What is
Common Rail technology
Common rail technology refers to a fuel supply method that completely separates the generation of injection pressure from the injection process in a closed-loop system composed of a high-pressure fuel pump, a pressure sensor and an ECU. The high-pressure fuel pump delivers high-pressure fuel to the public fuel supply pipe. Precise control of the oil pressure in the common oil supply pipe makes the pressure of the high pressure oil pipe independent of the engine speed, which can greatly reduce the variation of the oil supply pressure of the diesel engine with the engine speed, thus reducing the defects of the traditional diesel engine. The ECU controls the fuel injection volume of the injector, and the fuel injection volume depends on the pressure of the fuel rail (common fuel supply pipe) and the opening time of the solenoid valve.
High pressure common rail technology details
The
Common Rail System separates fuel pressure generation from fuel injection. If the unit pump diesel injection technology is compared to the revolution of diesel technology, the common rail can be called a rebellion because it deviates from the traditional diesel system and approximates in sequential gasoline injection systems. Common rail systems open up new ways to reduce diesel engine emissions and noise.
Europe can be said to be a paradise for diesel cars. In Germany, diesel cars account for 39%. Diesel cars have a history of nearly 70 years, and it can be said that diesel engines have developed by leaps and bounds in the last 10 years. In 1997, Bosch and Mercedes-Benz jointly developed the Common Rail Diesel Injection System (Common Rail System). Today in Europe, many brands of cars are equipped with common-rail diesel engines, such as Peugeot has HDI common-rail diesel engines, Fiat's JTD engines, and Delphi has developed Multec DCR diesel common-rail systems.
The difference between common rail system and diesel injection system
The common rail system is different from the previous diesel injection system driven by the camshaft. The common rail diesel injection system completely separates the generation of injection pressure and the injection process from each other. The injector controlled by solenoid valve replaces the traditional
Mechanical Injector. The fuel pressure in the fuel rail is generated by a radial plunger high-pressure pump. The pressure has nothing to do with the engine speed and can be set freely within a certain range. . The fuel pressure in the common rail is controlled by an electromagnetic pressure regulating valve, which continuously adjusts the pressure according to the working needs of the engine. The pulse signal that the electronic control unit acts on the solenoid valve of the fuel injector controls the fuel injection process. The amount of fuel injected depends on the oil pressure in the fuel rail, the opening time of the solenoid valve, and the fluid flow characteristics of the fuel injector. Fuel injection pressure is an important indicator of diesel engines, because it is related to engine power, fuel consumption, emissions, etc. At present, the common rail diesel injection system has increased the fuel injection pressure to 1800 bar.
Development in recent years
In the past two years, cars equipped with direct-injection diesel engines have developed significantly in Europe, featuring high efficiency, excellent fuel economy, and reduced engine noise. The direct-injection diesel engine uses a pump nozzle system, which is used in the domestically produced 1.9TDI Bora, and the maximum injection pressure can reach 1800 bar. The pump nozzle direct injection system is good, but the fuel pressure cannot be kept constant. As the emission control becomes more stringent, higher and constant diesel injection pressure and better electronic control are required, so many manufacturers make more advantages Many diesel common rail systems are used as the development direction of diesel engines. This system has a high fuel pressure and can provide flexible fuel distribution control, and the fuel distribution, fuel injection time, injection pressure and injection rate can be flexibly controlled through the ECU. Through the control of the above characteristics, the common rail has made the responsiveness and driving comfort of the diesel engine reach the level of the gasoline engine, and it has remarkable fuel economy and low emission characteristics.
Guaranteed high fuel pressure in all engine speed ranges, high injection pressure can obtain good combustion characteristics at low speed conditions Driven by the camshaft to control the axial plunger type distribution pump engine, the fuel system pressure is proportional to the engine speed The linear relationship creates insufficient fuel pressure at low engine speeds, while the common rail system can achieve very high fuel pressure in all engine speed ranges. The flexible electronic control system controls timing and injection pressure to achieve low emissions and high efficiency under various engine operating conditions. Since the pressure build-up is separated from the injection process, the engine designer has greater freedom when studying the combustion and injection process. The injection pressure and injection timing can be adjusted according to the requirements of the engine working conditions, so that the engine can achieve complete combustion at low speeds, so even at very low speeds, high torque can be obtained. The application of pre-injection technology has made further progress in reducing emissions and noise.
Precise control of oil supply system
The low-pressure fuel pump sucks the diesel oil out of the fuel tank, and supplies it to the high-pressure fuel pump after filtration. There is a solenoid valve in the low-pressure pump to control the fuel to the high-pressure pump chamber, and the fuel enters the tubular pressure accumulator-fuel rail. There is a pressure sensor on the common rail to monitor the fuel pressure from time to time, and transmit this signal to the ECU to control the fuel pressure in the common rail to the desired value by adjusting the flow. The injection pressure ranges from 200 to 1800 bar according to the different engine operating conditions, and then is injected into the cylinders separately through computer control. The common rail not only maintains the fuel pressure, but also eliminates pressure fluctuations.
Fuel injection is a very complex joint operation of mechanical, hydraulic and electronic systems. To adapt to the working environment of the engine under various working conditions, the fuel must be filtered and pressurized before combustion, and injected at a certain injection rate at an accurate time. inside each cylinder. The engine computer controls exhaust gas recirculation, boosting, and exhaust aftertreatment systems for optimum engine characteristics and exhaust emissions.
