汽车发动机机械要点培训.ppt

1、Chapter 10,Prepared by Martin Restoule Algonquin College,Short Blocks,10,Chapter 10,Objectives,List the parts that make up a short block and briefly describe their operation. Describe the major service and rebuilding procedures performed on cylinder block assemblies. Explain crankshaft construction,

2、 inspection, and rebuilding procedures. Explain the function of engine bearings.,continued,Chapter 10,Objectives,Explain the common service and assembly techniques used in connecting rod and piston servicing. Describe the procedure for installing pistons in their cylinder bores. Describe the purpose

3、, operation, and location of the camshaft.,continued,Chapter 10,Objectives,Describe the three types of camshaft drives. Inspect the camshaft and timing components. Describe how to install a camshaft and its bearings.,Chapter 10,Short Block Components,Cylinder block The cylinder block is normally a o

4、ne or two-piece casting that forms the lower section of the engine and houses most of the engines moving parts. Blocks may be cast from iron or aluminum.,continued,Chapter 10,Short Block Components,The cylinder block houses integral cylinder bores or removable cylinder sleeves. The block also houses

5、 a number of passageways and machined surfaces for various systems and components.,continued,Chapter 10,Short Block Components,Crankshaft Crankshafts are generally made of cast iron, forged steel, or nodular iron and machined for bearing fit and balance. Counterweights oppose the weight of the rod j

6、ournals and piston assemblies.,continued,Chapter 10,Short Block Components,Crankshaft Main bearing journals are machined at the crankshaft centreline and support the crankshaft in the block. Rod bearing journals are machined offset to allow the pistons force to rotate the crankshaft during operation

7、.,continued,Chapter 10,Short Block Components,Crankshaft bearings The crankshaft uses two-piece friction insert bearings that are fitted closely to the crankshaft to control proper clearance for oil pressure, and flow. These bearings are usually made from layers of different materials to retain thei

8、r shape and resist wear, expansion, corrosion, and fatigue. Aluminum alloy bearings are the most commonly used design,continued,Chapter 10,Short Block Components,Crankshaft bearings Bearing Spread is used to ensure positive positioning against the inside of the bearing bore to help keep the bearing

9、in place during assembly. Bearing spread means that the distance across the bearing parting edges is slightly larger than the bearing bore.,continued,Chapter 10,Short Block Components,Crankshaft bearings Each bearing half or insert is made slightly greater than exactly half which extends the parting

10、 faces slightly beyond the seat. This is called bearing crush which forces the bearing halves tightly into the housing bore. This allows for increased heat transfer and prevents the bearing from rotating during operation.,continued,Chapter 10,Short Block Components,Crankshaft bearings Most bearings

11、use locating lugs to locate the bearing in the correct position during installation. These lugs also prevent sideways movement of the bearing during operation.,continued,Chapter 10,Short Block Components,Connecting rods The connecting rod is used transmit the pistons force to the crankshaft. Connect

12、ing rods are generally forged high-strength steel, nodular steel or cast iron. The centre section is formed in an “I” shape for maximum strength and minimum weight. The small end is made to accept the piston pin and the big end is machined into two pieces to accept the connecting rod insert bearings

13、.,continued,Chapter 10,Short Block Components,Pistons The piston and piston rings forms the lower portion of the combustion chamber. The combustion pressure exerted on the piston forces it downward during the power stroke. Pistons must be strong enough to withstand this pressure and heat while being

14、 as light as possible. Most modern pistons are made of aluminum alloys.,continued,Chapter 10,Short Block Components,Piston Features and Terminology,continued,Chapter 10,Short Block Components,Piston Rings Seal the combustion chamber at the piston. Control the amount of oil on the cylinder walls to p

15、revent excess oil from entering the combustion chamber. Carry heat from the pistons to the cylinder walls to help cool the piston.,continued,Chapter 10,Short Block Components,Piston Rings Piston rings fall into two categories; oil control and compression. Compression rings are usually one-piece cast

16、 iron with oil control rings being a one-piece cast iron or a three-piece assembly (two steel scraper rings with one expander).,continued,Chapter 10,Short Block Components,Piston Ring Types Compression rings Along with engine oil form a seal between the piston and the cylinder wall. Most piston desi

17、gns use two compression rings. Oil control rings Remove the excess oil on the cylinder walls to maintain the correct amount to lubricate the cylinder walls and help seal the compression rings.,continued,Chapter 10,Short Block Components,Core plugs Core (frost or expansion) plugs are used to seal cor

18、e holes in all cast cylinder blocks. The purpose of the core holes is to allow the sand from the sand core moulds to exit the block after the blocks are cast.,continued,Chapter 10,Short Block Components,Cylinder Sleeves Commonly used in aluminum engine blocks or for integral cylinder repair. Removab

19、le sleeves are wet and supported at the top and bottom while dry sleeves are fitted inside an oversized bore. Both sleeves are pressed into the block.,Chapter 10,Short Block Service and Rebuilding,Prior to reconditioning or rebuilding; Clean threaded holes with the correct sized tap to remove dirt a

