Gearing Up for CVTs
With gasoline prices on the rise and automotive emissions standards becoming harder to meet, continuously variable transmissions provide manufacturers with the technology to make new improvements in fuel consumption and emissions reduction.
The average passenger car's fuel mileage consumption was 14 miles per gallon in 1970. Today, it is 27.5 mpg. As a practical consequence, the average driver can go nearly twice as far on a gallon of fuel as he once did. But in an era when fossil fuels are rapidly being depleted, gasoline prices are soaring, and lower automotive emissions levels have recently become law, even this virtual 100 percent improvement is not enough.
One hardly needs to be an automotive engineer to understand that the less fuel an engine consumes, the fewer pollutants produced, and the cleaner the air we breathe. Unfortunately, improving the variables in that equation is becoming increasingly difficult.
For the past 25 years, the Engine and Vehicle Research Division at Southwest Research Institute (SwRI) has been an instrumental force in helping vehicle manufacturers improve fuel economy and reduce polluting emissions.
Much of the initial improvement in fuel consumption resulted from enhancing engine efficiency, reducing vehicle weight, and making vehicle designs more aerodynamic. In general, the technical risks and engineering costs that led to these improvements were relatively low.
To achieve additional fuel economy improvements, automotive manufacturers have begun to focus on increasing efficiency in areas where improvements are much more difficult and costly to achieve - largely on powertrain components such as the transmission. Transmission efficiency presents a particularly elusive target for manufacturers. This stems from the fact that transmissions operate over a range of power conditions, such as low speed-high torque to high speed-low torque, as well as through a variety of gear ratios.
To achieve gains in this area, vehicle manufacturers have challenged the conventional thinking associated with powertrain functions and designs. Conventional powertrain configurations consist of an internal combustion engine operating across a wide range of torque and speed conditions and a transmission that has, by comparison, only a few discrete gear ratios. A typical gasoline engine has a dynamic torque range of 7:1 and a dynamic speed range of 9:1. The engine, by design, must constantly vary its operating condition for the vehicle to obtain the desired speed in response to changing road conditions. This sharply contrasts with automatic and manual transmissions, which have a predetermined number of gear ratios and remain in one gear for comparatively long periods.
The operational philosophy of conventional powertrains makes it difficult to reach maximum engine fuel efficiency because the opportunities for operating at the lowest fuel consumption or best "brake specific fuel consumption" are restricted and generally do not agree with the torque and speed conditions imposed on the engine by the vehicle.
During acceleration, the engine must operate between low and high speeds, typically at constant load. This means it operates between low and high power. However, constant power acceleration would be preferred to maximize engine performance and efficiency.
In contrast, a powertrain configured with a continuously variable transmission (CVT) allows the engine to operate at or near maximum load conditions by varying its operation to meet those continually changing load conditions. Within the CVT, a mechanism provides an infinite number of torque and speed conditions between two fixed ratios, thus supplanting the function the engine provides in current production powertrains. As another benefit, a continuously variable transmission can decouple torque and speed in ways not possible for a step ratio transmission.
Using a CVT-configured powertrain, the engine operates at or near maximum load conditions. This allows the engine to operate at or near its best brake specific fuel consumption rate, which means that the engine is operating at its highest average adiabatic efficiencies. For internal combustion engines this would be 36 percent, while for diesel engines it is 45 percent.
When evaluating whether a continuously variable transmission would be a good candidate for a particular powertrain configuration, the efficiency of a representative CVT is often questioned. It's good to keep in mind, though, that while high efficiency is always desirable, it should not be the critical factor in determining whether a CVT should be used. Rather it is important to remember that the most important feature this type of transmission brings to the powertrain is optimizing the engine's performance and efficiency across the whole spectrum of operating conditions.
The efficiency of a CVT, measured as an isolated component on a dynamometer test stand, is between 85-90 percent, lower than that of an automatic transmission measured under the same conditions. However, when an evaluation is carried out on the complete powertrain system - engine, transmission, and axle - the CVT-configured powertrain demonstrates much lower fuel consumption compared to an automatic- configured powertrain. At a minimum, such a vehicle will provide a fuel economy improvement of 7 percent. Fuel economy gains as high as 12 percent have been achieved over comparable automatic transmissions.
A number of efficient CVT designs are currently available, each with strengths and weaknesses that make them suitable for specific vehicles.
Metal Push-Pull Belt CVTs
The most common continuously variable transmission design is the belt-driven configuration, which consists of metal-banded belts that transmit drive torque. The belt-type transmission locates a metal belt between two pulleys - one on the crankshaft side of the transmission, the other on the driveshaft side. Each pulley's diameter is varied by mechanically squeezing together the sides of the pulley. As the pulley sides open or close, the "trough" in which the belt rides is widened or narrowed, varying the diameter at which the belt rides on the pulley. This diameter then determines the effective drive "ratio" at each end of the transmission.
