The high reliability, long life, and efficient operation of Free-Piston Stirling Engines (FPSEs) make them an attractive power system to meet future space power requirements with less mass, better efficiency, and less total heat exchanger area than other power convertor options. FPSEs are also flexible in configuration as they can be coupled with many potential heat sources and various heat input systems, heat rejection systems, and power management and distribution systems. Development of a 5-kWe Stirling Convertor Assembly (SCA) is underway to demonstrate the viability of an FPSE for space power. The design is a scaled-down version of the successful 12.5-kWe Component Test Power Converter (CTPC) developed under NAS3-25463. The ultimate efficiency target is 25% overall convertor efficiency (electrical power out over heat in). For the single cylinder prototype now in development, cost and time constraints required use of economical and readily available materials (steel versus beryllium) and components (a commercially available linear alternator) and thus lower efficiency. The working gas is helium at 150 bar mean pressure. The design consists of a displacer suspended on internally pumped gas bearings and a power piston/alternator supported on flexures. Non-contacting clearance seals are used between internal volumes. Heat to and from the prototype convertor is done via pumped liquid loops passing through shell and tube heat exchangers. The preliminary and detail designs of the convertor, controller, and support systems (heating loop, cooling loop, and helium supply system) are complete and all hardware is on order. Assembly and test of the prototype at Foster- Miller is planned for early 2008, when work will focus on characterizing convertor dynamics and steady-state operation to determine maximum power output and system efficiency. The device will then be delivered to Auburn University where assessments will include start-up and shutdown characterization and transient response to temperature and load variations. Future activities may include testing at NASA GRC

Chapman, Peter A.

Vitale, Nicholas A.

Walter, Thomas J.

