Unknown column when using WHERE (ERROR) codeigniter

I am trying to show a value of the Database in an input, however I get the following error. Why is this happening?

A Database Error Occurred

Error Number: 1054

Unknown column '17811942-4' in 'where clause'

SELECT 'horario'.'hrs_ini' FROM 'horario' JOIN 'usuarios' ON 'horario'.'rut_usu' = 'usuarios'.'rut_usu' WHERE 'usuarios'.'rut_usu' = '17811942-4' AND 'horario'.'lunes' = 'ATTE'. 'ESTUDIANTES'

Filename: C:/xampp/htdocs/SAE/system/database/DB_driver.php

Line Number: 691

Query in the database

Model

public function lunes(){


$this->db->select('horario.hrs_ini');
$this->db->from('horario');
$this->db->join('usuarios','horario.rut_usu = usuarios.rut_usu');
$this->db->where('usuarios.rut_usu=17811942-4');
$this->db->where('horario.lunes=ATTE. ESTUDIANTES');
$start_lunes = $this->db->get();


if($start_lunes->num_rows() > 0 ){

    return $start_lunes->result();

    }

}

Controller

public function index(){


        $this->load->view('layouts/header.php');
        $this->load->view('layouts/menu.php');


        $this->load->model('mCalendar');
        $data['start_lunes'] = $this->mCalendar->lunes();
        $data['usuarios'] = $this->mCalendar->get_usuarios();
        $data2['motivos'] = $this->mCalendar->get_motivos();
        $this->load->view('usuarios/vConsulta_Horarios.php',$data,$data2);
        $this->load->view('layouts/footer.php');




}