The Four Stroke Five Event Cycle Principle

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The Four Stroke Five Event Cycle Principle Powered By Docstoc
					                The Four-Stroke
           Five-Event-Cycle Principle
             The Intake or Admission Stroke

     During the intake or admission stroke, the piston moves
downward as a charge of combustible fuel and air is admitted into
 the cylinder through the open intake valve. At the completion of
      this stroke the intake valve closes. This is event No. 1.

                 The Compression Stroke
During the compression stroke, the crankshaft continues to rotate,
 the piston is forced upward in the cylinder, and both intake and
 exhaust valves are closed. The movement of the piston upward
       compresses the fuel-air mixture. This is event No. 2.

                Power or Expansion Stroke
 As the piston approaches the top of its stroke within the cylinder,
  an electric spark jumps across the points of the spark plugs and
ignites the compressed fuel-air mixture. This is the ignition event,
     or event No. 3. The intake and exhaust valves are closed.

 Having been ignited, the fuel-air mixture burns. It expands as it
burns and drives the piston downward. This causes the crankshaft
  to revolve. Since it is the only stroke and event that furnishes
   power to the crankshaft, it is usually called the power stroke,
although it is sometimes called the expansion stroke for purposes
of instruction. This is event No. 4. The intake and exhaust valves
                              are closed.

            The Exhaust or Scavenging Stroke
             During the power or expansion stroke, the hot gases obtained by
            combustion exert tremendous pressure on the piston to force it to
              move downward, but near the end of the stroke this pressure is
              greatly reduced because of the expansion of the gases. At this
               stage, the exhaust valve opens as the crankshaft continues to
             revolve and the piston is again moved upward in the cylinder by
             the connecting rod. The burning gases remaining in the cylinder
               are forced out through the exhaust valve, hence this stroke is
              usually called the exhaust stroke, although it may be called the
            scavenging stroke for purposes of instruction. This is event No. 5.
                           One engine cycle has been completed.

                             Summary of Events
To summarize the events, it is found that the charge of fuel and air was admitted into the
cylinder during the intake stroke (event No. 1); the piston compressed the fuel-air mixture
during the compression stroke (event No. 2); the electric spark ignited the compressed
fuel-air mixture as the piston approached the top of its stroke within the cylinder (event
No. 3); the fuel-air mixture burned and the expanding gases drove the piston downward
during the power stroke (event No. 4); the burned gasses were forced out of the cylinder
during the exhaust stroke (event No. 5)."
This five-event sequence of intake, compression, ignition, power, and exhaust, is a cycle
which must take place in the order given if the engine is to operate at all, and it must be
repeated over and over for the engine to continue operation.

None of the five events can be omitted, and each event must take place in the proper
sequence. For example, if the gasoline supply is shut off, there can be no power event,
but the mixture of gasoline and air must be admitted to the cylinder during the intake
stroke. Likewise, if the ignition switch is turned off, there can be no power event, but the
ignition must occur before the power stroke can take place.

                           Rotary Engine Theory
                 100 hp Gnome Monosoupe (The Science Museum, London)

Most often attributed to the American F.D.Farwell the rotary engine may have had an earlier
beginning in a compressed-air engine worked out by the Australian pioneer Lawrence
Hargrave some eight or nine years prior. It is certain, however, that the French brothers
Seguin brought the engine into commercial and mechanical life based on the conceptions of
one brother (Laurent). It was in 1907 that his 7-cyl rotary was born - and it came to be known
as the Gnome. This engine was followed by a succession of designs by many manufacturers,
most of which were successful.

We are accustomed to seeing someone swing the prop to start an engine. This was not always
necessary because a crank could be engaged to the gear on the rear of the thrust plate and
enough rotary motion could be generated to get the engine started. When you compare this
with the starting of a radial or an inline the reason why rotaries started easier can be seen. In
the case of the radial or inline, it was necessary to set the inards of the engine in motion. The
rotary was started by rotating the engine. The mass of the rotary added to the starting
function and assisted the effort. The rotary was its own inertial starter!

The rotary engine gained quick acceptance because of its remarkable power to weight ratio.
The only comparable ratios came from the brilliant mind of an American, Charles Manly. He
had, in the very early years of the 1900s, achieved P/W ratios that even rotaries did not match
until 1916. His 5-cyl 4-stroke static radial gave a ratio of 2.4 lb per hp dry and 4.0 with all of
its plumbing attached. How remarkable was his achievement can be seen in a comparison of
the Wright's engine which delivered one hp per 15 lbs and the 1912 Gnome rotary of 80hp
which had a 2.625 ratio. Manly did not produce his engine commercially: the brothers Seguin

That the rotary engine dominated the early years of aviation is evident - although there were
some very fine engines extant such as the twins of Duthiel-Chalmers and Darracq, the
Antoinette by Levavasseur, and those of Fiat.

