  A completely nonintegrable $2$-dimensional distribution on a $4$-manifold is
called an Engel structure. A $4$-manifold with an Engel structure is called an
Engel manifold. The developing map for an Engel manifold is very important tool
to determine the Engel structure. Montgomery used it to prove that an Engel
automorphism is determined by the values on a global slice. Moreover,
Montgomery constructed Engel manifolds whose automorphism group is small. In
this paper, we prove that the automorphism group of an Engel manifold is
embedded into the automorphism group of the Cartan prolongation of a contact
$3$-orbifold, if the developing map is not a covering map. As an application,
we will construct an Engel manifold whose automorphism group is trivial.
