BARC ME 2021 : IC Engine + Power Plant Study Notes

AIR STANDARD CYCLE

An engine is a device that transforms one form of energy into another form. Normally, most of the engines convert thermal energy into mechanical work and therefore they are called 'heat engines'.

Heat engine is a device that transforms the chemical energy of a fuel into thermal energy and utilizes this thermal energy to perform useful work. Thus, thermal energy is converted to mechanical energy in a heat engine.

Heat engines can be broadly classified into two categories:

 (i) Internal Combustion Engines (IC Engines)

(ii) External Combustion Engines (EC Engines)

(i) IC Engine

internal combustion engines are those engines in which combustion takes place within the engine.

Or in a simple way, if the product of combustion is directly supplied t the prime mover then it is classified as an IC engine.

Example – In the case of gasoline or diesel engines, the products of combustion generated by the combustion of fuel and air within the cylinder form the working fluid and are directly supplied to the piston.

BASIC ENGINE COMPONENTS

Figure: Cross Section of A Single Cylinder Spark-Ignition Engine

(i) Cylinder Block

The cylinder block is the main supporting structure for the various components. The cylinder of a multi-cylinder engine is cast as a single unit, called a cylinder block.

The cylinder head is mounted on the cylinder block & the bottom portion of the cylinder block is called a crankcase.

The cylinder head and cylinder block are provided with water jackets in the case of water cooling or with cooling fins in the case of air cooling.

(ii) Cylinder

It is a cylindrical vessel or space in which the piston makes a reciprocating motion.

The varying volume created in the cylinder during the operation of the engine is filled with the working fluid and subjected to different thermodynamic processes.

(iii) Piston

It is a cylindrical component fitted into the cylinder and do reciprocating motion under the action gas pressure force and allow the movement of the combustion system.

It fits perfectly into the cylinder providing a gas-tight space with the piston rings and the lubricant.

(iv) Combustion Chamber

The space between the cylinder head and the piston top in which combustion of fuels takes place, is known as the combustion chamber.

(v) Inlet Manifold

The pipe which connects the intake system to the inlet valve of the engine and through which air or air-fuel mixture or air is drawn into the cylinder is called the inlet manifold.

(vi) Exhaust Manifold

The pipe which connects the exhaust system to the exhaust valve of the engine and through which the products of combustion escape into the atmosphere is called the exhaust manifold.

(vii) Inlet and Exhaust Valves

These are the openings either on the cylinder head or on the side of the cylinder for regulating the amount of charge coming into the cylinder (inlet valve) and for discharging the products of combustion (exhaust valve) from the cylinder.

(viii) Spark Plug

It is a component to initiate the combustion by generating a spark through battery in Spark-Ignition (SI) engines and is usually located on the cylinder head.

(ix) Connecting Rod

It interconnects the piston and the crankshaft and transmits the gas forces from the piston to the crankshaft.

The two ends of the connecting rod are called as small end and the big end. Small end is connected to the piston by gudgeon pin and the big end is connected to the crankshaft by crankpin.

(x) Gudgeon Pin

It forms the link between the small end of the connecting rod and the piston.

(xi) Crankshaft

It converts the reciprocating motion of the piston into useful rotary motion of the output shaft.

The crankshaft is enclosed in a crankcase.

(xii) Piston Rings

Piston rings are slots around the piston to provide a tight seal between the piston and the cylinder wall thus preventing leakage of combustion gases.

(xiii) Camshaft

The camshaft and its associated parts control the opening and closing of the two valves.

This shaft also provides the drive to the ignition system & it is driven by the crankshaft through timing gears.

(xiv)    Cams

These are made as integral parts of the camshaft and are designed in such a way to open the valves at the correct timing and to keep them open for the necessary duration.

(xv) Flywheel

The net torque imparted to the crankshaft during one complete cycle of operation of the engine fluctuates causing a change in the angular velocity of the shaft. In order to achieve a uniform torque an inertia mass in the form of a wheel is attached to the output shaft and this wheel is called the flywheel.

IDEAL or AIR STANDARD CYCLE

The operating cycle of an internal combustion engine can be broken down into a sequence of separate processes viz., intake, compression, combustion, expansion and exhaust.

In order to understand them it is advantageous to analyze the performance of an idealized closed cycle that closely approximates the real cycle. One such approach is the Air standard cycle.

Air standard cycles are defined as cycles using a perfect gas as the working medium. Air is almost invariably used as the working fluid in internal combustion engines and is assumed to behave as a perfect gas.

The following assumptions are made in the analysis of air standard cycle.

(i) The working medium is assumed to be a perfect gas and follows the relation PV = mRT or P = pRT.

(ii) There is no change in the mass of the working medium.

