\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  \aligned 
    \dot \tau &= -a \tau + u_e \\
    y_e &= K \tau
  \endaligned
  \label{engine.eq}
\end{equation*}
\end{document}