BASIC STRUCTURE OF IC ENGINES

Even though reciprocating internal combustion engines look very simple in appearance, they are highly complex machines. There are a large number of components which have to perform their functions to produce power. Before going through the working principle of the complex machine, a brief description of the engine components is shown in Figure 6.2.

Figure 6.2 The Components of an IC Engine

• Cylinder: It is a hollow cylindrical structure closed at one end with cylinder head. The combustion of the fuel takes place inside the cylinder. This is known as heart of the engine. It is made of hard and high thermal conductivity materials by casting. A piston reciprocates inside the cylinder and produces power.
• Cylinder head: It covers one end of the cylinder and consists of valves/ports and spark plug/injector.
• Cylinder liner: The internal surface of the cinder is equipped with replaceable liner which can be easily replaced after wear and tear. Liner is used to protect the wear of the cylinder so that replacement of complete cylinder can be avoided.
• Piston: It is a cylindrical component which is fitted perfectly inside the cylinder providing a gas tight space with the piston rings and the lubricant. The piston is connected to connecting rod by hardened gudgeon pin. The main function of the piston is to transfer the power produced by combustion of the fuel to the crankshaft.
• Piston rings: The outer periphery of the piston is provided with several grooves into which piston rings are fitted. The piston is fitted with these rings. The upper ring is known as compression ring and the lower rings are known as oil rings. The function of the compression ring is to compress the air or air–fuel mixture and the function of the oil rings is to collect the surplus lubricating oil on the liner surface.
• Water jacket: Water jacket is an integral part of the cylinder through which cooling water is circulated to prevent the overheating of engine.
• Connecting rod: It connects the piston and the crankshaft. One end, called the small end, is connected to gudgeon pin located in piston and the other end, called big end, is connected to crank pin. The function of the connecting rod is to transfer the reciprocating motion of the piston into rotary motion of the crankshaft.
• Crankshaft: It is a principal rotating part of the engine which controls the sequence of reciprocating motion of the pistons. It consists of several bearings and crank pins.
• Valves: Normally, two valves are used for each cylinder which may be of mushroom shaped poppet type. They are provided either on the cylinder head or on the side of the cylinder for regulating the charge coming into the cylinder and for discharging the products of combustion from the cylinder. The valve mechanism consists of cams, cam follower, push rod, rocker arms, and spring.
• Inlet manifold: This is the pipe which connects the intake system to the inlet valve of the engine and through which air or air–fuel mixture is drawn into the cylinder.
• Exhaust manifold: This is the pipe which connects the exhaust system to the exhaust valve of the engine and through which products of combustion escape into the atmosphere.
• Cams and camshaft: Cam is mounted on a shaft which is known as camshaft. The function of the cam is to facilitate the control of the timing of opening and closing of the inlet and exhaust valve. It provides to and fro motion to the valve rods to open and close the valves.
• Spark plug: In a S.I. engine, a spark plug is located near the top of the cylinder and initiates the combustion of the fuel.
• Carburettor: Carburettor is a device which is used to control the fuel qualitatively in a S.I engine. It atomizes the fuel, mixes with air and vaporizes it and finally sends the air–fuel mixture inside the cylinder through inlet valve.
• Fuel pump and injector unit: This unit is used in CI engines (nowadays injection system is also used in S.I engine as multi-point fuel injection, MPFI). Its function is to supply the fuel to injector under pressure which consists of one or more orifices through which the fuel is sprayed into the cylinder.
• Crank case: It consists of cylinder, piston, and crankshaft. It helps in lubrication of different parts of the engine.
• Flywheel: It is a heavy wheel mounted on the crankshaft to minimize the cyclic variations in speed. It absorbs the energy during power stroke and releases it during non-power stroke. By employing a flywheel, the turning moment becomes uniform at crankshaft.