UNIVERSITY OF GUJRAT

STIRILING ENGINE

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CONSTRUCTION AND WORKING OF ALPHA BETA GAMMA ENGINE

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Stirling engine History Types of stirling engineEfficiency comparison of I.C & E.C

engineFive main components of stiriling engineApplication

STIRLING ENGINE

A Stirling engine is a external combustion  heat engine that operates by cyclic compression and expansion of air or other gas (the working fluid) at different temperatures, such that there is a net conversion of heat energy to mechanical work.

The Stirling engine is noted for high efficiency compared to steam engines, quiet operation, and its ability to use almost any heat source.

The Stirling Engine is one of the hot air engines. It was invented by Robert Stirling (1790-1878) and his brother James. His father was interesting in engine and he inherited it. He became a minister of the church at Scotland in 1816. At this period, he found the steam engines are dangerous for the workers. He decided to improve the design of an existing air engine. He hope it wound be safer alternative.

HISTORY

• 1-2: constant volume process

• 2-3: isothermal expansion process

• 3-4: constant volume process

• 4-1: isothermal compression process

PV DIAGRAM

CYCLE EFFICIENCY

Net work, Wnet = Wexp + Wcomp

Net work, Qtotal = Qheat + Qheat + Qexp( Recovered mechanical energy )

STIRLING ENGINE WORKING

STIRLING ENGINE WORKING

Heating: Heat source provides thermal energy to the engine so that it raises pressure and temperature of gas.

Expansion: in this phase the volume increases, but the pressure and temperature decrease, mechanical energy is produced from heat energy during this phase of cycle only.

STIRLING ENGINE WORKING

Cooling: the gas is cooled and temperature and pressure decrease, so the gas is prepared to be compressed during this cycle.

Compression: the pressure of gas increases whereas its volume decreases; a part of produced mechanical energy is used for processing of this phase, because it needs an amount of work to be done.

CYCLES

ALPHA TYPE

Alpha type Stirling engine

An Alpha Stirling contains two power pistons in separate cylinders, one hot and one cold. The hot cylinder is situated inside the high temperature heat exchanger and the cold cylinder is situated inside the low temperature heat exchanger

Action of an alpha type Stirling engineMost of the working gas is in contact with the hot cylinder walls, it has been heated and expansion has pushed thehot piston to the bottom of its travelin the cylinder. The expansion continues inthe cold cylinder, which is 90° behind the hot piston in its cycle, extracting more work from the hot gas.

The gas is now at its maximum volume. The hot cylinder piston begins to move most of the gas into the cold cylinder, where it cools and the pressure drops

Almost all the gas is now in the cold cylinder and cooling continues. The cold piston, powered by flywheel momentum (or other piston pairs on the same shaft) compresses the remaining part of the gas

The gas reaches its minimum volume, and it will now expand in the hot cylinder where it will be heated once more, driving the hot piston in its power stroke

BETA TYPE

Beta type Stirling EngineA beta Stirling has a single power piston arranged within the same cylinder on the same shaft as a displacer piston. The displacer piston is a loose fit and does not extract any power from the expanding gas but only serves to shuttle the working gas between the hot and cold heat exchangers

Action of a beta type Stirling engine

Power piston (dark grey) has compressed the gas, the displacer piston (light grey) has moved so that most of the gas is adjacent to the hot heat exchange

The heated gas increases in pressure and pushes the power piston to the farthest limit of the power stroke

The displacer piston now moves, shunting the gas to the cold end of the cylinder.

The cooled gas is now compressed by the flywheel momentum. This takes less energy, since its pressure drops when it is cooled

GAMMA TYPE

GAMMA TYPE

Efficiency comparison of I.C & E.C engine

Internal combustion engine: Most internal combustion engines are incredibly

inefficient at turning fuel burned into usable energy. The efficiency by which they do so is measured in terms of "thermal efficiency", and most gasoline combustion engines average around 20 percent thermal efficiency.

Internal Combustion Engines – Theorem Carnot cycle

Carnot's theorem is a formal Statement of this fact: No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between those same reservoirs.

Internal Combustion Engines – Carnot cycle

TH is the absolute of cold reservoir

TC is the absolute of hot reservoir

Efficiency comparison of I.C & E.C engine External combustion engine (Stiriling

Engine):

Inside the cylinder, the gas that expands and contracts is usually hydrogen. Stirling engines are one of the best options on the market to harvest solar power as they can reach a 31% efficiency compared to just 16% for parabolic trough technology or 14-18% achieved by photovoltaic panels.

External combustion engine (Stiriling Engine):

During each cycle: w is the net work done by the engine QH is the energy taken from the (hot) source QC is the energy given to the (cold) sink

External combustion engine (Stiriling Engine):

Efficiency Stirling Engine

FIVE MAIN COMPONENTS OF STIRLING ENGINE

Working Gas: The Stirling cycle is a closed cycle and the various

thermodynamic processes are carried out on a working gas that is trapped within the system.

Heat-Exchanger: Two heat exchangers are used to transfer heat

across the system boundary. A heat absorbing heat-exchanger transfers the heat from outside the system into the working gas, and a heat rejecting heat-exchanger transfers heat from the working gas to outside the system.

Displacer Mechanism: This moves(or displaces)the working gas between

the hot and cold ends of the machine (via generator).

Regenerator:This acts both as a thermal barrier between the hot

and cold ends of the machine, and also as a “thermal store”for the cycle. Physically a regenerator usually consists of a mesh materiaL and heat is transferred as the working gas is forced through the regenerator mesh

Expansion/Compression Meahanism: This expands and compresses the working

gas.In an engine this mechanism produces a net work output.

ADVANTAGES OF STIRLING ENGINE Silence of operation : There is no expansion in the atmosphere like in the case of an internal

combustion engine, combustion is continuous outside of the cylinders. In addition, its design is such as the engine is easy to balance and generates few vibrations.

The high efficiency : It is function of the temperatures of the hot and cold sources. As it is

possible to make it work in cogeneration (mechanical and caloric powers), the overall efficiency can be very high.

The multitude of possible “hot sources” : Combustion of various gases, wood, sawdust, waste, solar or geothermic

energy... Reliability and easy maintenance:  The technological simplicity makes it possible to have engines with a very

great reliability and requiring little maintenance.

DISADVANTAGES OF STIRLING ENGINE The price : its cost is probably the most important problem, it is not yet

competitive with other means well established. A generalization of its employment should solve this problem inherent in any novelty. 

The lack of flexibility : the fast and effective variations of power are difficult to obtain with a Stirling engine. This one is more qualified to run with a constant nominal output. This point is a great handicap for an utilization in car industry.

Stiriling engine requires a blower to force air through preheater and combustion chamber ,this reduces engine efficiency and noise