The demise of the rotary came about for several reasons. Among the most important of these
was the large rotating mass of the engine which produced gyroscopic forces. These forces
had their useful features - if the pilot could master them before something happened to lessen
his desire to fly. It provided the Sopwith Camel with remarkable turning power. However,
the engine also delivered sharp torque reversals when the ignition was cut which was tough
on the engine mounts and the airframe.

Another problem encountered by rotary engine designers was met when trying to meet the
demand for greater power. The size of the engine could be expanded in only two directions:
make it larger in circumference, make it more than one row (deeper). The problem with the
first solution was that this just made the gyroscopic forces even more unmanageable. The
second way out of the problem provided much the same effect and the rear bank of cylinders
were hard to cool.

There are other reasons that would have tended against the use of the rotary into more
modern times and the greatest of these would be its enormous appetite for oil. The fuel was
mixed with air as it was introduced through a primitive "carburetor" - usually in the tail end
of the crankshaft. Via this route it made its way to the crankcase where is picked up all of the
oil that was loose. When the fuel mixture was introduced to the combustion chamber it was
very much a mix of fuel, air, and castor oil.

The imperfect combustion of any engine is not equalled by that of a rotary. The castor oil,
being the least combustible of the two liquids, was spewed out into the atmosphere. It would
be but a short time before the whole of the slipstream area of the aeroplane would be well
coated with castor oil. The pilot would be soaking up oil at a fairly rapid rate as well. It is
arguable that the reason for cowling the engine had as much to do with trying to control the
wildly spewing oil as it was to do with the concepts of streamlining. The usual practice was
to direct the oil underneath the fuselage by opening up the bottom of the cowl.

However, a cowling is not a favorite item to a rotary. The cylinders are air-cooled. As has
been mentioned, the use of two banks of cylinders caused trouble enough. The cowling made
the engine much hotter that it liked. The reason for the cutout in the bottom of the cowl, then,
was to direct the spray of oil as well as to aid in cooling the engine. Some of the cowlings of
WWI aeroplanes show evidence of extra cooling openings being cut into them by mechanics
in the field.

Many people remark about the pleasantness of the odor of burnt castor oil. Out in the open
where one's exposure is contrasted with other scents, it can be an enjoyable sensation. It is
still nice if you are saying, "bye-bye" to the pilot before you go back to your mechanic's
tasks. But to sit behind an engine that is spraying you with unburnt - as well as burnt - castor
oil is quite another matter after a few hours. The oil is known for its purgative qualities. It
would be impossible to expose oneself to such an atmosphere and not experience certain

It is the need for cooling that is part of the reason that pilots 'blipped' their engines. One
could not use a throttle on them because they had such great need of motion to keep them
cool. That they were allowed to stop to decend is true but the combustion had ceased during
that time. (Of course, starting them up again could be an exciting experience. If they were not
too loaded with the explosive fuel mixture - they might do just that: explode. If badly loaded
in one or two cylinders, the rough running could cause considerable concern before it

Although the cowling did cause them to overheat, It also allowed them to produce greater
power as the air trapped within the cowl was easier to "stir" with the cylinders than would be
a stream of high velocity air directed at the front of the engine.

They were easy to start by diving to turn the prop - which turned the engine. And they have
been known to run with the most awesome damage inflicted on one or more cylinders.

There are many stories about the gyroscopic forces and their ability to turn a sorely pressed
pilot out of danger. The most engaging terms used to describe the turn of a Camel was said
by Dick Day: "Why, it puts both eyes on the same side of your nose!"

                           Rotary Engines and Specifications:

                                                  110 Hp
                         80 Hp                                           Oberrursel
    Model:                                       Le Rhône
                        Le Rhône                                          U.R. II
                                                  (Type J)
     Date:             Circa 1916               Circa 1916                Circa 1916
  Cylinders:                9                        9                        9
                         Rotary,                 Rotary,                   Rotary,
                        Air cooled              Air cooled                Air cooled
                          80 hp                   113 bhp                   110 hp
                         (59 kw)                  (84 kw)                  (82 kw)
    R.P.M.:             1,200 rpm                1,200 rpm                   NA
    Bore &            4.1 in x 5.5 in          4.4 in x 6.7 in           4.9 in x 5.9 in
    Stroke:         105mm x 140mm            112mm x 170mm             124mm x 150mm
Displacement:              NA                     920 in³               995 in³/16.3 L
    Weight:                NA                 330 lbs (149 kg)               NA

                         Inside The Radial Engine
                        Master-and-articulating-rod Assembly

The master-and-articulating-rod assembly is used on X-type engines, radial-type engines,
and on some V-type engines. The master rod is similar to any other connecting rod except
that it is constructed to provide for the attachment of the articulated rods on the big end.

