8-speed power train of automatic transmission

PURPOSE: An 8-speed power train of an automatic transmission improves fuel efficiency by forming a gear shift stage with forward 8-speed and backward 1-speed. CONSTITUTION: A first line gear (S1) is selectively connected to a second line gear through a first clutch (CL1). A first ring gear (R1) is always connected to a transmission case. A second ring gear is selectively connected to the first line gear through a second clutch. A second planetary carrier (PC2) is selectively connected to an input shaft through a third clutch. A third line gear is directly connected to the second planetary carrier, and a third ring gear is directly connected to the second line gear. A third planetary carrier (PC3) is selectively connected to the input shaft through a fourth clutch. A fourth line gear is directly connected to the second line gear and the third ring gear. A fourth ring gear (R4) is directly connected to the third planetary carrier.