STIRLING ENGINESukaran SinghSenior Undergraduate StudentDeptt. Of Chemical Engg., IIT Delhi*29/04/2014Supervised by: Prof. B. Pitchumani

HISTORY The Stirling Engine was invented by Rev. Robert Stirling in Edinburgh, Scotland.Robert Stirling was a minister of the Church of Scotland who was interested in the health of his parishioners bodies in addition to the well being of their souls. He invented the Stirling engine (he called it an "air engine") because steam engines of his day would often explode killing and maiming those who were unlucky enough to be standing close by. Robert Stirling's engines couldn't explode and produced more power than the steam engines then in use.*Source: http://en.wikipedia.org/wiki/Stirling_engine

HISTORY In 1816 he received his first patent for a new type of "air engine". The engines he built and those that followed eventually became known as "hot air engines" and continued to be known called hot air engines until the 1940's when other gases such as helium and hydrogen were used as the working fluid. Robert Stirling was an active minister and inventor all his life. Perhaps his most important invention was the "regenerator" or "economizer" as he called it. This is used today in Stirling engines and many other industrial processes to save heat and make industry more efficient.

*Source: http://en.wikipedia.org/wiki/Stirling_engine

OVERVIEWA Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical workThis is the first practical example of a closed cycle air engine*

MAIN COMPONENTSPower piston small tightly sealed piston that moves up when the gas inside the engine expandsDisplacer larger piston and it is very loose in its cylinder so air can move easily between the heated cooled sections of the engine as the displacer moves up and down The piston moves by the action of compression and expansion.Difference in pressure causes the piston to move and produce power. *

TYPES OF STIRLING ENGINE*

ANIMATED EXAMPLES*Alpha TypeBeta TypeGammaTypeSource: http://stirlingengineforum.com/viewtopic.php?f=1&t=1694

WORKING PRINCIPLEIsothermal Expansion: The expansion-space and associated heat exchanger are maintained at a constant high temperature, and the gas undergoes near-isothermal expansion absorbing heat from the hot source.Constant-Volume Heat-Removal: The gas is passed through the regenerator for use in the next cycle.Isothermal Compression: The compression space and associated heat exchanger are maintained at a constant low temperature so the gas undergoes near-isothermal compression rejecting heat to the cold sinkConstant-Volume Heat Addition: The gas passes back through the regenerator where it recovers much of the heat transferred in 2, heating up on its way to the expansion space.*

How do the Stirling Engines work?Every Stirling engine has a sealed cylinder with one part hot and the other cold. The working gas inside the engine (which is often air, helium, or hydrogen) is moved by a mechanism from the hot side to the cold side. When the gas is on the hot side it expands and pushes up on a piston. When it moves back to the cold side it contracts. *

How do they work? (contd.)Properly designed Stirling engines have two power pulses per revolution, which can make them very smooth running. Two of the more common types are two piston Stirling engines and displacer-type Stirling engines. The two piston type Stirling engine has two power pistons. The displacer type Stirling engine has one power piston and a displacer piston.

*

SummaryThere are four parts to the Stirling cycle:Heat is added to the gas inside the heated cylinder (left), causing pressure to build. This forces the piston to move down. This is the part of the Stirling cycle that does the work.The left piston moves up while the right piston moves down. This pushes the hot gas into the cooled cylinder, which quickly cools the gas to the temperature of the cooling source, lowering its pressure. This makes it easier to compress the gas in the next part of the cycle.

*

SummaryThe piston in the cooled cylinder (right) starts to compress the gas. Heat generated by this compression is removed by the cooling source.The right piston moves up while the left piston moves down. This forces the gas into the heated cylinder, where it quickly heats up, building pressure, at which point the cycle repeats.

*

Displacer Type Displacer Piston:When there is a temperature difference between upper displacer space and lower displacer space, the engine pressure is changed by the movement of the displacer. The pressure increases when the displacer is located in the upper part of the cylinder (and most of the air is on the hot lower side).The pressure decreases when the displacer is moved to the lower part of the cylinder. *

Displacer Type (contd.) Displacer Piston:The displacer only moves the air back and forth from the hot side to the cold side. It does not operate the crankshaft and the engine. In other words, the connecting rod to the displacer could be a string in this engine and it would still work.

*

Displacer Type (contd.) Power Piston:When the engine pressure reaches its maximum because of the motion of the displacer, a power piston is pushed by the expanding gas adding energy to the crankshaft. The power piston should ideally be 90 degrees out of phase with the displacer piston. The displacer type Stirling engine is operated by the power of the power piston.*

Gamma Mechanical Configuration*Source: http://www.ohio.edu/mechanical/stirling/engines/engines.html

Two-Piston Type Heating: If we see from the top dead center of the hot piston. The hot piston moves to the upper part of the cylinder and the cold piston moves to the lower part of the cylinder during the first 90 degrees of revolution. The working air is moved from the cold space to the hot space. And the pressure in the engine is increased.*

Two-Piston Type (contd.) Expansion: During the next 90 degrees of revolution, the two pistons both move the lower part accepting the air pressure. The engine gets its power during this portion of its cycle. Cooling: The crankshaft revolves by power stored in the flywheel for the next 90 degrees. The hot piston moves to the lower part and the cold piston moves to the upper part. The air is moved from the hot space to the cold space. And the pressure in the engine is decreased.*

