Fuel system leak detection

A vehicle fuel system with on-board diagnostics for leak testing comprising:

• a) fuel tank for containing fuel for delivery to an internal combustion engine;
• b) a purge canister connected to the space in the tank above the fuel;
• c) a canister vent valve (CVV) for connecting the purge canister to the atmosphere;
• d) a purge valve for connecting the purge coanister to the engine;
• and
• e) an electronic control unit (ECU) arranged for monitoring pressure and fuel level in the tank and other engine, vehicle and ambient conditions and for controlling opening and closing of the valves;
• f) the CVV and the purge valve being controlled by the ECU for venting the tank to atmosphere via the purge canister (purge valve closed, CVV open), and for purging vapor from the canister by allowing air to be drawn through the canister by manifold vacuum (both valves open);
• g) the ECU being arranged to carry out a periodic two stage leak test, when the engine is running;
• h) one stage of the leak test comprising:
• i) evacuation of the tank with purge valve open and the CVV closed;
• ii) monitoring pressure rise in the tank with both valves closed;
• and
• iii) recording the pressure rise dP_A over a predetermined period A following increase of pressure to a predetermined value p2;
• i) the other state of the leak test comprising:
• i) venting the tank to atmospheric pressure via the CVV then sealing the tank by closing the CVV;
• and
• ii) measuring the amount dP_B by which the pressure in the tank rises above atmospheric due to vapour generation over a period (period_B) following closure of the CVV;
• and
• h) the ECU being arranged to calculate a value X representative of leakage from the difference between dP_A and dP_B using a scaling factor K wherein K represents a stored value in a two dimensional map indexed by measured values of fuel level and ambient temperature.