def yk_from_phi(phi, c_x, h_y):
    """ 
    *Return the mass fraction of elements from a fuel aspect ratio and stoechio element coeff.*

    :param phi: the air-fuel aspect ratio
    :type phi: float
    :param c_x: stoechio coeff of Carbone
    :type c_x: float
    :param h_y: stoechio coeff of hydrogene
    :type h_y: float

    """
    y_k = dict()
    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
    if phi == 0.0:
        y_k['fuel'] = 0.0
    else:
        y_k['fuel'] = 1.0 / (1.0 + (1.0 + 3.76 * (mass_molar['N2'] /
            mass_molar['O2'])) * (y_o2_fuel_sto / phi))
    y_air = 1 - y_k['fuel']
    y_k['N2'] = y_air / 1.303794
    y_k['O2'] = 0.303794 * y_k['N2']
    return y_k
