Common Rail System

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, then the common rail can be called a rebellion, because it deviates from the traditional diesel system and is similar to the sequential gasoline injection system. Common rail systems open up new ways to reduce emissions and noise from diesel engines.

History
Europe can be said to be a paradise for diesel vehicles, with 39% of diesel cars in Germany. Diesel cars have a history of nearly 70 years, and in the last 10 years, it can be said that diesel engines have made rapid progress. In 1997, Bosch developed the common rail diesel injection system (Common Rail System) jointly with Mercedes-Benz. In Europe, many brands of cars are equipped with common rail diesel engines, such as Peugeot's HDI common rail diesel engine, Fiat's JTD engine, and Delphi's Multec DCR diesel common rail system.

Difference
The common rail system differs from the previous camshaft-driven diesel injection system in that the common rail diesel injection system completely separates the generation of the injection pressure and the injection process from each other. The fuel injector controlled by solenoid valve replaces the traditional mechanical fuel injector. The fuel pressure in the fuel rail is generated by a radial plunger high-pressure pump. The pressure is independent of 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, and the pressure is continuously regulated according to the working requirements of the engine. The electronic control unit acts on the pulse signal on the injector solenoid valve to control the fuel injection process. The amount of fuel injected depends on the fuel pressure in the fuel rail and the length of time the solenoid valve is open, as well as the fluid flow characteristics of the fuel injector.

Fuel injection pressure is an important indicator of diesel engine, because it is related to engine power, fuel consumption, emissions and so on. The common rail diesel injection system has increased the fuel injection pressure to 2600 bar.

Cars with direct injection diesel engines have been significantly developed in Europe, with high efficiency and excellent fuel economy, and reduced engine noise. The direct injection diesel engine uses a pump nozzle system, which is used in the 1.9 T di Bora produced in China, with a maximum injection pressure of 1800 bar.Although the pump nozzle direct injection system is good, the fuel pressure can not be kept constant. With the more stringent emission control, higher and constant diesel injection pressure and more perfect electronic control are needed. Therefore, many manufacturers regard the diesel common rail system with more advantages as the development direction of diesel engine.This system has high fuel pressure and can provide flexible fuel distribution control, which can flexibly control fuel distribution, fuel injection time, injection pressure and injection rate through ECU. Through the control of the above characteristics, common rail has made the response and driving comfort of diesel engine reach the level of gasoline engine, and it has significant fuel economy and low emissions.

In that engine of the axial plunge type distribution pump driven and controlled by the camshaft, the fuel system pressure and the engine rotate speed are in a linear relationship, and insufficient fuel pressure is formed at the low rotating speed of the engine, The common rail system can achieve very high fuel pressure in all engine speed ranges.The flexible electronic control system controls the timing and injection pressure to achieve low emissions and high efficiency under all engine operating conditions. The separation of pressure formation from the injection process gives the engine designer greater freedom in the study of the combustion and injection processes. The injection pressure and the injection timing can be adjusted according to the requirements of the working condition of the engine, so that the engine can realize complete combustion under the low-speed working condition, and high torque can be obtained even at a very low rotating speed.The application of pilot injection technology has made greater progress in reducing emissions and noise.

Precise Control
The diesel oil is sucked out from the oil tank by the low-pressure oil pump and is supplied to the high-pressure oil pump after being filtered, a solenoid valve is arranged in the low-pressure oil pump to control the fuel oil to reach the high-pressure oil pump chamber, and the fuel oil enters the tubular pressure accumulator-fuel rail. The pressure sensor on the common rail monitors the fuel pressure all the time and transmits the signal to the ECU. The fuel quantity entering the high-pressure pump is controlled by adjusting the driving current of the solenoid valve, so as to adjust the fuel pressure in the common rail to the desired value.The injection pressure varies from 200 to 1800 bar according to the operating conditions of the engine, and then is injected into the cylinder through computer control. The common rail not only maintains the fuel pressure, but also eliminates the pressure fluctuation.

Fuel injection is a complex combination of mechanical, hydraulic and electronic systems. In order to adapt to the working environment of the engine under various working conditions, the fuel must be filtered and pressurized before combustion, and injected into each cylinder at a certain injection rate at an accurate time. The engine computer controls the exhaust gas recirculation, supercharging, and exhaust aftertreatment systems for optimum engine performance and exhaust emissions.

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, Smart, equipped with a 3-cylinder common rail diesel engine, was born, with a displacement of only 799 mL, a maximum power of 30 kW, and a maximum output torque of 100 Nm at 1800 ~ 2800 rpm.
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 1000 rpm, 85% of the peak torque at 1400rpm, and a peak torque of 500 Nm in the wide area of 1800-2600rpm. The acceleration time from 0 to 100 km/H is only 7.7 seconds, and the maximum speed is 243 km/H. The combined fuel consumption is 6.9 L/100 km, and the fuel tank of 80 L makes the endurance reach 1000 km. The combined fuel consumption of E320 with gasoline engine is 9.9 L/100km.

A New Generation
The diesel common rail system has been developed for three generations, and it has great technical potential. The first generation common rail high-pressure pump is always kept at the highest pressure, resulting in energy waste and high fuel temperature. The second generation can change the output pressure according to the engine demand, and has the function of pre-injection and post-injection. Pre-injection reduces engine noise: a small amount of fuel is injected into the cylinder for compression ignition one millionth of a second before the main injection, pre-heating the combustion chamber.The preheated cylinder makes the compression ignition after main injection easier, and the pressure and temperature in the cylinder no longer increase suddenly, which is conducive to reducing combustion noise. In the expansion process, post-injection produces secondary combustion, which increases the temperature in the cylinder by 200 to 250 degrees Celsius and reduces the hydrocarbons in the exhaust gas.
Because of its strong technical potential, manufacturers have set their sights on the third generation of the common rail system, the piezo-electric (piezo) common rail system, which replaces the solenoid valve with a piezoelectric actuator, thus achieving more precise injection control. Without the oil return pipe, the structure is simpler. The pressure is elastically adjusted from 200 to 2000 bar. The minimum injection amount can be controlled at 0.5mm3, which reduces smoke and NOX emissions.

Maintenance
1. In order to ensure high-pressure injection and accurate flow control, the precision of each component of the high-pressure common rail system is very high, and the clearance control of the matching parts is quite strict. The straightness of some parts is below 0.8 microns, and the clearance of the matching parts is between 1.5-3.7 microns, so it puts forward high requirements for the cleanliness of diesel. Traditional diesel filters can only filter particles above 10 microns, and the filtering efficiency of particles above 3 microns is very poor.The high pressure common rail system requires the filter to provide 95% water separation efficiency and 98.6% 3-5 micron particle filtration efficiency. Diesel filters that meet the performance requirements are monopolized by foreign companies, and almost all the main engine factories are imported products of foreign companies or foreign-funded enterprises in China. There are also national brands entering the main engine, such as Duffit in Suzhou Industrial Park. The product quality of these enterprises is reliable, basically have: Foreign capital enterprise: Man.MH, Parker, Fleet guard, BOSCH, etc. Domestic brands such as DIFITE, etc.

2. The components of the high-pressure common rail system are expensive. If the filter is not replaced regularly according to the instructions, the high-pressure pump of the injector will be damaged, and the maintenance cost is quite expensive. Take the fuel injector body of a heavy truck as an example, it costs more than 1500 yuan.