Thermodynamics

Engine:

 Machine for converting thermal energy into mechanical energy or power to produce force and motion. An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines (such as steam engines) burn a fuel to create heat, which then creates motion. Electric motors convert electrical energy into mechanical motion.

Heat Engine:

In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical energy which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat "source" generates thermal energy that brings the working substance to the high temperature state.

The working substance generates work in the "working body" of the engine while transferring heat to the colder "sink" until it reaches a low temperature state. During this process some of the thermal energy is converted into work by exploiting the properties of the working substance. The working substance can be any system with a non-zero heat capacity, but it usually is a gas or liquid.

Examples of Heat Engine:

Most engines used in modern society are heat engines. This includes steam electric power generators, automobiles, trucks, many locomotives, refrigerators, air conditioners, heat pumps.

Efficiency of Heat Engine:

Note that the efficiency of heat engine is always less than unity. The efficiency of heat engine is related to the temperature of the boiler and temperature of the condenser in a typical power plant (or any other type of heat engine).

Types of heat Engine:

ü Internal Combustion Engine(ICE) 

ü Internal Combustion Engine(ICE) 

External Combustion Engine;

An external combustion engine (EC engine) is a heat engine where an internal working fluid is heated by combustion of an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine produces motion and usable work. The fluid is then cooled, compressed and reused (closed cycle), or (less commonly) dumped, and cool fluid pulled in (open cycle air engine).

Working fluid in External Combustion Engine:

The working fluid can be a gas as in a Sterling engine, or steam as in a steam engine or an organic liquid such as n-pentane in an Organic Rankin cycle. The fluid can be of any composition; gas is by far the most common, although even single-phase liquid is sometimes used. In the case of the steam engine, the fluid changes phases between liquid and gas.

Internal Combustion Engine:

The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with variants, such as the six-stroke piston engine. A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines

Principle of Internal combustion engine:

The internal combustion engine is an engine in which the combustion of a fuel (generally, fossil fuel) occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high temperature and high pressure gases, which are produced by the combustion, directly applies force to components of the engine, such as the pistons or turbine blades or a nozzle, and by moving it over a distance, generates useful mechanical energy.

File Triangular:

The fire triangle or combustion triangle is a simple model for understanding the necessary ingredients for most fires.

The triangle illustrates the three elements a fire needs to ignite: heat, fuel, and an oxidizing agent (usually oxygen). A fire naturally occurs when the elements are present and combined in the right mixture,^[2] and a fire can be prevented or extinguished by removing any one of the elements in the fire triangle. For example, covering a fire with a fire blanket removes the "oxygen" part of the triangle and can extinguish a fire.

Types of Internal Combustion Engine:

ü Petrol Engine (auto cycle process)

ü Diesel Engine(diesel cycle)

Diesel Engine_:

The diesel engine (also known as a compression-ignition engine) is an internal combustion engine that uses the heat of compression to initiate ignition and burn the fuel that has been injected into the combustion chamber. This contrasts with spark-ignition engines such as a petrol engine (gasoline engine) or gas engine (using a gaseous fuel as opposed to gasoline), which use a spark plug to ignite an air-fuel mixture. In a diesel engine, only air is compressed (and therefore heated), and the fuel is injected into very hot air at the end of the compression stroke, and self-ignites.

Efficiency of diesel engine:

The diesel engine has the highest thermal efficiency of any standard internal or external combustion engine due to its very high compression ratio. Low-speed diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) can have a thermal efficiency that exceeds 50%.

Types of Diesel Engine:

Diesel engines are manufactured in two-stroke and four-stroke versions.

Petrol Engine_:

A petrol engine (known as a gasoline engine in North America) is an internal combustion engine with spark-ignition, designed to run on petrol (gasoline) and similar volatile fuels.

Principle of Petrol Engine:

In most petrol engines, the fuel and air are usually pre-mixed before compression (although some modern petrol engines now use cylinder-direct petrol injection). The pre-mixing was formerly done in a carburetor, but now it is done by electronically controlled fuel injection, except in small engines where the cost/complication of electronics does not justify the added engine efficiency. The process differs from a diesel engine in the method of mixing the fuel and air, and in using spark plugs to initiate the combustion process.

Compression ratio:

With both air and fuel in a closed cylinder, compressing the mixture too much poses the danger of auto-ignition — or behaving like a diesel engine. Because of the difference in burn rates between the two different fuels, petrol engines are mechanically designed with different timing than diesels. In case of petrol engine the compression ratio is 1:8 to 1:12.

Characteristics of petrol engine:

Petrol engines run at higher speeds than diesels, partially due to their lighter pistons, connecting rods and crankshaft (a design efficiency made possible by lower compression ratios) and due to petrol burning faster than diesel. They also tend to have a much shorter stroke and therefore petrol engines pistons can move up & down much quicker than a diesel engine. However the lower compression ratios of a petrol engine give a lower efficiency than a diesel engine.

TWO stroke Petrol Engine:

A two-stroke, two-cycle, or two-cycle engine is a type of internal combustion engine which completes a power cycle in only one crankshaft revolution and with two strokes, or up and down movements, of the piston in comparison to a "four-stroke engine", which uses four strokes. This is accomplished by the end of the combustion stroke and the beginning of the compression stroke happening simultaneously and performing the intake and exhaust (or scavenging) functions at the same time.

