It's inevitable for truck drivers to have problems with the vehicles they drive all year round. Therefore, it is important to understand some basic knowledge of engine maintenance. The following is a brief analysis of common truck engine failures and maintenance practices.
1. Analysis of causes of cracks in machine body
During operation, cracks sometimes appear in housing (especially in cylinder block and cylinder head), reasons are as follows:
(1) Manufacturing defects. Such as trachoma, shrinkage porosity, pores, etc., there are also hull cracks caused by quality defects such as poor aging and stress concentration areas.
(2) Incorrect usage. For example, when engine is in a state of high temperature and lack of water, a large amount of cold water is suddenly added; when starting in morning in severe winter, a large amount of high-temperature water is added; cooling water is not drained, causing cooling water to freeze and expand cylinder.
(3) Improper maintenance and repair. Main bolts and auxiliary bolts not properly tightened, replaced
The self-made water plug has too much interference because there is no special maintenance tool, it cracks when fuselage is turned over, etc. During use and maintenance, above problems should be paid attention to prevent cracks in cylinders.
2. Engine crankshaft failure analysis
Crankshaft failure is a serious mechanical engine failure. Failure usually occurs at junction of crankpin, main journal, and crank arm, or at stress concentrations such as oil hole in journal.
The main cause of crankshaft failure
(1) Due to improper selection of engine oil by some users, or careless cleaning and replacement of "three filters", engine oil deteriorates after long-term use; The crankshaft was badly worn due to engine burnout. The crankshaft of WD615 series engine is repaired by replacement, that is, a new crankshaft is purchased for installation, and damaged crankshaft is sent to manufacturer for repair. After car had a crankshaft wear problem, some users found several small local factories for repair and processing due to cost and time. Due to repair methods and technical problems, connecting fillets of connecting rod journal, main journal and crank arm have changed, resulting in localized stress concentration, since crankshaft is forged with No. 45 precision steel, surfacing has caused metallographic weaving of crankshaft to occur variety. The above two points are main causes of crankshaft failure.
(2) After repair, engine has not passed break-in period before loading, that is, it is overloaded and overloaded, and engine is overloaded for a long time, so that load on crankshaft exceeds allowable one. allowable limit.
(3) When repairing crankshaft, surfacing welding is used, which destroys dynamic balance of crankshaft, and balance check is not carried out. The imbalance exceeds standard, causing severe vibration of engine and resulting in failure. crankshaft.
(4) Due to bad road conditions and severe overload of vehicle, engine often runs within critical torsional speed and shock absorber fails, which also leads to crankshaft failure due to torsional fatigue damage. .
3. Causes of early wear of engine cylinder liners
If wear of engine cylinders exceeds allowable rate, power and efficiency of car are seriously reduced. Therefore, understanding causes of cylinder liner wear and determining pattern of cylinder wear is convenient for proper and prudent use in order to slow down cylinder wear and prolong engine life.
The general rule is that along axial direction of cylinder above cylinder liner, amount of wear on first air ring is greatest at a position slightly below top dead center, and wear on piston ring is minimum at a position below bottom dead center. From worn section, cylinder is ground to non-standard cylindricity. Generally speaking, wear of first cylinder and sixth cylinder of engine is more severe than wear of other cylinders, mainly due to poor cooling.
Causes of engine cylinder wear
Mechanical wear, abrasive wear, chemical corrosion.
(1) Mechanical wear:
Mechanical wear is caused by mechanical friction, and degree of mechanical wear is affected by machining accuracy, fit quality, lubrication conditions, operating temperature and mutual pressure of cylinder and piston ring. When engine is running, temperature of upper part is high, lubricating oil suffers and thinner, quality of oil film is poor, back pressure of first piston ring (high pressure gas enters piston an annular groove, firmly pressing piston ring against cylinder wall) Therefore, first piston ring in upper parts of cylinder and a little lower, and under it wear becomes less.
Abrasion is most common type of wear. Abrasives are a mixture of metallic impurities mixed with engine oil and airborne impurities that can accelerate wear of parts. Because during intake process, compressed intake air rushes directly into cylinder wall opposite intake valve when engine is running at low temperature, atomization of diesel fuel is not good, and diesel fuel particles that are not completely burned dilute engine oil, thinning engine oil, destroying oil film.
