OTTO ENGINE

ENGINES

1. An engine or a motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including I.C. engines and E.C. engines burn a fuel to create heat, which then creates motion. Motors convert electrical energy into mechanical motion.

2. Engine was originally a term for any mechanical device which converts energy into motion. Engine comes from old French Ingenium meaning ability. In modern terms, engine is a device which burns or consumes fuel to perform mechanical work by exerting a torque or a linear force to drive machinery. A heat engine also works as a Prime Mover- a component that transforms the flow or changes in pressure of a fluid in to mechanical energy

TYPES OF ENGINES

Engines are classified on the following basis:

• Number of cylinders: single cylinder engines , multi cylinder engines.

• Arrangement of cylinders: Row giving-in-line engine, V type engine.

• Arrangement of valves: Overhead valve engine, T-head engine. • Number of strokes: Two stroke engine, Four stroke engine.• Type of cycle: Otto engine, Diesel engine.• Type of cooling: Air cooled engine, Water cooled engine.• Type of fuel used: Gas engine, Petrol engine etc.• Field of application: Marine engines, Stationary engines etc.

INTERNAL COMBUSTION ENGINE

1. The internal combustion engine is an engine in which the combustion of a fuel (normally a fossil fuel) occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine (ICE) the expansion of the high-temperature and high-pressure gases produced by combustion apply direct force to some component of the engine. The force is applied typically to pistons, turbine blades, or a nozzle. This force moves the component over a distance, transforming chemical energy into useful mechanical energy. The first commercially successful internal combustion engine was created by Étienne Lenoir.

2. 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 and the Wankel rotary engine. A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines on the same principle as previously described.

INTERNAL COMBUSTION ENGINE

• The ICE is quite different from external combustion engines(E.C.E.), such as steam or Stirling engines, in which the energy is delivered to a working fluid not consisting of, mixed with, or contaminated by combustion products. Working fluids can be air, hot water, pressurized water or even liquid sodium, heated in some kind of boiler. ICEs are usually powered by energy-dense fuels such as gasoline or diesel, liquids derived from fossil fuels. While there are many stationary applications, most ICEs are used in mobile applications and are the dominant power supply for cars, aircraft, and boats.

TYPES OF I.C. ENGINES

Common layouts of engines are:Reciprocating:• Two-stroke engine• Four-stroke engine (Otto cycle)• Six-stroke engine• Diesel engine• Atkinson cycle• Miller cycleRotary:• Wankel engineContinuous combustion:• Gas turbine• Jet engine (including turbojet, turbofan, ramjet , rocket etc.)•

Four Stroke Engine Two Stroke Engine

Wankel Engine Rocket Engine

FOUR STROKE ENGINE

1. 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.

2. The name four stroke refers to the intake, compression, combustion and exhaust stroke that occurs during two crankshaft rotations per power cycle. The cycle begins at Top Dead Centre, when the piston is farthest away from the crankshaft. A stroke refers to the full travel of the piston from Top Dead Centre to Bottom Dead Centre.

TYPES OF FOUR STROKE ENGINE

There are two types of four stroke engines. They are closely related to each other, but they have major differences in design.

1. First type of four stroke engine is known as petrol or gasoline engine named after the fuel. First created by Nikolaus A. Otto, they are called Otto engines. They require a spark plug to ignite the combustible material inside the chamber. So they are also called spark ignited engine (S.I.).

2. The other type of four stroke engine is the Diesel engine, named after the fuel and its inventor Rudolf Diesel. It employs the technique of self ignition by compressed air and so are also called compressed ignition engines (C.I.).

Nikolaus O. Otto And The Otto Engine

Rudolf Diesel And The Diesel Engine

FOUR STROKE CYCLE

1. As their name implies, four-stroke internal combustion engines have four basic steps that repeat with every two revolutions of the engine:

2. (1) Intake/suction stroke (2) Compression stroke (3) Power/expansion stroke and (4) Exhaust stroke

INTAKE STROKE

• The first stroke of the internal combustion engine is also known as the suction stroke because the piston moves to the maximum volume position (downward direction in the cylinder) creating a vacuum (negative pressure). The inlet valve opens as a result of the cam lobe pressing down on the valve stem, and the vaporized fuel mixture is sucked into the combustion chamber. The inlet valve closes at the end of this stroke.

COMPRESSION STROKE

• In this stroke, both valves are closed and the piston starts its movement to the minimum volume position (upward direction in the cylinder) and compresses the fuel mixture. During the compression process, pressure, temperature and the density of the fuel mixture increases.

