A bi-fuelled (CNG and petrol) vehicle is equipped with a bypass with ball valve. Its manual operation circumvents the internal check valve in front of the pressure vessel (natural gas tank). This subsequent conversion can be effected without problems by any vehicle converter. The CNG tank of the service vehicle will be refuelled at the CNG fuelling station. The service vehicle running for this purpose on petrol, drives to the broken down mono-fuelled vehicle. The broken down vehicle can be refuelled locally using the WEH^® TZ21 CNG emergency fuelling set.

The TK6 CNG service nozzle is placed onto the receptacle of the service vehicle. The TK4 CNG fuelling nozzle is connected to the broken down vehicle. Both vehicles are connected with a hose to each other. By rotating the opening spindle at the TK6 CNG service nozzle the check valve of the WEH^® TN1 CNG receptacle opens. The ball valve at the bypass of the service vehicle is opened slowly. At the TK4 CNG fuelling nozzle the natural gas is lead into the empty tank. After refuelling the ball valve at the bypass of the service vehicle is closed. The opening spindle of the TK6 CNG service nozzle is rotated back and the check valve of the WEH^® TN1 CNG receptacle is closed. Then the integrated venting valve at the TK6 CNG vents the TZ21 CNG emergency fuelling set. The TK6 CNG service nozzle and the TK4 CNG fuelling nozzle can be removed. Now the monofuelled CNG vehicle can drive away with a refuelled CNG tank.