Two-Piston Type (contd.) Contraction:The two pistons are moved to upper part by the contraction of the air during the next 90 degrees. The engine also gets power during this portion of its cycle. The two piston type Stirling engine then repeats this cycle.*

Are they really the most efficient?In the mid 1800s, Sadi Carnot figured out the maximum efficiency possible with any heat engine. It is a formula like this (Temperature of the hot side - Temperature of the cold side)/Temp of hot side x 100 equals the max theoretical efficiency. Of course the temperatures must be measured in degrees Kelvin or Rankine.Stirling engines (with perfect regeneration) match this cycle. Real Stirling engines can reach 50 percent of the maximum theoretical value. That is an incredibly high percentage!*

ADVANTAGESStirling engines can run directly on any available heat source, not just one.It can provide emission less power generation and is thus environmental friendly. No exhaust valves which vent high-pressure gasses.Heat is external, thus the creation of pollutants such as NOx can be avoided.Operates at relatively low pressure and uses a single-phase fluid and thus is much safer than typical steam engine.Stirling engines run very quietly and also very efficiently. In contrast to internal combustion engines, which are loud and only achieve efficiencies of around 18%, Stirling engines are capable of efficiencies of up to 40%.*

DISADVANTAGESThe engines require fairly high temperatures to operate efficiently. These temperatures are higher than typical factory waste heat temperatures. Since a primary purpose of using Stirling engines would be to capture waste heat from industry, this limits their effectiveness. Secondly, the engines are very large for the amount of energy they produce. This is due to limitations on heat transfer, which powers the device, imposed by heat transfer coefficients of the working gas. Thus they have extremely poor power density*

DISADVANTAGESThirdly, a gas or diesel engine just sucks in air/fuel, compresses it to the required level, then ignites it at maximum compression to get high power output. A Stirling engine needs to be permanently pressurised, and in order to match the power output of a gas engine, would need to be permanently pressurised to the maximum pressure reached inside the cylinder of a gas engine. The problem with permanently pressurising an engine to that sort of pressure is that the gas will leak out of the seals on any moving part. Operating at lower pressures mitigates the leakage problem, but destroys the power output of the engine, while friction losses stay the same; so proportionally, efficiency gets wrecked.What this means is that the moving parts and seals need to be made to exceptional tolerances (expensive) and have a short lifetime as even tiny amounts of wear will reduce their performance to unacceptable levels.*

APPLICATIONS The modern uses of Stirling engines are invisible to almost everyone. There have been many research engines built in recent years but there are only three areas where Stirling engines have made a dramaticimpact. There are Stirling engines in Submarines, stirling machines used as cryocoolers. Cryogenics is the science of things that are exceedingly cold and Stirling engines are one tool that can be used to make things exceedingly cold. Its not obvious but a Stirling engine is a reversible device. If you heat one end and cool the other, you get mechanical work out, but if you put mechanical work in, by connecting an electric motor, one end will get hot and the other end will get cold. *Source: http://en.wikipedia.org/wiki/Applications_of_the_Stirling_engine

APPLICATIONS (contd.)If one designs the machine correctly, the cold end will get extremely cold. In fact, Stirling coolers have been made that will cool below 10 degrees Kelvin. Micro Stirling coolers have been produced in large numbers for cooling infrared chips down to 80 degrees Kelvin for use in night vision devices.In nuclear power, replacing the steam turbines of the nuclear power plant with Stirling engine might simplify the plant, yield greater efficiency, and reduce the radioactivity of by products. *

Why dont we drive a Sterling Engine?Because the heat source is external, it takes a little while for the engine to respond to changes in the amount of heat being applied to the cylinder -- it takes time for the heat to be conducted through the cylinder walls and into the gas inside the engine. This means that:The engine requires some time to warm up before it can produce useful power.The engine can not change its power output quickly.

Further, low fuel prices work against efficient engine that does not start instantly

*Source: http://auto.howstuffworks.com/stirling-engine4.htm

THANKYOU!*

*due to the frequent explosion caused by unsustainable high pressure killings and injuring of workers*due to the frequent explosion caused by unsustainable high pressure killings and injuring of workers*It is very hard to heat a gas inside a metal container. You can throw flames at the outside of a big (or small) diameter tube but only the gas touching the surface of the metal gets hot. The gas molecules in the middle of the tube are being jostled by neighbouring molecules - but they do not transfer heat to each other. It is only when the molecules hit the metal surface that they pick up heat, need more space, create pressure and push pistons This leads Stirling engine makers to pass the gas through many thin heater tubes surrounded by flame. Or to put slots in the side of large diameter tubes - and then force the gas through these narrow slots. Even so, it is hard to get large quantities of heat through the metal and into the gas So - why not have a "heat store" inside the engine. A bundle of fine (usually stainless steel) wires pretty tightly mashed together. The gas can flow through it and there is a very large surface area of metal touching the gas. When the hot air leaves the hot chamber it passes through the wire mesh dropping heat to the wires. It emerges pretty cold, ready to go into the cold chamber for compression. When the gas returns from the cold chamber - and it passes through the still hot regenerator wire mesh - it picks up the heat. Finally, the now hot air, gets to those tubes or slots mentioned above for a final top up of temperature.*A number of breeder reactor designs use liquid sodium as coolant. If the heat is to be employed in a steam plant, a water/sodium heat exchanger is required, which raises some concern as sodium reacts violently with water. A Stirling engine eliminates the need for water anywhere in the cycle. This would have advantages for nuclear installations in dry regions.*