Analysis of the failure causes:
For diesel to burn inside a diesel engine, three conditions must be met: fuel, an ignition source (the temperature generated by the piston during the compression stroke), and combustion support (air). If a diesel engine fails to start, it is obvious that one or more of the above - mentioned conditions are not met.
① Influence of the starter: If the starter malfunctions and fails to rotate, the diesel engine cannot intake air, inject fuel, or generate the compression temperature, thus causing the engine to fail to start.
② Influence of the fuel supply system: According to the working principle of the engine, the fuel system should inject fuel into the cylinder at the right time and in the right amount and ensure good atomization quality to enable proper fuel combustion. The working quality of the fuel system is mainly reflected in the injection timing, injection quantity, and atomization quality. If there is a fault in the fuel system, it is difficult to ensure the correct timing, quantity, and atomization quality of the fuel injected into the cylinder, which leads to difficulty in starting the engine. A common phenomenon is that there is no smoke from the exhaust pipe (usually indicating a lack of fuel), which means there are no signs of starting.
③ Poor compression in the cylinder: Cylinder compression needs to be carried out under air - tight conditions. That is, when the piston moves upward from the bottom dead center, the volume above the piston decreases, and the pressure and temperature increase. If the components that contain the gas in the cylinder have poor sealing, air will leak during the compression stroke, resulting in poor compression. At the end of the compression stroke, neither the gas pressure nor the temperature can rise, lacking the ignition source for fuel combustion. As a result, the diesel injected into the cylinder cannot ignite and burn on its own, and the diesel engine fails to start.
④ Influence of temperature: When a diesel engine starts, the piston in the cylinder compresses the air to generate high temperature, which causes the atomized diesel injected into the cylinder to evaporate, mix thoroughly with air, and oxidize for combustion, thus pushing the piston to generate power. If the ambient temperature is low and the engine body is cold, the heat exchange rate between the compressed air entering the cylinder and the engine body increases, causing a sharp drop in the gas temperature in the cylinder. This not only deteriorates the evaporation effect of the fuel but also makes it difficult for the fuel to ignite and burn on its own (lacking the ignition source for fuel combustion). Therefore, the engine fails to start. The unburned atomized fuel is discharged from the engine body in the form of milky white smoke.
⑤ Incorrect valve timing: The opening and closing moments of the intake and exhaust valves of each cylinder of the engine are usually represented by the crankshaft angle, which is called the valve timing. If the valve timing deteriorates due to changes in the technical condition of the valve train, such as keyway or key damage and misalignment, excessive valve clearance, wear of the cam and timing gear, torsional deformation of the camshaft and crankshaft, etc., the original valve timing will be disrupted. In mild cases, it will affect the engine's power; in severe cases, it will make it difficult to start the diesel engine.
⑥ Improper installation: If the timing gear marks are not aligned correctly during maintenance, it will not only affect the valve timing but also the fuel supply timing of the fuel injection pump, resulting in the engine's failure to start.
⑦ Poor fuel quality: If there is water in the fuel or cooling water leaks into the cylinder, the engine will fail to start. The water entering the cylinder evaporates into steam when heated and is discharged from the engine body as white smoke.
Diagnosis and troubleshooting:
① If the crankshaft does not rotate when starting the engine, it indicates a starter failure. Diagnose and troubleshoot the starter according to the methods described in the electrical system section.
② If there is no smoke from the exhaust pipe when starting the engine, it usually means there is a fault in the fuel system. Diagnose and troubleshoot the fault according to the methods described in the fuel system section.
③ If the engine fails to start with white smoke from the exhaust pipe, it indicates that the engine temperature is too low, the fuel atomization is poor, or the temperature fails to reach the self - ignition point, preventing the fuel from burning and the engine from starting. Start the engine according to the low - temperature starting operation method, that is, pre - heat the engine (including the lubricating oil). If the temperature is normal and the exhaust pipe still emits white smoke when starting the engine, check whether there is water in the fuel or cooling water leaking into the cylinder.
If the temperature is normal, the diesel engine is approaching the end of its service life (i.e., the components are severely worn), or the engine has poor sealing due to other reasons, the engine's failure to start is caused by poor compression. Measure the cylinder pressure with a cylinder pressure gauge. If the cylinder pressure is too low, it indicates poor cylinder compression. Find out the cause of the poor compression and troubleshoot it.
If the diesel engine could start before maintenance but fails to start after maintenance, check whether the timing gear marks are aligned correctly.