-, 9- or 10-speed or similar. Toyota has created a clever Direct Shift-CVT for certain models, which includes a separate first gear set. Standing-start acceleration is through this gearset; meaning that initially there is linearity between the rise in engine revs and gain in vehicle speed before the transmission seamlessly moves into CVT operation. As the CVT mechanism does not need to transmit standing-start acceleration loads, the weight of certain components has been reduced resulting in better response and further improvements in fuel consumption.
CVTs work best when coupled with modern turbo engines with higher and flatter torque curves. Decent levels of acceleration in city traffic can be easily achieved using no more than an initial 3 500 r/min, quickly reducing this to 2 500 and even 2 000 r/min by easing back on the throttle as speed builds up. When cruising on the open road, routine overtaking and typical gradients are easily tackled with a slight opening of throttle to access additional torque, without the need for significantly higher engine speeds.
An interesting history
While the push for efficiency has seen CVTs achieving widespread use only in recent years, they have a long and interesting history. As early as the 16th century, a sketch by Leonardo da Vinci appears to depict the technology. An early version of the CVT appeared on the first automobile, patented by Karl Benz in 1886. The first mass-production CVT in a car was the DAF 600 Variomatic in 1958 using a separate rubber drive belt running over variable diameter pulleys for each rear wheel. Non-automotive applications for CVTs include drill presses and lathes as well as personal watercraft and snowmobiles.
Considered for F1
In 1993, a prototype F1 Williams Renault FW15C fitted with a CVT transmission was tested by then test driver, David Coulthard. It certainly sounded different to contemporary F1 cars with its 3,5-litre V10 running at a constant 18 000 r/min when accelerating; there is speculation it had instantly proved several seconds per lap quicker than the conventional Williams. Development was discontinued when the FIA moved quickly to outlaw the technology, introducing a clause specifically banning CVT from the following year. No reasons were ever provided and the ban came at the time active suspension and launch control were also banned but some insist it was because it just didn’t sound right.
In the correct applications and under many circumstances, CVT’s advantages outweigh its disadvantages, offering steady acceleration, smooth operation and the ability to adapt to varying road conditions, power demands and improve fuel consumption. You can expect to see more CVTs as technological advances further improve their functionality and the need for improved fuel economy.