Abstract
Keywords - shift schedule, automatic transmission, adaptive schedule, fuel economy, vehicle performance
Automatic transmissions are used in more than 85 percent of the automotive applications in North America. In most of these transmissions, a single shift schedule map is used to determine points when the transmission is shifted from one gear to another. The shift schedule is predetermined and is optimised for fuel economy/performance for a certain vehicle load and with acceptable driveability. These shift points are a function of throttle position and vehicle speed. The shift points do not take into consideration the load on the vehicle, be it extra passengers in the vehicle or extra load in cargo or towing a trailer or driving up a grade. In most cases, the shift points are not adapted based on the load on the vehicle. Some automakers do provide a tow/haul switch, which provides a different shift schedule. Again, this towing schedule is pre-programmed and hence is not a function of the actual load on the vehicle. Some automakers inhibit certain shifts when the engine power is deemed not sufficient in the new gear. In this paper, an adaptive shift schedule system is described in which the deviation of current transmission output acceleration (which is a direct reflection of the actual load on the vehicle) from a nominal value is used to obtain a new shift point.
The load on a vehicle can be indirectly estimated without computing the vehicle mass and/or the gradient of the road. The current transmission output (or vehicle) acceleration can be compared with the acceleration of a nominal vehicle to estimate the load on the vehicle. The nominal value of the acceleration is calculated in real time from a mathematical model of the nominal vehicle. The nominal could be defined as a vehicle with two passengers on a flat road. The deviation of the current acceleration from the nominal is used to compute a modification factor for the shift schedule. The adaptation method can be redesigned for fuel economy gains by suitably defining the nominal vehicle and appropriately designing the shift maps used in this method. Vehicle test results are presented to show that shift points can be adapted to obtain enhanced performance from the powertrain under certain loading conditions.