Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.

First Law. A body at rest tends to remain at rest and a body in motion tends to continue in motion in a straight line unless caused to change its state by an external force.

Second Law. The acceleration of a body is directly proportional to the force causing it and inversely proportional to the mass of body.

Third Law. For every action , there is an equal and opposite reaction.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.

The second and third laws of motion are the most applicable to the development of thrust and equations for jet thrust may be derived directly from the second law.

This law may be stated in words differing from those given before, however, the meaning is the same:

A force is created by a change of momentum, and this force is equal to the time rate of change momentum.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.

The momentum of a body is defined as the product of its mass and velocity.

From Newtons Law, it is apparent that if we change the velocity of a body (or mass), a force is required.

F= dM / dt

Where F is the force; M the momentum; and t the time.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.

To understand the operation of equations dealing with acceleration, the nature of acceleration must be understood.

Acceleration may be defined as a change in velocity.

Generally the word acceleration is used to indicate an increase in velocity.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.

It is known through studies in the laws of physics that a free-falling body will accelerate at 32.2 ft/(s)(s) or s2 [9.8 m/(s)(s)].

This means that the velocity of the body will increase 32.2 ft/s each second that it is in a free-falling state, discounting the effects of air friction.

Thus, 32.2 ft/s2 is called the acceleration of gravity, and the letter g is used to indicate this value.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.

If an automobile weighs 3,000 lb and we wish to know how much force is necessary to cause it to accelerate from 0 mph in 5 s, we can proceed as follow:The force needed is then determined by multiplying 3000 by 17.6/32.2.The basic equation which we may derive from Newtons second law is F = M aWhere: F is force, M is mass (weight), and a is acceleration.Mass is a basic property of matter, whereas weight is the effect of gravity on a mass.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.The equation for thrust may be writtenF = w / g (V2 V1) WhereF = force, lb w = flow rate, lb/s of air and fuelg = acceleration of gravity, (32.2 ft/s2)V2 = final velocity of gases V1 = initial velocity of gasesThe appropriate thrust of an engine may be determined by considering the weight of the air only because the fuel weight is a very small percentage of the air weight.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.The equation for thrust may be writtenF = wa / g (V2 V1) + wf / g (Vj) Wherewa = airflow through the engine, lb/swf= fuel flow, lb/s Vj = velocity of gases at the jet nozzle, ft/s

However, to obtain an accurate indication of thrust produced by the acceleration of the fuel-air mixture, it is necessary to include both fuel and air in the equation.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.In actual practice, all the pressure of the gases following from the nozzle of at jet engine cannot be converted to velocity.

This is particularly true of a jet nozzle in which the velocity of gases reaches the speed of the sound.

In these cases, the static pressure of the gases at the jet nozzle is above the ambient air pressure.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.The thrust generated at the jet nozzle is indicated by the equation.F j = Aj (Pj Pam)Where

F j = force (thrust)Aj = area of jet nozzle, ft2Pj = static pressure at jet nozzle, lb/ft2Pam = static pressure of the ambient air

This difference in pressure creates additional thrust proportional to the area of the jet nozzle.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.

Fn = wa / g (V2 V1) = wf / g (Vj) + Aj (Pj Pam)

When the jet nozzle is added to the reaction thrust created by acceleration of gases in the engine, the equation for the net thrust becomes.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.To better understand what takes place in a typical jet engine, it is well to examine the pressure and temperatures which exist within the engine during operation.

Figure 19-5 is a drawing showing the values of pressure and temperature within a twin-spool turbofan engine.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton real

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton real

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.It will be noted that there is a substantial rise in both pressure and temperature through the two compressor sections.

Also, the pressure has increased more than 13 to 1 and the temperature has increased from 59 to 715F [15 to 379C] in the diffuser section.

The air from the compressor enters the burner where fuel is added and burned.

Pressure drops slightly while velocity and temperature increase markedly.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.As the hot gases pass through the turbine section, pressure, temperature, and velocity all decrease.

This is because the turbines extract energy from the high-temperature, high-velocity gases.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.The effects of heat in a jet engine, or any engine, are explained by the laws of thermodynamics.

The first law states that energy is indestructible.

Heat energy is imparted to the air in a gas-turbine (jet) engine by the compressor, and additional heat is added when the fuel is burned.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.The heat energy is changed to thrust and the gases are cooled as they pass through the turbine section and out the jet nozzle.

In any energy cycle, the total quantity of energy involved is exactly equal to the energy which can be accounted for in any of the forms in which it may occur throughout the cycle, i.e., mechanical energy, heat energy, pressure energy, etc.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realNewtons Laws of Motion.The second law of thermodynamics in one form states that heat cannot be transferred from a colder body to a heater body.

The cooling of an engine involves this principle, in that heat is transferred from hotter bodies or substances to cooler bodies or substances.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realThe Brayton cycle.

Is the name given to the thermodynamic cycle of a gas turbine engine to produce thrust.

