刘 震涛 - 浙江大学动力机械及车辆工程研究所
Zhentao Liu - Power Machinery and Vehicular Engineering Institute, Zhejiang University
A diesel engine should achieve the highest maximum explosion pressure to provide good torque and power features and lower fuel consumption to meet emission standards. The higher the explosion pressure, the higher the mechanic load, which requires a higher-strength diesel engine. This also applies to the engine body, which is the main component of a diesel engine.
Chinese and international researchers have conducted theoretical and experimental studies to enhance engine body strength and reliability. We based the theoretical research on finite-elefiment computing, and the experimental on engine and simulation experiments. The engine experiment directly examines the strength of the engine body, but takes a long time and fails to simulate the strain of the body when the load is higher than the maximum explosion pressure. However, the simulation experiment can simulate the strain of the engine body under different load conditions by changing the body’s load. Using the fatigue reliability theory, we can obtain a quantitative evaluation of the body’s fatigue lifetime and safety factor. Therefore, the body fatigue simulation experiment is ideal to evaluate the strength and fatigue lifetime of the body.
In the 1980s, the China North Engine Research Institute adopted a series of hydraulic servo simulation experiment equipment developed by Schenck. The equipment could run simulation experiments on engine components such as pistons, crank arms, and bodies. Shandong Bohai Piston Co. Ltd. introduced a system for the hydraulic simulation experiment for the piston and pin boss, another Schenck technology. To determine engine body damage, an operator would stop the experiment. If there was a flaw, the body was thought to be damaged; otherwise, the experiment would continue. This method had many drawbacks including manual intervention and uncertainty of when the flaw occurred.
We developed a new engine hydraulic fatigue experiment system based on the CompactRIO platform to better evaluate engine body damage through strain changes and examine the fatigue lifetime of a particular engine. Developing the automatic control system did not require knowledge of any special hardware design language, which dramatically reduced the development time.
Experiment System Setup
The engine hydraulic fatigue experiment system works in single-cylinder mode in which only one cylinder is under experiment at a time. In the new engine hydraulic fatigue experiment, we mounted a virtual piston, crank arm, and crankshaft (direct axis) on the cylinder body and crankcase. The virtual crank arm includes a straight shaft and two pairs of bushings. The shaft is the same diameter as the main crankshaft at both ends, and the diameter gradually reduces to the crank pin diameter in the middle. With this approach, the crank shaft prototype is unnecessary and we have the freedom to select the number and locations of the cylinders under experiment.
We put a loading piston on top of the virtual piston to minimize the volume by compressing it with hydraulic oil during the experiment. A special sealing ring prevents any leakage of the hydraulic oil at the top of the virtual piston. The sealing should be located as high as possible to minimize the action on the cylinder wall caused by the hydraulic pressure and to prevent generating irrelevant flaws in the cylinder wall during the experiment. We replaced the cylinder cap with a steel plate, which is mounted with a cap pad prototype. The cap blots are screwed down. The steel plate offers connections for the hydraulic oil supply and drainage and temperature and pressure sensors. Figure 1 shows the loading mechanism.
During the experiment, the NI 9263 analog output module controls the action of the hydraulic servo solenoid valve by generating sinusoidal hydraulic pulse signals. Then, the hydraulic pressure acts on the virtual piston via the loading mechanism and passes to the engine body via the mechanic structure. As a result, the cylinder wall surface of the engine body bears alternating tension and compression, which simulates the alternating load that the engine would bear in a real operation environment. The hydraulic load is determined by the engine’s real explosion pressure and the safety factor of the body design.
An NI 9237 simultaneous bridge module acquires the output signal of the strain sheet attached to the root of the body’s main bearing blot where the body can easily be damaged. If the variation of the strain exceeds the configured threshold during monitoring, the CompactRIO controller outputs the corresponding signals, terminates the experiment, and indicates the existence of body damage. The threshold is usually set to 50 percent of the body damage evidence. If this is exceeded, there will be visible damage at the strain sheet location.
Designing the Control System
Introduction to the CompactRIO Embedded Controller
The highly reliable and deterministic CompactRIO controller includes a real-time controller and a field-programmable gate array (FPGA), which is ideal for an independent and reliable embedded system or for distributed application systems. The controller also includes a hot-swappable industrial I/O module that has a built-in signal conditioning circuit capable of directly connecting to sensors/actuators. CompactRIO can adapt to a temperature range of -40 to 70 ℃, to a shock of up to 50 g, and to dangerous or explosive environments. Most I/O modules can sustain a transient voltage up to 2,300 Vrms and a continuous voltage of 250 Vrms.
Hardware Design of the Control System
The experiment runs at a high frequency, usually 10 to 30 Hz, which can vary according to the unit under experiment. To strictly control the wave and amplitude of the signal loaded during the experiment, we have to control the hydraulic servo solenoid valve using a closed-loop structure. The duration of the fatigue experiment is long and takes about 4 million to 10 million loops. The experiment environment is harsh and the hydraulic pump station is about 2 to 3 m away from the experiment bench. The control system adopts an upper/lower computer architecture to improve the reliability of the whole experiment system.
In a traditional solution, the lower controller would consist of a programmable controller or a single-chip microcomputer. The former has high reliability but poor real-time performance, and the latter has good real-time performance but poor reliability. To integrate the advantages of these two methods, we used NI products to develop the experiment system: a CompactRIO embedded controller for the major control unit, two NI 9237 modules for strain acquisition, an NI 9263 module for the driver of the hydraulic servo solenoid valve, an NI 9401 high-speed bidirectional digital I/O module, and an NI 9485 8-channel relay module for the system display and switch signal control. We also added an NI 9201 8-channel C Series module to acquire the pressure signals regarding the common rail, accumulator, and loading oil channel. The real-time calculations in the system ran on a 3M gate FPGA, which offered high-calculation efficiency and reliability.
Software Development of the Control System
To reduce development time, we programmed our software using the LabVIEW development environment and the LabVIEW Real-Time and LabVIEW FPGA modules. Then we used the LabVIEW PID Control Toolkit to implement the closed-loop control of the hydraulic servo solenoid valve. We based the software on a three-layer architecture with the FPGA, host, and upper PC program. Figure 3 shows the function diagram.
Control System Application
After developing the experiment system, we conducted the engine body fatigue simulation experiment on a particular engine body. Figure 4 illustrates the overall architecture of the experiment system and the control system.
We configured the strain variation to be larger than 50 percent of the body damage evidence in the experiment so that flaws would be visible. The frequency used for this experiment was 16 Hz. Figure 5 shows the graphs used to acquire digital signals and monitor the interface.
We automatically logged the number of experiment loops, the variation of the experiment pressure signal, and the strain value into a database during the experiment. After 1,760,000 experiment loops, the system automatically stopped and indicated the engine body damage. We examined the stored strain value and discovered changes in the value of the strain sheet on the left-rear side of the cylinder body under experiment, which identified damage to the engine body surface. After examination, we found a visible flaw where the experiment data indicated, as shown in Figure 6, demonstrating the ability to determine the body damage by monitoring the change of the strain in the engine body.
The engine body fatigue hydraulic simulation experiment bench can run the hydraulic fatigue experiment on an engine body. Moreover, it can automatically determine the presence and localization of body damage by monitoring the change of strain.
Compared with traditional solutions that usually combine an industrial control computer and a dynamic strain gage, the experiment bench control system hardware developed with CompactRIO and NI modules delivered a simple architecture with high system reliability. Additionally, by programming the application with the LabVIEW development environment, we cut the original development cycle time in half compared to Visual C++ programming.