POWER STROKE

• When the piston reaches a point just before top dead center, the spark plug ignites the fuel mixture. The point at which the fuel ignites varies by engine; typically it is about 10 degrees before top dead center. This expansion of gases caused by ignition of the fuel produces the power that is transmitted to the crank shaft mechanism.

EXHAUST STROKE

• In the end of the power stroke, the exhaust valve opens. During this stroke, the piston starts its movement in the maximum volume position. The open exhaust valve allows the exhaust gases to escape the cylinder. At the end of this stroke, the exhaust valve closes, the inlet valve opens, and the sequence repeats in the next cycle. Four-stroke engines require two revolutions

OTTO CYCLE

An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical spark ignition reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines.

The Otto cycle is constructed out of:1. Top and bottom of the loop: a pair of quasi-

parallel adiabatic processes2. Left and right sides of the loop: a pair of

parallel isochoric processes3. The adiabatic processes are impermeable to heat:

heat flows into the loop through the left pressurizing process and some of it flows back out through the right depressurizing process, and the heat which remains does the work.

GRAPHS

P-V Diagram T-S Diagram

PROCESSES IN OTTO CYCLE1. Process 1-2 is an isentropic compression of the air as the

piston moves from bottom dead centre (BDC) to top dead centre (TDC).

2. Process 2-3 is a constant-volume heat transfer to the air from an external source while the piston is at top dead centre. This process is intended to represent the ignition of the fuel-air mixture and the subsequent rapid burning.

3. Process 3-4 is an isentropic expansion (power stroke).4. Process 4-1 completes the cycle by a constant-volume

process in which heat is rejected from the air while the piston is a bottom dead centre.1. The Otto cycle consists of adiabatic compression, heat

addition at constant volume, adiabatic expansion, and rejection of heat at constant volume.

2. In the case of a four-stroke Otto cycle, technically there are two additional processes: one for the exhaust of waste heat and combustion products (by isobaric compression), and one for the intake of cool oxygen-rich air (by isobaric expansion); however, these are often omitted in a simplified analysis.

Thermal Efficiency

first law is rewritten as:

• 1

Applying this to the Otto cycle the four process equations can be derived:

• 2• 3 • 4 • 5

The net work can also be found by evaluating the heat added minus the heat leaving or expelled.

INFERENCE• From analyzing efficiency equation it is evident that the Otto cycle efficiency

depends directly upon the compression ratio • Since the   for air is 1.4, an increase in    will produce an increase in . However,

the    for combustion products of the fuel/air mixture is often taken at approximately 1.3. The foregoing discussion implies that it is more efficient to have a high compression ratio.

• The standard ratio is approximately 10:1 for typical automobiles. Usually this does not increase much because of the possibility of auto ignition, or "knock", which places an upper limit on the compression ratio.

•  During the compression process 1-2 the temperature rises, therefore an increase in the compression ratio causes an increase in temperature. Auto ignition occurs when the temperature of the fuel/air mixture becomes too high before it is ignited by the flame front. The compression stroke is intended to compress the products before the flame ignites the mixture. If the compression ratio is increased, the mixture may auto-ignite before the compression stroke is complete, leading to "engine knocking". This can damage engine components and will decrease the brake horsepower of the engine.

•  

CONCLUSION• Otto engines are about 30% efficient; in other words, 30% of the energy

generated by combustion is converted into useful rotational energy at the output shaft of the engine, while the remainder being losses due to waste heat, friction and engine accessories.

• The maximum amount of power generated by an engine is determined by the maximum amount of air ingested. The amount of power generated by a piston engine is related to its size (cylinder volume), whether it is a two-stroke or four-stroke design, volumetric efficiency, losses, air-to-fuel ratio, the calorific value of the fuel, oxygen content of the air and speed (RPM).

• The speed is ultimately limited by material strength and lubrication. Valves, pistons and connecting rods suffer severe acceleration forces.

• At high engine speed, physical breakage and piston ring flutter can occur, resulting in power loss or even engine destruction.

• The Thermal efficiency of a Otto engine is directly related to the compression ratio  .

• Since the   for air is 1.4, an increase in   will produce an increase in  . However, the    for combustion products of the fuel/air mixture is often taken at approximately 1.3.

• The foregoing discussion implies that it is more efficient to have a high compression ratio.

• The standard ratio is approximately 10:1 for typical automobiles.