I think when the engine, in some circumstances, can bypass all the EV stuff and directly drive the transmission (parallel ev system) its more efficient (maybe t

I think when the engine, in some circumstances, can bypass all the EV stuff and directly drive the transmission (parallel ev system) its more efficient (maybe that happens under heavy load, like highway driving, or accelerating). It uses a mechanism I think is called something like a power split device. The serial ev system, which is basically a tesla with a portable gas generator in the trunk, is less efficient because the engine power always goes through the electric motor, rather than sometimes directly driving the transmission.