Stirling Engine Research at Lund University, Division of Combustion Engines

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Stirling Engine Research

Background

What is a Stirling engine?
The Stirling engine is a thermodynamic heat engine. It has pistons working in cylinders (like an Otto or a Diesel engine has). However, unlike what is the case for the other two engine types mentioned, the cycle medium is not exchanged once per cycle, but instead the cycle medium is retained in the cycle. The Stirling engine is characterised by the fact that the energy driving the engine is supplied as heat from outside the cylinder, i.e. the heat is heat-exchanged into the cycle gas and not supplied as chemically bound energy in a fuel (unlike in a Otto engine where the heat is supplied as fuel injected into the cycle gas). This means that in a Stirling engine the heat can be supplied any way at all as long as the heat can be transported into the cycle. This way the powering sources can be as diverse as the sun (focused onto the heat-exchanger with a dish mirror), a fire made with rice husks, or a high-tech low-emission combustor.

The principle of the Stirling engine
As mentioned above the Stirling engine working fluid is a gas that is not exchanged between cycles, but instead is retained in the system. The working fluid is usually helium, while it also might be hydrogen or air. Hydrogen is the best fluid, but helium is commonly used due to it being an inert gas. Air does not have very good properties as a cycle gas, however it is abundant and cheap and thus is used in some designs.
The cycle gas can occupy one of two spaces in the Stirling engine, either the cold (or compression) space or the hot (or expansion) space. Also the gas is alternately compressed and expanded.

The ideal cycle works like follows:

1: The cycle gas occupies the cold space, and thus the gas is cold and the cycle pressure is low.

1-2: Now the cold gas is isothermally compressed by a piston, i.e. cycle pressure increases, cycle volume decreases and cycle mean temperature is constant. Here work is added to the system!

2-3: The cycle gas is transferred to the hot space passing through first the cooler and then the regenerator (i.e. a heat buffer) on the way picking up some extra heat, then passing through the heater where heat is added until the cycle gas reaches its final (set) temperature. The volume is constant during the transfer, but cycle mean temperature and pressure increases. Note that no work is added to the system, apart from the small fraction needed to overcome flow losses.

3-4 The cycle gas is now isothermally expanded in the hot space, thus work is transferred to the expansion (or work) piston The temperature is constant during the expansion, but cycle pressure decreases and the volume increases. This is where we get useful work from the cycle!

4-1 The cycle gas is transferred back to the cold space passing through, in order, the heater, the regenerator (where excess heat is stored until the next cycle) and the cooler. The volume is constant during the transfer, but cycle mean temperature and pressure decreases. Note that no work is added to the system, apart from the small fraction needed to overcome flow losses.

The above described ideal cycle is realised more or less ideal through a system of pistons and some cranc/-shaft mechanism, e.g. the rombic drive or an ordinary crancshaft-piston rod arrangement. The pistons can be arranged in different ways, e.g. two pistons in one cylinder, two pistons and two cylinders per cycle, or four cylinders with four double acting pistons giving four separated cycles.
To enhance engine efficiency, a plate-type recuperator is used to preheat the inlet air with the exhaust. The engine efficiency is comparable with that of diesel engines.

Advantages of the Stirling engine
As mentioned above, the Stirling engine is fuel independent, it doesn't even need any fuel - the sun is enough! This of cause means that the engine will be very fuel versatile, but also the fact that no internal combustion is needed means that a well controlled low-emission continuous combustion can be used for heat supply. Thus an engine equipped with a modern low-emission combustion system will have very low emissions.
Another advantage is that the Stirling engine is a very quietly operating engine, what other engine can be run in the same room in which you plan to take a nap? As well the Stirling engine can be made operating virtually vithout any vibrations, which is one of the reasons why Kockums are supplying the Swedish Navy submarines with Stirling engines for underwater battery recharging.
A high percentage of the Stirling engine heat losses will go to the cooling fluid instead of into the exhausts, which makes the Stirling engine suitable for combined heat and power generation (so-called micro-CHP).
Possible long intervals between maintenance and major overhawls is another positive factor.

Disadvantages of the Stirling engine
First, the Stirling engine has a disadvantageous weight-to-power ratio. This means that not all mobile applications are possible. E.g. a lot of work was put into developing Stirling car engines in the seventies, but the concept couldn't beat the Otto and Diesel engines for this application.
The Stirling engine at todays level of development is too expensive, mainly due to the fact that no serial production yet exists. Expensive materials in the hot parts of the engine is another factor that tend to make the engine expensive.

