  We theoretically show how structural modifications and controlling quantum
coherency can enhance linear and nonlinear thermoelectric performance in
graphene nanostructure heat engines. Although graphene has emerged as a
promising material for a nanoscale heat engine due to its high coherency and
tunable electronic properties, its large lattice thermal transport often limits
its thermal efficiency. Using the density-functional tight-binding method, we
demonstrate that one can suppress lattice thermal transport degrading the
thermal efficiency by deliberately manipulating the junction's bending angle at
low temperatures. We further argue that applying an optimal local gate voltage
unleashes its great potential in achieving excellent efficiency and reasonably
high output power that persist in the fully nonlinear regime.