The articulated rods are fastened by knuckle pins to a flange around the master rod. Each
articulated connecting rod has a bushing of nonferrous metal, usually bronze, pressed or
shrunk into place to serve as a knuckle-pin bearing. The knuckle pins may be held tightly
in the master-rod holes by press fit and lock plates or they may be of the full-floating
If the big end of the master rod is made of two pieces, the cap and the rod, the crankshaft
is made of one solid piece. on the other hand, if the rod is made of one piece, then the
crankshaft may be of either two-piece or three-piece construction. Regardless of the type
of construction, the usual bearing surfaces must be supplied.

It should be understood that the type of connecting rod used in an engine depends largely
on the cylinder arrangement. If the cylinders are arranged in a line parallel to the
crankshaft, the connecting rod is similar to that used in most automobile engines.
However, certain types of aircraft engines have a system of connecting rods connected to
the same crankshaft bearing, called an articulating connecting-rod assembly. The main
rod or master rod joins one of the pistons with the crankshaft, and the other rods, called
articulating rods or link rods, connect the other pistons to this same master connecting
Different Types of Jet Engines
While every type of turbojet engine shares a basic core - compressor, combustor, turbine
- there are variations for different applications.

Turbo prop
Turbo prop engines can be found on small commuter aircraft. While they may look like
the standard piston-driven propeller engines found on recreational planes, turbo props are
much more powerful.

Energy from the turbine is used to spin the large front-mounted propeller. The shaft that
connects the propeller to the turbine is also linked to a gearbox that controls the
propeller's speed. The propeller is most efficient and quiet, when the tips are spinning at
just under supersonic speed (the speed of sound).

Turbo shaft
Turbo shaft engines are extremely versatile and are used in helicopters, electric power
plants, offshore oil drilling, even the mighty M1 tank! In theory, it works just like the
turbo prop. But instead of the turbine's force driving a propeller or creating thrust directly
behind the engine, power can be routed via the shaft to a variety of devices; pumps,
generators, wheels, helicopter blades, a ship's propeller -- just about anything that spins.

Turbofan (High Bypass)
Large commercial airliners use a turbofan jet engine. Turbofans use the same compressor,
combustor and turbine common to all turbojet engines. The difference is the addition of a
large fan mounted to the front of the engine. These fans, some as large as 10' in diameter,
draw air into the engine. Some of the air is sent to the compressor and the combustor,
while the rest bypasses these components through ducts on the outside of the engine.

Most turbofans in use today are designated as high-bypass turbofans, where the ratio of
bypass air to the air directed into the compressor is 5:1 or greater. At subsonic speed,
high-bypass turbofans are more fuel efficient and quieter than other types of jet engines,
making them ideally suited for commercial aircraft.

Accelerating a vehicle heavier than a locomotive from 0 to 200 mph in less than 60
seconds requires a lot of thrust. It is the fan, and the high volume of air it pulls in which
creates most of the engine's thrust at takeoff!

                                Jet Engine Theory

Centuries ago in 100 A.D., Hero, a Greek philosopher and mathematician, demonstrated
jet power in a machine called an "aeolipile." A heated, water filled steel ball with nozzles
spun as steam escaped.

Over the course of the past half a century, jet-powered flight has vastly changed the way
we all live. However, the basic principle of jet propulsion is neither new nor complicated.

Centuries ago in 100 A.D., Hero, a Greek philosopher and mathematician, demonstrated
jet power in a machine called an "aeolipile." A heated, water filled steel ball with nozzles
spun as steam escaped. Why? The principle behind this phenomenon was not fully
understood until 1690 A.D. when Sir Isaac Newton in England formulated the principle
of Hero's jet propulsion "aeolipile" in scientific terms. His Third Law of Motion stated:
"Every action produces a reaction ... equal in force and opposite in direction."

The jet engine of today operates according to this same basic principle. Jet engines
contain three common components: the compressor, the combustor, and the turbine. To
this basic engine, other components may be added, including:
      A nozzle to recover and direct the gas energy and possibly divert the thrust for
       vertical takeoff and landing as well as changing direction of aircraft flight.
      An afterburneror augmentor, a long "tailpipe" behind the turbine into which
       additional fuel is sprayed and burned to provide additional thrust.
      A thrust reverser, which blocks the gas rushing toward the rear of the engine,
       thus forcing the gases forward to provide additional braking of aircraft.
      A fan in front of the compressor to increase thrust and reduce fuel consumption.
      An additional turbine that can be utilized to drive a propeller or helicopter

                            The Turbojet Engine

                                    A turbojet engine.

The turbojet is the basic engine of the jet age. Air is drawn into the engine through the
front intake. The compressor squeezes the air to many times normal atmospheric pressure
and forces it into the combustor. Here, fuel is sprayed into the compressed air, is ignited
and burned continuously like a blowtorch. The burning gases expand rapidly rearward
and pass through the turbine. The turbine extracts energy from the expanding gases to
drive the compressor, which intakes more air. After leaving the turbine, the hot gases exit
at the rear of the engine, giving the aircraft its forward push ... action, reaction !

