Technically it is not proper to say that the force generated by the engine accelerates the car because the engine does not generate a force along the acceleration. Even if it did so internal reaction force would have balanced it.
Since the movement of the piston of the engine makes a rotational motion in the wheels which in turn is used to push back the road and due to frictional force (External force) by the road, the car accelerates, hence in common language, such statement is made.