The CVT offers high fuel economy, presumably because it ensures a low BSFC driving condition with its continuously variable ratio characteristics. Focusing solely on BSFC values, i.e., indices that represent the engine's thermal efficiency, is not enough, as substantial work loss occurs in the drivetrain, including the transmission and engine accessories, from the engine to wheels. Therefore, attention needs to be paid to the efficiency of the entire powerplant, including engine and transmission, rather than just focusing on engine thermal efficiency, as has been conventionally done. Engine and CVT controls should be integrated as in addition to the stoichiometric operation, an engine may be operated in modes of lean burn and exhaust-gas recirculation.
This paper describes a newly developed algorithm for the calculation of the combinations between the engine torque and CVT ratio in order to achieve the highest overall efficiency for the engine and transmission system. An integrated engine-CVT control system based on the algorithm and its performance in vehicle tests are also reported.