In order to reduce the fuel consumption of vehicle, it is
required, as a supplemental effort, to adapt the specification of
powertrain to the engine characteristics. For the manual
transmission, it has been well known that the specification of
powertrain should be chosen so as to operate the engine under its
condition of minimum brake specific fuel consumption for required
power. For the automatic transmission with torque converter,
however, the criteria on the selection of specification of
powertrain to reduce the fuel consumption have not yet been
clarified.
In this paper, such criteria are analytically examined by
considering the torque converter characteristics. It is shown that
the optimum condition of engine operation for automatic
transmission is different from that for manual transmission,
particularly at lower engine speeds. In practice, the total gear
ratio of each step of auxiliary gear including the final gear and
the gear shift points of automatic transmission should be carefully
chosen by the obtained criteria.
By using the computer simulation based on the above analytical
consideration, effects of optimum specification of automatic
transmission to engine on the fuel consumption are investigated. As
an example of the results, the fuel consumption of refined vehicle
on the EPA City Driving Cycle is improved by 13% in case of optimum
selection of total gear ratio, compared with the vehicle with
current automatic transmission.