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INTRODUCTIONAn 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."Engine" was originally a term for any mechanical device that converts force into motion. The word derives from Old French engin, from the Latin ingenium, which is also the root of the word ingenious, meaning ability.
Exploded view of petrol engine
PETROL ENGINEA petrol engine (also known as a gasoline engine) is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels. It was invented in 1876 in Germany by German inventor Nicolaus August Otto. In most petrol engines, the fuel and air are usually pre-mixed before compression. 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.
COMPRESSION RATIOThe compression ratio is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its lower capacity.
SPEED AND EFFICIENCYPetrol 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 engines.However the lower compression ratios of a petrol engine give a lower efficiency than a diesel engine. To give an example, a petrol engine is like operating a bicycle in its lowest gear where each push from your feet adds little energy to the system, but you still expend energy to move your legs back to the TDC position.
APPLICATIONSPetrol engines have many applications, including:Motor carsMotorcyclesAircraftMotorboatsSmall engines, such as lawn mowers, chainsaws and portable engine-generators
DESIGNWORKING CYCLESPetrol engines may run on the four-stroke cycle or the two-stroke cycle. For details of working cycles :Four-stroke cycleTwo-stroke cycle
FOUR-STROKE ENGINEA four-stroke engine (also known as four-cycle) is an internal combustion engine in which the piston completes four separate strokesintake, compression, power, and exhaustduring two separate revolutions of the engine's crankshaft, and one single thermodynamic cycle.There are two common types of four-stroke engines. They are closely related to each other, but have major differences in design and behaviour. The earliest of these to be developed is the Otto cycle engine developed in 1876 by Nikolaus August Otto in Cologne, Germany, after the operation principle described by Alphonse Beau de Rochas in 1861. This engine is most often referred to as a petrol engine or gasoline engine, after the fuel that powers it.
The four strokes refer to intake, compression, combustion (power) and exhaust strokes that occur during two crankshaft rotations per power cycle1.INTAKE or INDUCTION stroke2.COMPRESSION stroke3. POWER stroke4. EXHAUST stroke
INTAKE or INDUCTION strokeOn the intake or induction stroke of the piston, 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. The intake valve(s) then closes. The volume of air/fuel mixture that is drawn into the cylinder, relative to the maximum volume of the cylinder, is called the volumetric efficiency of the engine.
COMPRESSION strokeWith both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the air or fuel-air mixture into the combustion chamber of the cylinder head. During the compression stroke the temperature of the air or fuel-air mixture rises by several hundred degrees.
POWER strokeThis is the start of the second revolution of the cycle. While the piston is close to Top Dead Centre, the compressed airfuel mixture in a petrol engine is ignited, by a spark plug. The resulting pressure from the combustion of the compressed fuel-air mixture forces the piston back down toward bottom dead centre.
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).
GRAPHS FOR OTTO CYCLE
P-V GRAPHT-S GRAPH
Process 1-2 is anisentropiccompression of the air.Process 2-3 is a constant-volume heat transfer to the air from an external source.Process 3-4 is an isentropic expansion (power stroke).Process 4-1 completes the cycle by a constant-volume process in which heat is rejected from the air.PROCESSES IN OTTO CYCLE
first lawis rewritten as:
Applying this to the Otto cycle the four process equations can be derived: Thermal Efficiency
TWO-STROKE ENGINEA 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 to do so. 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", and, compared to 4-stroke engines, have a greatly reduced number of moving parts, are more compact and significantly lighter.
ADVANTAGES AND DISADVANTAGES OF TWO STROKE ENGINESAdvantages: It has no valves or camshaft mechanism, hence simplifying its mechanism and construction For one complete revolution of the crankshaft, the engine executes one cyclethe 4-stroke executes one cycle per two crankshafts revolutions. Less weight and easier to manufacture.High power-to-weight ratioDisadvantages:The lack of lubrication system that protects the engine parts from wear. Accordingly, the 2-stroke engines have a shorter life. 2-stroke engines do not consume fuel efficiently. 2-stroke engines produce lots of pollution.
CONCLUSIONAfter reading the paper, we come to the know that Petrol 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), 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 andlubrication. Valves, pistons andconnecting rodssuffer severe acceleration forces. At high engine speed, physical breakage andpiston ringflutter can occur, resulting in power loss or even engine destruction.The Thermal efficiency of a Petrol engine is directly related to the compression ratio.