This is a varying-volume, constant pressure cycle of events and is commonly called the constant-pressure cycle, open cycle or continuous-combustion cycle).

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realThe Brayton cycle.

The four continuous and constant events are: intake, compression, expansion (includes power), and exhaust.

In the intake cycle air enters at ambient pressure and in constant volume and leaves at an increased pressure and decreased volume.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realThe Brayton cycle.

In the compressor section air is received from the intake at increased pressure slightly above ambient and slightly decreased volume. Air enters the compressor where it is compressed, leaving with a large increase in pressure and decrease in volume. This is caused by the mechanical action of the compressor.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realThe Brayton cycle.

The next step (expansion) takes place in the combustion chamber by burning fuel, which expands the air. Pressure remains relatively constant, but a marked increase in volume takes place.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realThe Brayton cycle.

The expanding gases move rearward through the turbine assembly and are converted from velocity energy to mechanical energy by the turbine.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBernoullis theorem.When a gas or fluid is flowing through a convergent duet (as in nozzle stator vanes or venture) its speed will increase and its temperature and pressure will decrease.

If this area is a divergent duct (as in a diffuser or rotor blade), its speed will slow, and its temperature and pressure will increase.

The total energy in a flowing gas is made up of static and dynamic temperatures and pressures.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBernoullis theorem.A nozzle or a diffuser does not change the total energy level but rather changes one form of energy to another.By varying the area of a pipe, velocity can be changed into pressure and pressure into velocity.

A turbine engine is just such a pipe, with areas where air pressure and velocity are constantly being changed to achieve desired results.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBernoullis theorem.To state this principle simply: the divergent duct increases velocity and decreases pressure.

The convergent duct can be associated with the compressor where the air is slowing and pressurizing.

In the combustion area the opposite is true. There, the velocity is increasing and the pressure is decreasing.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBoyles Law and Charless Law.The pressure and temperatures of gases follow the principles set forth in Boyles Law and Charless Law.

Boyles Law states that the volume of a confined body of gas varies inversely as its absolute pressure, the temperature remaining constant.

Charless Law states that the volume of a gas varies in direct proportion to the absolute temperature.

This law explains the expansion of gases when heat is added by the burning of fuel in the engine.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBoyles Law.This law states that if the temperature of a confined gas is not changed, the pressure will increase in direct relationship to a decrease in volume.

The opposite is also true The pressure will decrease as the volume is increased.

A simple demonstration of how this works may be made with a toy balloon. If you squeeze the balloon, its volume is reduced, and the pressure of air inside the balloon is increased.

If you squeeze hard enough, the pressure will burst the balloon.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBoyles Law.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realCharless Law.This law states that if a gas under constant pressure is so confined that it may expand, and increase in the temperature will cause an increase in volume.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realCharless Law.If you hold the inflated balloon over a stove, the increase in temperature will cause the air to expand and, if the heat is sufficiently great, the balloon will burst.

Thus, the heat of combustion expands the air available within the combustion chamber of a gas turbine engine.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realPressure and Velocity.Air is normally thought of in relation to its temperature, pressure, and volume.

Within a gas turbine engine the air is put into motion and another factor must be considered, velocity.

Consider a constant airflow through a duct. As long as the duct cross-sectional area remaining unchanged, air will continue to flow at the same time rate (disregard frictional loss).

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realPressure and Velocity.If the cross-sectional area of the duet should become smaller (convergent area), the airflow must increase velocity if it is to continue to flow the same number of pounds per second of airflow (Bernoullis Principle).

In order to obtain the necessary velocity energy to accomplish this. The air must give up some pressure and temperature energy (law of conservation of energy).

The net result of flow through this restriction would be a decrease in pressure and temperature and an increase in velocity.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realPressure and Velocity.The opposite would be true if air were to flow from a smaller into a larger duct (divergent area); velocity would then decrease, and pressure and temperature would increase.

The throat of an automobile carburettor is a good example of the effect of airflow through a restriction (venture); even on the hottest day the centre portion of the carburettor feels cool.

Convergent and divergent areas are used through a gas turbine to control pressure and velocity of the air-gas stream as it flows through the engine.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realGas Turbine Engine vs Reciprocating Engine.A cycle is a process that begins with certain conditions and ends with those same conditions.

Reciprocating and turbine engines have similarities.Both power plants are sir-breathing engines. Both engines have the same series of events (intake, compression, power, and exhaust).

The difference is that in a turbine engine all of these events happen simultaneously, whereas in the reciprocating engine each event must follow the preceding event.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realGas Turbine Engine vs Reciprocating Engine.Another difference is that in a turbine engine each operation in the cycle is performed by a separate component designed for its particular function; in the reciprocating engine all of the functions are performed in one component.

Note that the reciprocating engine obtains its work output by employing very high pressures (as much as 1000 poudns per square inch psi-) in the cylinder during combustion.

