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INTRODUCTION: For us to get a clear picture of what we are going to discuss we must consider these terms
The term reciprocates:
Move backwards and forwards: to move backward and forward in an alternating motion, or move something in this way.
Engines:
Engine is machine for converting energy into motion or mechanical work. The energy is usually supplied in the form of a chemical fuel, such as oil or gasoline, steam, or electricity, and the mechanical work is most commonly delivered in the form of rotary motion of a shaft. Engines are usually classified according to
• the form of energy they utilize, as steam, compressed air, and gasoline;
• the type of motion of their principal parts, as reciprocating and rotary;
• the place where the exchange from chemical to heat energy takes place, as internal combustion and external combustion;
• the method by which the engine is cooled, as air-cooled or water-cooled;
• the position of the cylinders of the engine, as V, in-line, and radial;
• the number of strokes of the piston for a complete cycle, as two-stroke and four-stroke;
• the type of cycle, as Otto (in ordinary gasoline engines) and diesel; and
• the use for which the engine is intended, as automobile and airplane engines.
Other specialized engines are the windmill, gas turbine, steam turbine, and rocket and jet engines.
Fig. 1: A Typical Engine
Some Components of Engines
Piston
Cylinder
Connecting Rod
Crank Shaft
Intake Valve
Spark Plug (For petrol engine)
Cam Shaft
Injector or Carburetor
Radiator
Piston
Piston is a solid cylinder or disk that fits snugly inside a hollow cylinder and slides back and forth. The fit is loose enough to allow the piston to move, but tight enough that virtually no air or fluid in the cylinder can leak past it. Pistons are used in a variety of machines to convert one form of energy to another, or to transfer fluids (such as water or air) or energy from one place to another. In an automobile, pistons are found in the internal-combustion engine, the braking system, the water pump, and the air conditioner.
HOW IT WORKS
Pistons can perform work in several ways when they are forced to slide back and forth inside their cylinders. They can compress or expand fluids and gases. They can push fluids out of cylinders or draw them in. In internal-combustion engines, pistons convert heat energy caused by fuel combustion into mechanical energy that turns a crankshaft.
A tight seal between the sides of the piston and the cylinder wall improves the piston’s performance, unless it is so tight it interferes with the piston’s movement. Lubricants such as oil improve the seal and allow smoother movement. Lubricated gaskets or metal rings sometimes are fitted around the piston to improve the seal.
Cylinder
The cylinder block or engine block is a machined casting (or sometimes an assembly of modules) containing cylindrically bored holes for the pistons of a multi-cylinder reciprocating internal combustion engine, or for a similarly constructed device such as a pump. It is a complex part at the heart of an engine, with adaptions to attach the cylinder head, crankcase, engine mounts, drive housing and engine ancillaries, with passages for coolants and lubricants. The distance between the cylinder bores (midpoint to midpoint) cannot easily be changed since the machining facilities would require extensive modification. Instead, the bore (diameter) is commonly varied to obtain different engine displacements. This and the minimum thickness of material required between two cylinders are a limiting factor concerning the potential displacement because the bore to stroke ratio has to stay within certain limits. Engine blocks are usually made from cast iron or, in modern engines, aluminum and magnesium.
A wet liner cylinder are block features removable cylinder bores which fit into the block by means of special gaskets and offer the advantage of being easily replaced without the need to re-machine the entire casting. Wet Liner designs are popular with European manufacturers, most notably Renault and Peugeot who continue to use them to the present, while
A dry-liner engine has the cylinder liners attached to the cylinder block.
DIESEL ENGINES
Theoretically, the diesel cycle differs from the Otto cycle in that combustion takes place at constant volume rather than at constant pressure. Most diesels are also four-stroke engines but they operate differently than the four-stroke Otto-cycle engines. The first, or suction, stroke draws air, but no fuel, into the combustion chamber through an intake valve. On the second, or compression, stroke the air is compressed to a small fraction of its former volume and is heated to approximately 440°C (approximately 820°F) by this compression. At the end of the compression stroke, vaporized fuel is injected into the combustion chamber and burns instantly because of the high temperature of the air in the chamber. Some diesels have auxiliary electrical ignition systems to ignite the fuel when the engine starts and until it warms up. This combustion drives the piston back on the third, or power, stroke of the cycle. The fourth stroke, as in the Otto-cycle engine, is an exhaust stroke.