20、nd burrs that can affect bolt torquing. Chamfer any holes that have signs of thread pulls or jagged edges. Damaged threads in aluminum blocks can be repaired by installing a thread insert.,continued,Chapter 10,Short Block Service and Rebuilding,Deck Flatness The top of the engine block where the cyl

21、inder head mounts, is called the deck. To check deck warpage, use a precision straightedge and feeler gauge. With the straightedge positioned lengthwise and diagonally across the deck, the amount of warpage is determined by the size of feeler gauge that fits into the gap between the deck and the str

22、aightedge.,continued,Chapter 10,Short Block Service and Rebuilding,Cylinder Bore Inspection Taper is the difference in diameter between the bottom of the cylinder bore and the top of the bore just below the ridge. Subtract the smaller diameter from the larger diameter gives the cylinder taper. Some

23、taper is permissible, but normally not more than 0.1524 mm (0.006 in.).,continued,Chapter 10,Short Block Service and Rebuilding,Cylinder Bore Inspection Cylinder out-of-roundness is the difference of the cylinders diameter when measured parallel with the crank and then perpendicular to the crank. Ou

24、t-of-round is measured at the top of the cylinder just below the ridge. Typically, the maximum allowable specification is 0.0381 mm (0.0015 in.).,continued,Chapter 10,Short Block Service and Rebuilding,Cylinder Measurement A dial bore gauge and a micrometer can be used to measure bore diameter, out-

25、of-roundness and taper. A telescoping gauge can also be used with a micrometer.,continued,Chapter 10,Short Block Service and Rebuilding,Cylinder Measurement When using a dial bore gauge or a telescoping gauge to check a cylinders bore, make sure the measuring arms are parallel to the plane of the cr

26、ankshaft. The best way to do this is to rock the gauge until the smallest reading is obtained.,continued,Chapter 10,Short Block Service and Rebuilding,Cylinder Bore Surface Finish The desired cylinder wall finish is comprised of many small crisscross grooves that ideally cross at 50- to 60-degree an

27、gles. This finish leaves millions of tiny diamond-shaped areas, which serve as lubricant reservoirs. This finish also leaves small flat areas on the surface that a film of oil adheres to and acts as a bearing surface for the piston rings.,Chapter 10,Short Block Service and Rebuilding,Cylinder Deglaz

28、ing If the cylinder measurements are within acceptable limits, the cylinder walls may only need to be deglazed. Combustion heat, engine oil, and piston movement combine to form a thin residue on the cylinder walls commonly called glaze. A ball hone with fine honing stones, often referred to as degla

29、zing or honing stones remove the glaze and leave the walls with the desired finish.,continued,Chapter 10,Short Block Service and Rebuilding,Desired Cylinder Surface Finish A Ball Hone,continued,Chapter 10,Short Block Service and Rebuilding,Cylinder Boring When cylinder surfaces are badly worn or exc

30、essively scored or tapered, a boring bar or boring machine is used to cut the cylinders for oversize pistons or sleeves. A boring bar leaves a pattern on the cylinder wall similar to uneven screw threads.,continued,Chapter 10,Short Block Service and Rebuilding,Cylinder Boring To remove boring marks,

31、 you should hone the bore after it has been bored. Using a rigid Hone will slightly enlarge the cylinder so the boring should only be performed to the exact size of the new pistons. The honing procedure will produce the required piston clearance.,continued,Chapter 10,Short Block Service and Rebuildi

32、ng,Crankshaft Saddle Alignment If the block is warped and its main bearing bores are out of alignment, the crankshaft will inflict heavy loads on one side of the main bearings. Engine blocks that are not severely warped can be repaired by an operation called line boring or honing, a machining operat

33、ion in which the main bearing housing bores are bored to standard size and alignment Badly warped blocks must be replaced.,continued,Chapter 10,Short Block Service and Rebuilding,Crankshaft Saddle Alignment Saddle alignment can be checked by trying to rotate a precisely ground arbor in the bearing b

34、ores or by using a straightedge and feeler gauge. If a feeler gauge that is half the maximum oil clearance will fit under the straightedge, the saddles are out of align- ment and the block must be line-bored.,Chapter 10,Crankshaft Inspection and Service,Crankshaft Inspection Visually inspect the cra

35、nkshaft for signs of damage to the vibration damper and flywheel mounting surfaces, damage or discolouration of the bearing journals, worn or sharply ridged sealing surfaces, and signs of surface cracks or hardness distress. If any or all of these conditions are present, the parts need to be repaire

36、d or replaced.,continued,Chapter 10,Crankshaft Inspection and Service,Crankshaft Measurements An outside micrometer should be used to measure the journals for size, out-of-roundness, and taper. Taper is measured from one side of the journal to the other with a maximum taper of 0.0254 mm (0.001 in.).

