Getting the auto summary from a node body
node_view() is appropriate when you need to render whole node or many of it's fields.
But sometimes you have to render the only field you need. It can be done with field_view_field() or field_view_value():
$node = node_load($nid);
// Render array of body field.
$build_body = field_view_field('node', $node, 'body', 'teaser');
// Render it in common way.
echo render($build_body);

The interesting thing is that you can pass your own field formatter settings instead of view mode with the fourth parameter.
Please, read API docs for more info.