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2 Stroke Vs 4Stroke engine

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2 Stroke Vs 4Stroke engine Powered By Docstoc
					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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