The feature that best characterizes this type of CVT is that it transmits power under compression through a metal V-belt. The belt consists of segmented, thick stamped steel blocks configured with horizontal cutouts on both sides that contain stacked ribbons of steel, known as bands. These bands contain and shape the segments into an overall belt assembly. The load path depends on a complex interaction of friction and contact forces between the bands, block slots, block-to-block interfaces, and block sidewalls to pulley faces. The amount of power that can be transmitted through the belt is determined by the tensile strength in the bands as the belt is squeezed between the two halves of the sheave.
In the unsupported section of the belt, between the driving and the driven sheaves, there are no gaps between the blocks because of the presence of block compression forces induced at the entrance to the driven sheave. In the unsupported section of belt, between the driven and the driving sheaves, the blocks become unloaded because of the inability of the block elements to transmit tensile force. As the blocks approach the exit of the driving sheave, they continue to back around the arc and begin to develop compressive forces that transmit force back through the belt. This is similar to what happens when a locomotive applies its brakes and the cars behind it begin to press against it from the rear.
This type of transmission is limited by the tensile strength of the steel bands. To date, it has been used in vehicles that produce engine torque below 150 foot-pounds. Some of the vehicles using this CVT are the Subaru Justy, Nissan Primera, Nissan Mira, Ford Fiesta, and the Honda Civic.
The toroidal transmission consists of two sets of planetary type, steerable rollers housed between an inner and outer toroidal-shaped disc, one driving and the other driven. By tilting the steerable rollers, the relative diameters of engagement of the input and output toroidal discs can be varied to achieve a desired speed ratio. Very high contact pressures exist at the point of contact between the steerable rollers and the toroidal discs. Torque is transferred at the point of contact under a high shear stiffness traction fluid placed under extremely high pressures, causing the fluid to become glass-like. In this mode, its behavior is described as elastrohydrodynamic. With the fluid operating in the elastrohydrodynamic region, metal to metal contact between the rollers and toroidal discs is prevented.
A unique feature of the toroidal CVT is that it is able to handle extremely high torques at high efficiencies. Because of this efficient power transfer at the contact point, toroidal transmissions provide an average efficiency of 91.6 percent.
Such transmissions are being used in the manufacture of the Nissan Cedric and Gloria passenger cars equipped with 3.0-liter engines that have the ability to handle high torque values. These transmissions are also excellent candidates for pickup trucks, vans, and sport utility vehicles. In even higher horsepower applications, they are used in Hovercraft and the Harrier "Jump" Jet.
The CVT technologies available for heavy-duty applications include the toroidal transmission described above and the hydromechanical transmission, which consists of a hydraulic pump and motor coupled with a planetary gear set and clutches. These transmissions have their foundation in hydrostatic transmissions, which have a narrow speed range over which high efficiencies can be produced. To overcome this characteristic, hydrostatic transmissions are combined with planetary gear sets.
The planetary allows the speed range to be increased by controlling one member of the planetary to be at zero speed. This allows the engine speed to be driven directly through the planetary. Such a device provides a dual power path through the transmission that is parallel in nature, permitting it to transfer power in any of three modes - purely hydraulic, combined hydraulic and mechanical, or purely mechanical.
At the low end, all power is transmitted hydraulically. At the high end, all power is transmitted mechanically. Between these two extremes, power is transmitted as a mixture of hydraulic and mechanical with the ratio continually favoring mechanical power as it progresses upward through the range.
This type of CVT is used in several current production European agricultural tractors manufactured by Fendt, Claas, and Steyr-Puch, and in Japanese construction equipment produced by Komatsu.
Over the past 10 years, SwRI engineers have been actively involved and working closely with inventors and manufacturers in the development of such continuously variable transmissions as push-pull belt, variable geometry-epicyclic, toroidal, hydrostatic, and hydromechanical. The work has included modeling, analysis, sizing, design, fabrication, and extensive laboratory and dynamometer testing.
Several recent projects include performing a kinematic analysis of a variable geometry CVT for a compact car, designing a half-toroidal CVT for a heavy-duty off-road vehicle, designing and fabricating a hydromechanical CVT for a light-duty on-road vehicle, validating pre-production CVTs, and developing a comprehensive set of CVT test procedures. As continuously variable transmission technology continues to mature, SwRI is actively involved in increasing efficiency by developing improved fluids used in belt and toroidal transmissions.
CVTs are an important component that can easily be integrated into the drivetrain to achieve significant fuel economy improvements. As mentioned earlier, such improvements will be far more difficult to achieve than were previous-generation improvements in fuel consumption. But the final laps of any critical race are always the most arduous and hard-won. It is also the only way to make it to the finish line.
Comments about this article? Contact Kluger at (210) 522-3095 or firstname.lastname@example.org.
Published in the Summer 2000 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Stothoff.