Quantum heat engines and refrigerators are useful probes to study thermodynamic features of quantum systems . In this talk, we consider coupled quantum systems (spins or oscillators) as the working media of quantum thermodynamic machines [2,3]. I will discuss bounds of efficiency in these models. We also show the impact of quantum correlations in optimal work extraction . Further, we compare the performances of coupled spin-1/2 systems and coupled quantum oscillators with similar interactions. I will also discuss friction in quantum heat engines .