Improvements To Automatic Transmissions

The transmission in a vehicle matches the power requirements of the automobile to the power output available from an engine or motor; the automatic transmission’s selection of different gears keeps the engine operating in speed ranges that allow high levels of efficiency to be achieved. Most modem transmissions operate at efficiencies of over 85 percent on the city cycle and 92 to 94 percent on the highway cycle. The efficiency losses that do occur are caused primarily by:

· Hydraulic losses in the torque converter (current automatic transmissions use a hydraulic system to transmit the engine power to the drivetrain).

· Designs that avoid the operating point that would maximize fuel economy. If fuel economy were the only concern, the optimum point would maximize torque and minimize engine speed (rpm), which reduces throttling and fiction losses. Designing the transmission for maximum efficiency leaves little or no reserve power, however, so that even modest changes in road load horsepower may require a downshift-and frequent downshifts are considered undesirable for customer satisfaction. In addition, operating at too low an rpm causes excessive driveline harshness and poor accelerator response.

Improvements to current transmissions can occur in the following areas:

· reduction in flow losses in the torque converter for automatic transmissions;

· increase in the ratio spread between top and first gear;

· increase in the number of gear steps between the available limits (that is, moving to five or more speeds in an automatic transmission), with continuous variable transmissions (CVTs) being the extreme limit; and

· electronic control of transmission shift points and torque converter lockup.

All of these improvements have been adopted, in some form, by automakers, but their penetration of the fleet is incomplete and, in some cases, further technical improvements are possible. For example, Mercedes-Benz and Nissan have recently (1993) introduced a five-speed automatic transmission, while GM introduced a six-speed manual transmission. Product plans reveal that such transmissions are likely to be more widely adopted by 2005. CVTs have been introduced in Europe and Japan, and in the United States in one car model that has been since discontinued.

Torque converter improvements

Redesign of the torque converter to reduce flow losses will yield improved fuel economy. Toyota has introduced a new “Super Flow” converter in its Lexus LS400 vehicle.157 The new converter was computer designed to optimize impeller blade angle and blade shape to reduce loss of oil flow. In addition, new manufacturing techniques were developed for the impeller to increase rigidity. As a result, Toyota claims the converter efficiency is the world’s best, and is 3 percent to 5 percent higher than other torque converters.158 Such an improvement is expected to provide a 0.5 percent benefit in composite fuel economy.

Greater number of gears

Increasing the number of transmission gears can be used to provide a wider ratio spread between first and top gears, or else to increase the number of steps with a constant ratio spread for improved drivability and reduced shift shock. In addition, the wider ratio spread can be utilized to provide higher performance in the first few gears while keeping the ratio of engine speed to car speed in top gear constant, or else to maintain the same performance in the first few gears and to reduce engine speed in top gear. Because the manufacturer is able to select among these tradeoffs, different manufacturers have chosen different strategies in selecting gear ratios; therefore, any fuel economy gain from increasing the number of gears is dependent upon these strategies.

Five-speed automatic transmissions have only recently been commercialized in Japan and Europe. Nissan has provided a comprehensive analysis of the effect of numbers of gears and choice of first gear and top gear ratios on fuel economy.159 They found declining benefits with increasing numbers of gears, with little or no benefit above six gears. With a first gear ratio of 3.0 (similar to that of current automatics) they found no benefits in fuel economy in using overdrive ratios lower than about 0.7. Increasing the first gear ratio to about 4.0, however, provided better standing start performance. The Nissan production five-speed transmission uses a 3.85 first gear ratio and a 0.69 overdrive ratio for a 5.56 ratio spread. At constant performance, Nissan showed fuel economy gains in the 3 percent range. 160 Mercedes, the only other manufacturer to have introduced a five-speed automatic, confirmed that the fuel economy benefit over a four-speed automatic was in the 2 to 3 percent range. Ford estimates that their planned five-speed automatic would provide a 2.5 percent fuel economy benefit at current performance levels, but could have much smaller benefits at other levels.

A 2.5 percent fuel economy benefit appears representative of a five-speed automatic over a four-speed automatic. With either a six-speed or seven-speed transmission, complexity and weight increases appear to offset fuel efficiency benefits.

