  In this paper we analyze the cycle-to-cycle variations of maximum pressure
$p_{max}$ and peak pressure angle $\alpha_{pmax}$ in a four-cylinder spark
ignition engine. We examine the experimental time series of $p_{max}$ and
$\alpha_{pmax}$ for three different spark advance angles. Using standard
statistical techniques such as return maps and histograms we show that
depending on the spark advance angle, there are significant differences in the
fluctuations of $p_{max}$ and
  $\alpha_{pmax}$. We also calculate the multiscale entropy of the various time
series to estimate the effect of randomness in these fluctuations. Finally, we
explain how the information on both $p_{max}$ and $\alpha_{pmax}$ can be used
to develop optimal strategies for controlling the combustion process and
improving engine performance.
