What is a Graph Query Engine
A query engine takes input queries written in a specific query language (e.g. SQL statements), performs the necessary operations in the database, and then outputs the data of interest to the user application. You may also view a query engine as an abstraction layer such that the user can design queries simply in the supported query language and let the query engine do the rest.
In the case of a graph query engine, the output data is a graph (wiki).