Since it's an "ensemble" (= assembly), you are looking at at least 2 things, and I feel this should be written "l'ensemble rampe/durit retour gazole" or "l'ensemble (rampe et durit retour gazole)".

Hard to know without context what this "rampe" is. Depending on its position in the system (upline or downline), I imagine it could be either a nozzle or a manifold. However, given that it is on the return line, I imagine it is a manifold leading from each cylinder (or a point near thereto) to the common return line, which presumably takes excess diesel back to the tank.