  Inflation in the early Universe can be followed by a brief period of
preheating, resulting in rapid and non-equilibrium particle production through
the dynamics of parametric resonance. However, the parametric resonance effect
is very sensitive to the linearity of the reheating sector. Additional
self-interactions in the reheating sector, such as non-canonical kinetic terms
like the DBI Lagrangian, may enhance or frustrate the parametric resonance
effect of preheating. In the case of a DBI reheating sector, preheating is
described by parametric resonance of a damped relativistic harmonic oscillator.
In this paper, we illustrate how the non-linear terms in the relativistic
oscillator shut down the parametric resonance effect. This limits the
effectiveness of preheating when there are non-linear self-interactions.
