During the test, select four points of ambient temperature, namely 5℃, 0℃, -5℃, -10℃, and the test shall be carried out completely in accordance with the standard . In order to ensure the reliability of the test data, this test uses TEAC tape recorder to transiently record the speed signal, the pressure signal of each cylinder, the starting current and the voltage signal of the starting process, and the exhaust temperature signal. Then use AVL646 digital analyzer to process and analyze each signal. Use SC16 type light oscilloscope to monitor the whole process of the starting of the testing machine. In order to make the test results more in line with the actual situation, the accessories of the vehicle diesel engine (such as: generator, fan, water pump, filter, etc.) are not removed. In the test, the diesel engine used the lubricating oil produced by Lanzhou Refinery QF?CD, SAE10W/30, the antifreeze was prepared with ethylene glycol ten water; the specific gravity was 1.073g/cm3, and the freezing point was -42℃.
The cold start process analysis is the change of the starting speed of the test machine with time under different ambient temperatures, which is the cold start performance index. The results show that the testing machine can start smoothly at +5℃ and 0℃, but the average index of starting time and stable operation time at +5℃ is smaller than that at 0℃, which proves that the testing machine is at an ambient temperature greater than 0. At ℃, the starting performance is good without taking any measures.
Schematic diagram of test device +5℃, diesel engine starting process at 0℃, diesel engine starting process at -5℃, diesel engine starting process at -10℃, diesel engine starting process when the ambient temperature drops to -5℃, although the start can be successful. However, the stable operation time is significantly increased (164s). The result shows that if the testing machine does not take auxiliary measures at this time, the cold start performance will deteriorate. When the ambient temperature is further reduced to -10°C, the starting process fails, as shown. This indicates that the starting boundary state temperature of the testing machine is -5°C.
Dynamic analysis of cylinder pressure during cold start It is found from the experiment of cold start process that vehicle diesel engine is an extremely unstable process when starting at the boundary state. Therefore, it is necessary to dynamically analyze the pressure change of each cylinder of the diesel engine and discuss the cold start process of the diesel engine in depth. In order to highlight the instability of the combustion of each cylinder when the diesel engine is started at low temperature, the starting process test at room temperature (24℃) was first carried out, which is the maximum cylinder cycle pressure peak. It was found that during the entire starting process, the peak cycle pressure of the second, third, and fourth cylinders was almost the same as that of the first cylinder, and there was a pressure increase in each working cycle, and there was a fire in each working cycle. There has never been a fire. Moreover, the peak combustion pressure of each cycle is almost equal. About 66bar. This shows that the compression conditions and fuel supply conditions of each cylinder of the 4D105 diesel engine after careful adjustment are basically the same.
The peak pressure of each cylinder per working cycle at 24°C is -5°C** the peak cycle pressure of the cylinder is -5°C The peak cycle pressure of the second cylinder is 0-5°C The peak pressure of each cycle in the third cylinder ~ 1 is the boundary state temperature -5, each The change of the pressure peak value of each cycle during the starting process of the cylinder, the result is that it is obviously different from the normal temperature state, and the ignition situation of each cylinder is quite different. The fourth cylinder catches fire early in the 38 working cycle*, and then it misfires. After a period of intermittent ignition cycle, continuous ignition occurs after 69 cycles. Although the second and third cylinders have fires, there is no continuous ignition during the starting time, and the pressure peaks are also different.
And ** The cylinder has not caught fire during the starting time. This shows that: at -5°C, the speed of the testing machine changes greatly during the starting time. The fundamental reason is that each cylinder is intermittently fired at the peak pressure of each cycle of the fourth cylinder at 1-5°C. After a stable operation time of 164 seconds, the final test machine started successfully. From the analysis of the pressure peaks of each cylinder, the reason for the successful start of the test machine is because the fourth cylinder has continuous ignition during the start, which drives the other three cylinders to move through the fuel. , Heat accumulation effect, each cylinder will ignite one after another until it reaches stable operation.
Although the working conditions of all cylinders of the 4D105 diesel engine are the same, under low temperature conditions, the fourth cylinder first ignites continuously and the first cylinder does not have any ignition. The reason is that after the diesel engine antifreeze fluid is cooled by the diesel engine cooling system, it first enters the ** cylinder, so that the temperature of the ** cylinder antifreeze is lower, resulting in a lower temperature in the cylinder and not easy to catch fire. The antifreeze in the fourth cylinder is heated by the first three cylinders and then enters the fourth cylinder and flows out of the water outlet. The temperature of the antifreeze rises, resulting in a higher temperature in the fourth cylinder, which is conducive to fire. This phenomenon has guiding significance for optimizing diesel engine design and improving the cold start process.
Conclusion There is a big difference between the cold start process of a diesel engine and the normal temperature start process. During the cold start, the combustion in each cylinder is extremely unstable and misfire will occur. To make a successful cold start of a multi-cylinder diesel engine, at least one cylinder must be continuously fired during the starting time. During the starting time, there is a big difference in the pressure peak of intermittent ignition, which makes the operation of the diesel engine extremely unstable and may cause cold start failure.
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