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The demand for the fuel metering unit

Author:Site Editor     Publish Time: 2021-09-06      Origin:Site

The fuel metering unit, or fuel quantity control valve, is kind of a solenoid valve. In Delphi systems, the fuel metering unit is called IMV valves; in Denso systems, SCV or PCV valves; in Bosch systems, is known by the acronyms ZME, MEUN.


The drive software of the fuel metering unit can flexibly regulate the frequency and duty cycle of the control signal, which helps to control the opening angle of the valve in order to adjust the rail pressure in different conditions.


The fuel from the tank must pass through the fuel metering unit before entering the high-pressure pump. The quality of the metering unit directly affects the amount of fuel quantity that enters the high-pressure pump, which in turn affects the acceleration performance and the fuel economy of the vehicle.


Common Rail Pressure Limiter Valve


Bosch's first generation of the high-pressure common rail system has the pressure relief valve installed on the common rail. The valve, directly controlled by the ECU computer, is used to control the feeding and the returning of the fuel, which indirectly controls the fuel flow from the piston pump to the rail. This process helps to regulate the pressure of the common rail system. The valve combined with the rail pressure sensor forms the closed loop to control the rail pressure. Yet, the system has been eliminated.


Rail Functions and Composition


  • The function of the rail is to distribute fuel pressurized by the supply pump to each cylinder injector.


  • The shape of the rail depends on the model and the component parts vary accordingly.


  • The component parts are the rail pressure sensor (Pc sensor), pressure limiter, and for some models a flow damper and pressure discharge valve.



Rail Functions and Composition


The pressure limiter opens to release the pressure if abnormally high pressure is generated. If pressure within the rail becomes abnormally high, the pressure limiter operates (opens). It resumes operation (closes) after the pressure falls to a certain level. Fuel released by the pressure limiter returns to the fuel tank. 


Rail Pressure


The operating pressures for the pressure limiter depend on the vehicle model and are approximately 140-230MPa for the valve opening pressure, and approximately 30-50MPa for the valve closing pressure.


The operating pressures

Inlet Metering Valve & Suction Control Valve


The fuel metering unit, also called the fuel proportional valve, is an important part of the common rail system. The fuel proportional valve, installed in the inlet of the high-pressure fuel pump and controlled by the ECU, is used to adjust the fuel supply and fuel pressure. In other words, the fuel proportional valve is a throttle valve or a tap that can control the fuel supply. 


The fuel metering unit

Bosch's second-generation common rail system has a "Y" shaped high-pressure triplex position throttle pump.  The opening angle of the pump is adjusted by controlling the opening angle of the fuel metering unit. The normally closed valve is utilized in this system. This valve is closed when no electric current passes through it, hence no fuel flow. When the electric current passes through it, the valve opens and allows the fuel to enter the pump; the opening angle of the valve becomes larger as the electric current passing through increases. 


The second generation of common rail engine is characterised by a multiple injection system that allows more injections per cycle. The secret of the multiple injection system lies in the design of the control unit and injector nozzle, which can produce a series of injections at very short intervals.


Bosch's third-generation common rail system


In order to save energy and reduce emissions, great progress has been made in electronically controlled fuel injection technology for diesel engines. The application of common rail injection technology means a great leap in the development of the diesel engine. In addition to reducing lubricant consumption, optimizing the boost system and the exhaust after treatment system, further improvements to the combustion process are of paramount importance in order to maintain the fuel economy of direct injection diesel engines while meeting the increasingly stringent emission regulation.


Bosch-Diesel Common Rail System (1)

The fourth generation of high pressure common rail injection systems with coaxial variable nozzle and pressure expander technology will have a maximum injection pressure of over 200MPa.


Denso - Suction Control Valve (SCV)


In contrast to the HP2, the SCV for the HP3 supply pump is equipped with a linear solenoid valve. The fuel flow volume supplied to the high-pressure plunger is controlled by adjusting the engine ECU supplies power to the SCV (duty ratio control). 


When current flows to the SCV, the internal armature moves according to the duty ratio. The armature moves the needle valve, controlling the fuel flow volume according to the amount that the valve body fuel path is blocked. Control is performed so that the supply pump suctions only the necessary fuel quantity to achieve the target rail pressure. As a result, the supply pump actuation load is reduced. 


There are two types of HP3 SCV: the normally open type (the suction valve opens when not energized) and the normally closed type (the suction valve is closed when not energized). The operation of each type is the reverse of that of the other. 


In recent years, a compact SCV has been developed. Compared to the conventional SCV, the position of the return spring and needle valve in the compact SCV are reversed. For this reason, the operation is also reversed. 


Normally Open Type 


  • When the solenoid is not energized, the return spring pushes against the needle valve, completely opening the fuel passage and supplying fuel to the plungers. (Total quantity suctioned → Total quantity discharged)


  • When the solenoid is energized, the armature pushes the needle valve, which compresses the return spring andcloses the fuel passage. In contrast, the needle valve in the compact SCV is pulled upon, which compresses the return spring and closes the fuel passage.


  • The solenoid ON/OFF is actuated by duty ratio control. Fuel is supplied in an amount corresponding to the opensurface area of the passage, which depends on the duty ratio, and then is discharged by the plungers.


SCV1

SCV2


Duty Ratio Control


The engine ECU outputs sawtooth wave signals with a constant frequency. The value of the current is the effective (average) value of these signals. As the effective value increases, the valve opening decreases, and as the effective value decreases, the valve opening increases.


LOW


When the SCV Energized Duration (Duty ON Time) is Short

 

When the SCV energization time is short, the average current flowing through the solenoid is small. As a result, the needle valve is returned by spring force, creating a large valve opening. Subsequently, the fuel suction quantity increases


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With the continuous progress of the technology of the electronically controlled common rail fuel injection system, the diesel engine fuel injection system will develop in the direction of flexible control of the pressure, quantity, time and rate of the injection.


The research of the high-pressure common rail injection system for the diesel engine should focus on the development of parts manufacturing technology


  1. Increase the common rail pressure is crucial to meet exhaust emission standards.


  2. Smaller spray hole diameters, shorter response times and lower power consumption for increased reliability and longevity of critical components.


  3. Develop sensors with higher accuracy and quicker response.


  4. Develop the parts manufacturing technology. The increasing injection pressure has higher requirements for the solenoid valve and the fuel injector, that is to say, the solenoid valve should be quicker in response, be more accurate in controlling the fuel flow, be more reliable in repetitive work and have a longer service life.


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