unset($portfolio_args['ai1ec_events']);

function custom_conference_in_home_loop( $query ) {
 if ( is_home() && $query->is_main_query() )
    $query->set( 'post_type', 'ai1ec_event' );
    $query->set( 'orderby', 'start' );
        return $query;
 }

 add_filter( 'pre_get_posts', 'custom_conference_in_home_loop' );