MG University Syllabus

1. (ii) The cyclic flow of working fluid within the engine is
achieved solely through geometric volume changes and
without the use of inter-mittently-closed valves or ports.
(iii)
An intermittent flow heat exchanger stores a large
portion of the heatof the working fluid after expansion and
subsequently returns it to the working fluid after
compression, thereby, accomplishing thermal regeneration.
(Q34)
(Q23)
regeneration
However, the gas is heated in a radically
different manner. The conven-tional engines burn fuel
internally and intermittently whereas the Stirling engine burns
its fuel outside the engine itself and continuously, i.e. it is an
external continuous combustion engine. The distinctive
features of the Stirl-ing engine which set it apart from other
heat engines are:
(i) The fuel, the combustion air and the products of
combustion do not enter the engine cylinder to become the
working gas, on the other hand the working medium is
completely sealed within the active space of the engine and
repeatedly undergoes a thermodynamic cycle to operate the
engine. The working medium may be air, hydrogen or helium
or any other suitable gas.
regeneration
In all internal combustion engines the air is
compressed at low temperature_ in the cylinder, before (in case
of diesel) or after (in case of petrol)] the addition of the fuel, the
mixture is burned and the products of combustion allowed to
expand at a high temperature, and power is obtained from the
engine. The same principle, the alternate compression at low
temperature and expansion at a high temperature of a confined
working fluid, is the basis for the Stirling engine.
(Q23)
(Q12)
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)

2. TDC
regeneration
Cooler
(Q34)
ODC
IDC
BDC
regeneration
(Q23)
(Q23)
(Q12)
Heater
Alpha Stirling Engine
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)

3. Performance
The 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 Requirement
Since the Stirling engine is an external combustion engine, it
possesses multi-fuel capability. It can use any petroleum
fraction such as gasoline, diesel, broad-cut distillate, methanolgasoline blends with no cetane or octane number
requirements. Thus the Stirling engine has the desirable
characteristics of adaptability to changing fuel availability.
Exhaust Emissions
One of the major attractions of a Stirling engine is its lower
exhaust emis-sions. Use of overall lean mixtures and exhaust
gas recirculation is resorted to for controlling exhaust
emissions from Stirling engine.
ADVANTAGES OF STIRLING ENGINE
The 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
(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 of
41 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/litre
displacement 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 Stirling
engine 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 the
working 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 suita-ble for a variety of applications such as
automobiles, ship propulsion, hospi-tals, 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.

4. DISADVANTAGES OF STIRLING ENGINE
The following are main disadvantages of the Stirling engine:
• The engine design is complex due to use of rhombic drive, regene-rator, 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 for
automotive 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.