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1、Chapter 2 Internal Combustion Engine 2.3 Engine block and head,New Words,Chapter 2 Internal Combustion Engine 2.3 Engine block and head,2.3.1 Engine block The engine block (or cylinder block) serves as a rigid metal foundation for all parts of an engine (Fig.2-4). It contains the cylinders and suppo
2、rts the crankshaft and camshaft. Accessory units and the clutch housing are bolted to it. Blocks are made of either cast iron or aluminum. The cylinder is a round hole formed in the block. It forms a guide for the piston and acts as a container for taking in, compressing, firing, and exhausting the
3、air-fuel charge. Cylinders have been made of both steel and cast iron. Cast iron is by far the most popular. When steel cylinders are desired in an aluminum block, they are installed in the form of cylinder sleeves (round, pipe-like liners), These sleeves may be either cast or pressed into the block
4、. Some engines use removable cylinder sleeves. When the cylinder becomes worn, the old sleeves can be pulled out and new sleeves can be pressed in.,Chapter 2 Internal Combustion Engine 2.3 Engine block and head,Fig.2-4 Cylinder block, cylinder head and camshaft,Chapter 2 Internal Combustion Engine 2
5、.3 Engine block and head,2.3.2 Cylinder head The cylinder head fastens to the top of the block (Fig.2-4). The underside forms the combustion chamber with the top of the piston. In-line engines of light vehicles have just one cylinder head for all cylinders; larger in-line engines can have two or mor
6、e. Just as with engine blocks, cylinder heads can be made of cast iron or aluminum alloy.,Chapter 2 Internal Combustion Engine 2.4 Pistons, Connecting Rods and Crankshaft,New Words,2.4.1 Piston and connecting rod The piston must move down through the cylinder to produce a vacuum to draw a fuel charg
7、e into the cylinder. It then travels up in the cylinder and compresses the mixture. When the mixture is fired, the pressure of the expanding gas is transmitted to the top of the piston. This drives the piston back down through the cylinder with great force, transmitting the energy of the expanding g
8、as to the crankshaft. The piston then travels up through the cylinder and exhausts the burned fuel charge. Pistons are usually made of aluminum. Often, aluminum pistons are tin-plated to allow proper break-in when the engine is started. Aluminum pistons can be forged, but they are more commonly cast
9、. Cast iron is a good material for pistons used in a slow-speed engine. It has excellent wear characteristics and will provide good performance.,Chapter 2 Internal Combustion Engine 2.4 Pistons, Connecting Rods and Crankshaft,Fig.2-5 Pistons, connecting rods and crankshaft,As the name implies, conne
10、cting rods are used to connect pistons to the crankshaft (Fig.2-5). The upper end of the rod has a hole through it for the piston pin. The bottom of the large end of the connecting rod (Fig.2-6) must be removed so the rod can be installed on the crankshaft journal. The section that is removed is cal
11、led the connecting rod cap. Connecting rods are normally made of alloy steel. They are drop-forged to shape and then machined.,Fig.2-6 Connecting rod,Chapter 2 Internal Combustion Engine 2.4 Pistons, Connecting Rods and Crankshaft,2.4.2 Crankshaft The engine crankshaft (Fig.2-7) provides a constant
12、turning force to the wheels. It has throws to which connecting rods are attached, and its function is to change the reciprocating motion of the piston to a rotary motion to drive the wheels. Crankshafts are made of alloy steel or cast iron. The crankshaft is held in position by a series of main bear
13、ings. The maximum number of main bearings for a crankshaft is one more than the number of cylinders. It may have fewer main bearings than cylinders.,Chapter 2 Internal Combustion Engine 2.4 Pistons, Connecting Rods and Crankshaft,Most engines use precision insert bearings that are constructed like t
14、he connecting rod bearings, but are somewhat larger. In addition to supporting the crankshaft, one of the main bearings must control the forward and backward movement.,Fig.2-7 Crankshaft,Chapter 2 Internal Combustion Engine 2.5 Valve System,New Words,The camshaft must turn at one-half crankshaft spe
15、ed. The camshaft is linked with crankshaft by one of three ways: belt drive, chain drive and gear drive. The main components of the valve system are: Camshaft. It is used to open and close the valves. Generally only one camshaft is used in most engines. Newer engines are increasingly equipped with t
16、wo or more camshafts (Figure 2-8). Valves. Each engine cylinder ordinarily has two valves. However, modern engines often use four valves per cylinder (two intake valves and two exhaust valves). In order to prevent burning, the valve must give off heat to the valve seat and to the valve guide. The valve must make good contact with the seat and must run with minimum clearance in the guide.,Chapter 2 Internal Combustion Engine 2.5 Valve System,Valve lifters. Mech
