Stirling Engine
Prof. S. L. Bapat
Mechanical Engineering Department
Department of Energy Science and Engineering
Indian Institute of Technology Bombay,
Mumbai  400 076
April 28, 2008
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Indian Scenario
" Shortage of Electrical Power
Thermal power plants
Nuclear power plants
Hydel power plants
" Solar P-V cells
" Solar Thermal
Rankine Cycle
Stirling Cycle
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Choice of Capacity for Stirling Engine
" ~ 43,000 villages to be electrified
" Features of these villages (Sastry, 2003):

Difficult terrain

3-30 km away from grid

No. of household 2 to 200

Average population ~ 500

Power demand quite low (Supply for 4-6 hrs/day)

facilities are minimal (TV, Refrigerators etc.)

Income levels & paying capacity low
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Cost of P-V plants ranges from Rs. 3.6 lakh- 4.8 lakh for 1.5 kW
(Sastry, 2003)
Applications:
- For a group of 3-4 households having enough cattle to
supply bio-gas for gas based systems or hybrid systems
- Use for small capacity pumps for irrigation application
Investment $ Rs.
required (in 2003)
per village 60,000 24,00,000
for India 1.20 billion 48 billion
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Ideal Stirling cycle
(Normally explained by using Ä…  type)
" Assumptions
" Working

Compress the gas, heat the gas, and
then expand to get power output

Internal heat transfer in regenerator
(a) P-V and T-S diagrams (b) Piston arrangements at the terminal
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points of the cycle (c) Displacement-time diagram
Basic arrangements of Stirling engine
Piston  Displacer
Piston  Displacer
Two piston machine
in separate
in same cylinder
(Ä…)
cylinder (Å‚)
(²)
Different types of Stirling engine
- Free piston - Free displacer engine
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- Disciplined (Kinematic) engine
Selection of drive mechanism
Kinematic mechanisms for reciprocating motion
(a) Simple slider Crank b) Cross head Crank c) Rhombic Drive
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Operating conditions
Sr. no. Parameter Unit Value
1 Speed rpm 1440
2 Mean Pressure bar 30
3 Temperature hot side (th) K 750
4 Phase angle (motion) ° 78.5
5 Specific heat ratio -- 1.43
6 Fluid  Hydrogen /Helium -- --
7 Minimum gas temperature (tc) K 350
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Engine and receiver arrangement
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Issues Involved
1. Obtaining heat at temperature level of
about 750 K or more
-
Gas Flame (Bio-gas, CNG, LPG)
-
Circulation of burnt gases for reuse in preheating
of combustion air
-
Dish Concentrator
-
Size of the dish depends on heat input
requirements
-
Hybrid System using solar concentrator
and gas flame
-
Arrangement for switching over from solar to gas
flame and vice-versa
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Issues Involved
2. Requirement:
- Engine of the capacity of 1.5 kWe should
satisfy most requirements
3. Materials:
- Some special materials to be chosen based on specific
requirements (properties) e.g.
O-rings at higher temperature (soft metal- Indium,
Copper)
Sealing rings:

Compression rings

Oil scrapper rings
 Sealing of displacer
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4. Lubrication Problems:
Issues Involved
5. Operating and Resulting Parameters
1. Working spaces and dead volumes in
cooler, regenerator and heater tubes
decide the pressure ratio
2. Large value of pressure ratio leads to
higher value of peak pressure. The
enclosing components such as compression and
expansion cylinders have to be stronger from
mechanical design point of view. So this
parameter needs to be decided
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Issues Involved
6. Manufacturing processes:
This will depend on scale of manufacture
- If number crosses 1000, the fabrication processes,
rejection schemes need to be worked out
- Interchangeability is required and hence tolerances
will have to be really fine
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Issues Involved
7. Mechanized assembly:
i) Components such as crankcase can be using castings or
made out of plates by welding
ii) leak proof ness /porosity has to be checked for enclosing
components
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Issues Involved
8. Testing Procedures:
i) Test engine with electrical heating to determine
minimum heat input
ii) Testing with gas flame to find fuel consumption
iii) Design dish and test to provide suitable dish
area for a given heat input
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Worldwide Scenario

Engines upto 30 kW capacity have been
made as a Single Cylinder Engine

Some of these have been coupled with dish
systems
iii) Testing over very large number of hours is
done with a very small number of units
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Bio-gas requirement : 1 m3 / h or so.
Calorific Value = 20 MJ/m3
Net heat input required is 5000 Watt for 1.5 kWe
capacity
Qgas = 5000 Watt
Combustion efficiency = 90 %
http://www.mct.gov.jm/energy_7.htm date: 23/03/07]
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Conclusions
1. Stirling engines seems to be viable option
2. Capacity needs to be at least 1.5 kW
3. Major heat input should be through gas flame or
solar energy
4. Bio-gas requirement will be about one cubic meter/
hour
5. Hybrid system will be the ideal option if suitable
arrangement is possible
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Thank you !
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