At same time, due to damage to air filter element, frequentEggs of dust and sand carried by intake air flow adhere to cylinder wall opposite intake valve, causing severe abrasive wear. Abrasive wear not only causes more wear on part just below top dead center of first piston ring at top of cylinder, but also makes cross section of cylinder elliptical. The main causes of abrasion are: damaged air filter element, hose from supercharger to intercooler or from supercharger to inlet pipe disconnected, cracked or disconnected, and short circuit of air into cylinder with formation of severe abrasion; there are also some users who pay too much attention to changing engine oil according to its quality. if engine oil is used for too long, it deteriorates, becomes thinner, and contaminants increase. contaminants cause severe abrasive wear. In units that do not have instrumentation, it is best to change engine oil on a regular basis. Changing engine oil, oil filter element and air filter element is best maintenance method for every secondary maintenance (12000 km).
(3) Chemical corrosion:
When engine cylinder wall temperature is low and cylinder pressure is high, water vapor in cylinder forms water droplets on cylinder wall. These water droplets come into contact with exhaust gases, forming acidic substances that attach to cylinder wall and cause cylinder corrosion. The lower working temperature of cylinder, more acidic substances are formed and greater corrosive effect. Corrosive wear is mainly caused by incorrect operation. The correct operation is to start starter motor and let it idle for a few minutes. After water temperature and oil pressure are normal, gradually increase engine speed to maintain normal engine temperature. The water temperature in engine while driving is preferably 80 ~90°С. Driving at low temperatures is very unfavorable for engine, and corrosion and wear will also increase.
In order to ensure normal operation of engine and prevent premature wear of cylinder, following suggestions are put forward:
(1) After starting engine, let it idle for a few minutes, then increase engine speed, increase load, maintain normal operating temperature of engine, and decisively end cold driving.
(2) Clean air filter element in time, and replace it in time if it is damaged; it is best to replace air filter element every time second warranty expires (mileage 12,000 kilometers).
(3) Be sure to use a certified supercharged diesel engine oil of CD level and regularlyo change oil to ensure its purity and quality. Change engine oil and oil filter every time.
(4) Frequently check connection between supercharger and intercooler and intake pipe to prevent connecting hose from loosening or breaking, and make sure that air enters cylinder after being filtered.
In short, pay attention to proper operation and maintenance, pay attention to use of car's break-in period, and early cylinder wear can be effectively prevented.
4. Causes and treatment of excessive cylinder liner wear
Due to different piston positions and different operating conditions, wear of each part of cylinder liner is also different. Cylinder liner wear is most severe at contact between first piston ring and cylinder wall, and amount of wear is greatly reduced from top to bottom. This up and down wear makes cylinder tapered. If there are impurities in oil without filtering, impurities contained in oil and metal chips will be brought to surface of cylinder wall, which will cause abrasive wear, and cylinder liner will become drum-shaped after wear. .
Severe wear of cylinder liner in engine is caused by following reasons:
(1) Filter failed. In particular, air filter is broken and unfiltered air enters cylinder through a short circuit. Due to dust and impurities in air entering cylinder, wear of cylinder liner increases.
(2) Wrong grade of engine oil, resulting in poor lubrication and serious cylinder liner wear.
(3) The use of poor quality diesel fuel causes severe coking after combustion, which causes severe wear of cylinder liner.
(4) Problems with work. When starting a cold car, especially in winter, due to high viscosity and poor fluidity of oil, oil supply of oil pump is insufficient, while oil flows along original cylinder wall along cylinder wall after parking, and cannot work at time of starting. Such lubrication increases during cylinder wall wear.
(5) Driving in cold weather. Due to low temperature, it can not maintain good lubrication and increase cylinder wear.
(6) The quality and assembly of cylinder liner.
To prolong life of engine, you must do following:
(1) Start engine correctly. To increase lubrication and reduce wear, when starting engine for first time, it is recommended to use starter motor to idle engine for a few laps and then apply oil to start engine past friction surface. lubricated. After starting, it should idle (650~700 rpm) for 3~min, standby oil pressure is normal, and water temperature reaches 40°C before starting. Start in a low gear and drive slowly for about one kilometer, thenSlowly return to speed after oil pressure and water temperature return to normal.
(2) Maintain normal engine temperature. At low temperature, water vapor in cylinder easily condenses into water droplets, and water droplets dissolve acid gas molecules in exhaust gases, easily forming acidic substances that corrode cylinder; at high temperature, strength of the cylinder liner will be reduced, and wear will increase. Normal engine water temperature should be maintained at 80~90°C.