  A strained graphene monolayer is shown to operate as a highly efficient
quantum heat engine delivering maximum power. The efficiency and power of the
proposed device exceeds that of recent proposals. The reason for these
excellent characteristics is that strain enables complete valley separation in
transmittance through the device, implying that increasing strain leads to very
high Seebeck coefficient as well as lower conductance. In addition, since
time-reversal symmetry is unbroken in our system, the proposed strained
graphene quantum heat engine can also act as a high performance refrigerator.
