Why diesel cars don't come with CVTs

Diesel engines have made a lot of progress over the past few years. They are no longer the gruff and loud clattering engines that they used to be especially with the newer models. They have become more refined, quiet, and are now producing more power than ever before. However, the transmission options for these engines have always either been a manual or a traditional automatic transmission. With  Continuously Variable Transmission's (CVT) rise in popularity over the past few years for gasoline-powered vehicles, the question now arises, why don’t diesel-powered vehicles come with this new transmission option?

Before we answer that question, we have to look at why diesel engines have become so popular in the Philippines and how CVTs work. In the case of diesel engines, it all boils down to the power output that these motors can produce. Since diesel engines are paired with turbochargers they produce a whole lot more torque giving them the pull needed to accelerate quickly even at lower speeds. This works well with the design of a diesel engine as its powerband tends to be located within the lower RPMs. This makes it easy for the engine to get a car going especially in traffic. 

Moving on to CVTs, these are essentially belt-driven transmissions that come with a more compact form factor compared to their torque converter counterparts. CVTs come with a basic design, two cone-shaped pulleys, and a steel belt that goes in between them. The input pulley turns the belt and the belt, in turn, turns the output pulley. Thanks to the cone-shape design, these pulleys can then expand or retract independently, effectively changing the ratio and filling the role of traditional gears. This gives them an advantage over their fixed ratio automatic transmission counterparts. When power is needed, a CVT can change the ratio of the pulleys so that the engine is always within the peak of its powerband, which is typically towards the higher end of the rev range.

While CVTs do provide the distinct advantage of letting a motor always be within its powerband, there is a limit in its design. This limit is in the contact patch between the steel belt and the cones. If the motor paired to the CVT is too powerful for it, the engine can easily overpower the contact patch resulting in slip and less power translated to the wheels. 

Compared to a traditional torque converter transmission, however, they can handle a lot more power as their “contact patch” is greater thanks to the use of a liquid coupling. Liquid coupling works by having two turbines inside the transmission spin each other using fluid. This is what gets your car moving in a conventional automatic transmission. Torque converters also have the benefit of further multiplying the torque that an engine produces. This is also why torque converter transmissions are often paired with turbo-diesel engines as they play well to the strong points of the engine. Paired with hard gears, this type of transmission only slips when the engine produces way more power and torque than it can handle. Thankfully most torque converters, on average, handle bigger loads better than CVTs.

CVTs rely on the friction between the two cones and the belt between them to deliver power to the wheels. Overpowering this friction will lead to losses in terms of power delivery.  CVTs also scale poorly with torque, if the torque is greater than the friction, the steel belt in a CVT will just spin without putting any of the power to good use. When this happens the cones can get ground down, further damaging the transmission.

Another reason why CVTs and diesel engines don't mix is that the operating rev range of a diesel engine is much lower than that of a gasoline engine. This means that peak torque comes in much earlier compared to its gas-powered counterparts. This also means the risk of the belts spinning is much higher since peak torque comes in earlier. 

While diesel engines would benefit greatly from having near-infinite gears and always being in the powerband, the technology isn't mature enough to deal with the torque they produce.

There are vehicles on sale on the international market today that come with a diesel engine and a CVT pairing. While the combination of the two does work, it is important to note that the power output especially in the torque area is lower than what you would find in the bigger SUVs and pickup trucks available today. Examples of these vehicles would be the Subaru Forester diesel that has since been phased out in favor of a gasoline engine and the Honda Amaze which is sold in India. While there are still other examples out there these still adhere to the limits in the design of a CVT.

As technology continues to evolve, we might soon see a turbo-diesel-powered vehicle that comes with a CVT. However, it may take some time as the longevity and reliability of the pairing will still need to be tried and tested.