The efficiency of the diesel engine, which is in general governed by the same factors that control the efficiency of Otto-cycle engines, is inherently greater than that of any Otto-cycle engine and in actual engines today is slightly more than 40 percent. Diesels are, in general, slow-speed engines with crankshaft speeds of 100 to 750 revolutions per minute (rpm) as compared to 2500 to 5000 rpm for typical Otto-cycle engines. Some types of diesel, however, have speeds up to 2000 rpm. Because diesels use compression ratios of 14 or more to 1, they are generally more heavily built than Otto-cycle engines, but this disadvantage is counterbalanced by their greater efficiency and the fact that they can be operated on less expensive fuel oils.
OTTO-CYCLE ENGINES
The ordinary Otto-cycle engine is a four-stroke engine; that is, in a complete power cycle, its pistons make four strokes, two toward the head (closed head) of the cylinder and two away from the head. During the first stroke of the cycle, the piston moves away from the cylinder head while simultaneously the intake valve is opened. The motion of the piston during this stroke sucks a quantity of a fuel and air mixture into the combustion chamber. During the next stroke, the piston moves toward the cylinder head and compresses the fuel mixture in the combustion chamber. At the moment when the piston reaches the end of this stroke and the volume of the combustion chamber is at a minimum, the fuel mixture is ignited by the spark plug and burns, expanding and exerting a pressure on the piston, which is then driven away from the cylinder head in the third stroke. During the final stroke, the exhaust valve is opened and the piston moves toward the cylinder head, driving the exhaust gases out of the combustion chamber and leaving the cylinder ready to repeat the cycle.
The efficiency of a modern Otto-cycle engine is limited by a number of factors, including losses by cooling and by friction. In general, the efficiency of such engines is determined by the compression ratio of the engine. The compression ratio (the ratio between the maximum and minimum volumes of the combustion chamber) is usually about 8 to 1 or 10 to 1 in most modern Otto-cycle engines. Higher compression ratios, up to about 15 to 1, with a resulting increase of efficiency, are possible with the use of high-octane antiknock fuels. The efficiencies of good modern Otto-cycle engines range between 20 and 25 percent—in other words, only this percentage of the heat energy of the fuel is transformed into mechanical energy.
The p-v diagram of 4 stroke engine
The steps involved here are:
1. Intake stroke: Air and vaporized fuel are drawn in.
2. Compression stroke: Fuel vapor and air are compressed and ignited.
3. Combustion stroke: Fuel combusts and piston is pushed downwards.
4. Exhaust stroke: Exhaust is driven out. During the 1st, 2nd, and 4th stroke the piston is relying on power and the momentum generated by the other pistons. In that case, a four-cylinder engine would be less powerful than a six or eight cylinder engine.
TWO-STROKE ENGINES
By suitable design it is possible to operate an Otto-cycle or diesel as a two-stroke or two-cycle engine with a power stroke. The power of a two-stroke engine is usually double that of a four-stroke engine of comparable size.
The general principle of the two-stroke engine is to shorten the periods in which fuel is introduced to the combustion chamber and in which the spent gases are exhausted to a small fraction of the duration of a stroke instead of allowing each of these operations to occupy a full stroke. In the simplest type of two-stroke engine, the poppet valves are replaced by sleeve valves or ports (openings in the cylinder wall that are uncovered by the piston at the end of its outward travel). In the two-stroke cycle, the fuel mixture or air is introduced through the intake port when the piston is fully withdrawn from the cylinder. The compression stroke follows, and the charge is ignited when the piston reaches the end of this stroke. The piston then moves outward on the power stroke, uncovering the exhaust port and permitting the gases to escape from the combustion chamber.
The steps involved here are:
1. Intake and exhaust occur at bottom dead center. Some form of pressure is needed, either crankcase compression or super-charging.
