def phi_from_far(far, c_x, h_y):
    """
    *Return phi coefficient with the fuel air ratior coeff + fuel composition.*

    :param far: the air-fuel ratio
    :type far: float
    :param c_x: stoechio coeff of Carbone
    :type c_x: float
    :param h_y: stoechio coeff of hydrogene
    :type h_y: float
    """
    mass_molar = {'C': 0.0120107, 'H': 0.00100797, 'O2': 0.0319988, 'N2': 
        0.0280134}
    mass_mol_fuel = c_x * mass_molar['C'] + h_y * mass_molar['H']
    coeff_o2 = c_x + h_y / 4
    y_o2_fuel_sto = coeff_o2 * mass_molar['O2'] / mass_mol_fuel
    phi = far / y_o2_fuel_sto
    return phi
