Designing for fuel economy without sacrificing performance has become a greater and greater challenge for automotive design engineers. While little can be done about driver-related issues as fuel-efficient acceleration and other driving techniques (or the use of air conditioning and under-inflated tires that increase drag and fuel consumption), there are a number of things automobile designers can do. These run the gamut from reducing vehicle weight through use of materials such as aluminum and carbon fiber, to optimizing engine combustion strategies with such technologies as variable-valve timing and HCCI (Homogeneous Charge-Compressed Ignition) combustion.
Fuel efficiency can also be improved through use of precisely controlled automatic transmissions that have a greater number of gears than the three or four that traditionally have been available. The wider span between the highest and lowest gear ratios enables better use of engine power, smoother operation, and improved fuel economy.
For example, a 6-speed transmission jointly developed by Ford and General Motors enables up to 8% improved performance, and up to 4% better fuel economy when compared with a 4-speed automatic. A 6-speed transmission from Aisin AW has been shown to improve fuel efficiency by 9% over a 5-speed transmission, and a 6-speed transmission from zF Friedrichshafen reduces fuel consumption by as much as 7%, with up to 5% better acceleration, compared to a 5-speed transmission.
A variety of wide-gear-ratio transmissions has been developed. However, along with more gears, additional, more-complex electronics are required to control the transmission and ensure smooth operation. This has led to greater cost and complexity that has limited their use to high-end luxury-car models.
Now, developments in control electronics offer designers the flexibility to engineer 6-, 7- and 8-speed transmissions that can cost-effectively be used in mass market platforms. Among these developments is the emergence of highly integrated, flexible ICs (integrated circuits) that provide the precise control needed to activate the Variable Force Solenoids (VFS) used in the new multi-ratio automatic transmissions.
Automatic transmission operation
Automatic transmissions primarily use hydraulics to select gears, depending on pressure exerted by fluid within the transmission assembly. Rather than using a clutch to engage the transmission, a torque converter is placed between the engine and transmission. Today's transmissions use electronically controlled actuators, or solenoids, to manage the fluid pressure to shift gears and manipulate the clutch.
Control of the shift solenoids that are activated to change gears ranges from very simplistic in older transmission designs to extremely complex controls in very new designs. The manner and time of shift-solenoid activation (shift profile) are chosen by the transmission manufacturer, and can vary greatly from vehicle to vehicle.
The exacting control of engaging and disengaging the clutches in today's transmissions is dependent on precisely controlling the transmission fluid pressure applied to these clutches. The better the control of the transmission fluid pressure, the better the shift quality, and the better the fuel efficiency and drivability, as well as greater reliability and lifetime of the transmission components. Older 3- and 4-speed designs typically use a single VFS to modify pressure across the entire transmission, while newer wide-gear-ratio designs use many solenoids. A 6-speed transmission, for example, has as many as seven. The figure below illustrates the projected increase in worldwide demand for variable-force solenoids in different transmission types through 2011 (the data was compiled from a variety of sources and evaluated by Infineon).