Criteria for Coasting on Highways for Passenger Cars 2014-01-1157

This study quantifies fuel savings and conditions for the application of coasting phases, i.e. the vehicle rolling without traction force in an automated driving strategy, herein named economic cruise control ECC. Under the presumption of a driver input lead velocity and a limit of acceptable delta speed deviation, fuel savings can achieve values of 5 % to just above 10 % on highways traveling with a conventional, non-hybridized powertrain, assuming that the ICE is stopped upon coasting. Lower mean speed driving yields relatively higher savings. Reference is constant speed driving at the identical mean velocity, such that an ECC function may obtain somewhat higher savings in real traffic environment. For a first version of driving and powertrain control strategy it could be shown, that the fuel economy of a hybridized powertrain can benefit from coasting, too.

For the initiation of the coasting phase, a criterion has been derived by defining a ‘coasting preview length’ CPL function, which can be analytically calculated at each point of the road, if its altitude profile is known. Since downhill coasting seems to be the naturally acceptable case, the CPL function shows a rather sharp increase when approaching the hill summit. The point of inflection of the CPL function was identified as criterion for the earliest reasonable point to start coasting.