A continuously variable transmission (CVT) offers an infinite choice of gear ratios between fixed limits, allowing optimization of engine operating conditions to maximize fuel economy. Currently, Subaru is the only manufacturer that has offered a CVT in a small car in the United States. Although there are several designs being tested, the CVT that is in production features two conical pulleys driven by metal belts. The position of the belts on the conical pulleys determines the gear ratio between input and output shafts. Under steady-state conditions, the metal belt system can be less efficient than a conventional system, but the fuel used over a complete driving cycle is decreased because of the optimized speed/load conditions for the engine. Nissan and Ford have developed CVTs using rollers under radial loads that may be more efficient than metal belt designs.

Shift performance of the CVT should be equal to, or somewhat better than, conventional automatic transmissions, with its main benefit the absence of shift shock associated with discrete gear changes. However, a CVT can produce unexpected changes in engine speed--that is engine speed dropping while the vehicle speed is increasing--which may deter consumer acceptance. Moreover, attaining acceptable start-up vehicle performance could require the use of a lockup torque converter or a conventional planetary gear set, or both, which would add to cost and complexity. Nevertheless, developments in the metal belt system coupled with weight reduction of future cars are expected to enhance the availability of the CVT for use in all classes of cars and trucks in the 2005 time frame.

During the early 1980s, CVTs were expected to provide substantial fuel economy benefits over three-speed automatic transmissions. Researchers from Ford161 showed that an Escort with a CVT of 82 percent efficiency would have a fuel economy 14 percent higher than the fuel economy with a three-speed automatic; at a CVT efficiency of 91 percent, the fuel economy benefit was computed to be 27 percent (91 percent was considered to be an upper limit of potential efficiency). Similarly, Gates Corporation installed a CVT in a Plymouth Horizon and found a fuel economy improvement of 15.5 percent over a conventional three-speed automatic with lockup, at almost identical performance levels. 162 Design compromises for drivability, however, as well as improvements to the base (three speed) automatic since the time these papers were published (1982), have resulted in lowered expectations of benefits. A more recent test conducted by the Netherlands Testing Organization on a Plymouth Voyager van with a 3.3 LV6 and a four-speed automatic replaced by a Van Doorne CVT showed fuel economy benefits of 13 percent on the city cycle and 5 percent on the highway cycle for a 9.5 percent improvement (over a four-speed automatic). These figures, however, may be unrepresentative of more average applications as supplier companies usually provide the best possible benefit estimates. The current consensus among auto manufacturers is that the CVT will be 4 to 8 percent more efficient than current fourspeed automatics with lockup. A 6 percent improvement, including the benefit of the electronic control required to maximize CVT benefits, would be consistent with the measured results from the Subaru Justy CVT sold in the United States.

The benefits for the CVT, however, are associated with current engine technology. Reduction of fuel consumption is associated with two effects: reduced friction losses owing to lower engine rpm, and reduced pumping losses owing to operation at higher load. In the future, engines equipped with variable valve timing and direct injection stratified charge engines will have much lower pumping losses than current engines, thus reducing part of the CVT fuel economy reduction potential. Typically, this would reduce the benefits of CVTs to about half the value estimated for current engines, or to approximately 3 percent.

Electronic transmission control (ETC)

ETC systems to control shift schedules and torque converter lockup can replace the hydraulic controls used in most transmissions. Such systems were first introduced in Toyota’s A43DE transmission in 1982. The benefit of the ETC system lies in the potential to maximize fuel economy by tailoring shifts and torque converter lockup to the driving schedule. Domestic auto manufacturers, however, claim that the measured benefits are small, because most modem nonelectronic transmissions have been optimized for the FTP test cycle. In 1994, more than half of all vehicles had ETC. Although several electronically controlled transmissions are available, “paired sample” comparisons are impossible as no example is available of the same car/engine combination with nonelectronic and electronic transmissions. Regression studies across different models of similar weight and performance show a 0.9 percent advantage164 for the electronic transmission. However, it appears there is potential for greater improvement with some loss of smoothness or “feel.”

Estimates by Ross and DeCicco165 have claimed very large benefits for ETC by following an aggressive shift profile, and they estimate fuel economy benefits as great as 9 percent. These benefits have been estimated from simulation models, although detailed documentation of the input assumptions and shift schedule followed is unavailable. Clearly, shifting very early into a high gear (such as by shifting from second gear to fourth gear directly) and operating the engine at very low rpm and high torque can produce significant gains in fuel economy--but at a great cost to drivability and vibration. Operating the engine at very low rpm leads to conditions known as “lugging” that causes a very jerky ride. Current industry trends, however, are to maximize smoothness, so that it is difficult to envision a strategy similar to the one advocated by Ross and DeCicco being introduced without incentives strong enough to override performance and comfort considerations.