- Detail

Modern analysis methods for diesel engine design

(I) crankshaft

1, fine analysis of the whole crankshaft

the stress concentration near the fillet of connecting rod journal and main journal and the lubrication of raw materials on the crankshaft, but one valuable oil hole a day (mainly on the connecting rod journal) is an important reason for the damage of the crankshaft

2. Calculation model:

1) continuous beam and spatial rigid frame model: the calculation accuracy is very poor

2) two-dimensional and three-dimensional models of single cranks: the degree of stress concentration can be calculated, but it cannot reflect the influence of adjacent cranks

3) space rigid frame model of equivalent stiffness crank: inconvenient to use

3. Treatment method of connecting crankshaft and engine block

use master-slave relationship

the matching relationship between the nodes on the circumference of the outer surface of the center of each main journal and the corresponding nodes of the main bearing bush of the corresponding main bearing hole is treated as a contact problem (not included in the oil film)

the matching relationship between all nodes on the outer surface of each main journal and the corresponding nodes of the main bearing bush of the corresponding main bearing hole is treated as a contact problem (not included in the oil film)

4. Calculation working condition of crankshaft

due to the influence of ignition sequence, the dangerous working condition cannot be determined at a glance

the only correct way to determine the crankshaft dangerous working condition is to calculate each corner of the crankshaft separately and calculate the full stress amplitude of each node. The crankshaft corners that produce the first few maximum full stress amplitudes are dangerous working conditions

in order to determine dangerous working conditions, a simplified model can be used. The characteristic of the simplified model is that the lattice distribution is relatively uniform, which can greatly reduce the number of nodes, thus greatly reducing the computational workload

5. Dynamic calculation of crankshaft

only the dynamic response calculation can truly calculate the maximum and minimum stress of each node of the crankshaft and make a correct strength evaluation

in the dynamic analysis model of the crankshaft, the oil film should be considered between the main journal and the main bearing and between the connecting rod journal and the connecting rod bearing, that is, in the calculation model, the crankshaft is considered to be supported on the oil film

initial condition: for high-power diesel engine, the dynamic analysis of crankshaft needs to calculate 3 working cycles

for medium and small power diesel engines, the number of working cycles should be more

(II) connecting rod

1. Fine analysis of connecting rod:

the deformation of the connecting rod big end hole, the strength of the connecting rod screw, the parts where the geometric shape of the connecting rod changes violently and the stress of the root of the big end split surface are the key factors affecting the reliability of the connecting rod

2. Calculation model of connecting rod scheme design

1) the analysis of plane model during connecting rod scheme design can meet the needs of design

2) the calculation of connecting rod should consider pre tightening condition, tension condition and compression condition

3. Application of various assembly preloads of connecting rod

the screw assembly preload can be applied to the contact surface between the screw head and the bearing cap in the form of interference. The screw interference value is the sum of its pre elongation and the compression of the pad cover

the interference between the big end bearing bush and the small end bushing of the connecting rod can be applied by the conventional calculation method with friction contact problem

4. The calculation model of the connecting rod in the final check calculation

1) adopts the three-dimensional model

2) crankshaft connecting rod journal and piston pin are included in the model

3) the matching relationship between the five parts of the connecting rod body, bearing cap, screw, bearing bush and bushing, the matching between the connecting rod neck and the big end bearing bush, and the matching between the piston pin and the small end bushing are the same as the plane model, which are all treated as friction contact problems

5. Working condition and load

the calculation working condition of connecting rod is still pre tightening working condition, tension working condition and compression working condition. The calculated load of each working condition is also the same as that of the plane model

(III) piston

1. Fine separation of piston. Function of bolt tray shear strength testing equipment: analyze the top of piston, mainly studying temperature stress

the strength of the piston skirt, especially its pin seat, is mainly determined by the mechanical load

features of combined piston design: optimize the gap between the top and skirt

2. Analysis of piston temperature field

the calculation of piston stable temperature field should establish a three-dimensional model, and adopt mixed boundary conditions (Class 3 and class 1) according to the ambient temperature

the transient temperature field is usually calculated by the second kind of boundary conditions, which can also be done in the same way as the steady-state temperature field. When adopting the latter, it is necessary to provide the temperature of some boundary nodes at each calculation instant as the convergence criterion during iteration

3. Calculation of combined piston temperature field

the difference between the calculation of combined piston temperature field and the overall piston is that there are two iterations in the whole temperature field, one is to calculate the overall temperature field of the piston, and the other is to calculate the contact state of each point on the top and skirt interface. The two are carried out alternately, and the calculation results are the premise of another calculation

4. Static analysis of the piston

the static analysis of the piston includes the calculation of temperature deformation and temperature stress, the calculation of mechanical deformation and mechanical stress caused by the maximum tension condition and the maximum compression condition of the diesel engine, and the analysis of the comprehensive stress after the superposition of the two

5. The calculation model

adopts three-dimensional solid model

6. Calculated load

1) temperature field

2) maximum tensile mechanical load

3) mechanical load of maximum compression

4) pre tightening force of piston screw of combined piston

5) the piston is also subjected to the lateral reaction force of the cylinder liner, which is relatively small and usually ignored

