[ Eq(Derivative(x1, t), ((-1)*k13*k5*x1 + (-1)*k13*k11*k10*x3*x1 + 1*k13*k11*x2*x3*x1)/k13),
  Eq(Derivative(x2, t), ((-1)*k13*k2*x2 + (-1)*k13*Piecewise((1, Lt(t, 1)), (Piecewise((1, Gt(t, 15)), (21/5000, True)), True))*k8*x2*x3 + 1*k13*k1)/k13),
  Eq(Derivative(x3, t), ((-1)*k13*k3*x3 + 1*k13*Piecewise((1, Lt(t, 1)), (Piecewise((1, Gt(t, 15)), (21/5000, True)), True))*k8*x2*x3 + (-1)*k13*k9*x3*x4)/k13),
  Eq(Derivative(x4, t), ((-1)*k13*k7*x4 + 1*k13*k11*k10*x3*x1)/k13) ]