The latest generation common rail engine
The compact structure of the injector makes the common rail system a practical solution even for small displacement 4-valve engines. At the end of 1999, the Smart equipped with a 3-cylinder common-rail diesel engine was born. Its displacement is only 799mL, its maximum power is 30kW, and its maximum torque is 100Nm at 1800-2800rpm. the
The second-generation common rail engine is installed on the E320 launched by Mercedes-Benz, with a maximum power of 150kW, an output torque of 250Nm at 1000rpm, and 85% of the peak torque at 1400rpm, and a peak value of 500Nm in a wide range of 1800-2600rpm torque. The acceleration time from 0 to 100km/h is only 7.7 seconds, and the maximum speed is 243km/h. The comprehensive fuel consumption is 6.9L/100km, and the 80L fuel tank makes the battery life reach 1000km. The combined fuel consumption of the E320 equipped with a gasoline engine is 9.9L/100km.
Three generations of diesel common rail system
Diesel common rail system has been developed for 3 generations, it has strong technical potential
The first-generation common rail high-pressure pump always maintains the highest pressure, resulting in waste of energy and high fuel temperature. The second generation can vary the output pressure according to the engine demand, and has pre-injection and post-injection functions. Pre-injection reduces engine noise: a small amount of fuel is injected into the cylinder for a millionth of a second prior to main injection, pre-heating the combustion chamber. The preheated cylinder makes the compression ignition after the main injection easier, and the pressure and temperature in the cylinder no longer increase suddenly, which is beneficial to reduce the combustion noise. Post-injection is performed during the expansion process to generate secondary combustion, increase the temperature in the cylinder by 200-250°C, and reduce the hydrocarbons in the exhaust.
Due to its strong technical potential, today's manufacturers have set their sights on the third generation of the common rail system - the piezoelectric (piezo) common rail system. The piezoelectric actuator replaces the solenoid valve, so a more accurate Spray control. Without the oil return pipe, the structure is simpler. The pressure is elastically adjustable from 200 to 2000 bar. The minimum injection volume can be controlled at 0.5mm3, reducing smoke and NOX emissions.
"Electronic control" means that the fuel injection system is controlled by a computer, and the ECU (commonly known as a computer) can precisely control the fuel injection quantity and fuel injection timing of each fuel injector, so that the fuel economy and power of the diesel engine can reach the best Balance, while the traditional diesel engine is controlled mechanically, and the control accuracy cannot be guaranteed.
"High pressure" means that the pressure of the fuel injection system is three times higher than that of a traditional diesel engine, up to 200 MPa (while the fuel injection pressure of a traditional diesel engine is 60-70 MPa), the pressure is large and the atomization is good enough to burn, thereby improving the power performance. Finally, the goal of saving fuel is achieved.
"Common rail" is to supply each fuel injector at the same time through the common fuel supply pipe. The fuel injection quantity is accurately calculated by ECU, and at the same time, fuel of the same quality and pressure is provided to each fuel injector to make the engine run more smoothly, thereby optimizing the diesel engine. Comprehensive performance. The traditional diesel engine injects fuel from each cylinder separately, the fuel injection quantity and pressure are inconsistent, and the operation is uneven, resulting in unstable combustion, loud noise and high fuel consumption.
At present, domestically produced diesel engines with internationally advanced electronically controlled high-pressure common rail technology adopt the latest core technology of European and American diesel engines, which are obviously superior to traditional supercharged diesel engines. Compared with traditional supercharged diesel engines, it has 8% higher combustion efficiency, 10% lower carbon dioxide emissions, and 15% lower noise, completely changing the image of diesel engines as "noisy and black smoke" in people's minds.
Structural principle
The high-pressure common rail system uses a large-volume common-rail cavity to accumulate the high-pressure fuel output by the oil pump, eliminate pressure fluctuations in the fuel, and then deliver it to each injector, and realize injection by controlling the solenoid valve on the injector start and end.
features
Its main features can be summarized as follows:
The high pressure in the common rail cavity is directly used for injection, which can save the booster mechanism in the injector; moreover, the high pressure in the common rail cavity is continuously high, and the driving torque required by the high pressure oil pump is much smaller than that of traditional oil pumps.
Through the pressure regulating solenoid valve on the high-pressure oil pump, the oil pressure in the common rail cavity can be flexibly adjusted according to the engine load condition, economy and emission requirements, especially optimizing the low-speed performance of the engine.
The injection timing, injection oil quantity and injection rate are controlled by the solenoid valve on the injector, and the injection oil quantity of pre-injection and post-injection and the interval with main injection can be flexibly adjusted under different working conditions.
The high-pressure common rail system consists of five parts, namely high-pressure oil pump, common-rail cavity and high-pressure oil pipe, fuel injector, electronic control unit, various sensors and actuators. The fuel supply pump pumps the fuel from the fuel tank into the fuel inlet of the high-pressure fuel pump. The high-pressure fuel pump driven by the engine pressurizes the fuel and sends it into the common rail cavity, and then the solenoid valve controls the fuel injectors of each cylinder to inject fuel at the corresponding time. Before the main injection, the pre-injection injects a small part of fuel into the cylinder, and premixing or partial combustion occurs in the cylinder, shortening the ignition delay period of the main injection. In this way, both the increase rate of the cylinder pressure and the peak pressure will decrease, the engine will work more gently, and the temperature in the cylinder will decrease to reduce NOx emissions. Pre-injection can also reduce the possibility of misfire and improve the cold start performance of high pressure common rail system.
Reducing the injection rate at the beginning of the main injection can also reduce the amount of fuel injected into the cylinder during the ignition delay period. Increasing the injection rate in the middle stage of the main injection can shorten the injection time and shorten the slow combustion period, so that the combustion can be completed within the more effective crank angle range of the engine, increase the output power, reduce fuel consumption, and reduce soot emissions. Rapid fuel cut-off at the end of main injection can reduce incomplete combustion of fuel, reduce smoke and hydrocarbon emissions.
