CVT vs. Automatic Transmission Fluid: What’s the Difference – Jalopnik

There are seven core fluids (eight, including differential, which isn’t always on the list) that keep most gasoline-powered cars running smoothly: fuel, coolant, power steering fluid, windshield wiper fluid, brake fluid, engine oil, and transmission fluid. In certain cars, some fluids may have dual functions when similar properties are required, such as brakes and clutches that use a common reservoir. In general, however, it is best to assume that different liquids are not interchangeable and should not be mixed; For example, pouring oil into the windshield wiper fluid reservoir is a terrible idea.

Pouring automatic transmission fluid (ATF) into a continuously variable transmission (CVT) is also a terrible idea, but it happens. That’s understandable, because CVTs and automatics both shift gears on your behalf, and for many people that’s where the story ends. There are several cars that you probably didn’t even know had a CVT. Yes, they are automatic, but they are not the same as the usual automatics.

ATF is usually painted red to distinguish it. CVT fluids can be green or yellow. But the material difference between ATF and CVT fluid lies in the way each transmission does its job, and the forces involved – especially friction. Automatics don’t like it, but CVTs need some to function properly. If you’re not sure which fluid to use or whether your car has a CVT or an automatic, check the manufacturer’s specifications for your car. It is the easiest way to avoid possible transmission failures.

Uncontrolled heat and rust are not good for transmission guts. CVT and automatic transmission fluid both need to withstand high temperatures to keep everything running smoothly, and they may contain additives to protect against oxidation. From here things diverge as we get into fluid coupling, where fluid (in this case ATF) is used to provide torque instead of a mechanical connection. We recently discussed how your automatic transmission knows when to shift gears. At a high level, traditional automatic transmissions have a fixed set of planetary gears that cannot operate without automatic transmission fluid, which also serves as a hydraulic medium that engages the gears and a lubricant that keeps them sliding freely. The less friction here, the better.

Automatic transmission fluid flows through a torque converter. The converter pressurizes an oil pump, which directs the slick ATF through a series of valves, which operate clutches and tires that select gears based on the amount of pressure in the system. Some torque converters will lock up, meaning they connect directly to the engine’s output shaft to bypass the fluid coupling and reduce heat.

As temperatures rise, the volume of automatic transmission fluid increases and volume affects flow. This increase in volume is accompanied by a decrease in density, which changes the compression properties of the fluid. As a hydraulic fluid, ATF must have the right viscosity to compress just enough to flow properly and pressurize the system without breaking down.

Torque converters are also used part-time in many cars with CVTs – such as some Subarus – but there are no gears in a traditional CVT, and therefore no hydraulic gear changes so that CVT fluid can be pressurized or processed. Typically, they use a belt or chain stretched around two pulleys (a drive pulley and a driven pulley) to simulate gear ratios. It’s easier to visualize how CVTs work with LEGO, but the gist is that the belt is squeezed between two conical surfaces on each pulley. As the faces compress, the belt pushes up toward the edges of the cones; when they expand, the belt slides toward the center. The variance between the pulleys determines how fast the belt turns.

The belt itself never changes size, but it does three things at once that CVT fluid is supposed to help manage: grip the front of the pulley, slide up and down the pulley, and transmit shear force through the ever-changing tension of those pulleys. The higher viscosity of CVT fluid allows it to handle these tasks. In short: each liquid is designed for opposite results. If you put automatic transmission fluid in a CVT, you won’t have enough friction for the belt and pulleys to engage each other. Add CVT fluid to an automatic transmission and your gears don’t slide, they drag.