(IV) cylinder head

1. Issues of concern:

1) the "bridge" between each two air valve holes on the flame plate and the stress around the injector hole, especially the temperature stress

2) contact state of each point on the plane of the sealing ring between the cylinder gasket and the cylinder head and cylinder sleeve

3) lateral deformation of air valve guide

2, calculation model

1) the calculation of cylinder head can only be a three-dimensional model using block elements

2) the calculation model of the cylinder head must include the latter, cylinder head bolts and cylinder gaskets, and use the contact model to simulate the matching relationship of these five parts. The matching relationship between the engine block and the cylinder liner should also be treated with contact problems

3. Calculation of cylinder head

calculation of stable temperature field

the mechanical load should mainly consider the gas explosion pressure and the pre tightening force of the cylinder head bolt assembly. The force of the air valve on the valve seat is usually ignored due to its small value

4. Application of cylinder head bolt assembly preload

the bolt assembly preload can be applied to the contact surface between the nut and the cylinder head top plate in the form of interference, and the interference value is the sum of its pre elongation and the compression of the cylinder head

5. Mechanical load calculation condition of cylinder head

preload condition and burst condition

(V) body

1. Fine analysis of the body

the body is the foundation of the diesel engine. The main parts of the diesel engine such as crankshaft, connecting rod, piston, cylinder head and cylinder liner are directly or indirectly fixed on the body, so the strength and especially the stiffness of the body need to be carefully analyzed in the design

the problem of stress concentration exists in a large number in the body

2. Calculation model of the machine body

1) only a single main bearing seat partition wall is calculated

2) space rigid frame model

3) simple machine body (including main bearing cap)

4) the connection between the main bearing cover and the main bearing seat, the connection between the cylinder cover and the gasket and the cylinder sleeve, the connection between the cylinder sleeve and the machine body, the connection between the main bearing bush and the main bearing seat and the main bearing cover, the connection between the crankshaft and the main bearing bush, and the connection of various bolts are all treated as contact problems

3. Calculated load of the engine block

1) the main shaft load acting on the main bearing bush by each main journal of the crankshaft

2) the gas pressure acting on the flame plate of each cylinder head and the inner wall of the cylinder liner

3) piston side pressure acting on the inner wall of each cylinder liner

4) various bolt forces

5) camshaft force: relatively small in value, usually ignored

6) influence of temperature field: temperature stress is usually ignored in calculation

4. Calculation condition of the engine block

the selection of calculation condition can be carried out according to one of the following two principles:

1) take the instantaneous explosion of the cylinders on both sides of the partition wall of the middle main bearing seat as the calculation condition

2) the instantaneous explosion of the cylinders on both sides of the partition wall of the main bearing seat that produces the maximum main bearing load that affects the measurement accuracy is taken as the calculation condition

(VI) turbocharger turbine rotor

1. Detailed analysis of turbocharger turbine rotor

the working speed is very high, and the compressor and turbine of the rotor have heavy working load, resulting in great stress

the frequent structural problem of turbine rotor in the working process is blade flying off

contact problem of cyclic symmetric structure

2. Acting load

the loads borne by the turbine include aerodynamic force, centrifugal force caused by high-speed rotation and temperature load, and centrifugal force is the main force

3, calculation model

1) establish a calculation model for the whole rotor

2) the model is established according to the contact problem for the assembly of the mortise and groove of one blade and the mortise and groove of the wheel disc, and the connection between the other mortise and groove of the wheel disc is calculated according to the continuum

3) variable thickness leveling can be used in scheme design. We all know that large equipment installation will be more troublesome

4) the three-dimensional model should be used in the final check of strength

(VII) turbocharger compressor rotor

1. Fine analysis of turbocharger compressor rotor

the compressor is assembled by impeller, shaft sleeve and main shaft through interference

there are two types of impellers: integral and split

2. Characteristics of compressor calculation

the structural analysis of impeller is an elastoplastic problem

the analysis of compressor is an elastoplastic contact problem

3. Acting load

the acting load of the compressor is the centrifugal force generated by aerodynamic force and high-speed rotation, and the centrifugal force is the main force

the interference fit between the impeller and the shaft sleeve and between the shaft sleeve and the main shaft must also be considered in the calculation

4. Calculation model

the overall three-dimensional elastoplastic contact model including impeller, shaft sleeve and spindle

(VIII) impeller

1, elastoplastic calculation of impeller

1) the constitutive relationship curve of aluminum alloy can be regarded as a linear strengthening curve

2) the incremental theory must be used in the elastoplastic calculation of compressor, and the loading step should be reasonably selected in the calculation

2. Pre overspeed manufacturing process

1) before the impeller is processed and not assembled, it is preloaded with an overspeed speed greater than the rated speed, unloaded after stable operation for a period of time, and then assembled

2) for the impeller manufactured by pre overspeed process, the whole process calculation must be carried out according to the order of preloading, unloading and final loading of overspeed speed. The residual deformation of each node after pre overspeed loading and unloading should be treated as the initial deformation of final loading

(end)

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