The External Combustion Engines are those engines in which the combustion occurs separately from the work output device such as the turbine. Hence in many of the cases the working fluid is heated at a specific place and then ported to another chamber where positive work output device is installed such as turbine.

The External Combustion Engines can be classified on the basis of different factors which are as follows:

a) Transportation

b) Industrial purposes

c) Aeroplanes / Commercial Aviation

d) Fighter jets

e) Shuttles and Rocket Propulsion

f) Racing Automobiles.

a) Coal

b) Furnace Oil

c) LPG

d) Kerosene

a) Steam

b) Oxy-Fuel Mixture

The steam turbine is considered one of the most important components in the Industrial World. Following are the salient features of a steam turbine.

In the steam turbine, the working fluid is steam. The steam cycle usually works on Rankine Cycle. The main components of a Steam Turbine working on Rankine cycle are Boiler, Condenser, Turbine, Pump.

iv) The water is then pumped to the boiler from where the cycle continues.

The steam turbine described above the most simple and the very basic of all steam turbines. However by some auxilliary installations in the steam plant and by some changes in the basic design higher efficiencies can be attained.

The Internal Combustion Engines are those engines in which the combustion occurs insider the work output device such as the petrol, diesel engine and gas turbines.

The Otto Cycle is the ideal air standard cycle for the petrol engine, the gas engine, and the high speed oil engine. The cycle is shown on p-v diagram in Figure (1)

Process 1 to 2 is isentropic compression

Process 2 to 3 is reversible constant volume heating

Process 3 to 4 is isentropic expansion

Process 4 to 1 is reversible constant volume cooling.

Diesel worked on the idea of spontaneous ignition of powdered coal, which was blasted into the cylinder by compressed air. Oil became the accepted fuel used in compression ignition engines, and the oil was originally blasted into the cylinder in the same way that Diesel had intended to inject the powdered coal. Fig. (2) shows the p-v diagram of the cycle.

Process 1 to 2 is isentropic compression

Process 2 to 3 is reversible constant pressure heating

Process 3 to 4 is isentropic expansion

Process 4 to 1 is reversible constant volume heating

Fig. (3) shows the process of the Dual Combustion Cycle.

Process 1 to 2 is isentropic compression

Process 2 to 3 is reversible constant volume heating

Process 3 to 4 is reversible constant pressure heating

Process 4 to 5 is isentropic expansion

Process 5 to 1 is reversible constant volume cooling.

d) Four Stroke (Naturally Aspirated)

a) Over head (I-head) Valves

b) Under Head Valves (L-Head)

c) Rotary Valves

d) Cross Scavanged

a) Petrol Gasoline Engine

b) Diesel Engine

c) Dual Fuel Engine

d) LPG (Liquid Petroleum Gas)

e) CNG

a) Carburation

b) Fuel Injection into the intake ports or intake manifold

c) Fuel injection into the engine cylinder

a) SI (Spark Ignition)

b) CI (Compression Ignition)

c) Electronic Fuel Ignition (EFI)

a) Open Chamber

b) Divided Chamber

a) Throttling of fuel and air flow together (SI)

b) Control of fuel flow alone (CI)

c) A combination of the two

a) Air Cooled

b) Water Cooled

A simplest engine requires at least following basic parts.

1) Cylinder

2) Piston and piston rings

3) Connecting Rod

4) Crank Shaft

5) Valves and valves drive

The cylinder is round, hollow tube closed off at one end open at the other end. It is the stationary part of the engine; all other parts move in one way or other. The cylinder in the automobile engine is part of basic frame work known as the cylinder block. The block, usually made of iron or aluminum, contains four or more cylinders.

The piston is a round piece of aluminum designed so that it will fit into a cylinder very snugly, yet be able to slide up and down with some freedom. The piston is commonly flat or domed shaped at one end and open at the other end. A large hollow in which a steel pin running from side to side in the piston allows for the attachment of the connecting rod. When the piston moves within the cylinder until it reaches the upper limit f its travel. The space remaining is known as combustion chamber.

There are three or five piston rings in a cylinder, which are fixed in a groove so that the system can be airtight. Suppose a three ring piston. Here, the upper two rings are compression rings and the last one is oil ring.

Connecting rod is used to transmit power from piston to the crank shaft. It usually has a small end, and the large end is attached to the connecting rod journal of the crankshaft. The connecting rod itself is made from steel forged into an I-beam shape. This method of manufacture and shape give it the strength which it need to transmit all the downward force from the piston to the crankshaft.

The job of Crank Shaft is to turn up and down motion to rotary motion. This is the basic operation of most automotive engines in use today.

The crank shaft are made by the forging process. In which heating and the hammering is done to convert the steel or cast rod or bar into crankshaft shape. In small engines it can also be made by casting.

