  We report our results on non-periodic experimental time series of pressure in
a spark ignition engine. The experiments were performed for a low rotational
velocity of a crankshaft and a relatively large spark advance angle. We show
that the combustion process has many chaotic features. Surprisingly, the
reconstructed attractor has a characteristic butterfly shape similar to a
chaotic attractor of Lorentz type. The suitable recurrence plot shows that the
dynamics of the combustion is a nonlinear multidimensional process mediated by
stochastic noise.
