  Unlike classical systems, a measurement performed on a quantum system always
alters its state. In this work, the impacts of two diagnostic schemes to
determine the performance of quantum Otto heat engines are compared: In one
scheme, the energy of the engine's working substance is measured after each
stroke (repeated measurements), and in the other one, the energies after each
stroke are recorded in one or two pointer states and measured only after the
completion of a prescribed number of cycles (repeated contacts). A single
pointer state suffices if one is only interested in either work or heat. For
joint work and heat diagnostics, two pointers are needed. These schemes are
applied to Otto engines, whose working substance consists of a two-level
system. Depending on the engine protocol, the duration of a single cycle may be
infinite or finite. Because in the repeated contact scheme, the number of
measurements is drastically reduced compared to the repeated measurement
scheme, the quantum coherence after and during the contact diagnostics is much
better maintained than repeated measurements that destroy any coherence at the
end of each stroke. We demonstrate that maximum power, reliability, and
efficiency of the engine in the presence of repeated contacts typically
outperform these figures of merit of repeated measurements. Due to the improved
coherence persistence, heat engines with a finite cycle duration require a
larger number of cycles to reach a periodically asymptotic state. Overall, our
results document the importance of taking into account the particular nature of
diagnostic tools for monitoring and testing purposes but also for feedback
control, both in theory and experiment.
