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 did. 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 difficulties. 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 cleared.) 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, Configuration: Air cooled Air cooled Air cooled 80 hp 113 bhp 110 hp Horsepower: (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 type. 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 rod. 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 rotor. 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 push. 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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