Causes of difficulties in starting diesel engines
It can be known from the working principle of the engine that the fuel system should spray fuel into the cylinder regularly and quantitatively and ensure the quality of atomization to make the fuel burn well. The working quality of the fuel system is concentrated in the injection time, fuel injection quantity and atomization quality.If the fuel system is faulty, it is difficult to ensure the time, quantity and atomization quality of fuel injection into the cylinder, which makes the engine difficult start up. The most common phenomenon is that the exhaust pipe is smokeless (mostly without fuel), that is, there is no sign of starting. The analysis reasons are mainly in the following aspects:
(1) Causes of low-pressure oil circuit (lack of fuel supply, etc.)
1. There is no oil in the fuel tank or insufficient oil storage, cracked tubing intake, loose fittings, water in the fuel, clogged fuel filter and overflow valve, clogged filter, etc. ;
2. The fuel tank switch is not turned on or the air hole of the fuel tank cap is blocked;
3. The pipeline between the fuel tank and the fuel injection pump is blocked;
4. There is an air leakage part in the pipeline between the oil tank and the oil pump, which allows air to enter the oil circuit;
5. The diesel engine filter or fuel pump filter screen is clogged;
6. The overflow valve of the fuel injection pump is not sealed;
7. Water is infiltrated in the oil circuit or the diesel grade used is wrong.
(2) Reasons for high-pressure oil circuit:
1. The plunger of the fuel injection pump is worn too much and the internal leakage is large, so that the fuel supply cannot meet the needs at the start;
2. The fuel injection pump oil volume adjustment mechanism is stuck, so that the plunger cannot rotate or the amount of rotation is too small;
3. Poor sealing of the oil outlet valve, resulting in no or insufficient oil supply;
4. The needle valve of the injector is deposited or sintered and cannot be opened
5. The opening pressure of the injector needle valve is adjusted too high;
6. The nozzle of the fuel injector is blocked;
7. There is air in the high-pressure oil pipe or its joints are loose.
8. Low injection pressure and poor atomization of injection, stuck needle valve of multilayer injection nozzle, broken spring of injection nozzle adjustment spring, etc.
(3) Other reasons:
1. The timing of fuel injection is not allowed. The reasons include the wear of the pusher roller and cam, the damage of the fuel pump drive connecting parts and the improper adjustment.
2. The low temperature starting preheating device fails, and the temperature in the cylinder of the diesel engine is too low;
3. The fuel supply time is too early or too late;
4. The compression pressure of the cylinder is too low, and the temperature and pressure at the end of the compression cannot reach the temperature that will cause the diesel to spontaneously ignite.
5. The cylinder does not intake air or the intake air is seriously insufficient. The reasons are serious clogging of the air filter, clogging of the exhaust pipe and too large valve clearance.
Fault diagnosis and troubleshooting
(1). Low-pressure oil supply is not smooth.
The inlet and return lines of the diesel engine from the fuel tank to the fuel injection pump's oil inlet are low-pressure oil paths. When the pipeline joints, gaskets and oil pipes leak oil due to damage, air will enter the oil path and cause air resistance , Resulting in poor fuel supply, difficulty in starting the engine, slow acceleration and other failures, it will automatically shut down when serious. When the aging, deformation and clogging of the oil pipe reduce the cross-sectional area of the oil passage, or the dirty oil blocks the oil inlet filter and the diesel filter, it will cause insufficient fuel supply, which will cause the engine power to decrease and start difficult. When checking the fault, pump the oil to a certain pressure with a hand oil pump, loosen the air screw, if there are bubbles overflowing and the exhaust is always exhausted, it means that the air is entering the air; if there are no bubbles, but the diesel oil overflows from the bleed screw, It means that the oil passage is blocked.
The method of troubleshooting is to find damaged or aging gaskets, joints or oil pipes to replace; to prevent such failures is to clean the oil filter and diesel filter, check the pipeline, and find the problem and solve it in time.
(2) The piston spring of the oil pump is broken.
The engine suddenly stopped during the operation and could not be started. After the exhausted air was restarted, the engine was automatically stopped again after returning to normal driving for a certain distance. It is likely that the piston spring of the fuel pump was broken. The troubleshooting method is to unscrew the screw and replace it with a spring.
(3), the check valve of the oil pump is not tightly sealed.
It is difficult to start after the engine is turned off and stopped for a certain period of time. After loosening the bleed screw, there will be bubbles overflowing, and the air must be discharged again to start. This kind of failure is mostly caused by the tightness of the check valve of the oil pump. The inspection method is to unscrew the oil pump screw, and use the oil pump to make the fuel fill the oil connector cavity. If the oil level in the connector drops quickly within 3 minutes, the unidirectional seal is not good. Remove the check valve to check whether the sealing surface is intact, whether the check valve spring is broken or deformed, and whether there is particulate impurities on the sealing seat surface. Grind the sealing surface, replace the check valve or check valve spring to remove the fault.
(Four), the high-pressure fuel pipe is blocked.
When a high-pressure fuel pipe of a cylinder is blocked due to deformation or impurities, there is a significant knocking sound in the fuel pipe after starting the engine, and the engine power decreases because the cylinder cannot work normally. The inspection method is to loosen the nuts of the oil inlet end of the high-pressure oil pipe cylinder by cylinder. When the knock sound disappears after loosening a certain cylinder, it can be concluded that the cylinder is a malfunction cylinder, and the malfunction can be eliminated after replacing the oil pipe.
(5). The fuel injection nozzle is stuck.
When the needle valve of the fuel injection nozzle is stuck in the closed position, there will be a regular knocking sound because the pressure wave of the fuel injection pump impacts the injector. The judgment method is: loosen the high-pressure fuel pipe connected to the injector end, if the knocking sound disappears immediately, it can be concluded that the needle valve of the fuel injection nozzle of this cylinder is stuck.
