In all internal combustion engines the air iscompressed at low temperature_ in the cylinder, before (in caseof diesel) or after (in case of petrol)] the addition of the fuel, themixture is burned and the products of combustion allowed toexpand at a high temperature, and power is obtained from theengine. The same principle, the alternate compression at lowtemperature and expansion at a high temperature of a confinedworking fluid, is the basis for the Stirling engine.

However, the gas is heated in a radicallydifferent manner. The conven­tional engines burn fuelinternally and intermittently whereas the Stirling engine burnsits fuel outside the engine itself and continuously, i.e. it is anexternal continuous combustion engine. The distinctivefeatures of the Stirl­ing engine which set it apart from otherheat engines are:

(i) The fuel, the combustion air and the products ofcombustion do not enter the engine cylinder to become theworking gas, on the other hand the working medium iscompletely sealed within the active space of the engine andrepeatedly undergoes a thermodynamic cycle to operate theengine. The working medium may be air, hydrogen or heliumor any other suitable gas.

(ii) The cyclic flow of working fluid within the engine isachieved solely through geometric volume changes andwithout the use of inter-mittently-closed valves or ports.

(iii) An intermittent flow heat exchanger stores a largeportion of the heatof the working fluid after expansion andsubsequently returns it to the working fluid aftercompression, thereby, accomplishing thermal regeneration.

(Q34)

(Q12)

(Q23)

(Q23)

Ideal thermodynamic cycle of a Stirling engine consists of four processes• Isothermal compression (W12) with simultaneous heat rejection(Q12).• Constant Volume heat addition (Q23) (by regeneration)• Isothermal expansion (W34) with simultaneous heat addition (Q34).• Constant Volume heat addition (Q41) (by regeneration)

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Alpha Stirling Engine

Heater

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TDC

BDCODC IDC

(Q34)

(Q12)

(Q23)

(Q23)

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2-3 : Crank rotates 90 o clockwise, H- Piston (Horizontal) moves towards IDC & V-piston (Vertical) moves to TDC. Working fluid is transferred from the hot space to the cold space. Working fluid absorbs heat from regenerator. Almost constant volume.3-4 : Crank again rotates 90 o clockwise, H-piston moves to IDC & V-piston moves towards BDC. (Isothermal heat addition)4-1 : Crank again rotates 90 o clockwise, H-piston moves towards ODC & V-piston moves to BDC. Working fluid rejects heat to regenerator. Almost constant volume.1-2 : Crank rotates 90 o clockwise, H-piston moves to ODC & V-piston moves towards TDC. (Isothermal heat rejection)

PerformanceThe Stirling cycle engine is specially suitable for automotive applications because of(i) high brake thermal efficiency,(ii) desirable output shaft maximum to ideal speed ratio,(iii) low noise and low emissions, and(iv) specific power output comparable to Otto engine.

Fuel RequirementSince the Stirling engine is an external combustion engine, itpossesses multi-fuel capability. It can use any petroleumfraction such as gasoline, diesel, broad-cut distillate, methanol-gasoline blends with no cetane or octane numberrequirements. Thus the Stirling engine has the desirablecharacteristics of adaptability to changing fuel availability.

Exhaust EmissionsOne of the major attractions of a Stirling engine is its lower exhaust emis­sions. Use of overall lean mixtures and exhaust gas recirculation is resorted to for controlling exhaust emissions from Stirling engine.

(iv) Efficiency and size. Maximum thermal efficiency of the Striling engine is greater than that of diesel and petrol engines. It has a value of 44 per cent at 300°C and 112 bar of working gas pressure compared to that of41 per cent for a diesel engine. The output per litre of swept volume is much higher for a Stirling engine. Typical values for diesel and automotive petrol engines are 0.49 and 0.65 bhp/litredisplacement volume.

(v) Smooth and noise-free engine operation. The external heating of the Stirling engine makes possible continuous combustion in the burner and continuous discharge of the exhaust gases. Accordingly, the Stirlingengine is inherently quiet in operation. Good balancing provided by the rhombic drive and torque-smoothing effect of buffer space makes the engine smooth and noise-free.

(vi) Overload capacity. Since the temperatures of various engine parts remain constant when the power output is controlled, the engine can be safely overloaded for a brief period by increasing the pressure of theworking gas. This is because the life of the engine is chiefly determined by creep due to the maximum temperatures.

(vii) Reliable starting. The starting of the engine depends only on the ignition of fuel in the burner. If the ignition system is alright, reliable starting would be obtained.(viii) Flexibility. Almost constant torque characteristics over the entire range of engine speed and pressure control of output, make it suita­ble for a variety of applications such as automobiles, ship propulsion, hospi­tals, etc. It can be used as a total energy system also.(ix) No lubricating oil needed. The use of rolisock seal has eliminated the use of lubricating oil. Even if this seal is not used, the blowby past the piston and the thermal deterioration of lubricating oil is small because of the fact that the power piston operates in a comparatively lower temperature zone.

ADVANTAGES OF STIRLING ENGINEThe Stirling engine, because of being an external combustion

engine, has certain special characteristics altogether different from that of conventional Otto engine. The following is a brief discussion of the advantages of Striling engine.

(i) Multifuel capability. This engine requires hot gas for its working. Thus any heat source ranging from refuse to radioactive fuels can be used to operate it.(ii) Perfect balancing. the rhombic drive mechanism allows perfect balancing of even a single cylinder engine by suitable placement of balance weights. This coupled to the fact that there is no horizontal thrust on the bearings, allows vibration-free operation of the engine.

(iii) Reduced exhaust emissions

DISADVANTAGES OF STIRLING ENGINEThe following are main disadvantages of the Stirling engine:

• The engine design is complex due to use of rhombic drive, regene­rator, heaters and coolers.• It needs a large amount of cooling water as compared to petrol or diesel. This increases the size of the radiator to be used forautomotive applications. Rapid developments in radiator technology have, however, reduced the importance of this.• The greatest disadvantage is the high cost of the engine.• Stirling engine requires a blower to force the air through the pre-heater and combustion chamber. This reduces the engine efficiency and increases the noise.