Due to the implementation of a new control system and new control hardware, an initial test sequence was executed during which the PID settings of the entire control system were set. Preliminary testing was then performed, over multiple days, in order to monitor the control stability of the test setup. These tests also served the purpose of identifying the most suitable speed and load points at which to conduct future engine tests.
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In order to obtain an accurate indication of the engine’s performance, the aim was to select speed and load points which were representative of the engine’s normal operating range. The final test points were identified by evaluating the engine’s performance curves (see Figure 53 in Appendix I) and selecting the load points such that they all fall within the engine’s torque output capability over the selected speed range. Following this selection procedure, preliminary test points were identified as being: 5, 10, 15 and 20 N·m at 2400 rpm, 2800 rpm and 3200 rpm respectively. These test points therefore also cover the 3000 rpm operating point of a 50 Hz generator, which is a typical application in which the test engine is commonly used. The test points are shown in Figure 25 where the torque is plotted against engine speed (shown in rpm on the horizontal axis). In addition, Figure 25 also displays the peak torque curve of the test engine as obtained from the engine manufacturer’s data.
An automated test sequence was then programmed in ETA and executed using constant torque mode. The engine was started and allowed to reach operating temperature, after which the automated test was initiated. The automated test was started at a load of 5 N·m and a speed of 2400 rpm (test point 1 in Figure 25). The engine was run at each this test point for 2 minutes (the amount of time required for the engine’s exhaust gas temperature to stabilise), after which sixty data points were then captured over a period of five minutes. Upon completion of the data capturing, the automated test moved the test engine on to the second test point (10 N·m at 2400 rpm). After the test point was reached, the engine was run once more until the exhaust gas temperature stabilised and the sixty data points were captured again. This process was repeated to sequentially complete all the test points in the order shown in Figure 25, without any intervention from the operator.
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The results of the test are presented in Figure 26, where the measured torque is plotted for each captured data point. The data presented are the raw data points as they were captured (over a period of 5 minutes) and without correcting the torque for ambient conditions. From Figure 26 it is clear that the test setup exhibits very good control stability across all test points, especially considering the fact that the test engine used is a single-cylinder, compression-ignition engine, which is known for torsional oscillations.
The largest deviation in load, can clearly be noticed for all test points conducted at a test speed of 2800 rpm, with the highest deviation being recorded at test point number 5 (20 N·m at 2800 rpm). These more noticeable deviations in load can be attributed to the engine’s torque characteristic displaying significant torque backup around 2800 rpm (as can be seen from the steep gradient in the full load torque curve presented in Figure 25 above). As a result, when running at an operating speed of 2800 rpm, any small deviation in speed, results in a significant change in fuel delivery and therefore torque output of the engine. The change in torque output of the engine causes the engine’s speed to change even more, forcing the control system to respond by changing the load on the engine, in order to bring it back to the set point values for the specific test point. The repetition of the above sequence of events, is the cause of the larger deviations in torque output for test points conducted at speeds of 2800 rpm.
Figure 26: Test setup control stability over period of 5 minutes 0 5 10 15 20 0 10 20 30 40 50 60 Tor qu e [ N ·m ]
Data Point Number
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Table 5 was constructed in order to quantify the accuracy of the data presented graphically in Figure 26. Statistical analysis was performed which yielded the mean and standard deviation values (for both load and speed) for each of the recorded data sets. The results of which can be seen in the last four columns of Table 5. Further inspection of the results reveals that the mean torque values are all within the initial stipulated ± 0,5 N·m tolerance band, listed in Table 1. This also holds for test point 5, where even with the larger deviations due to the reasons discussed in the paragraph preceding Figure 26, the value of the mean torque recorded is still within 0,4 N·m of the set point value of 20 N·m. Table 5 also indicates that the mean speed values obtained are all within 5 rpm of the set point value and therefore far exceed the ±1 % speed control accuracy requirement, listed in Table 1. Overall, the stability and accuracy of the entire test setup (including the control system) proved satisfactory and the stipulated design requirements were met successfully. Note that test point 1 was repeated at the end of the test cycle in order to compare the test setup accuracy before and after completing the entire test cycle.
Table 5: Control system accuracy Test Point # Test Point (Set Point Values) Mean Torque [N·m] Mean Speed [rpm] Load Speed Standard Deviation [N·m] Standard Deviation [rpm] 1 5 N·m at 2400 rpm 5,0 2400 0,1 0,6 2 10 N·m at 2400 rpm 9,9 2402 0,1 0,7 3 15 N·m at 2400 rpm 14,9 2403 0,1 0,9 4 20 N·m at 2400 rpm 19,7 2404 0,2 1,0 5 20 N·m at 2800 rpm 19,6 2804 0,9 2,7 6 15 N·m at 2800 rpm 14,9 2805 0,3 0,5 7 10 N·m at 2800 rpm 9,9 2802 0,3 0,4 8 5 N·m at 2800 rpm 5,0 2800 0,2 0,3 9 5 N·m at 3200 rpm 5,0 3198 0,2 0,2 10 10 N·m at 3200 rpm 9,9 3200 0,2 0,2 11 15 N·m at 3200 rpm 14,9 3202 0,3 0,2 12 20 N·m at 3200 rpm 19,7 3205 0,4 0,3 13 5 N·m at 2400 rpm 5,0 2402 0,1 0,6
Data obtained by Corbett (2017), who used the test bench developed in this project to perform testing of the larger Honda GX670 spark-ignition engine,
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shows that the dynamometer also displays very good control stability when testing at higher load points, as can be seen in Figure 27 below. This verifies that the developed test bench and dynamometer system meets the control accuracy requirements initially stipulated in Table 1, where the test setup was designed with the purpose of also being suitable to test the larger Honda, spark-ignition engine.
Figure 27: Test setup stability when testing Honda, spark-ignition engine (Source: Corbett, 2017)