Definition
The common rail system separates the fuel pressure generation from the fuel injection. If the single pump diesel injection technology is compared to the diesel technology revolution, the common rail can be called a rebellion because it deviates from the traditional diesel system and approximates for sequential gasoline injection systems. The common rail system opens up new avenues to reduce diesel engine emissions and noise.
Working principle
The common rail injection type oil supply system consists of a high pressure oil pump, a common fuel supply pipe, a fuel injector, an electronic control unit (ECU) and some pipeline pressure sensors. Each injector in the system passes its respective high pressure oil pipe and common oil supply. The pipes are connected, and the common oil supply pipe acts as a hydraulic pressure accumulator for the injector. During operation, the high-pressure oil pump delivers fuel to the common fuel supply pipe at high pressure. The high-pressure oil pump, pressure sensor and ECU form a closed-loop operation, which precisely controls the oil pressure in the common fuel supply pipe and completely changes the oil supply pressure with the engine speed. phenomenon.
Differences
Common Rail System unlike the diesel injection system previously driven by a camshaft, the common rail diesel injection system completely separates the injection pressure generation and injection processes from each other. The solenoid-controlled injector replaces the traditional mechanical injector. The fuel pressure in the fuel rail is generated by a radial piston high-pressure pump. The pressure is independent on 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 performs continuous pressure regulation according to the working needs of the engine. The electronic control unit acts on the pulse signal of the injector solenoid valve to control the injection process of the fuel. The amount of fuel injected depends on the oil pressure in the fuel rail and the length of time the solenoid valve is open, and the liquid flow characteristics of the injector.
Fuel injection pressure is an important indicator of diesel engines because it relates to engine power, fuel consumption, emissions, and so on. The common rail diesel injection system has increased the fuel injection pressure to 2600 bar.
Precise control
Fuel injection is a complex mechanical, hydraulic, and electronic system. It must be adapted to the working environment of the engine under various working conditions. Before combustion, the fuel must be filtered and pressurized, and injected at a certain injection rate at an accurate time. Inside each cylinder. The engine computer controls the exhaust gas recirculation, boost, and exhaust aftertreatment systems for optimum engine characteristics and exhaust emissions.
New generation
The diesel common rail system has been developed for 3 generations and it has strong technical potential. The first generation of common rail high pressure pumps are always kept at the highest pressure, resulting in wasted energy and high fuel temperature. The second generation can vary the output pressure depending on the engine demand and has a pre-injection and post-injection function. Pre-injection reduces engine noise: a small amount of fuel is injected into the cylinder for compression in a millionth of a second before the main injection, preheating 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 abruptly, which is beneficial to reduce the combustion noise. The post-injection is performed during the expansion process to generate secondary combustion, and the temperature in the cylinder is increased by 200 to 250 ° C to lower the hydrocarbon in the exhaust gas.
Due to its strong technical potential, manufacturers have set their sights on the third generation of the common rail system, the piezo common rail system. The piezoelectric actuator replaces the solenoid valve, so more accurate injection is obtained. control. Without the return line, it is simpler in structure. The pressure is adjusted from 200 to 2000 bar. The minimum injection volume can be controlled at 0.5mm3, reducing smoke and NOx emissions.
Maintenance
1. High-pressure common rail system guarantees high-pressure injection and precise flow control. The accuracy of each component is very high. The gap control of the even parts is quite strict. The partial straightness is below 0.8 microns, and the even part gap is between 1.5-3.7 microns. Therefore, there is a high demand for diesel cleanliness. Conventional diesel filters can only filter particles above 10 microns, and 3 micron particles are very inefficient. The high pressure common rail system requires a filter to provide 95% water separation efficiency and 98.6% 3-5 micron particle filtration efficiency.
2. High-pressure common rail system components are expensive. If the filter is not replaced regularly according to the instructions, the injector high-pressure pump will be damaged and the maintenance cost is quite expensive. Take the fuel injector body of a heavy truck as an example, it costs about 1,500 yuan or more.
