The main causes of piston burnout in Cummins K38 engine
Piston erosion is a common phenomenon in diesel engine use, with most of the erosion occurring at the top of the piston, the first and second piston ring grooves, and the circumference of the piston head. Generally, the main forms are the piston top surface melt hole, perforation, and keyway shaped notch and honeycomb eye at the head circumference. The main fault phenomenon is the increase of exhaust gas under the engine, and even the oil rushing out of the breathing hole.
Burnout of the piston will cause abnormal operation of the diesel engine, directly leading to a decrease in cylinder pressure and power, indirectly causing cylinder pulling, brake holding, and damage to components such as the turbocharger and cylinder head.
Below, based on the experience of repairing piston erosion in Cummins K38 engine and relevant technical information, the author analyzes the causes of piston erosion in Cummins K38 engine.
The piston erosion is shown in Figure 1.
Excessive exhaust back pressure
Exhaust back pressure refers to the resistance pressure of the engine's exhaust.
The exhaust pressure of K38 engine is less than 0.09kPa. If the muffler is blocked or the exhaust pipe is improperly modified, it will increase the exhaust resistance, leading to excessive exhaust backpressure.
Due to the high exhaust back pressure of the engine, the exhaust gas generated by the combustion of the mixture in the cylinder is difficult to discharge, and the exhaust gas can only flow back. Heat relatively accumulates in the cylinder, resulting in high cylinder temperature and ultimately causing piston burnout.
Poor quality piston
The piston moves back and forth in a straight line under harsh conditions of high temperature, high pressure, high speed, and poor lubrication, directly in contact with high-temperature gas. The instantaneous temperature can reach over 2500 ℃, which is severely heated and has poor heat dissipation conditions. Therefore, the temperature of the piston during operation is very high, with the top reaching 600-700 ℃, and the temperature distribution is very uneven;
The top of the piston bears a large amount of gas pressure, especially the maximum pressure during the power stroke, which can reach 6-9 MPa in diesel engines. This causes the piston to experience impact and bear the effect of lateral pressure;
The piston moves back and forth in the cylinder at a high speed (8-12m/s), and the speed is constantly changing, generating a large inertial force, which puts a significant additional load on the piston.
Pistons working under such harsh conditions will deform and accelerate wear, as well as generate additional loads and thermal stress.
If the quality of the piston is not up to standard and there are defects such as pores, looseness, microcracks, and slag inclusions during casting, these pores, looseness, and microcracks will cause fatigue damage under high temperature and pressure; The slag inclusion in the piston first melts, causing the piston to melt and leading to piston erosion failure.
Burning black smoke and severe carbon buildup on the piston
The generation of carbon deposits is quite complex and closely related to the engine structure, the type of fuel and lubricating oil used, as well as the working conditions and working conditions of the engine.
In the combustion chamber, the oxygen supply is insufficient, and the fuel and lubricating oil entering the combustion chamber cannot be completely burned, resulting in oil smoke and tar particles. After mixing with the lubricating oil, they further oxidize into a viscous gel like liquid hydroxy acid, which further oxidizes into a semi fluid resin like resin, firmly adhering to the parts. Then, under the continuous action of high temperature, the resin polymerizes into a more complex polymer, forming a hard cemented carbon, that is, carbon deposition.
The components of carbon deposits include lubricating oil, hydroxy acids, asphalt, oil coke, carbon blue, sulfates, silicon compounds (from ash and sand in the intake), and trace amounts of metal shavings and their compounds.
The higher the engine temperature, the harder and tighter the carbon deposits formed, and the stronger the adhesion with the metal.
The carbon deposits in the piston ring groove can cause the piston ring to lose elasticity and become stuck, resulting in a decrease in the sealing performance of the piston ring and causing oil burning, thereby exacerbating the generation of carbon deposits.
Carbon deposits on the intake and exhaust valves can cause the valves to not close tightly, and high-temperature particulate carbon deposits adhering to the valves can also cause valve and valve seat erosion, exacerbating valve leakage.
Valve leakage causes high-temperature gas to wash away the valve and valve seat, further causing the valve and valve seat to burn and leak, ultimately leading to a decrease in cylinder pressure. The large amount of combustion smoke promotes the generation of carbon deposits in the piston.
The carbon deposition on the piston weakens its heat dissipation effect and increases the temperature. When the temperature exceeds the thermal endurance limit of the piston, it will cause piston erosion.
Carbon deposits on the piston are shown in Figure 2
The main reasons for the large amount of black smoke and severe carbon accumulation in engine combustion are:
If the intake and exhaust valves are not tightly closed, it will cause the high-temperature and high-pressure combustible mixture to erode the valve and valve seat working surface, causing pitting, carbon accumulation, and erosion on both working surfaces. Pitting, carbon accumulation, and erosion will accelerate the lax closure of the intake and exhaust valves, forming a vicious cycle.
Loose valve closure, decreased cylinder pressure, poor combustion, excessive carbon buildup in the cylinder, resulting in a decrease in engine power and economy.
Pump nozzle mismatch, excessive fuel injection
There are two models on the K38 engine, CPL844 and CPL1628. There are differences in the fuel pumps and injectors of the two control numbers, CPL1628 and CPL844. Among them, the BA94 fuel pump and 3077760 injector are used to match CPL1628, while the B844 fuel pump and 3058802 or 3076132 injector are used to match CPL844.
Compared to the B844 pump, the BA94 pump has a higher fuel consumption, and the 3058802 or 3076132 injector has a higher fuel consumption compared to the 3077760 injector.
To meet strict emission requirements, Cummins engines have developed a new type of hydraulic driven variable fuel injection timing control system called STC (Step Timing Control).
The STC system divides the engine fuel injection timing into two parts: mechanical timing (controlled by the timing gear and camshaft), also known as the "normal timing mode", and mechanical hydraulic timing (controlled by the engine fuel pressure, also known as the "fuel injection advance timing mode").
Under start-up and light load conditions, the "fuel injection advance timing method" is adopted to inject fuel earlier in the compression cycle;
In medium and heavy load conditions, the "normal timing mode" is adopted, and the fuel is injected later in the compression cycle.
The STC valve acts as a directional control valve, with fuel pressure equivalent to pilot oil pressure. The opening pressure of the STC valve is 27 Psi, and the closing pressure is 65 Psi.
If the STC valve is not working properly, the engine injection timing will change, the fuel combustion will not be good, the post combustion period will be extended, a large amount of carbon deposits will be generated in the cylinder, the piston heat dissipation will be poor, and long-term operation will lead to final piston erosion, cylinder head explosion and other faults.
Poor cooling causes high temperature
The normal operating temperature of the engine is between 82~93 ℃. If there is insufficient coolant or other oil mixed in, the radiator is blocked, or the fan is not working properly, it will cause the engine cylinder temperature to be too high.
In addition, the engine piston and cylinder liner are mainly carried away by the oil sprayed out by the oil cooling nozzle.
If the cooling nozzle nozzle has deformation, sand holes, incorrect injection position or low oil pressure, it will cause a decrease in the amount of injected oil, directly leading to high temperatures of the piston and cylinder liner.