why diesels dont come with CVTs

Diesel engines have made a lot of progress over the years. They are no long gruff and loud, clattering engines. They have become more refined, quiet and they’ve gone up in power too, and so have continuously variable transmissions (CVT). They are now the go-to transmission for efficiency and are now a common sight in gasoline cars in the Philippines. 

But, why haven’t they shown up in diesel-fed vehicles? 

Toyota Innova enginebay

Diesel engines have always made more torque than their gasoline counterparts. This is partly thanks to diesel engines being paired with turbos. This is what gives the humble diesel engine quite a lot of pull on the road and in the Philippine market. Diesel engines tend to accelerate much quicker and are more responsive at low speeds. This works well with a diesel engine’s powerband which can also be found at lower RPMs. This makes it easy for the engine to get the car going especially in traffic.

CVT transmission

A CVT is essentially a belt driven transmission. It is a lot smaller than its conventional torque converter counter parts. CVTs are designed with two cone shaped pulleys and a steel belt that goes in between them. This is what makes up a basic CVT design. The input pulley turns the belt and the belt in turn turns the output pulley. These pulleys can expand or retract independently effectively changing ratio at which power is produced. CVTs effectively have near infinite gear ratio options compared to their fixed ratio automatic counter parts. When power is needed a CVT can change the ratio of the pulleys so that the engine can turn at the RPM where it produces the most power. This is typically at the higher end of the rev range.

CVT transmission

The contact patch between the cones in a CVT is smaller than their other conventional torque converter counterparts as torque converters use liquid coupling to translate the power from the engine to the wheels. Liquid coupling works by having two turbines inside the transmission spin each other using fluid. This is what gets your car going in a conventional automatic transmission. Torque converter transmissions also have the benefit of further multiplying torque, further enhancing the capabilities of diesel powered vehicles.

CVTs rely on the friction between the two cones and 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 burnt out further damaging the transmission.

Another reason why CVTs and Diesel engines don't mix is because 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, technology hasn’t quite found a solution yet to deal with the torque they produce.

Honda Brio Amaze

The Honda Amaze with its diesel variant is one of the few cars out there that have a CVT mated to a diesel engine. However, this small turbo diesel engine only makes about 78.9 hp and 160 Nm of torque. This is far smaller and weaker than what powers bigger SUVs. Technology has advanced enough for this to happen and work reliably. While it is possible to apply a CVT to a diesel vehicle applications, so far, are limited.

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