Starting from the conditions required for the smooth start of the diesel engine, this paper analyzes the reasons for the difficult start of the diesel engine. Taking the single cylinder engine S195 as an example, this paper discusses the fault diagnosis method for the difficult start.
Difficult starting of diesel engine is a common comprehensive fault on diesel engine. The cause of the fault is complex and often occurs in diesel engine that has been used for a period of time. Because after the diesel engine has been running for a period of time, due to natural wear and human factors, the technical state will deteriorate, and it is easy to have difficulty in starting. Difficulty in starting means that the diesel engine cannot be started for more than 3 consecutive times according to the starting procedure, or even the diesel engine cannot be started by means of auxiliary means, such as heating water in the water tank.
Conditions for successful start of diesel engine
The successful start of diesel engine must meet the following three conditions, and none of them is indispensable.
01
The inhaled air shall form a strong swirl, and the compression end temperature at the compression end point shall be 200 ~ 400 ℃ higher than the diesel spontaneous combustion point, i.e. 500 ~ 700 ℃. If air leakage occurs in the cylinder or valve, the pressure in the combustion chamber will be insufficient, and the temperature and pressure at the end of compression will not meet the requirements, which will lead to the failure of normal combustion and explosion of diesel, resulting in the failure to start the diesel engine.
02
Spray fine atomized diesel oil into the combustion chamber at high pressure. The structure of the fuel system is complex and the parts are precise. Once a fault occurs in the middle part, it will lead to abnormal fuel supply, which will make it difficult or impossible to start the diesel engine.
03
The crankshaft rotation speed shall be higher than 110 r/min. If the starter of the starting device has insufficient power and works abnormally, and the crankshaft speed is lower than 110 r/min, the normal starting of the diesel engine will be directly affected.
Starting speed too low
01
The air vortex is weak, the mixing of fuel and air becomes worse, the temperature in the cylinder decreases, the fuel is not easy to evaporate, and the ignition conditions are not met;
02
The fuel injection pressure is low, the fuel injection quality becomes worse, the fuel injection quantity is significantly reduced, and the fuel supply quantity required for startup cannot be reached;
03
The compressed gas is easy to leak through the loose seal of the cylinder, causing the temperature and pressure of the compression end point to decrease, which makes it difficult to start the diesel engine. In addition, the low ambient temperature and the failure of the lubrication system cause poor lubrication of the mating motion pair, which increases the motion resistance of the crank connecting rod mechanism, makes it difficult to rotate the crankshaft, and also makes it difficult to start the diesel engine.
From the above analysis, it can be seen that the main fault parts that cause difficulties in starting the diesel engine are the compression system and the oil supply system; The secondary fault parts include lubrication system and starting device; There are also environmental factors. In case of difficult starting faults of the diesel engine, we can certainly restore the good starting performance of the diesel engine as long as we carefully find and diagnose these aspects and eliminate them one by one.
Reasons for difficult starting of diesel engine
01
Failure of the compression system. The main reason is that the pressure at the end of cylinder compression cannot reach the specified value, and the mixture in the cylinder cannot ignite and burn normally. This is mainly caused by the wear of inlet and exhaust valves, carbon deposition, incorrect adjustment of valve clearance, etc., resulting in air leakage due to lax valve closure, or excessive wear clearance between piston ring and piston and cylinder liner, resulting in poor sealing of combustion chamber and air leakage.
