CLASSIFICATION OF IC ENGINES

There are several bases for classification of IC engines, some of the important bases can be explained as

• Number of strokes per cycle.
• Nature of thermodynamic cycle.
• Ignition systems.
• Fuel used.
• Arrangement of cylinders.
• Cooling systems.
• Fuel supply systems.

Number of Strokes per Cycle: IC engines can be classified as four stroke engines (4S) and two stroke engines (2s). In four stroke engines, the thermodynamic cycle is completed in four strokes of the piston or two revolutions of the crankshaft whereas in two stroke engines, the thermodynamic cycle is completed in two strokes of the piston or one revolution of the crankshaft.

Nature of Thermodynamic Cycle: IC engines can be classified as Otto cycle, diesel cycle, and dual cycle engine. In Otto cycle engine, heat addition, and heat rejection occur at constant volume; therefore, this is also known as constant volume engine whereas in diesel cycle engine, heat addition occurs at constant pressure, and heat rejection occurs at constant volume. In dual cycle, heat addition occurs partly at constant volume and partly at constant pressure but heat rejection occurs fully at constant volume.

Ignition Systems: There are two modes of ignition of fuel inside the cylinder: spark ignition and self or compressed ignition. In spark ignition, sparking starts at the end of compression stroke from spark plug while in compressed ignition the temperature of the fuel is increased to the self-ignition point by compressing the air alone and at the end of compression fuel is injected inside the cylinder.

Fuel Used: On the basis of fuel used, IC engines can be classified as:

• Gas engines like CNG, natural gas, etc.
• Petrol engine.
• Diesel engine.
• Bi-fuel engine.

In bi-fuel engine two types of fuel are used like gaseous fuel and liquid fuel.

Arrangement of Cylinders: According to arrangement of cylinders (Figure 6.1), IC engines can be classified as:

• In-line engines.
• V-engines.
• Opposed cylinder engines.
• Opposed piston engines.
• X-type engines.
• Radial engines.

Figure 6.1 Classification of IC Engines on the Basis of Cylinders Arrangement

In an in-line cylinder engine, all the cylinders are arranged linearly and transmit power through a single crankshaft. V-engines have two banks of cylinders arranged in the shape of English letter V and single crankcase and crankshaft is used to transmit the power. In opposed cylinder engines, all the cylinders lie in same plane but the cylinders are arranged on both sides of the crankshaft at 180°. It is inherently well balanced. When a single cylinder houses two pistons each of which drives a separate crankshaft, it is called an opposed piston engine. The moments of the pistons are synchronized by coupling two crankshafts. It is also inherently well balanced. X-type engines have four cylinders with single crankcase and single crankshaft. The cylinders are arranged in the shape of English letter X. In radial engine, cylinders are arranged in radial directions like the spokes of a wheel and are connected to a single crankshaft. These engines are used in conventional air-cooled aircraft engines.

Cooling Systems: There are two types of cooling systems in IC engines: water cooling and air cooling. In water cooling, coolant, and radiators are provided to cool the cylinder. In air cooling, fins are provided on the surface of the cylinder to radiate the heat into the atmosphere. Low power engines like motorbikes are equipped with air cooling systems whereas large power producing engines like car, bus, truck, etc. are equipped with water cooling systems.

Fuel Supply Systems: On the basis of fuel supply systems, IC engines can be classified as:

• Carburettor engine.
• Air injection engine.
• Airless or solid or mechanical injection engines.

In a carburettor engine, air, and fuel are properly mixed into the carburettor and then fed into the cylinder. In air injection engines, fuel is supplied to the cylinder with the help of compressed air. In mechanical injection engines, the fuel is injected inside the cylinder with the help of mechanical pump and nozzle.

6.3  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.

6.3.1  Nomenclature

There are various terms which are frequently used in an IC engine and are discussed below.

• Cylinder bore (d): The nominal inner diameter of a cylinder is called cylinder bore which is designated by an English letter ‘d’ and expressed in millimetre (mm).
• Piston area (A): The area of the inner diameter of a cylinder is known as piston area. It is measured in terms of square centimetre (cm²) or square millimetre (mm²).
• Stroke (L): The axial distance for which a piston moves inside a cylinder in one stroke is known as stroke or stroke length (Figure 6.3) which is designated by an English letter ‘L’ and measured in terms of millimetre (mm). Figure 6.3 Stroke Length and Stroke Volume in an IC Engine
• Dead centres: The positions of the piston, at the moments when the direction of the piston motion is reversed are known as dead centres. There are two dead centres—top dead centre (TDC) and bottom dead centre (BDC). The farthest position of the piston head from the crankshaft is known as TDC and nearest position of the piston head from the crankshaft is known as BDC as shown in Figure 6.3.
• Displacement/stroke/swept volume (V_s): The nominal volume swept by the working piston when travelling from one dead centre to the other is called the displacement volume. It is expressed in terms of cubic centimetre (cc) and is given by,  
• Clearance volume (V_c): The nominal volume of the combustion chamber above the piston when it is at the TDC is known as clearance volume (V_c) and is expressed in cc.
• Compression ratio (r_v): It is the ratio of the total cylinder volume when the piston is at BDC to the clearance volume.