Potential of the Stirling engine
An appropriate field of operation is in micro-CHP plants. Another area where the Stirling engine even today beats the competition is in solar energy plants. New submarines in the Swedish Navy are equipped with Stirling engines to give underwater battery charging capability.

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Projects

The Stirling engine group at the division of Combustion Engines is mainly working with the hot parts of the engine, specialising in ultra-low emission LPP combustors with internal combustion gas recirculation (CGR).

The Stirling group is currently involved in a project developing a Stirling SOLO 161 10kWe (@1500rpm) CHP (Combined Heat and Power) plant for use in Gothenburg by Gothenburg Energy (Göteborgs Energi). The division is responsible for creating a ultra-low emission lean premix preheated air combustion system for the CHP plant.
In early spring 2000 and a year forward the plant will be field tested by Gothenburg Energy.

The research group has earlier experiences of making this type of combustors, and have a United Stirling V160 Stirling engine equipped with a prototype LPP combustor running in the lab since 1993. Since 1997 the combustor is converted to CGR (internal Combustion Gas Recirculation).
The research group has also been involved in a EC project in corporation with the German Aerospace Research Establishment (DLR), supplying a backup burner for a hybrid solar receiver for the SOLO 161 Stirling engine. The hybrid receiver was tested for mor than a year in Plataforma Solar (PSA) in Almeria, Spain, with good results.

The combustion research has been aiming to:

Convert the lab combustor to internal CGR

Getting stable operation in a broad operating range (2-40 kW thermal )

Study the influence of AFR on emissions

Study the influence of CGR-rate on emissions

Study flame stability at increased temperatures

Study the mixing process of air, fuel and CGR gas

Study control strategies for determining AFR

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Future research

In the future the research will be directed towards examining the recirculation and mixing processes by use of CFD calculations using the STAR-CD code. Also further work concerning AFR (Air/Fuel Ratio) control and detection will be made.

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Staff

Assistant Professor Magnus Pålsson (PhD, Mech. Eng.) is the supervisor of the Stirling engine research at the Division of Combustion Engines. As an engineering resource, engineer Jan-Erik Everitt is assigned to this project.

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Publications

Development of a Wood Powder Fuelled 35 kW Stirling CHP Unit,
M. Pålsson, H. Carlsen, ST_TA2_5, Proceedings of the 11th International Stirling Engine Conference, 19-21 November 2003, Rome, Italy
(download as pdf-file - 750 kb)

The Experimental V4X Stirling Engine - A Pioneering Development,
G. Lundholm, ST_TA2_1, Proceedings of the 11th International Stirling Engine Conference, 19-21 November 2003, Rome, Italy
(download as pdf-file - 1.69 Mb)

On Premixed Gas Fuelled Stirling Engine Combustors with Combustion Gas Recirculation,
M. Pålsson, Doctoral Thesis, Department of Heat and Power Engineering, Lund Institute of Technology, Lund, Sweden 2002, ISBN 91-628-5227-2

Overview Speech about Combustion Systems and Fuels,
G. Lundholm, Proceedings of the 10th International Stirling Engine Conference, 24-26 September 2001, Osnabrück, Germany, ISBN 3-931384-38-1

Development and Field Test of a SOLO 161 Stirling Engine based Micro-CHP unit with Ultra-Low Emissions,
M. Pålsson, Proceedings of the 10th International Stirling Engine Conference, 24-26 September 2001, Osnabrück, Germany, ISBN 3-931384-38-1
(download as pdf-file - 649 kb)

Development of a LPP CGR Combustion System with Ultra-Low Emissions for a SOLO 161 Stirling Engine based Micro-CHP unit,
M. Pålsson, Proceedings of the 10th International Stirling Engine Conference, 24-26 September 2001, Osnabrück, Germany, ISBN 3-931384-38-1
(download as pdf-file - 825 kb)

First Trials with Direct Catalytic Combustion on the Heater Surface of a V 160 Stirling Engine,
G. Lundholm, M. Pålsson, Proceedings of the 10th International Stirling Engine Conference, 24-26 September 2001, Osnabrück, Germany, ISBN 3-931384-38-1
(download as pdf-file - 901 kb)

Hybrid Dish/Stirling Systems: Combustor and Heat Pipe Receiver Development,
D. Laing, M. Pålsson, J. Solar Energy Eng.