For additional thrust or power, an afterburner or augmentor can be added. Additional fuel
is introduced into the hot exhaust and burned with a resultant increase of up to 50 percent
in engine thrust by way of even higher velocity and more push.

                 The Turboprop/Turboshaft Engine
                            A turboprop, or turboshaft engine.

A turboprop engine uses thrust to turn a propeller. As in a turbojet, hot gases flowing
through the engine rotate a turbine wheel that drives the compressor. The gases then pass
through another turbine, called a power turbine. This power turbine is coupled to the
shaft, which drives the propeller through gear connections.

A turboshaft is similar to a turboprop engine, differing primarily in the function of the
turbine shaft. Instead of driving a propeller, the turbine shaft is connected to a
transmission system that drives helicopter rotor blades; electrical generators, compressors
and pumps; and marine propulsion drives for naval vessels, cargo ships, high speed
passenger ships, hydrofoils and other vessels.

                            The Turbofan Engine

                              A high bypass turbofan engine.

A turbofan engine is basically a turbojet to which a fan has been added. Large fans can be
placed at either the front or rear of the engine to create high bypass ratios for subsonic
flight. In the case of a front fan, the fan is driven by a second turbine, located behind the
primary turbine that drives the main compressor. The fan causes more air to flow around
(bypass) the engine. This produces greater thrust and reduces specific fuel consumption.
                               A low bypass turbofan engine.

For supersonic flight, a low bypass fan is utilized, and an augmentor is added for
additional thrust.

                      The Ramjet/Scramjet Engine

                                      A ramjet engine.

A ramjet has no moving parts and achieves compression of intake air by the forward
speed of the air vehicle. Air entering the intake of a supersonic aircraft is slowed by
aerodynamic diffusion created by the inlet and diffuser to velocities comparable to those
in a turbojet augmentor. The expansion of hot gases after fuel injection and combustion
accelerates the exhaust air to a velocity higher than that at the inlet and creates positive
                                     A scramjet engine.

Scramjet is an acronym for Supersonic Combustion Ramjet. The scramjet differs from
the ramjet in that combustion takes place at supersonic air velocities through the engine.
It is mechanically simple, but vastly more complex aerodynamically than a jet engine.
Hydrogen is normally the fuel used.

                     The Ultra High Bypass Engine

                              An ultra high bypass jet engine.

A logical approach to improving fuel consumption is even higher bypass technology.
Mechanical arrangements can vary. During the 1980s, GE developed the Unducted Fan
UDF® engine which eliminated the need for a gearbox to drive a large fan. The jet
exhaust drives two counter-rotating turbines that are directly coupled to the fan blades.
These large span fan blades, made of composite materials, have variable pitch to provide
the proper blade angle of attack to meet varying aircraft speed and power requirements.
Powerplants such as the UDF® engine are capable of reducing specific fuel consumption
another 20-30 percent below current subsonic turbofans.

How a High-Bypass Turbofan Works
More Oxygen for More Efficient Combustion
The large front-mounted fan of a high-bypass turbofan draws in huge volumes of air, on
the order of 2400 pounds/second. That's enough to vacuum all the air out of Madison
Square Garden in 4 seconds.

Turning a Little Bit of Atmosphere into a Lot of Hot Air
Of the total volume of air that is drawn in by the fan, as little as 10% of it will actually
pass through the engine's core (compressor, combustor, turbine).

In the compressor, the air's pressure will increase by up to 40 times and the temperature
will also greatly increase.

How Do You Keep a Flame Burning in a 400mph Wind?
The hot, pressurized air from the compressor is mixed with a steady stream of fuel and
then ignited in the combustor to create a flame. The shape and perforations of the
combustor allow the pressurized air to then expand, slowing it down enough to sustain
the flame continuously (as long as fuel is supplied).

Like a Pinwheel in a Hurricane
There are two turbines that are driven by the gases from the combustor. The high-
pressure turbine, located just behind the combustor, uses the energy of the exhaust gases
to power the compressor. The low-pressure turbine located after the high-pressure unit
powers the front-mounted fan as speeds of 5,000 rpm for large fans. Generally, the
smaller the fan, the faster it moves.

Where Thrust is Created
Up to 90% of the air pulled in by the fan is channeled around the major components of
the engine, bypassing the compressor, combustor and turbine. This bypass air flows into
the narrow part of the nacelle (the engine casing) and then through the nozzle, where it
meets up with the hot exhaust created during combustion.

The exhaust and the bypass air will exit the engine through another nozzle - the core
nozzle. The net difference between the speed of the air entering the engine at the fan and
the speed at which it leaves at the nozzle is thrust.

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