  Collective behavior where a set of elements interact and generate effects
that are beyond the reach of the individual non interacting elements, are
always of great interest in physics. Quantum collective effects that have no
classical analogue are even more intriguing. In this work we show how to
construct collective quantum heat machines and explore their performance boosts
with respect to regular machines. Without interactions between the machines the
individual units operate in a stochastic, non-quantum manner. The construction
of the collective machine becomes possible by introducing two simple quantum
operations: coherence extraction and coherence injection. Together these
operations can harvest coherence from one engine and use it to boost the
performance of a slightly different engine. For weakly driven engines we show
that the collective work output scales quadratically with the number of engines
rather than linearly. Eventually, the boost saturates and the scaling becomes
linear. Nevertheless, even in saturation, work is still significantly boosted
compared to individual operation. To study the reversibility of the collective
machine we introduce the 'entropy pollution' measure. It is shown that there is
a regime where the collective machine is N times more reversible while
producing N times more work, compared to the individual operation of N units.
Moreover, the collective machine can even be more reversible than the most
reversible unit in the collective. This high level of reversibility becomes
possible due to a special symbiotic mechanism between engine pairs.
