Abstract: Burning tiles is one of the major mechanical failures of diesel engines. The causes of diesel engine burning include improper assembly, insufficient oil pressure, oil deterioration, etc. This article analyzes the causes of these failures to remind users to pay attention to prevention.
Burning is caused by the deterioration of lubrication between the engine crankshaft and the bearing bush, resulting in dry friction between the bearing bush and the journal. The high temperature generated by the friction softens or even melts the alloy layer of the bearing bush, and sticks to the journal. Burning tiles is one of the major mechanical failures of diesel engines. Burning tiles not only damages the bearings, but also causes scratches on the journals, deformation of the crankshaft, connecting rods and body, causing the diesel engine to lose its working ability.
1 Judgment of burning tiles
If a diesel engine's speed suddenly drops during operation without a change in load, black smoke emerges from the exhaust, oil pressure falls, pronounced cylinder knocking is heard, and the engine soon stops on its own, then feels heavy or impossible to turn when the decompression lever is lowered and the flywheel is cranked, this indicates a burned bearing shell. Generally, bearing-shell damage can be diagnosed when the following three conditions occur together: ① the oil temperature rises sharply above 95°C; ② previously normal oil pressure suddenly drops substantially; ③ bearing-alloy particles are found when inspecting the oil filter and sump.
2 The reason for burning tiles
2.1 Improper assembly
(1) Strict requirements apply to the assembly clearance between a bearing shell and its journal. If the clearance is too small, engine oil cannot enter easily and an oil film cannot form. If it is too large, oil escapes easily, again making oil-film formation difficult. For example, in an S1100 diesel engine, the fit clearance between the crankshaft main journals and main bearing shells is 0.08–0.12 mm, while that between the connecting-rod journals and connecting-rod bearing shells is 0.06–0.105 mm. If these clearances are below 0.05 mm and 0.04 mm respectively and the diesel engine operates at full load or overload, frictional heat caused by inadequate lubrication can score and scuff the bearing shells, resulting in bearing burnout.
If the fitting gap is too large, and some operators do not replace worn parts in time, the fitting gap will greatly exceed the limit (the two limit fitting gaps are 0.25 mm), and a large amount of oil will leak from the fitting parts, which will also cause tile burning. If the axial clearance of the crankshaft is less than 0.05 mm (specified as 0.10 to 0.25 mm), and the diesel engine is operating at an excessively high temperature, it is very easy to burn tiles. Seized parts often occur at the end face of the main bearing pad and the journal close to the end face of the main bearing pad.
(2) The roughness of the bearing or journal surface is poor. In this case, poor roughness often destroys the lubricating oil film, causing the bearing bush and journal to seize. For this requirement, the mating contact area between the bearing bush and the journal should not be less than 75% of the total area of the journal, and the contact points should be evenly distributed.
2.2 The oil supply is insufficient or the oil pressure is insufficient.
Both excessively low and excessively high oil pressure can lead to bearing-bush seizure. If pressure is too low, oil cannot reach the friction surfaces between the shaft and bush, causing inadequate lubrication and, in severe cases, loss of bearing alloy and seizure. Excessively high pressure can also make it difficult to maintain an effective oil film between the journal and bush. Causes of low pressure include: (1) diesel engines stored for long periods are prone to this failure. Before starting an engine after storage, turn the crankshaft until the oil-indicator valve rises.
If the oil pressure does not come up, you can remove the back cover and use an oil gun to press some oil into the oil hole leading to the pressure reducing valve. After the oil flows back to the oil pump, the oil pump can work normally. You can also remove the flywheel, remove the oil pump, reinstall it and start it again. (2) If there is insufficient oil in the engine, the amount of oil pumped by the oil pump will decrease, or even the pump will not oil due to air entering, causing the oil pressure to drop. (3) The oil pump rotor is broken, or the oil pump rotor is ruptured and cannot pump oil. In this case, the oil pump rotor should be replaced.
(4) Excessive impurities in the engine oil can completely block the strainer assembly, preventing oil intake. Remove and clean the oil strainer assembly and replace the engine oil. (5) At low ambient temperatures, excessively viscous engine oil may not be drawn in, causing bearing seizure. Warm the oil or replace it with new oil. (6) Omitting the copper washer when reinstalling the oil pickup strainer, or failing to tighten a pipe-connection screw, can also cause zero or insufficient oil pressure and bearing seizure. (7) Severe wear of the oil pump's inner and outer rotors, or wear that creates excessive clearance between the pump shaft and bushing, between the gear end face and pump cover, or in the tooth backlash or radial clearance, will reduce oil delivery and cause oil pressure to fall.
At this time, worn parts should be replaced, and the inner surface of the pump cover should be ground to restore the gap between it and the gear end face to the standard range.
2.3 Water has entered the engine oil or diesel oil has leaked into it.
Water or diesel fuel entering engine oil destroys its lubricating properties, degrades the oil, and causes bearing shell burnout. The main causes are: (1) Water leakage through casting pores in the engine block or cylinder head. This occurs in new engines or newly replaced blocks and cylinder heads. Replace the affected component promptly when leakage is found. (2) Leakage from water sealing rings. A sealing ring cut during cylinder liner installation or an aged sealing ring can cause water leakage. Replace it promptly upon discovery. (3) Leakage from water jacket plugs. Drive the plug tight again, or install and firmly seat a new plug, then fill with water for testing and check for leakage.
(4) The cylinder gasket is leaking. Because the cylinder head nut is not tight or the cylinder head nut loosens after the diesel engine has been running for a long time, the cylinder gasket is damaged and water leaks. (5) The positioning screw of the plunger sleeve of the fuel injection pump is not tightened, the copper pad is missing, the copper pad is uneven, and the fuel injection pump body in contact with the shoulder of the plunger sleeve is defective, etc., resulting in diesel oil leaking into the crankcase to dilute the oil, and also causing tile burning.
2.4 There are too many impurities in the oil.
The oil container and refueling funnel are not clean; the engine body or other parts are not cleaned properly after replacement; the inner cavity of the diesel engine is not cleaned after the tiles are bitten; the repair work is carried out in the open air, and sediment enters the crankcase.
2.5 The crankshaft is of poor quality or not maintained in time.
(1) Some crankshaft oil holes have not been drilled through. When replacing the crankshaft, check whether the oil holes are fully open. (2) Sand particles have not been completely removed from the crankshaft's centrifugal cleaning chamber, or the chamber has not been maintained for a long time. Thoroughly clean the centrifugal cleaning chamber before assembly; clean and maintain it after every 500 h of diesel engine operation. (3) The surface roughness of a newly replaced crankshaft's connecting-rod journals exceeds Ra=1.6 μm, and some connecting-rod journals have an obviously polygonal shape. In this case, use No. 0 abrasive cloth to remove sharp edges, then polish the crankshaft and connecting-rod journals until surface roughness reaches Ra=0.4 μm or the polygonal shape has been substantially eliminated.
2.6 The replacement bearing was not run in as required.
After replacing the bearing bush, it is operated under load without running in. Since the working surface of the replaced new bearing bush (especially after scraping) is not very smooth and flat, it cannot form a good full-area cooperation with the crankshaft journal. At this time, if it is operated under load without running in, under the action of high speed and heavy load, the bearing bush and the journal will produce severe friction and ablation. Therefore, after replacing the bearing bush, be sure to run it at idle speed for a period of time to protect the bearing bush and journal before working under load.
(Source: Diesel Master)