37、 Journal out-of-round is measured with two readings 90 from each other at both sides of the journal.,continued,Chapter 10,Crankshaft Inspection and Service,Crankshaft Measurements Compare these measurements to specifications to determine if the crankshaft needs to be reground or replaced. If the jou

38、rnals are within specifications, the journal area needs only to be cleaned up.,continued,Chapter 10,Crankshaft Inspection and Service,Crankshaft Reconditioning A crankshaft that has minor damage to the journal surface or measurements out of specifications may be reusable after the journals are refin

39、ished. This process grinds away some of the metal on the journals to provide an even and mar-free surface. When a crankshaft has been ground, undersize bearings are fitted to the crank to provide for the proper oil clearances.,continued,Chapter 10,Crankshaft Inspection and Service,Crankshaft Recondi

40、tioning At times, minor damage to the journals can be corrected by polishing the journals with a very fine crocus cloth. A polishing tool rotates a long loop of crocus cloth against the journals as the crankshaft is rotated by a stand. A crankshaft with severely damaged journals should be replaced.,

41、Chapter 10,Crankshaft Bearing Service,Crankshaft Bearing Inspection It is a common and recommended practice to replace all crankshaft bearings during engine rebuilding. Inspect all bearings for signs of uneven wear or damage which could indicate bent or damaged components.,continued,Chapter 10,Crank

42、shaft Bearing Service,Reconditioned Crankshaft Bearings Undersize bearings are available in .0254 mm (0.001 in.) or .0508 mm (0.002 in.) sizes for shafts that are uniformly worn by that amount. Undersize bearings are also available in standard sizes, such as .2540 mm (0.010 in.), .5080 mm (0.020 in.

43、), and .7620 mm (0.030 in.), for use with crankshafts that have been refinished (or reground).,continued,Chapter 10,Crankshaft Bearing Service,Reconditioned Crankshaft Bearings The difference in thickness of the bearing is normally stamped onto the back of the bearing and some may also be colour-cod

44、ed to indicate their size.,Chapter 10,Piston and Connecting Rod Service,Connecting Rods Connecting rods suspected of being bent or distorted can be checked with a rod alignment fixture. Normally damaged rods are replaced, although equipment is available to straighten them and to bore the small and b

45、ig ends. Replace any rods that show signs of cracks, no matter how small.,continued,Chapter 10,Piston and Connecting Rod Service,Piston Installation To ensure that the piston is installed correctly and has the correct offset, the top of the piston has a mark. The most common mark is a notch, machine

46、d into the top edge of the piston. Always refer to the appropriate service manual to determine the correct direction and position of the mark. It is important that the front of the piston match the front of the connecting rod,continued,Chapter 10,Piston and Connecting Rod Service,Piston Installation

47、 Install the rings on the piston as specified and place crankshaft protectors on the connecting rod bolts.,continued,Chapter 10,Piston and Connecting Rod Service,Piston Installation Install new connecting rod bearing inserts into the connecting rods and rod caps and make sure the tang on the bearing

48、 fits snugly into the matching notch. Be sure to coat the crankshaft assembly with clean lubricant or engine oil. The piston and rod can be assembled in the block according to recommended procedures.,continued,Chapter 10,Piston and Connecting Rod Service,Piston Installation When all the pistons and

49、rods have been installed, connecting rod side clearance can be measured. After each piston assembly is installed in the block, rotate the crankshaft and check its freedom of movement.,Chapter 10,Camshafts,A camshaft is a shaft with a cam for each exhaust and intake valve, each one placed to allow fo

50、r the proper timing of each valve. A cam is a device that changes rotary motion into reciprocating motion. Each cam has a high spot or lobe that controls the opening and closing (timing) of the valves. The height of the lobe is proportional to the amount the valve will open.,continued,Chapter 10,Cam

51、shafts,Camshaft Location The camshaft can be located in either the cylinder block or cylinder head(s). The camshaft fits into a bore next to the crankshaft on some engines, unless the engine has overhead camshafts where it is mounted in bearing caps in the head.,Chapter 10,Camshaft Drives,Gear drive

52、 A gear on the crankshaft meshes directly with a gear on the camshaft. Chain drive Sprockets on the crankshaft and the camshaft are linked by a continuous chain. Belt drive Sprockets on the crankshaft and the camshaft are linked by a neoprene belt.,continued,Chapter 10,Camshaft Drive Mechanisms,Chap

53、ter 10,Valve Lifter Types,Hydraulic lifters Use oil to maintain zero valve lash in the valve train movement. Solid lifters Require a clearance between parts of the valve train. Roller lifters Use a roller to minimize friction and can be either hydraulic or solid.,Chapter 10,Valve Lifter Types,contin

54、ued,Chapter 10,Valve Train Inspection,Check the timing components for damage or wear. Check the bottom of lifters for scoring or pitting. Check camshaft lobes for scoring, scuffing, cracks, and wear. Use a dial indicator or outside micrometer to check camshaft lobes.,continued,Chapter 10,Camshaft Bearings,The camshaft and balance shafts are supported by several friction-type bearings, or bushings. They are a one piece design and are typically pressed into the camshaft bore in the engine block. Overhead camshaf