Two-stroke engines often provide high power-to-weight ratio, usually in a narrow range of rotational speeds called the "power band". Compared to 4-stroke engines, they have a greatly reduced number of moving parts, are more compact and significantly lighter.

The heat transfer from the engine to the cooling system is less in a two-stroke engine than in a four-stroke. This adds to the overall engine efficiency. Two-stroke engines have higher exhaust emissions than four-stroke engines.

Four stroke Petrol Engine:

A four-stroke engine (also known as four-cycle) is an internal combustion engine in which the piston completes four separate strokes which comprise a single thermodynamic cycle. A stroke refers to the full travel of the piston along the cylinder, in either direction. While risqué slang among some automotive enthusiasts names these respectively the "suck," "squeeze," "bang" and "blow" strokes.

Ø PROCESS:

Any change that a system undergoes from one equilibrium state to another equilibrium state is called a process, and the series of state through which a system passes during a process is called the path of the process.

Ø Cycle:

A system is said to undergone a cycle if it returns to its initial state at the end of the process. This is for a cycle the initial and final states are identical.

Ø STROKE:

The stroke of the piston describes the distance in                millimetres or inches, a piston travels from either the top of the cylinder to the bottom of the cylinder, or from bottom of the cylinder back to the top of cylinder. Either direction up or down travelled by the piston is considered to be a single stroke.

Parts of 4 stroke engine:

4 stroke petrol engines comprises of various parts, which are along with their functions, are given below:

§  SPARK PLUG:

    A spark plug (sometimes in British English a sparking plug, colloquially a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator.

·       CRANK SHAFT:

The crankshaft, sometimes abbreviated to crank, is responsible for conversion between reciprocating motion and rotational motion. In a reciprocating engine, it translates reciprocating linear piston motion into rotational motion, whereas in a reciprocating compressor, it converts the rotational motion into reciprocating motion. In order to do the conversion between two motions, the crankshaft has "crank throws" or "crankpins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach.

·       EXHAUST VALUES:

An exhaust valve is a valve through which burned gases from a cylinder escape into the exhaust manifold. It usually consists of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem.

·       CAM SHAFT:

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part. In internal combustion engines with pistons, the camshaft is used to operate poppet valves. It then consists of a cylindrical rod running the length of the cylinder bank with a number of oblong lobes protruding from it, one for each valve. The cam lobes force the valves open by pressing on the valve, or on some intermediate mechanism as they rotate.

·       PISTON:

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall.

·       CARBURETOR:

A carburettor is a device that blends air and fuel for an internal combustion engine.

Carburet means to combine with carbon. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fluid by mixing it with a volatile hydrocarbon.

·       CONNECTING ROD:

In a reciprocating piston engine, the connecting rod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts reciprocating motion into rotating motion. Connecting rods may also convert rotating motion into reciprocating motion.

Other parts include:

·       Intake valve: for air and fuel entering.

·       Piston pin/rest pin

·       Intake manifold: when intake valve open, mixture fill the groove of manifold.

·       Exhaust manifold: before exhausting from valve, for store in manifold.

·       Piston ring

·       Fly wheel

Basic Process in 4-stroke petrol engine:

An internal-combustion engine in which an explosive   mixture is drawn into the cylinder on the first stroke and is compressed and ignited on the second stroke; work is done on the third   stroke and the products of combustion are exhausted on the fourth stroke.

As the name suggest the Four Stroke Petrol Engine uses a cycle of four strokes and petrol as the fuel. Each cycle includes 2 rotations of the crankshaft and four strokes, namely:

1.INTAKE STROKE: This stroke of the piston begins at top dead centre. The piston descends from the top of the cylinder to the bottom of the cylinder, increasing the volume of the cylinder. A mixture of fuel and air is forced by atmospheric (or greater) pressure into the cylinder through the intake port.

2.COMPRESSION STROKE: This stroke is known as compression stroke because the compression of the fuel mixture takes place at this stage. When the intake valve closes (exhaust valve is already closed), the piston forced back to the top of the cylinder and the fuel mixture gets compressed. The compression is around 1/8th of the original volume. An engine is considered more efficient if its compression ratio is higher.

3.POWER/COMBUSTION STROKE: this is the start of the second revolution of the cycle. While the piston is close to Top Dead Centre, the compressed air–fuel mixture in a gasoline engine is ignited, by a spark plug in gasoline engines, or which ignites due to the heat generated by compression in a diesel engine. The resulting pressure from the combustion of the compressed fuel-air mixture forces the piston back down toward bottom dead centre.

4.EXHAUST STROKE: during the exhaust stroke, the piston once again returns to top dead centre while the exhaust valve is open. This action expels the spent fuel-air mixture through the exhaust valve(s).

Applications of Petrol Engine:

Petrol engines have many applications, including:

• Motor cars
• Motorcycles
• Aircraft
• Motorboats

Small engines, such as lawn mowers, chainsaws and portable engine-generators