02
Fuel supply system failure. It can be analyzed according to low-pressure oil circuit and high-pressure oil circuit. Low pressure oil circuit faults include: insufficient oil in the oil tank or the oil tank switch is not opened, and there is air or water in the oil circuit; The vent hole of the fuel tank cover is blocked; The oil circuit or diesel filter is blocked and does not supply oil; The filter screen of the oil transfer pump is blocked; The oil valve of the oil transfer pump is stuck or not sealed; The upper oil pipe in the oil tank is broken or loose; Oil pump connecting oil pipe locking nut intake; The oil transfer pump is not working properly. High pressure oil circuit faults include: the plunger and sleeve of the fuel injection pump are worn beyond the limit; The tight seat of the oil outlet valve is loose, sticky, leaking oil or its spring is broken; The throttle lever or plunger is stuck in the non oil supply position; The needle valve of the fuel injector is blocked due to carbon deposition or sintering; Less oil injection, low pressure and poor atomization; Nozzle blockage; The pressure regulating spring is too tight or broken; The high-pressure oil pipe is broken, the joint is loose or there is air in the pipe.
03
Problems with the starting system and the electrical system. It is mainly due to poor circuit contact, which shows that the connector is loose; The battery power is insufficient, and the starting motor power drops; The carbon brush of the starting motor has poor contact with the commutator or the spring force of the carbon brush holder is too weak; Carbon brush of starting motor is seriously worn; The gear of the starting motor cannot be embedded into the flywheel gear ring to mesh with each other.
04
Lubrication system failure. There is air in the oil circuit; The oil strainer is blocked; The oil filter is blocked; Low oil pressure of oil pump or insufficient oil supply; The pressure oil pipeline and oil passage are blocked or leaking.
05
other. The air filter is seriously blocked, so that the air cylinder cannot suck in air; The cylinder head exhaust passage or exhaust pipe is blocked, making the exhaust resistance too large; The starting hole on the swirl chamber insert is blocked.
Fault diagnosis (take single cylinder 5195 as an example)
01
Turn off the throttle, depressurize, and rotate the flywheel quickly by shaking the handle. If the flywheel rotates slowly, laboriously and feels blocked, it indicates that there are too many impurities and thick oil, or the fitting clearance between connecting rod bearing and main bearing is too small. If the flywheel rotates freely, easily, and has inertia, it indicates that the fitting clearance is normal. If there is a "click, click" sound of horseshoe or piston knock, it is mostly because the tightening nut on the rocker shaft seat is loose and the valve clearance is incorrect.
02
Turn off the throttle without reducing the pressure, and rotate the flywheel slowly by shaking the handle. When you feel heavy, take out the handle, change the direction, and then depressurize your hands downward. If the flywheel does not rotate, it indicates that the air hold of the valve is good, and there is no air leakage at the cylinder gasket and injector copper gasket; If the flywheel rotates freely and effortlessly, and the cylinder head emits a "hissing, hissing" sound, it indicates that the valve is leaking. Pour a small amount of engine oil into the intake pipe to lubricate the valve, and drain the engine oil from the oil pan at the same time. Open the rear cover plate of the machine body, and rotate the flywheel with the rocker. If the cylinder liner and piston skirt leak, there will be mist like gas flowing outward.
03
Open the accelerator and depressurize. If you hear the "Pu, Pu" clear oil injection sound during startup, and the exhaust pipe continuously emits thick smoke, it indicates that the oil circuit is smooth and the oil supply is normal; If the fuel injection sound is weak and the exhaust pipe emits blue smoke, it is mostly due to poor atomization of the fuel injector, wear of the oil pump plunger and weak pump oil; If the exhaust pipe emits white smoke, the oil supply time is incorrect or the oil supply is not available. The reasons are: wrong diesel grade in winter, oil condensation, blockage of oil pump outlet valve hole, blockage of air filter, and blockage of starting hole on swirl chamber insert.
04
Hot car ignition. If the diesel engine is hot started, it needs to be ignited for many times, which indicates that the fuel supply advance angle error of the oil pump is large, the fuel injection atomization is poor, the plunger sleeve of the oil pump is seriously worn, and the compression ratio is insufficient (the air leakage of the valve, piston ring and cylinder gasket, the subsidence of the valve seat exceeds the specified value, the piston is eccentric worn, and the cylinder liner is out of round). It indicates that the diesel engine is in poor technical condition, with high fuel consumption and low power. It should be repaired in time.
