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Diagnosis plan based on common rail pressure signal of gasoline direct injection system

In the field of gasoline direct injection, electromagnetic injectors are widely used because of their low cost. Vitesco Technology Company has developed a diagnostic program based on the common rail pressure signal for this injector, and improved the fuel injection volume through this program, thus using it as a development tool.

In the field of gasoline direct injection, electromagnetic injectors are widely used because of their low cost. Vitesco Technology Company has developed a diagnostic program based on the common rail pressure signal for this injector, and improved the fuel injection volume through this program, thus using it as a development tool.

0 Preface

In the context of further tightening of fuel consumption and emission standards, researchers have put forward new requirements for the fuel measurement accuracy of the fuel injection system. The original emission of the engine and the conversion of pollutants in the exhaust gas aftertreatment depend on the excess air coefficient λ in the cylinder of the internal combustion engine. Researchers usually use a wideband lambda sensor between the engine and the three-way catalytic converter to adjust the intake air volume, and adjust the ratio of air to fuel injection volume in the engine. In addition to the uniform distribution of the air quality in each cylinder, the engine also needs to use the fuel injection system to more accurately calculate the fuel injection volume (Figure 1).

Diagnosis plan based on common rail pressure signal of gasoline direct injection system
Figure 1 Schematic diagram of the layout of the high-pressure fuel system

Under normal circumstances, researchers can determine the timing of fuel injection valve switching by controlling the voltage and current, and can compensate for the difference in hydraulic opening duration by adjusting the Electronic control duration of each injector, but this type of method The deviation caused by the nozzle flow rate is not considered. If the nozzle has carbon deposits during its service life due to manufacturing errors or other reasons, the nozzle flow rate will also change accordingly. In order to achieve good fuel injection accuracy, researchers should not only consider the hydraulic opening duration, but also diagnose and compensate for the difference in nozzle flow.

The parameters that researchers need to diagnose are mainly the physical relationship between the fuel injection volume and its interference with the common rail pressure. Researchers use the signal processing method introduced in this article to determine the pressure interference and hydraulic natural frequency through the common rail pressure signal, so as to obtain the correlation between the fuel injection volume under the wide pressure and temperature boundary conditions. Subsequently, researchers can diagnose the injector flow rate by applying the linear working process of the injector valve.

The common rail pressure sensor with higher resolution makes the application of the new calibration tool possible. The researchers took a typical operating point for heating the catalytic converter as an example, and carried out key research on the relationship between combustion stability and the smoothness of engine operation and fuel injection. The resolution of the cycle accuracy can ensure that the combustible mixture is fully combusted, and the pressure in the cylinder can be optimized accordingly.

1 Principle of pressure difference measurement

In the high-pressure fuel injection system, the volume formed by the common rail, fuel pipe, and fuel injector functions as a pressure accumulator, and can provide high-pressure fuel through a high-pressure fuel pump. According to the working principle, the fuel supply process and the fuel injection process can be operated independently in time, and the common rail fuel injection system can flexibly adjust parameters such as fuel injection start point and fuel injection times.

The fuel injection valve can work in a leak-free state, and the total amount of fuel injected into the cylinder will cause the pressure to drop. On the one hand, the compressibility of the entire system mainly depends on the compressibility of the fuel. Therefore, the high-pressure common rail, high-pressure fuel pipe and injector must have sufficiently high rigidity, and the volume share of gas must be reduced as much as possible. . Some researchers believe that the high-pressure volume is a single-phase state, and the pressure in the entire fuel system is at a constant state; on the other hand, there is a certain temperature gradient between the injector installed in the hot cylinder head and the external common rail . This measurement principle is used in pressure indicators such as HAD and Mexus2.0 to show the increase in pressure caused by the injected fuel.

2 From pressure signal to fuel injection volume

In order to apply this type of measurement principle to the fuel system, the researcher only needs to provide a signal for pressure regulation. In order to determine the value of the speed of sound, the researchers used an ultrasonic sensor and a measurement method based on the characteristic curve field on the test bench, which can be used to measure the temperature of the medium.

Figure 2 shows the common rail pressure curves of three different single injection qualities when the common rail pressure is 35 MPa. In order to determine the magnitude of the pressure drop, the researchers divided the pressure signal into two areas before and after injection, each including a pressure platform formed by the superimposition of pressure fluctuations.

Diagnosis plan based on common rail pressure signal of gasoline direct injection system
Figure 2 Common rail pressure cycle and its probability tightness function

In order to confirm the height of the pressure platform, the researchers used the core tightness evaluation program to convert such signals into a probability tightness function, in which a triangular core with a bandwidth of 0.05 MPa was applied. Researchers can set the height of each pressure platform and its pressure drop as the platform difference by determining the maximum value of the distribution function. Unlike the simple average calculation method, this method is more reliable.