2. Compression stroke: Fuel-air mix compressed and ignited. In case of Diesel: Air compressed, fuel injected and self ignited
3. Power stroke: piston is pushed downwards by the hot exhaust gases.
Stroke" refers to the movement of the piston in the engine. 2 Stroke means one stroke in each direction. A 2 stoke engine will have a compression stroke followed by an explosion of the compressed fuel. On the return stroke new fuel mixture is inserted into the cylinder.
A 4 stroke engine has 1 compression stroke and 1 exhaust stoke. Each is followed by a return stroke. The compression stroke compresses the fuel air mixture prior to the gas explosion. The exhaust stroke simply pushes the burnt gases out the exhaust.
A 4 stroke engine usually has a distributor that supplies a spark to the cylinder only when its piston is near TDC (top dead center) on the fuel compression stroke, i.e. one spark every two turns of the crank shaft. Some 4 stroke engines do away with the distributor and make sparks every turn of the crank. This means a spark happens in a cylinder that just has burnt gasses in it which just means the sparkplug wears out faster.
Advantages of the two stroke:
• Has more get-up-and-go because it fires once every revolution, giving it twice the power of a four stroke, which only fires once every other revolution.
• Packs a higher weight-to-power ratio because it is much lighter.
• Is less expensive because of its simpler design.
• Can be operated in any orientation because it lacks the oil sump of a four stroke engine, which has limited orientation if oil is to be retained in the sump.
Disadvantages of the two stroke:
• Faster wear and shorter engine life than a four stroke due to the lack of a dedicated lubricating system.
• Requires special two stroke oil ("premix") with every tank of gas, adding expense and at least a minimal amount of hassle.
• Heavily pollutes because of the simpler design and the gas/oil mixture that is released prior to, and in the exhaust (also creates an unpleasant smell).
• Is fuel-inefficient because of the simpler design, resulting in poorer mileage than a four stroke engine.
• Has a high-decibel whine that may exceed legal noise limits in some areas, depending on the product and local applicable laws.
Automobile Radiator
Automobile Radiator is part of the cooling system in a liquid-cooled engine that removes excess heat from the engine. The radiator is important in internal combustion engines because the engine cannot run properly when it is overheated. At high enough temperatures, oil lubricating the engine’s moving parts breaks down and burns away. Eventually, some of those parts jam or melt and the engine stops running.
A radiator works because heat in metal flows from the hottest region to the coolest, and heat in any substance radiates into cooler air surrounding it (see Heat Transfer). Most cars and trucks use a liquid to transfer heat away from the engine. Engines used in lawn mowers, chain saws, airplanes, most motorcycles, and some cars rely only on the transfer of heat from hot metal to the air. They are called air-cooled engines.
In a liquid-cooled engine, a coolant—usually a mixture of water and chemicals—circulates through hollow chambers that surround the engine’s cylinders. Heat produced by burning fuel is transferred from the metal into the coolant. A pump circulates coolant through the engine to the radiator. Hot coolant arrives at the radiator, which exposes the coolant to cooler metal and lowers its temperature. Afterwards, the coolant is pumped through the engine to repeat the cycle.
Inside the radiator, coolant is pumped through air-cooled, hollow tubes. Heat is transferred from the coolant to the tubes, then to the air. The typical radiator consists of a large number of small tubes that carry coolant from a tank at one end of the radiator to another tank at the other end. The rate at which the radiator’s tubes can transfer heat from the coolant depends on how much metal surface is in contact with the coolant and the air.
CONCLUSION:
Diesel and Otto cycle engines are designed both two and four strokes but diesel engines are heavily built due to its high compression ratio i.e. 14 is to 1 as against 8 is to 1 of the Otto cycle engine.
REFERENCES
"History of Technology: Internal Combustion
engines". Encyclopædia Britannica. Britannica.com. Retrieved
2012-03-20.
Pulkrabek, Willard W. (1997). Engineering Fundamentals of the
Internal Combustion Engine. Prentice Hall. p. 2. ISBN 9780135708545.
James, Fales. Technology Today and Tomorrow. p. 344