The operation of the internal combustion engine necessities the admission to the trapping in and the exhausting of the working medium from the engine cylinder all of which is accomplished with valves and valves mechanism. The valves must be open and close at definite crank angle and with the minimum of noise and wear.

Valve opening is should be sufficient to obtain ample breathing capacity. They must seat tightly and prevent the escape of gases, during the working process. Also, the valve should be made of material that will not be made of material that will not be seriously effected by the various temperature, erosive and corrosive conditions.

The valve mechanism consist of a crankshaft, cams (usually integrated with crankshaft), tappets and springs (also push rod and rocker arms for over head valves). The entire mechanism is an elastic system will deflects when subjected to the various forces applied during operation.

There are two types of it.

1) Suction valve

2) Exhaust valve.

Every Engine has Auxiliary systems which are required to make it run or keep it running. Those listed below are the most basic Auxiliary systems.

1) Fuel System

2) Ignition System

3) Lubrication system

4) Cooling System

5) Starting System

6) Charging System

These systems often rely upon each other for assistance to perform properly. As an example, for the lubricating system to do its job, the cooling system must keep the engine oil within normal operating temperatures. When the oil is t cool, it does not lubricate as well as it should; when the oil become over heated, it burns (oxidizes), which causes it lose much of its slippery quality.

The purpose of an engine fuel system is to provide the cylinder with a mixture of air and fuel in the correct proportions for the engine requirements at any particular instant. There are basically two methods available, one is called carburation and is used for petrol engines and the other is a type of fuel injection which is a characteristic method for diesel engines.

The fuel system comprises the fuel tank, lines or tubing, fuel pump, fuel filter, and carburetor or injectors. Its prime function is to deliver the correct amount of fuel to the intake manifold or cylinder under all conditions of engine speed and load requirements.

The ignition system consists of ignition switch, wiring harness, resistance or ballast resistor, ignition coil, distributor, high tension (secondary) wiring, spark plugs, and low tension (primary) wiring. Electrical energy to operate the ignition system is obtained is from the battery while cranking, and from the charging system while running.

The lubrication system consists of the oil pan, oil pump, oil filter, oil galleries (oil passages in the engine block), and the oil itself. When an engine is run, great internal friction develops. The natural characteristics of oil reduce friction and the resulting heat. The pump picks up oil from the pan and forces it through the filter. From the point, the oil goes to each of the bearing and moving parts requiring lubrication. Oil is splashed on those surfaces that are not directly fed with pressure oil.

The cooling system keeps the engine from overheating. The engine's burning fuel creates heat so great that it could melt the metal cylinders. The cooling system uses either air-cooled water or cool air to regulate the temperature of the cylinders.

Most Automobile engines are water-cooled. The water circulates through the Radiator and the engine. It is cooled by air passing around the Radiator's tubes and fins. A pump sends the cooled water to the water jacket surrounds the cylinders. The water absorbs some of the engine's heat and returns to the Radiator to be re-cooled. An engine-driven or electric fan helps move air through the Radiator when the car is stopped or moving at low speed.

The parts of liquid cooling system include the radiator and its hoses, water pump, water jackets (passages within the block where coolant flows), and thermostat. As an engine is run, the heat that was not used in creating power in the cylinder must be taken away. This job is performed by the cooling system. The water pump takes cooling liquid from the radiator and pushes it through the engine block. A thermostat places near the temperature of the coolant. The radiator serves to reduce the temperature of the coolant. In the air-cooled engine a fan pushes cool air across fins on the cylinders and cylinder heads to lower the temperature to acceptable levels. Some air-cooled engines have a thermostat to regulate the air flow which brings the temperature to the correct level.

The part that make up the staring (cranking) system are the battery, battery cables, starter motor, solenoid and/or relay, and starting switch (usually part of the ignition switch).

The battery is the source of electricity used to crank the engine. A chemical with in the battery produces electrical energy. The starting system will operate when the starting switch is turned on and the solenoid or relay is activated, causing the motor to revolve. As the motor turns, it engages the fly wheel of the engine to make it rotate. The cables provide the path of electricity from the battery to the starter motor.

The charging system consist of the battery, the alternator and its regulator, and the connecting wiring. The functions of charging system are to recharge the battery and provide electrical energy for all purposes when the engine is running.

To charge the battery properly as the engine runs, a regulator is needed to sense the state of charge. The regulator also senses the amount of electricity required by the rest of the vehicle while the battery is being charged.

Designs and types of charging system vary widely, but there functions same.

Q4. Differentiate between a Four Stroke Engine and a Two Stroke Engine?

STROKE :

The stroke of the piston is the distance it moves from the position most extreme from the crankshaft to that nearest it.