In addition, with the help of fast Fourier transform, the researchers can detect the natural frequency of the hydraulic system in a calibratable frequency window through the pressure signal, and make it possible to avoid additional interference frequencies. In the case of pressure increase and temperature decrease, the determined natural frequency is similar to sound waves and will increase.

Through formula (2), researchers can determine the amount of fuel injected during work, and measurements on a temperature-adjustable system test bench confirm the effect of this method (Figure 3). Each point is equivalent to one injection process, while the common rail pressure, temperature and injection duration are in a state of continuous change. The benchmark fuel injection volume can be measured by the Akribis fuel injection volume indicator, but for different fuel injection systems and the entire data record, the constant is the only definite value.

Diagnosis plan based on common rail pressure signal of gasoline direct injection system
Figure 3 Diagnosis results of fuel injection volume relative to reference fuel injection volume

3 Diagnosis of injector flow

Researchers use the pressure drop to identify the relative flow difference between the injectors, so that they can be corrected during their service life. As the experimental research shows, researchers can diagnose the mass-produced pressure sensor with a 16 kHz scanning detection process.

In the characteristic curve field used to describe the duration of the electronic control, the pressure-dependent injector flow rate is continuously increasing within the linear working range. To compare them, researchers need to observe the pressure drop and the duration of the electronic control. Figure 4 shows the characteristic curve field of a fuel injector in the system at different pressure levels. The pressure drop is used to replace the fuel injection volume in the figure. The researchers used regression calculations on several values ​​composed of electronic control duration and pressure drop to find out the slope of each injector’s characteristic curve. This value is closely related to the injection quantity Qp. During the duration of the diagnosis, the researcher must maintain the boundary conditions, especially the average pressure level should be in a constant state.

Diagnosis plan based on common rail pressure signal of gasoline direct injection system
Figure 4 Injector characteristic curve field derived from common rail pressure drop

In order to simulate the flow deviation between fuel injectors on the hydraulic test bench, the researcher needs to install four fuel injection valves with different flow rates in the fuel injection system. The static flow rate of these 4 injectors is between 7.5 and 9.0 g/s. This value is measured by the researcher under the condition that the common rail pressure is 10 MPa and the test medium temperature is 20 ℃ by measuring the alternative fuel n-heptane. As shown in Figure 5, a is the correction factor, R is a constant, and the Qp value can be used to determine the pressure level and is proportional to the flow rate. On this basis, the researchers compensated by adjusting the electronic control duration in the engine electronic control system, and can also determine the reference fuel injection quantity Qref with the aid of the fuel injection quantity indicator.

Diagnosis plan based on common rail pressure signal of gasoline direct injection system
Figure 5 The diagnostic value of the flow rate of different pressures and injectors and the measured reference fuel injection volume

4 Fuel injection quantity detection

The researchers obtained another application possibility by expanding the indicating system on the engine test bench and using a high-resolution common rail pressure sensor. As long as proper fuel injection timing is adopted, the researcher can distinguish the relationship between engine combustion status and fuel injection volume according to the cycle, and the four of the 4-cylinder turbocharged engines with a displacement of 2.0 L are in low load and low speed conditions. Measure under the operating conditions.

As shown in Figure 6, the researchers plotted the fuel injection volume and average indicated pressure of a certain cylinder in every 100 cycles. Each operating mode point was operated at a stoichiometric ratio, but in order to enable the fuel injection volume to be able to Shows a significant change, and the researchers trigger it through the lambda regulator.

Diagnosis plan based on common rail pressure signal of gasoline direct injection system
Figure 6 The fuel injection quantity detected on the engine test bench and the cylinder deactivation process without flow correction

The three operating conditions with higher loads present normal combustion conditions. At this time, the load changes are related to the amount of fuel injected in the cylinder, and the maximum power can be identified when the excess air coefficient is thick, but the load is greatly reduced The cylinder will stop at times, which will cause the engine to run unstable. Related analysis shows that due to load changes, the mixture will be relatively thin, and the necessity of flow compensation becomes more important at this time. Researchers can prevent individual cylinders from running at a super-stoichiometric ratio by improving the accuracy of the fuel injection volume, thereby avoiding the occurrence of cylinder deactivation.

5 Conclusion

Similar to the method used to increase the pressure of the fuel injection system in measurement technology, the pressure drop generated during the fuel injection process is usually used to diagnose the required fuel injection volume. This article introduces its principle and corresponding signal processing methods. Researchers make full use of this method to identify difficult-to-find flow differences, and at the same time use it in the pressure adjustment process of pressure sensors, and improve the diagnostic accuracy by increasing the pressure and time resolution of mass-produced sensors. In the process of development and calibration, researchers use this method to study the working process in the cylinder, and determine the required fuel injection volume through cycle accuracy without using expensive measuring equipment.

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