With these high pressures a greater amount of work can be obtained from a given quantity of fuel, thereby raising the thermal efficiency of this type engine.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realGas Turbine Engine vs Reciprocating Engine.On the other hand, a turbine engines thermal efficiency is limited by the ability of its compressor to build up high pressures without excessive temperature rises.

Ideally, a turbine engine should burn as much fuel as possible in order to raise the gas temperature and increase the useful output.

Energy added in the form of fuel is more than enough to drive the compressor. The remaining energy produces the thrust or power for useful work.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBrayton cycle.From the foregoing discussion, it is seen that many physical laws are involved in the operation of a gas-turbine engine.

The operation of some of these laws is apparent when we examine the Brayton cycle, which defines the events that take place in the turbine engine.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBrayton cycle.Point 1 in the drawing indicates the condition of the air in front on the engine before it is affected by the inlet duct of the engine.

After the air enters the inlet duct, it is diffused and the static pressure increases.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBrayton cycle.This is indicated by point 2, which represents the air condition at the entrance to the compressor.

Through the compressor, the air volume is decreased and the pressure is increased substantially as shown by the curve from point 2 to point 3.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBrayton cycle.At point 3, fuel is injected and burned, causing a rapid increase in volume and temperature. Because of the design of the combustion chamber, the pressure drops slightly as the velocity of the hot gas mixture increases to the rear.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBrayton cycle.At point 4, the heated gases enter the turbine where energy is extracted, causing a decrease in both pressure and temperature.

The curve from point 5 to point 6 represents the condition in the exhaust nozzle as the gases flow out to ambient pressure.

The difference between the positions of point 1 and point 6 indicates the expansion of the air caused by the addition of heat from the burning fuel.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBrayton cycle.The temperature diagram for the Brayton cycle, is shown in fig. 19-6b.

It will be noted that the temperature increases because of compression.

During combustion, the pressure drops slightly and the temperature increases at a rapid rate as a result of the burning of fuel.

Fundamentos de Motores de Combustin Interna2.1 Ciclo Brayton realBrayton cycle.During expansion of the gases through the turbine, the temperature and pressure of the gases are reduced to the point where they enter the atmosphere.

At this time, the temperature is still considerably above the temperature of the ambient atmosphere, but it decreases rapidly as the gases leave the jet nozzle and go into the atmosphere behind the engine.

1st. Primera parte: un libro sobre una mesa. Segunda parte, ms difcil de visualizar: bloque de madera sobre el piso, bloque de hielo; sin friccin tericamente podra moverse libremente.2nd. Un desbalance de fuerzas sobre un cuerpo produce o tiende a producir una aceleracin en la direccin de la fuerza mayor. La aceleracin es directamente proportcional a la fuerza e inversmente proporcional a su masa.Dos bloques idnticos se mueven sobre el piso, al aplicar una mayor fuerza a una de ellas se mueven ms que la otra, tendr una mayor aceleracin. Si una caja est llena y otra vaca, y se aplica la misma fuerza a ambas, la ms pesada tendr una menor aceleracin.3rd. Globo y pistola.**This equation means that the force created by a change in the velocity of a body is equal to the amount of change in velocity divided by the change in time when appropriate units of measurement are used in the equation.*This is positive acceleration. A decrease in velocity is negative acceleration, or deceleration.

*This is positive acceleration. A decrease in velocity is negative acceleration, or deceleration.

*Since 60 mph is equal to 88 ft/s, to reach this velocity, the automobile must accelerate at 17.6 ft/s each second.Since the automobile would accelerate at 32.2 ft/s2 if the force applied were equal to its weight, we know that the needed force is less than 3,000 lb.

The force needed for the requires acceleration is then 1639.75 lb.

When considering matter on the Surface of th earth, mass and weight are approximately the same.**In this equation, Vj represents the acceleration of the fuel because the initial velocity of the fuel is the same as that of the engine.*****As the hot gases pass through the turbine section, pressure, temperature, and velocity all decrease.

This is because the turbines extract energy from the high-temperature, high-velocity gases.*As the hot gases pass through the turbine section, pressure, temperature, and velocity all decrease.

This is because the turbines extract energy from the high-temperature, high-velocity gases.*No presentar equivale a la 14.*No presentar equivale a la 15.*****Note: Thermal efficiency is defined as the relationship between the ptencial heat energy output of the engine.**The figure 19-6a is a diagram to represent the events of the Brayton cycle.The actual values represented by the curves may vary considerably with different engines however, the principles is the same for all.

*The figure 19-6a is a diagram to represent the events of the Brayton cycle.The actual values represented by the curves may vary considerably with different engines however, the principles is the same for all.

*The figure 19-6a is a diagram to represent the events of the Brayton cycle.The actual values represented by the curves may vary considerably with different engines however, the principles is the same for all.

*The figure 19-6a is a diagram to represent the events of the Brayton cycle.The actual values represented by the curves may vary considerably with different engines however, the principles is the same for all.

***