(iii) All the processes that constitute the cycle are reversible i.e. the compression and expansion processes are reversible adiabatic & there are no loss or gain of entropy.

(iv) Heat is assumed to be supplied from a constant high temperature source and not from chemical reactions during the cycle.

(v) Heat is assumed to be rejected to a constant low temperature sink during the cycle.

(vi) It is assumed that there are no heat losses from the system to the surroundings.

(vii) The physical constants C_p, C_v, γ and M (molecular weight) of working medium are the same as those of air at standard atmospheric conditions.

The three cycles of great practical importance in the analysis of piston engine performance are

(i) Constant Volume or Otto Cycle

(ii) Constant Pressure or Diesel Cycle

(iii) Dual Combustion or Limited pressure Cycle.

COMBUSTION IN SI & CI ENGINES

Spark-ignition engines normally use volatile liquid fuels. Preparation of fuel-air mixture is done outside the engine cylinder and formation of a homogeneous mixture is normally not completed in the inlet manifold. Fuel droplets which remain in suspension continue to evaporate and mix with air even during suction and compression processes.

CARBURETION

The process of formation of a combustible fuel-air mixture by mixing the proper amount of fuel with air before admission to engine cylinder is called carburetion and the device which does this job is called a carburetor.

FACTORS AFFECTING CARBURETION

Of the various factors the process of carburetion is influenced by

• The engine speeds
• The vapourization characteristics of the fuel
• The temperature of the incoming air

AIR-FUEL MIXTURES

An engine is generally operated at different loads and speeds. For this, proper air-fuel mixture should be supplied to the engine cylinder. Fuel and air are mixed to form three different types of mixtures.

(i) chemically correct mixture

(ii) rich mixture and

(iii) lean mixture

Chemically correct or stoichiometric mixture is one in which there is just enough air for complete combustion of the fuel. For example, to burn one kg of octane (C₈H₁₈) completely 15.12 kg of air is required. Hence chemically correct A/F ratio for C₈H₁₈ is 15.12:1; usually approximated to 15:1.It is always computed from the chemical equation for complete combustion for a particular fuel. Complete combustion means all carbon in the fuel is converted to CO₂ and all hydrogen to H₂O.

A mixture which contains less air than the stoichiometric requirement is called a rich mixture (example, A/F ratio of 12:1, 10:1 etc.).

A mixture which contains more air than the stoichiometric requirement is called a lean mixture (example, A/F ratio of 17:1, 20:1 etc.)

There is, however, a limited range of A/F ratios in a homogeneous mixture, only within which combustion in an SI engine will occur. Outside this range, the ratio is either too rich or too lean to sustain flame propagation.

Figure: Power output vs A/F ratio (based on full throttle operation)

ABNORMAL COMBUSTION

In normal combustion, the flame initiated by the spark travels across the combustion chamber in a fairly uniform manner. Under certain operating conditions the combustion deviates front its normal course leading to loss of performance and possible damage to the engine. This type of combustion may be termed as abnormal combustion or knocking combustion. The consequences of this abnormal combustion process are the loss of power, recurring preignition and mechanical damage to the engine.

COMPARISON OF KNOCK IN SI AND CI ENGINES

PERFORMANCE PARAMETERS
& TESTING OF ENGINE

The difference between the indicated and the brake power is known as friction power. The friction loss is made up of the friction between the piston and cylinder walls, piston rings and cylinder walls, and between the crankshaft and camshaft and their bearings, as well as by the loss incurred by driving  the essential accessories, such as the water pump. ignition unit etc.

It should be the aim of the designer to have minimum loss of friction. Friction power is used for the evaluation of indicated power in mechanical efficiency. Following methods are used to find the friction Power to estimate the performance of the engine.

(i) Willan's line method

(ii) Morse test

(iii) Motoring test

(iv) From the measurement of indicated and brake power

(v) Retardation test

INDICATED POWER

Indicated power of an engine tells about the health of the engine and also gives an indication regarding the conversion of chemical energy in the fuel into heat energy. Indicated power is an important variable because it is the potential output of the cycle. Therefore, to justify the measurement of indicated power, it must be more accurate than motoring and other indirect methods of measuring frictional power. For obtaining indicated power the cycle pressure must be determined as a function of cylinder volume.

BRAKE POWER

It involves the determination of the torque and the angular speed of the engine output shaft. The torque mean device is called a dynamometer. Figure 4 shows the basic principle of  a dynamometer.

ENGINE EFFICIENCIES

Apart from expressing engine performance in terms of power, it is also essential to express in terms of efficiencies. Various engine efficiencies are:

AIR-STANDARD EFFICIENCY

The air-standard efficiency is also known as thermodynamic efficient mainly a function of compression ratio and other parameters. It is upper limit of the efficiency obtainable from an engine.

