Stirling Engine - Seminar Report

  

ABSTRACT
The quest of human beings to develop engines with high power, high torque, less vibration and most essentially with no pollution is on since the discovery and development of engine. Stirling engine is just one step forward towards the creation of a noise free and pollution less engine.

          The Stirling engine is the engine, which uses a fixed amount of gas sealed inside a cylinder. The expansion and contraction of the gas, using heat from external source, creates the useful work. The main advantage of this engine is its capability to use any type of fuel and the emission of no exhaust gases.

Due to this pollution free and use of any type of fuel characteristics the Stirling engine shows a greater potential over any other type of engine existing today. To consolidate this claim an effort has been made to develop a working model of Stirling engine.

                                                                 INTRODUCTION

                              All of us including the lamest of laymen would have at one time or another experienced problems with our vehicles engine and most of the time after moaning and cursing finally in line with the universe and accepting our doom we would have coughed up the cash for repairs and parts and insistently taken old parts home, disregarding the fact that no descent human would have wanted them and during this exercise in existence it is doubtful that anybody would have chance to miss seeing a piston or two, this ubiquitous creatures that scurry up and down in an enclosed cylindrical space, getting their crowns slammed regularly and unceremoniously…eventually to be thrown aside and replaced by a marginally wider chap. Doubtful that any of us would be able to imagine modern travel without an internal combustion cycle. Hell if we ask our mechanic if he has ever seen a vehicle without any exhaust then he is probably going to refer to you to the nearest shrink.

                             So an engine without any exhaust gas is a mirage or it can be a reality? The answer for this is definitely yes. We can have an engine without any exhaust gas and that is  a Stirling engine.

            STIRLING ENGINE

The Stirling engine is a heat engine that is vastly different from an internal combustion engine. Stirling engines have two pistons that create a 90-degree phase angle and two different temperature spaces. The working gas in the engine is perfectly sealed, and doesn't go in and out to the atmosphere. The Stirling engine uses a Stirling cycle, which is unlike the cycles used in normal internal combustion engines.

PARTS OF A STIRLING ENGINE

Main chamber: It is the chamber where a fixed amount of gas is sealed initially. Inside this chamber only expansion and compression of gas takes place.

Output Shaft: It is horizontal and mounted several inches above the main chamber. The end of shaft centered over the main chamber has a crank and connecting rod, which drives the displacer.

Displacer:  This is the large piston in the drawing. This piston is very loose in its cylinder, so air can move easily between the heated and cooled sections of the engine as the piston moves up and down.

Power piston: This is the smaller piston at the top of the engine. It is a tightly sealed piston that moves up as the gas inside the engine expands.

Crankshaft: The crankshaft is made up of steel, which passes through the bearing and receives the crank disc .the crank disc forms mount for flywheel.

Regenerator: The regenerator is constructed of material that readily conducts heat and has a high surface area. When hot gas is transferred to the cool cylinder, it is first driven through the regenerator, where a portion of the heat is deposited.  When the cool gas is transferred back, this heat is reclaimed; thus the regenerator "pre heats" and "pre cools" the working gas, dramatically improving efficiency.

Operation

• Since the Stirling engine is a closed cycle, it contains a fixed mass of gas called the "working fluid", most commonly air, hydrogen or helium. In normal operation, the engine is sealed and no gas enters or leaves the engine. No valves are required, unlike other types of piston engines.
• The Stirling engine, like most heat engines, cycles through four main processes: cooling, compression, heating and expansion. This is accomplished by moving the gas back and forth between hot and cold heat exchangers, often with a regenerator between the heater and cooler.
• The hot heat exchanger is in thermal contact with an external heat source, such as a fuel burner, and the cold heat exchanger being in thermal contact with an external heat sink, such as air fins. A change in gas temperature will cause a corresponding change in gas pressure, while the motion of the piston causes the gas to be alternately expanded and compressed.
• The gas follows the behaviour described by the gas laws which describe how a gas' pressure, temperature and volume are related. When the gas is heated, because it is in a sealed chamber, the pressure rises and this then acts on the power piston to produce a power stroke. When the gas is cooled the pressure drops and this means that less work needs to be done by the piston to compress the gas on the return stroke, thus yielding a net power output.
• The ideal Stirling cycle is unattainable in the real world, and the actual Striling cycle is inherently less efficient than the Otto cycle of internal combustion engines.
• The efficiency of Stirling machines is linked to the environmental temperature; a higher efficiency is obtained when the weather is cooler, thus making this type of engine less interesting in places with warmer climates.
• As with other external combustion engines, Stirling engines can use heat sources other than from combustion of fuels.
• When one side of the piston is open to the atmosphere, the operation is slightly different. As the sealed volume of working gas comes in contact with the hot side, it expands, doing work on both the piston and on the atmosphere.
• When the working gas contacts the cold side, its pressure drops below atmospheric pressure and the atmosphere pushes on the piston and does work on the gas.
• To summarize, the Stirling engine uses the temperature difference between its hot end and cold end to establish a cycle of a fixed mass of gas, heated and expanded, and cooled and compressed, thus converting thermal energy into mechanical energy.
• The greater the temperature difference between the hot and cold sources, the greater the thermal efficiency. The maximum theoretical efficiency is equivalent to the Carnot cycle, however the efficiency of real engines is less than this value due to friction and other losses.

ADVANTAGES

• There is no exhaust of gases as in case of normal piston engines.
• These engines are quieter in operation.
• Gas inside the engine never leaves the engine.
• The external heat source could be anything-even heat from our hand.

• No combustion takes place inside cylinder of engine. Hence there is no explosion.

Waste heat is easily harvested (compared to waste heat from an internal combustion engine) making Stirling engines useful for dual-output heat and power systems.

Disadvantages

Stirling engine designs require heat exchangers for heat input and for heat output, and these must contain the pressure of the working fluid, where the pressure is proportional to the engine power output.

In addition, the expansion-side heat exchanger is often at very high temperature, so the materials must resist the corrosive effects of the heat source, and have low creep

.

Typically these material requirements substantially increase the cost of the engine. The materials and assembly costs for a high temperature heat exchanger typically accounts for 40% of the total engine cost.

APPLICATIONS

Stirling Thermal Motor

This motor uses heater tubes, combustor, and piston, to convert thermal energy into mechanical work.

Four-Piston Wobble-Yoke Machine

This machine uses four pistons and wobbles yoke drive. This machine is especially used where high power generation is required.

Four Cylinders Free-Piston Alpha Engine

The four cylinders are physically arranged with a 90-degree phase difference with each piston connected to a gas compressor. The gas compressors are then used to drive a gas turbine expander .The main advantage of this system is the promise of a high specific power and most important, high reliability and life resulting from the absence of heavily loaded moving parts.

Conclusion

The Stirling engine is the engine, which uses the gas sealed inside the cylinder. The expansion and contraction of the gas, using heat from external source, creates the useful work. This engine can work on any type of fuel whether it is conventional or non-conventional fuel. And the basic advantage of this engine is that the amount of gas inside the chamber remains fixed that is there is no exhaust and hence this engine is pollution less.

Due to this pollution free and use of any type of fuel characteristics the Stirling engine has greater potential over any other type of engine existing today. Hence this engine is highly preferred in automobile sector finding its application in submarines to hybrid cars. Due to the above specified advantages the striling engine is giving a cutting edge to all other engine existing today and is viewed as an answer to the existing energy crisis.