Neural Networks for Air-Fuel Estimation and Burner Control in a Micro-Cogen System,
P. Jansson, M. Pålsson, T. Rögnvaldsson, A. Törner, ISEC99049, Pilanesburg, South Africa, 1999
(download as pdf-file - 267 kb)

Hybrid Sodium Heat Pipe Receiver for Dish/Stirling Systems - Design and Test Results,
D. Laing, M. Pålsson, M. Reusch, ISEC99047, Pilanesburg, South Africa 1999
(download as pdf-file - 3.76 Mb)

Design and Testing of Stirling Engine Premix CGR Combustor for Ultra Low Emissions,
M. Pålsson, ISEC97001, Ancona, Italy, 1997
(download as pdf-file - 111 kb)

Prestandamätning av Aequiteas 2kW stirlingaggregat TRE 60,
M. Pålsson, Thesis for the degree of Master of Science, Department of Heat and Power Engineering, Lund Institute of Technology, Lund, Sweden 1995 (Written in Swedish)

A Study on a Low Emission Combustor - Lean Premix Prevaporize Concept,
J. Nilsson, Licentiate Thesis, Department of Heat and Power Engineering, Lund Institute of Technology, Lund, Sweden 1993, ISSN 0282-1990

Theoretical and Experimental Results from an Ultra-Low Emission Combustor for Stirling Engines,
J. Nilsson, 6th International Stirling Engine Conference, ISEC-93058, 1993

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Links

Automotive engineering, car and engine companies:
Aston Martin Lagonda Limited
Daniel's Empire - Automotive Engineering
Jaguar Cars Global Home Page
Mazda
Society of Automotive Engineers (SAE)
Volvo Group
Volvo Car Corporation

CFD, computers and programming:
CFD Online
Computational Fluid Dynamics HomePage (Cranfield University -UK)
Computational Dynamics (STAR-CD)
Delphi Download Page
Delphi Programming - About.com Delphi programming home page

Engineering, general:
ASME - The American Society of Mechanical Engineers
Deutsches Museum
DLR - German Aerospace Research Establishment
Fundamental Physical Constants from NIST
Halmstad University, Centre for Computer Systems Architecture
IBM Intellectual Property Network
NUTEK
Ny Teknik

Links to links:
Division of Combustion Engines - links page
Encyclopaedia Britannica: Search Results -- combustion engine
Energy Related Websides
Galaxy: Automotive Engineering
SGC Links
Solar Links
More Solar Links
Stirling Engine web sites

Natural gas related:
DGC - Dansk Gasteknisk Center a/s
Gas quality/composition (in the Swedish natural gas grid)
SGC - Svenskt Gastekniskt Center AB
Sydgas AB

Solar energy (especially Stirling engine related):
American Solar Energy Society
Compendium of SolarDish/Stirling technology (online version)
International Solar Energy Society
Overview of Solar Thermal Technologies - 296kb pdf file from Sun Lab
Solar Dish Engine Technology Description - 888kb pdf file from Sun Lab
Solar Energy Industries Association
Thermo-Mechanical Systems Branch Homepage (NASA)

Stirling engines:
American Stirling Company
Animation of a Stirling design similar to an aircraft engine
Encyclopaedia Britannica: hot-air engine
Encyclopaedia Britannica: Stirling hot-air engine (mech. engin.)
Encyclopaedia Britannica: Stirling, Robert
Hot air, caloric and Stirling engines
Naval Technology - The Stirling AIP System (Kockums)
Quiet Revolution Motor Company L.L.C.
Schlaich Bergermann und Partner (Solar Dish/Stirling Powerplants)
Sigma Elektroteknisk A.S.
SOLO Kleinmotoren GmbH
Stirling cycle engines (text in Danish)
Stirling Engine, an Encarta Encyclopedia Article Titled "Stirling Engine"
Stirling Engine and Steam Engine
Stirling Engine History
Stirling engine home page (Kiochi Hirata, Japan)
Stirling Engine Team
Stirling engines in submarines
Stirling engine research and computer program development/ Peter Fette home page
Stirling owered Gotland class submarines (Swedish Navy)
Stirling Technology Branch: Background (NASA)
Stirling Technology (NASA)
Stirling Technology, Inc.
Sunpower, Inc.
TEM, Stirling engines
What is Stirling? or, The Stirling Cycle
Whispergen home page

NOTE! The fact that a link appears above does not vouch for its scientific value!

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Cooperative partners

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This homepage was updated 2004-01-23.
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