• It can be divided into three parts: crown, ring groove and skirt (see Fig.2-4).
• Its reciprocating movements in the cylinder transform the energy of the expanding gases into mechanical energy．
• Crankshaft and flywheel group includes the • following components (see Fig. 2-5):
• Crankshaftis one of the most important parts in the engine．
• It is held in the crankcase in the cylinder block and converts the reciprocating motion of the pistons into a rotary motion for the drive wheels．
Unit Two Engine Construction
Passage A Crank Mechanism and Its Main
Components
• Crank mechanism is one of the major motion mechanisms of the engine．
• Its function is to transform the reciprocating movement of pistons into rotary movement of the crankshaft, offering torque to make the drive wheels turn (see Fig.2-1).
• In a 4-cylinder engine, the crankshaft usually has three main journals and four crank pins attached to the big ends of the connecting rods．
• Every crankshaft has counter weights whose purpose is to balance the weight of the connecting rods．
• Its front end is used to drive the valve train, water pump and cooling fan, and its rear end is used to install the flywheel．
• Flywheel is a rotating disk located on one end of the crankshaft．
• Cylinder head is bolted to the top of the cylinder block with a gasket in between sealing the block and forming the combustion chambers with the cylinders．
• Piston and connecting-rod group has some main components as follows (indrical shaped hollow part made of aluminum alloy．
• It has a small end and a big end．
• The small end attaches to the piston with the piston pin and the big end connects to the crank pin of the crankshaft．
• Piston rings are inserted in the ring grooves on the top of the piston．
• Most pistons have three rings: the top two are compression rings and the lower ring is an oil ring．
• Cylinder head holds camshafts, valves, spark plugs and injectors．
• Oil panis at the bottom of the cylinder block．
• It is a container for storage of engine oil．
• They function as sealing between the piston and cylinder wall, lubricating, heat transferring and piston supporting in the cylinder．
• Connecting rod connects the piston to the crankshaft．
• Crank mechanism is divided into 3 groups: block group; piston and connecting-rod group; and crankshaft and flywheel group．
• The block group contains the following main components (see Fig. 2-2):
• It has a ring gear around the disk．
• Its purposes are to reduce vibration caused by the power stroke through its inertia, and to start the engine when the starter meshes with it．