Part 1: Principles of the CVT.
In any application, whether it be on- or off-highway, a continuously variable transmission (CVT) can be used as a mechanism to provide the vehicle with the precise spread of ratios it needs. With this type of transmission, the goal is to try and keep the engine speed at a specific point or sweet spot, whether the priority is to optimize fuel economy or to maximize the power or torque delivered to the drive wheels or the implement. To illustrate what a CVT is, and how it operates, Lubrizol has created two videos — one detailing a chain CVT, the other explaining a push-belt CVT , both passenger car CVT examples.
As a general principle, the CVT’s overriding advantage is its ability to keep engine speed as near constant as possible. Most engines prefer steady-state running. Unnecessary extra fuel is burned during transients and when engines speed up and slow down during gear changes; precious rotational and longitudinal momentum is also lost during these processes. Having a CVT in place of a conventional manual gearbox can help eliminate those issues.
Variations of on- and off-highway CVTs
The theoretical advantages of a continuously variable drive are so appealing that countless devices have been developed and patented by automakers, T1 suppliers and lone inventors. Of these, only a few basic types have made it into series production, though several more claim to be close to production. At their most basic, CVTs can be either mechanical, with belts, pulleys, cones or toroidal sections; torque-blending between ICE and electric power sources, as on the Toyota Prius and other hybrid passenger cars; or hydraulic. The latter, used principally in the heavy off-highway sector but also on light machinery such as garden tractors and mowers, vary the vehicle’s speed and direction using a system of hydraulic pumps and motors.
On-highway CVT applications
For on-highway CVT applications, typically small and medium-sized passenger cars, the favored device is a push-belt or chain type of transmission. A segmented metal belt or chain runs between two sets of conical pulleys, each pair being able to move inward or apart to adjust its effective radius. Hydraulic fluid pressure is needed to keep the belt clamped against the pulley faces so that it is able to transfer torque from the input pair of pulleys to the belt and then back to the output pulley set. The differing diameters of the two sets of pulleys provides the continuous ratio variation for the transmission.
The fluid in a CVT has another vital function: to help control the friction in the belt/pulley interface. Here, there is an uneasy trade-off for the fluid engineers. Fluids with high metal-to-metal friction properties allow the transmission designer to lower the clamping pressure and still achieve the same torque transmission across that interface. The lower the belt clamping forces, the better the variator efficiency becomes, for there is a two-fold benefit: reduction of parasitic loss at the pump and reduction of frictional losses in the variator system.
However, with the increase in friction there are drawbacks in other parts of the transmission, including the launch device. Whether it is a launch clutch or conventional torque converter with a converter lock-out clutch, the higher metal-on-metal friction at the belt-pulley interface means a greater sensitivity to increased friction in these launch devices.
CVTs for agricultural equipment
Farm tractor CVTs can be either hydrostatic, mechanical or a combination of the two. Equipment manufacturers tend to favor one or the other – the differences will be explored further in part 2 of this series.
The main advantage of a CVT in the agricultural context is its ability to operate the internal combustion power unit at a fixed optimized speed, allowing the engine to remain in its sweet spot to improve fuel consumption and reduce exhaust emissions. However, there are important drawbacks, such as the high parasitic losses involved in operating the several hydraulic pumps and motors within the transmission.
CVTs for construction equipment
There are similar benefits in the construction sector, although in this case the main benefit is not so much fuel economy, but more that of maintaining a constant ground speed to aid operational efficiency and reduce driver fatigue. CVTs allow ground speed and implement speed to be independent of engine rpm. This is in stark contrast to traditional applications, where gear ratios are in fixed steps and the operator has to make constant adjustments and compromises. The CVT’s infinite range of ratios can always match the engine speed with what is required by the vehicle.
Passenger car and off-highway developments
For many years, the belt-and-pulley systems that characterized passenger car CVT applications suffered limitations in terms of torque capacity, confining their use to smaller models with lower torque outputs. More recently, however, systems from suppliers such as Jatco have begun to appear with higher torque ratings, making them suitable for many of today’s front-drive based crossovers and minivans, especially those marketed by Japanese brands. Traditional body-on-frame rear-wheel drive (RWD) pickups and SUVs will still rely on stepped planetary automatics for some time, however.
Overall, the popularity of CVTs is on the rise among off-highway machinery manufacturers as well as in the light vehicle market, especially in Japan, the US and China. One notable development in the off-highway sector is the Dana Rexroth HVT, a powersplit hydromechanical variable transmission that seeks to combine the advantages of both hydrostatic drive and mechanical power transfer. Smooth-acting hydrostatics handle launch, direction changing and low speed work, handing over to the more efficient mechanical torque path for transit between work zones or higher speed road sectors. The result, say the companies, is a fuel savings of 25 percent in mixed operation.