2011 I.C. ENGINE 2 Stroke Vs 4 Stroke Engine The internal combustion engine is an engine in which the combustion of fuel (normally fossil fuel) occurs with an oxidizer (usually air) in combustion chamber. The expansion of the high- temperature and pressure gases produced by combustion applies direct force to some component of the engine, such as pistons, turbine blades, or nozzle. This force moves the component over distance, generating useful mechanical energy. large number of different designs for ICEs have been developed and built, with variety of different strengths and weaknesses. For instance four- stroke and two-stroke piston engines, along with variants, such as the six-stroke piston engine and the Wankel rotary engine. 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. Here in this report we will deal with Two and Four Stroke IC Engine. Prashant Pratap Singh P2008ME1119 INTRODUCTION two-stroke engine is combustion engine that completes the thermodynamic cycle in two movements of the piston compared to twice that number for four-stroke engine. This increased efficiency is accomplished by using the beginning of the compression stroke and the end of the combustion stroke to perform simultaneously the intake and exhaust (or scavenging) functions. In this way two-stroke engines often provide strikingly high specific power, but still due to environmental issues and low fuel efficiency four stroke engine is preferred over stroke. Invention of the two-stroke cycle is attributed to Scottish engineer Dugald Clerk who in 1881 patented his design, his engine having separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as charging pump, is generally credited to Englishman Joseph Day (and Frederick Cock for the piston-controlled inlet port). The four-stroke engine was first patented by Alphonse Beau de Rochas in 1861. The first person to actually build car with this engine was German engineer Nikolaus Otto. That is why the four-stroke principle today is commonly known as the Otto cycle and four-stroke engines using spark plugs often are called Otto engines. FOUR STROKE ENGINE: Engines based on the four-stroke ("Otto cycle") have one power stroke for every four strokes (up-down-up-down) and employ spark plug ignition. Combustion occurs rapidly, and during combustion the volume varies little ("constant volume"). The Otto Cycle consists of adiabatic compression, heat addition at constant volume, adiabatic expansion and rejection of heat at constant volume. They are used in cars, larger boats, some motorcycles, and many light aircraft. They are generally quieter, more efficient, and larger than their two-stroke counterparts. i. Compression Stroke The piston goes up and compresses fuel/air mixture (in gas engine) or just air (in diesel). ii. Power Stroke The fuel is ignited (by spark in gas engine, by being injected into high temperature air in diesel.) The energy released drives the piston down. This provides the momentum necessary to keep the crankshaft turning and make the other three strokes happen. iii. Exhaust Stroke The piston goes up and pushes the burned gases out the exhaust valve. iv. Intake Stroke The piston goes down and draws in new air, or fuel/air mixture, ready for the next compression stroke. TWO STROKE ENGINE: In 2-stroke engine, the last three strokes are combined into one. i. Compression Stroke The piston goes up and compresses fuel/air mixture (in gas engine) or just air (in diesel.) ii. Power Stroke The fuel is ignited and drives the piston down. As the piston goes down it not only turns the crankshaft but also pressurizes the fuel/air mixture in the crankcase which is about to be admitted to the piston for the next cycle. Near the bottom of the stroke, an outlet opens and the exhaust gases are released. Even nearer the bottom of the stroke, an inlet opens and new fuel/air (which was just pressurized by the piston) rushes in ready for compression. PROBLEMS OF THE TWO-STROKE ENGINE: Actually the two-stroke engine should perform twice the performance of four-stroke engine with the same cubic capacity. Though it is just possible to gain performance that is about 50% better. The reasons are obvious: The cylinder can't be filled up with the same amount of fuel as in the four-stroke engine, because the individual strokes are separated not so clearly. If more fuel is induced, it leaves the combustion chamber through the ejection pipe without being burnt. Many concepts were developed to provide better expulsion of the exhaust in way that the fresh gas doesn't leave the combustion chamber (as for example the "nosepiston" which causes turbulences of certain type). Though all these inventions, the filling of the two-stroke engine is always worse than in the four- stroke engine, which loses fresh fuel only because of the "overlap" of the valve times (both valves are open for an instant). Beside these performance-technical problems, there are also increasing difficulties with the environment. The fuel mixture of the two-stroke engine often gets shifted with certain quantity of oil because of the necessary lubrication. Unfortunately the oil gets burnt partly, too, and harmful gases are expulsed by the engine. Also, the 2-stroke engine lets fuel into the crankcase, where the piston can pressurize it prior to intake. In order to keep the crankcase lubricated, you have to add expensive lubricants to the fuel, and even so, 2-stroke engines don't last very long. ADVANTAGES OF TWO-STROKES CYCLE OVER FOUR-STROKE. The two-stroke engines in such devices as chain saws and jet skis because two-stroke engines have three important advantages over four-stroke engines: i. Two-stroke engines do not have valves, which simplifies their construction and lowers their weight. ii. Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost. iii. Two-stroke engines can work in any orientation, which can be important in something like a chainsaw. A standard four-stroke engine may have problems with oil flow unless it is upright, and solving this problem can add complexity to the engine. These advantages make two-stroke engines lighter, simpler and less expensive to manufacture. Two-stroke engines also have the potential to pack about twice the power into the same space because there are twice as many power strokes per revolution. The combination of light weight and twice the power gives two-stroke engines a great power-to-weight ratio compared to many four-stroke engine designs. DISADVANTAGES OF THE TWO-STROKE OVER FOUR STROKE: You can now see that two-stroke engines have two important advantages over four-stroke engines: They are simpler and lighter, and they produce about twice as much power. So why do cars and trucks use four-stroke engines? There are four main reasons: i. Two-stroke engines don't last nearly as long as four-stroke engines. The lack of dedicated lubrication system means that the parts of two-stroke engine wear lot faster. ii. Two-stroke oil is expensive, and you need about ounces of it per gallon of gas. You would burn about gallon of oil every 1,000 miles if you used two-stroke engine in car. iii. Two-stroke engines do not use fuel efficiently, so you would get fewer miles per gallon. iv. Two-stroke engines produce lot of pollution (air as well as noise) -- so much, in fact, that it is likely that you won't see them around too much longer. The pollution comes from two sources. The first is the combustion of the oil. The oil makes all two-stroke engines smoky to some extent, and badly worn two-stroke engine can emit huge clouds of oily smoke. The second reason is less obvious but can be seen in the following figure: Each time new charge of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port. That's why you see sheen of oil around any two-stroke boat motor. The leaking hydrocarbons from the fresh fuel combined with the leaking oil is real mess for the environment. These disadvantages mean that two-stroke engines are used only in applications where the motor is not used very often and fantastic power-to-weight ratio is important. In the meantime, manufacturers have been working to shrink and lighten four-stroke engines, and we can see that research coming to market in variety of new marine and lawn-care products. CONCLUSION: Though two stroke engine emits more power and torque, they are not suited for the day to day activity. Moreover, they are not fuel efficient, have short life, polluting agent and also noisier than stroke ones. Therefore, stoke engines should be preferred as they are more fuel efficient, less polluting, and affordable. stroke bikes are ideal for day to day activities.
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