four stroke and two stroke


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Two Stroke and Four Stroke Engines


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Two Stroke and Four Stroke Engines

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Four Stroke Engine A four-stroke engine, also known as four-cycle, is an internal combustion engine in which the piston completes four separate strokes—intake, compression, power, and exhaust—during two separate revolutions of the engine's crankshaft, and one single thermodynamic cycle.

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1.Intake stroke :- The Piston is going down so thevolume of the cylinder is getting larger     The Intake valve is open. A vacuum, or pressure of lessthan atmospheric is created in the cylinder.     Atmospheric pressure forcesair into the cylinder. 14.7 psi at sea level.     Fuel is mixed with the air, and the cylinder is filled with the mixture of air and fuel. Four Stroke Cycle

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2. Compression Stroke

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3. Power Stroke The Piston is going down. Volume of the cylinder is increasing. Both valves closed. Spark plug fires, causing compressed air/fuel mixture to burn. The flame front travels across the  combustion chamber. The burning fuel creates tremendous pressure which forces the piston down. turning the  crankshaft and producing power. This is not an explosion, but a smooth, controlled burn.

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4. Exhaust Stroke The Piston is going up, so the volume of the cylinder is getting smaller. The Piston is going up, so the volume of the cylinder is getting smaller. Both valves are open for a short period of time at TDC. This is called "Valve Overlap" The intake stroke of the new cycle follows this exhaust stroke.

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Working of Four Stroke Engine

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Applications of Four Stroke Engine The continuously used engines in cars and trucks are 4-stroke engines. Car Engine Truck Engine

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Advantages : Last longer than two stroke engines More efficient use of gas Pollute less than two stroke engines More efficient burning process .As size increases, power-to-weight ratio improves Four Stroke Engine Disadvantages: More complicated. Many more parts to worry about. Half as powerful as two stroke engines (for equivalent engines) Fires once every two revolutions.

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A two-stroke engine is an internal combustion engine that completes the process cycle in one revolution of the crankshaft (an up stroke and a down stroke of the piston, compared to twice that number for a four-stroke engine). This is accomplished by using the end of the combustion stroke and the beginning of the compression stroke to perform simultaneously the intake and exhaust (or scavenging) functions. In this way, two-stroke engines often provide high specific power, at least in a narrow range of rotational speeds. The functions of some or all of the valves required by a four-stroke engine are usually served in a two-stroke engine by ports that are opened and closed by the motion of the piston(s), greatly reducing the number of moving parts. Gasoline (spark ignition) versions are particularly useful in lightweight (portable) applications, such as chainsaws, and the concept is also used in diesel compression ignition engines in large and weight insensitive applications, such as ships and locomotives. Invention of the two-stroke cycle is attributed to Scottish engineer Dugald Clerk, who in 1881 patented his design, his engine having a separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Englishman Joseph Day. Two Stroke Engine

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Two Stroke Cycle

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Working two stroke engine

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Two Stroke Engine Advantages : - Two-stroke engines do not have valves, simplifying their construction. - Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost. - Two-stroke engines are lighter, and cost less to manufacture. - Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution. Disadvantages: - Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.- Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon. - Two-stroke engines produce more pollution.

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Applications Of Two Stroke Engine The two-stroke engine was very popular throughout the 20th century in motorcycles and small-engined devices, such as chainsaws and outboard motors, and was also used in some cars, a few tractors and many ships. Part of their appeal was their simple design (and resulting low cost) and often high power-to-weight ratio. Because of the lower cost to rebuild and maintain, the two stroke engine was incredibly popular in the dirt bike and motocross industry for many years, until recently when the EPA required the industry to switch to four-stroke engines because they emit less pollution than two stroke engines. Many designs use total-loss lubrication, with the oil being burned in the combustion chamber, causing "blue smoke" and other types of exhaust pollution. This is a major reason for two-stroke engines being replaced by four-stroke engines in many applications. Two-stroke engines continue to be commonly used in high-power, handheld applications such as string trimmers and chainsaws. The light overall weight, and light-weight spinning parts give important operational and even safety advantages. For example, only a two-stroke engine that uses a gasoline-oil mixture can power a chainsaw operating in any position.

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Comparison Two stroke Engine Two stroke engines produce twice as much power as four stroke engines They have ports instead of valves The engines have fewer parts as compared to 4 stroke Fuel efficiency is much less These engines are highly polluting Two stroke engines fire once every revolution Used in chainsaws etc Four stroke Engine Four stroke engines produce half the power as two stroke engines They have valves and not ports Four stroke engines have many parts and hence have more complex construction The fuel efficiency is high They produce less pollution Four stroke engines fire once in every two revolutions Used in cars and trucks’

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There are two common types of engines, which are closely related to each other but have major differences in their design and behavior. The earliest of these to be developed is the Otto cycle engine which was developed in 1876 by Nikolaus August Otto in Cologne, Germany. This engine is most often referred to as a petrol engine or gasoline engine, after the fuel that powers it. The second type of four-cycle engine is the Diesel engine developed in 1893 by Rudolph Diesel, also of Germany. Diesel created his engine to maximize efficiency which was lacking in the Otto engine. There are several major differences between the Otto cycle engine and the four cycle diesel engine. The diesel engine is made in both a two-cycle and a four-cycle version. Ironically Otto's company Deutz AG produces primarily diesel engines in the modern era. Types of Four Stroke Engine

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OTTO CYCLE Pressure-Volume diagram Temperature-Piston position diagram An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines

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Process Description: Process 1-2 is an isentropic compression of the air as the piston moves from bottom dead center to top dead center. Process 2-3 is a constant-volume heat transfer to the air from an external source while the piston is at top dead center. This process is intended to represent the ignition of the fuel-air mixture and the subsequent rapid burning. 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 while the piston is a bottom dead center. Exhaust stroke-ejection of the gaseous mixture via an exhaust valve through the cylinder head. Induction stroke-intake of the next air charge into the cylinder. The volume of the exhaust gasses is the same as the air charge

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The fuel used in four cycle engines are most typically fractions of crude oil, coal tar, oil shale, or sands which are produced in a process called Petroleum Cracking. The ignition temperature of the fuel that is refracted is related to its weight. It is separated by being heating and is extracted at different heights in the refractory tower. The higher the fuel vapour rises in the tower the lower its weight and the less energy it contains. In refracting petroleum, there is a standard weight of fuels and products that is withdraw and which is associated with a specific extracted material. Gasoline is a light refractory product and is called a light fraction. The idealized four-stroke Otto cycle p-V diagram: the intake (A)  stroke is performed by an isobaric expansion, followed by the  compression (B)  stroke, performed by an adiabatic compression. Through the combustion of fuel an isochoric process is produced, followed by an adiabatic expansion, characterizing the  power (C)  stroke. The cycle is closed by an isochoric process and an isobaric compression, characterizing the exhaust (D)  stroke.

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Otto Cycle vs Diesel Cycle

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