Motor-vehicle hybrid propulsion system including a double-clutch transmission with gear shift device comprising a rotary drum

The hybrid propulsion system comprises an internal combustion engine, a double-clutch transmission with a mechanical gearbox (10) having at least five forward gears (I, II, III, IV, V, VI) and one reverse gear (R), and an electric machine (80). The gearbox (10) comprises a first primary shaft (12) carrying driving gearwheels (20, 22, 24) associated to the odd gears (I, III, V), the highest gear (VI) and the reverse gear (R), a second primary shaft (14) coaxial with the first primary shaft (12) and carrying driving gearwheels (28, 30) associated to the second gear (II) and the fourth gear (IV), and at least one secondary shaft (16, 18) carrying a plurality of idle driven gearwheels (36, 38, 40, 42, 50, 52) directly or indirectly meshing with the driving gearwheels (20, 22, 24, 28, 30) carried by the two primary shafts (12, 14). The gearbox (10) is also provided with a gear shift device comprising a plurality of sliding engagement sleeves (44, 46, 56, 58), each arranged to connect each time a driven gearwheel (36, 38, 40, 42, 50, 52) for rotation with the respective secondary shaft (16, 18), a corresponding plurality of sliding shift forks (66, 68, 70, 72), each arranged to cause a respective engagement sleeve (44, 46, 56, 58) to slide between a neutral position and at least one shift position, a rotary drum (60) having on its outer cylindrical surface a corresponding plurality of guide grooves (61, 62, 63, 64) in each of which a respective stud (65, 67, 69, 71) slidably engages, which stud is rigidly connected for translation with a respective shift fork (66, 68, 70, 72) in the sliding direction of this latter, and an actuation unit arranged to cause the drum (60) to rotate stepwise among a plurality of angular positions (R, R/II, N, I/II, II/III, III/IV, IV/V, IV/VI, VI) each corresponding to predetermined positions of the engagement sleeves (44, 46, 56, 58). The electric machine (80) is permanently cinematically connected to the second primary shaft (14). The guide grooves (61, 62, 63, 64) of the drum (60) are shaped in such a manner that in a first angular position (IV/VI) of the drum (60) the engagement sleeves (44, 46, 56, 58) are positioned so as to engage at the same time the highest gear (VI) and the fourth gear (IV).