What is a CVT? - Autoblog - Latest Global News

What is a CVT? – Autoblog

CVT stands for Continuously Variable Transmission, a type of automatic transmission that lacks a set number of gears or “speeds.” Instead, this type of automatic can vary the gear ratio continuously between the lowest starting gear and the highest cruising gear. This allows the engine to run at the most efficient speed depending on the vehicle speed, road gradient or load condition.

However, this makes the sound of CVT-equipped powertrains fundamentally different, as the engine speed tends to remain constant while the transmission continuously and seamlessly changes its ratio to keep the car accelerating. Anyone who expects or prefers stepped gear changes is more likely to come across CVT drives.

Why is a CVT a good idea?

It’s all about reducing fuel consumption and having the right gear at the right time. The first automatic transmissions were two-speed automatic transmissions, which were quickly replaced by three-speed and then four-speed automatic transmissions. Each successive change improved fuel economy and drivability, and so the trend in numbers continued, to the point where most current automatic transmissions generally have eight, nine or even eleven speeds. The obvious endpoint of this trend is infinite speeds, and that’s exactly what a CVT offers.

A higher number of gears reduces the size of the gaps between adjacent gears, which has several advantages. Fuel economy improves because the engine is more likely to run at its most efficient speed. Likewise, more gears reduce the likelihood of rocking back and forth on inclines, as smaller gears keep the engine closer to its sweet spot in terms of power and torque. The presence of smaller stages also reduces the need for a squishy torque converter, a generally inefficient, power-sapping device that smooths out the otherwise annoying shift shocks with each gear change.

A CVT maximizes the potential of this concept. An infinite number of gears means there are no steps at all, which is why CVT transmissions are sometimes referred to as “continuously variable” transmissions. Fuel efficiency and hill drivability are theoretically maximized with continuously variable gears, and no torque converter is required to ensure smooth shifting.

Why don’t people like CVTs?

The engine may roar if the exhaust system and interior noise cancellation functions are not properly designed. And of course, a CVT can only exploit the potential described above as well as the software that controls it. But control strategies have improved immeasurably in the decade-plus they have been in widespread use, and further improvements have been made as engines and cabins have been re-optimized with a CVT in mind.

The biggest remaining drawback is one of perception and emotion, as a CVT changes the sound of an engine and a powertrain responds to acceleration. We’re all used to a steadily rising engine sound that’s interrupted by gear changes. However, this is largely not the case with CVT drives, as they tend to keep the speed constant and change gears instead. Many drivers don’t care about this because this tendency is not so pronounced when the vehicle is driven in a normal, relaxed manner. However, many found the lack of traditional gear changes unpleasantly strange, particularly aggressive drivers who tend to put a lot of strain on the engine. The car may sound like it is constantly stuck in a low gear.

For this reason, many newer CVTs create artificially stepped gear changes – usually in “Sport” driving mode, but sometimes also in normal operation. At the detail level, there are still CVT actions, but such examples can create a more vivid and familiar experience – albeit at some cost in fuel economy.

How does a CVT work?

Imagine a bicycle with numerous gears – say eight of them on the rear wheel. There is a smallest and a largest sprocket, with six in between. Now place the same set of gears on the pedals, but flip them over. We set it up so that the largest front sprocket only works with the smallest rear sprocket, with similar one-to-one private relationships across all gear sets. This strategy helps our chain maintain a fixed length across eight gear combinations. There is no need for an S-shaped tensioner to lengthen and shorten the chain like a real bicycle chain, as any front sprocket can work with any rear sprocket. That’s not allowed here.

Now replace the chain with a V-belt and turn the gear sets into pulleys with V-flanges that can be clamped together or spread apart. By varying the gap between the V-flanges, the V-belt is raised or lowered between them, creating different belt operating diameters. There is still a smallest diameter and a largest diameter, but there are now infinite possible diameters in between. One of the pulleys is controlled by a computer, but the other is passive because the flanges are held together by spring tension. As the flanges of the controlled pulley expand or compress, the diameter of the V-belt shrinks or grows accordingly, and in response the spring-loaded pulley flanges and the belt diameter at the other end are forced to do the opposite because the length of the belt is fixed. The result is a continuously variable transmission with infinite gears or “speeds,” also called a CVT. The materials and design details are much more sophisticated, but that’s the basic idea.

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