Inspection and Diagnosis of Cylinder Leakage Faults in Single-Cylinder Diesel Engines

⑴ Symptom: When cranking the crankshaft without opening the throttle or engaging the decompression lever, the resistance felt during the compression stroke is weak or insufficient as the piston approaches Top Dead Center (TDC), and a "hissing" sound of air leakage can be heard around the cylinder; additionally, when rotation stops, the flywheel comes to a halt abruptly without any oscillating back-and-forth movement.

⑵ Cause: Air leakage in a diesel engine leads to reduced power output and increased fuel consumption; in severe cases, it causes the engine to burn and expel lubricating oil or even fail to start, thereby disrupting normal operation. The primary causes of leakage are: 

① Severe wear on the piston, piston rings, and cylinder liner, resulting in a poor seal. This occurs when wear eliminates the clearance fit between the piston and the cylinder liner. Severe piston ring wear causes oil to bypass into the combustion chamber due to the ring's pumping action, leading to oil burning, blue smoke from the exhaust, and heavy carbon buildup.

② Poor sealing between the valve and the valve seat. Leakage here can result from component wear, excessive valve clearance (often caused by frequent use of the decompression mechanism without timely adjustment), or damage to the sealing surfaces of the valve and seat due to carbon deposits, burning, or wear. When this happens, combustion gases leak past the intake and exhaust valves during operation, causing the exhaust pipe temperature to rise and resulting in popping sounds from the exhaust.

⑶ Diagnostic Method: 

① Remove the fuel injector from the engine block and inject a small amount of lubricating oil into the cylinder through the injector port. Reinstall the injector and crank the flywheel; if a distinct resistance (compression feel) is detected, it indicates a poor seal between the cylinder and piston. The piston assembly should be removed for inspection, and parts replaced if necessary. This method can also be used for an emergency start if leakage between the cylinder and piston assembly is making the engine difficult to start.

② Remove the dust-separating cyclone hood and the air cleaner, then inject a small amount of engine oil directly into the cylinder head through the intake port and rotate the flywheel. If distinct compression resistance is felt, it indicates poor sealing between the valve and the valve seat; the valve clearance and decompression mechanism should be inspected and adjusted. If the fault persists, further inspect the sealing contact surfaces of the valve and valve seat, and perform matched lapping if necessary. If wear or burning is severe, or if repeated lapping causes the valve sinking depth to exceed the wear limit, the parts must be replaced. This method can also be used for emergency starting when air leakage between the valve and valve seat prevents the diesel engine from starting.