INDICATED AND BRAKE THERMAL EFFICIENCIES

The indicated and brake thermal efficiencies are based on the ip and bp of the engine respectively. In modern engines an indicated thermal efficiency of almost 28 percent is obtainable with gas and gasoline spark-ignition engines having a moderate compression ratio and as high as 36 per cent or even more with high compression ratio oil engines.

MECHANICAL EFFICIENCY

The mechanical efficiency, ⴄ_m of the engine can be expressed as the ratio of bmep to imep.

Mechanical efficiency takes into account the mechanical losses in an engine. Mechanical

In general, mechanical efficiency of engines varies from 65 to 85%.

RELATIVE EFFICIENCY

The relative efficiency or efficiency ratio as it is sometimes called is the ratio of the actual efficiency obtained from an engine to the theoretical efficiency of the engine cycle. Hence,

Relative efficiency for most of the engines varies from 75 to 95% with theoretical air and decreases rapidly with insufficient air to about 75% with 90% air.

VOLUMETRIC EFFICIENCY

Volumetric efficiency is defined as the ratio of the actual mass of air drawn in to the engine during a given period of time to the theoretical mass which should have been drawn in during that same period of time, based upon the total piston displacement of the engine, and the temperature and pressure of the surrounding atmosphere.

where,                    m_th = ρ_anV_s

where n is the number of intake strokes per minute. For a four-stroke engine n = N/2 and for a two-stroke engine n = N, where N is the speed of the engine in rev/min. The actual mass is a measured quantity. The theoretical mass is computed from the geometry of the cylinder, the number of cylinders, and the speed of the engine, in conjunction with the density of the surrounding atmosphere.

Volumetric efficiency is a measure of the success with which the air supply and thus the charge, is inducted in to the engine. It is very important parameter, since it indicates the breathing capacity of the engine.

SCAVENGING EFFICIENCY

In case of two-stroke engines scavenging efficiency is defined as the ratio of the amount of air or gas-air mixture, which remains in the cylinder, at the actual beginning of the com-pression to the product of the total volume and air density of the inlet. Scavenging efficiency for most of the two-stroke engines varies from 40 to 95 per cent depending upon the type of scavenging provided.

The charge efficiency shows how well the piston displacement of a four Stoke engine is utilizes.

COMBUSTION EFFICIENCY

Combustion efficiency is the ratio of heat liberated to the theoretical heat in the fuel. The amount of heat liberated is less than the theoretical value because of incomplete combustion either due to dissociation or due to lack of available oxygen. Combustion efficiency in a well-adjusted engine varies from 92% to 97%.

Powerplant Engineering

STEAM TURBINES & NOZZLE

A steam turbine converts the energy of high-pressure, high temperature steam produced by a steam generator into shaft work. The energy conversion is brought about in the following ways:

1. The high-pressure, high-temperature steam first expands in the nozzles emanates as a high velocity fluid stream.
2. The high velocity steam coming out of the nozzles impinges on the blades mounted on a wheel. The fluid stream suffers a loss of momentum while flowing past the blades that is absorbed by the rotating wheel entailing production of torque.
3. The moving blades move as a result of the impulse of steam (caused by the change of momentum) and also as a result of expansion and acceleration of the steam relative to them. In other words, they also act as the nozzles.

A steam turbine is basically an assembly of nozzles fixed to a stationary casing and rotating blades mounted on the wheels attached on a shaft in a row-wise manner.

COMPOUNDING IN IMPULSE TURBINE

If high velocity of steam is allowed to flow through one row of moving blades, it produces a rotor speed of about 30000 rpm which is too high for practical use.

It is therefore essential to incorporate some improvements for practical use and also to achieve high performance. This is possible by making use of more than one set of nozzles, and rotors, in a series, keyed to the shaft so that either the steam pressure or the jet velocity is absorbed by the turbine in stages. This called compounding. Two types of compounding can be accomplished:

 (a) velocity compounding and (b) pressure compounding

REACTION TURBINE

A reaction turbine, therefore, is one that is constructed of rows of fixed and moving blades. The fixed blades act as nozzles. The moving blades move as a result of the Impulse of steam received (caused by change in momentum) and also as a result of expansion and acceleration of the steam relative to them. In other words, they also act as nozzles.

STEAM TURBINE GOVERNING AND CONTROL

The objective of governing is to keep the turbine speed fairly constant irrespective of load. The principal methods of steam turbine governing arc as follows:

1. Throttle governing
2. Nozzle governing
3. By-pass governing
4. Combination of 1 and 2 and 1 and 3.

COMPARISON OF THROTTLE AND NOZZLE CONTROL GOVERNING

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