FOUR STROKE ENGINE:

An engine which requires four strokes of the piston (i.e two revolutions of the crankshaft) to complete its cycle is called a Four Stroke Cycle Engine. It consist of four strokes which are as follows:

1) INDUCTION STROKE:

The first stroke of four stroke cycle engine is the downward movement of the piston. This create the vacuum in the combustion chamber area which draws the burnable air-fuel mixture from the carburetor. A more technical explanation is that the downward moving piston creates an area of less then atmospheric pressure in the combustion chamber. Since a fluid (liquid or gas) will travel from an area of high pressure to one of low pressure, the higher pressure above the carburetor causes the air to mix with the fuel at the carburetor on its way to the combustion chamber and cylinder.

Two valves on the top of the cylinder, the intake and exhaust valve, which regulates the flow of gases that enter and leave the cylinder. The valve movement are controlled by the camshaft, which, in turn, is controlled by the crankshaft movement. During the intake stroke (piston moving downward), the intake valve must be open to the entry of the air fuel mixture. During the intake valve must be open to allow the entry of the fuel mixture. During the intake stroke, the crankshaft has traveled 180 degrees, halfway through one complete 360 degrees revolution, one stroke equals on half of a crankshaft revolution.

2) COMPRESSION STROKE:

After the piston has traveled to the bottom of the cylinder (known as Bottom Dead Center, or BDC), the spring on the intake valve closes the opening to the carburetor for sealing the cylinder. As the crankshaft continues to turn the piston begins its travel to the top of the cylinder. The area above the piston become smaller and smaller as the piston continues to move upward, squeezing (compressing) the air-fuel mixture into a smaller space. The particles of air and fuel move close together, which increases the temperature of the mixture.

When the piston reaches its extreme to limit of travel (called TDC), the mixture fo air and fuel is confined to fraction of 1/8 or less of the space it originally occupied at BDC. The pressure of gases at this point is often 120-180 psi or, in Metric, 825-1240 kpa. This complete the compression stroke, which has move the crankshaft another one-half turn or 180 degrees.

3) POWER STROKE:

Just before the piston reaches the TDC, an electric spark from the coil jumps from the center of the spark plug to its ground electrode. This spark ignite the compressed air-fuel mixture, causing it t burn very rapidly (about 3/1000ths of a second). The burning of this mixture produces gases whose pressure to expand, forces the piston downward. The crankshaft changes the gas expansion piston movements into relative motion. The amount of force produced at the crankshaft has now turned another 180 degrees, a total of 540 degrees since the beginning of intake stroke.

4) EXHAUST STROKE:

At the end of power stroke, the crankshaft begins to open the exhaust valve and the piston moves upward in this last of the four strokes. The main job of this stroke is to force out all burned gases from the cylinder so a fresh charge can be brought in during the next cycle's intake stroke. The burned gases are forced out past the open exhaust valve, through the exhaust manifold, and eventually into the atmosphere. When the piston reaches TDC the exhaust stroke is completed, a total of two complete crankshaft revolution (720 degrees) have taken place.

TWO STROKE ENGINE:

Figure ( ) represents the cylinder of a two-stroke petrol engine with crankcase compression. As the piston ascends on the compression stroke the next charge is drawn into the crankcase, C, through the spring-loaded automatic valve, S. Ignition occurs before TDC, and at TDC the working stroke begins. As the piston descends through about 80% of the working stroke, the exhaust port, E, is uncovered by the piston and exhaust begins. The transfer port, T, is uncovered later in the stroke due to the shape of the piston or the position of the port is relation to the port E, and the charge in the crankcase, C, which has been compressed by the descending piston, enters the cylinder through the port T.

The piston can be shaped to deflect the fresh gas across the cylinder to assist the scavenging of the cylinder; this is called cross-flow scavenge. As the piston rises, the transfer port, T, is closed slightly before the exhaust port E, and after E is closed compression of the charge in the cylinder begins.

Instead of the spring loaded S, a design with a third port may be used. This is an induction port controlled by the piston, and through which the mixture is drawn into the crankcase.

The above description of the two stroke cycle applies also to CI engines with the exception that air only is compressed, and the sparking plug is replaced by a fuel injector.

In engines which have simple inlet ports and poppet ro sleeve valve controlled exhaust ports, the inlet and exhaust ports are placed at opposite ends of the cylinder and the fresh charge sweeps along the cylinder towards the exhaust port. This is called uni-flow scavenge and is applied with great mechanical simplicity in opposed piston engines.

In all reciprocating internal combustion engines the gases are induced into and exhausted from the cylinder through ports, the opening and closing of which are related to the piston position. In a two stroke engine the ports can be opened or closed by the piston itself, but in the four stroke engine, a separate shaft, called the camshaft, is required; this is driven from the crankshaft through a 2 to 1 speed reduction. The cams n this shaft operate valves, called poppet valves, either directly or by means of push rods. Modern high-speed petrol engines have two camshafts, one operating the exhaust valves, and the other operating the inlet valves. The timing of the valves and the point of ignition are fundamental to the engine performance.