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Common rail Euro-III engine controls fuel pressure with one control valve which controls inlet or outlet of fuel line. Inlet control type has disadvantage of activating fast increase of fuel pressure when initial starting or sudden acceleration because the procedure to get high fuel pressure takes many steps like low pressure pump ? control valve ? high pressure pump ? common rail. However, outlet control type has advantage of quick increase of fuel pressure but there are weak point such as loss of engine power and fuel temperatures up. Therefore, CRDi Euro-IV has been adopted "Dual Pressure Control type" so that precise and quick increase of fuel pressure can be easily established with various engine condition. Dual Pressure Control" type means that CRDi Euro-IV engine has the inlet pressure from high pressure pump and outlet pressure of common rail simultaneously.

Fuel pressure gets high if fuel pressure control valve(P-PRV, Pump Pressure Regulating Valve) installed between low pressure pump and high pressure pump(inlet side) closes the fuel line with increased duty values controlled by ECM. With rail pressure control valve(PRV, Pressure Regulator Valve), the higher duty values from ECM the higher rail pressure is set because it closes outlet of common rail.

ECM controls PRV installed on common rail with operating current by duty control after sensing RPS signal installed on common rail, engine RPM and Accelerator pedal sensor etc. corresponding to current engine RPM and load condition in order to get optimum rail pressure control. PRV installed on common rail controls fuel return amount feeded at rail pressure in order to not only get more rapid and accurate rail pressure at initial starting that needs quick rail pressure increase but, release rail pressure as quickly as possible at sudden deceleration. The more current increase to PRV, the smaller fuel return amount feeded at common rail that make increase common rail pressure and vise verse. PRV has been adopted on S engine so that cold starting can be enhanced.

With solenoid injector system, fuel pressure is maintained by closing fuel passage of PRV with inner spring force of PRV right after engine off but fuel pressure is decreased due to mechanical leakage of solenoid injector as time goes by. However, regarding PIEZO type injector system which has not any mechanical leakage, if the same PRV installed on the solenoid type injector is used for PIEZO type injector system, Bubble is generated inside of common rail because the volume of fuel in the common rail shut is decreased due to density increase when cold. This bubble makes engine hard to start result from difficulty to maintain the initial rail pressure. With this reason, PIEZO PRV equipped on S engine enhances cold starting to prevent bubbles from generation in common rail as opening its fuel passage.

P0091 is set when "0"A in control circuit of PRV (integrated with common rail) is detected for more than specified duration. This code is due to open or short to ground in control circuit or PRV internal open.