NASA Technical Reports Server

5-kWe Free-Piston Stirling Engine Convertor 
Peter A. Chapman1, Nicholas A. Vitale, and Thomas J. Walter 
1Foster-Miller, Inc., Albany, NY 12205 
518-456-9919; pchapman@foster-miller.com 
Abstract.  The high reliability, long life, and efficient operation of Free-Piston Stirling Engines (FPSEs) make them an 
attractive power system to meet future space power requirements with less mass, better efficiency, and less total heat exchanger 
area than other power convertor options.  FPSEs are also flexible in configuration as they can be coupled with many potential 
heat sources and various heat input systems, heat rejection systems, and power management and distribution systems.  
Development of a 5-kWe Stirling Convertor Assembly (SCA) is underway to demonstrate the viability of an FPSE for space 
power.  The design is a scaled-down version of the successful 12.5-kWe Component Test Power Converter (CTPC) developed 
under NAS3-25463.  The ultimate efficiency target is 25% overall convertor efficiency (electrical power out over heat in).  
For the single cylinder prototype now in development, cost and time constraints required use of economical and readily 
available materials (steel versus beryllium) and components (a commercially available linear alternator) and thus lower 
efficiency.  The working gas is helium at 150 bar mean pressure.  The design consists of a displacer suspended on internally 
pumped gas bearings and a power piston/alternator supported on flexures.  Non-contacting clearance seals are used between 
internal volumes.  Heat to and from the prototype convertor is done via pumped liquid loops passing through shell and tube 
heat exchangers.  The preliminary and detail designs of the convertor, controller, and support systems (heating loop, cooling 
loop, and helium supply system) are complete and all hardware is on order.  Assembly and test of the prototype at Foster-
Miller is planned for early 2008, when work will focus on characterizing convertor dynamics and steady-state operation to 
determine maximum power output and system efficiency.  The device will then be delivered to Auburn University where 
assessments will include start-up and shutdown characterization and transient response to temperature and load variations.  
Future activities may include testing at NASA GRC. 
Keywords: Free-piston Stirling engine, space power 
PACS: 84.60.-h 
INTRODUCTION 
From 1984 through 1996 under NASA contract, Mechanical Technology Inc. (MTI) conducted a two-phase program 
to develop a 25-kWe free-piston Stirling engine (FPSE) with a hot end temperature of 1050 K and a temperature 
ratio of 2.0 (525 K cold end temperature).  The goal of Phase I was to demonstrate the feasibility of using a FPSE 
directly coupled with a permanent magnet linear alternator to meet the requirements of a space power generating 
system.  The resulting Space Power Demonstrator Engine (SPDE) was successfully tested in October 1986, 
configured as a double-ended FPSE with half the total power generated per side.  A lower head temperature of 
650 K was selected while maintaining the temperature ratio of 2.0 to reduce development time and cost.  A pumped 
molten-salt loop provided heat to the engine and a conventional water/glycol loop was used for the cold end.  The 
goal of Phase II was to demonstrate engine operation at design temperatures of 1050 K/525 K and to perform 
component development.  Between April 1990 and September 1996, the Component Test Power Converter (CTPC) 
was successfully tested (Dhar, 1999).  The CTPC used a sodium-filled heat pipe heated by electric radiant heaters to 
supply heat to the engine and a pumped mineral oil loop to extract heat from the engine. 
Since that time, requirements for the power convertor were changed based on a proposed power system that uses a 
lower temperature reactor (Mason, 2006).  The power level was reduced from 25 kWe to 10 kWe per convertor and 
the hot end temperature was reduced to 830 K, but the temperature ratio of 2.0 was maintained.  In 2007, Foster-
Miller was contracted by Auburn University Space Research Institute to develop a 5-kWe single-cylinder FPSE 
demonstrator convertor.  The design of this Stirling Convertor Assembly (SCA), described herein, is based on the 
successful designs of the SPDE and CTPC with certain differences due to cost and time constraints. 
https://ntrs.nasa.gov/search.jsp?R=20080012733 2019-08-30T03:59:48+00:00Z
DEMONSTRATOR CONVERTOR DESIGN 
The SCA is designed to operate at a hot end temperature of 830 K, a cold end temperature of 415 K, with a nominal 
power output of 5 kWe, a peak power output of 6 kWe with overstroke, and a design life of 5 years (44,000 hr) of 
continuous operation at 100% power level.  High pressure (15 MPa) helium serves as the working gas, and the 
convertor operates at a resonant frequency of 85 Hz.  The final design goal is to achieve a minimum of 25% overall 
efficiency (electrical power out over heat in) and a specific mass of 6 kg/kWe.  Substitution of materials such as 
alloy steel for beryllium, the use of a commercially available linear alternator, and inclusion of bolted joints for 
convenience will preclude achievement of the efficiency and specific mass goals at this time.  Facilities limitations 
will also constrain testing of the hot end to 617 K at Foster-Miller and 650 K at Auburn University.   
As described below, the SCA consists of three primary subassemblies: the hot end assembly, the displacer drive 
assembly, and the cold end assembly (see Figure 1). 
Hot End Assembly 
The hot end assembly consists of the three heat exchangers: the heater head, regenerator, and cooler (see Figures 2 
through 4).  The heater head and cooler are both shell and tube heat exchangers, each with 1800 small diameter 
tubes (0.89 mm ID, 1.65 mm OD).  The ends of each tube are electron-beam welded to the tube sheets.  The heater 
shell, cooler shell, and tubes are constructed of Inconel 718.  The regenerator is constructed of a porous metal matrix 
with 80% porosity (20% density) brazed to an inner and outer metal liner.  The matrix is a sintered structure of 
randomly arranged metal fibers 0.022 mm in diameter.  The matrix is type 316 stainless steel; the inner and outer 
liners are Inconel 718. 
Displacer Drive Assembly 
The displacer drive (Figure 5) pumps the helium through the heat exchangers.  It consists of the displacer (dynamic 
component) and the post and flange (stationary component).  The displacer consists of a central rod, a displacer 
dome and its support on the hot end, and the gas spring piston on the cold end.  Two gas springs, one on each end, 
provide the spring stiffness needed to establish resonance.  The forward spring is relatively weak (24% of stiffness) 
versus the aft spring (76% of stiffness).  The rod provides the bearing support for the displacer drive.  An internally 
pumped hydrostatic gas bearing is used in the SCA design for long life.  All components in the displacer drive are 
alloy steel, with the exception of the displacer dome, which is Inconel 718 due to exposure of the dome to the hot 
expansion space.  In the CTPC, the equivalent steel parts were fabricated from beryllium for its low density and high 
stiffness.  Because of the additional losses in the gas springs suspending a heavy steel displacer, a penalty of 
approximately 3.5 points in efficiency is predicted.  Since the bearings are not active during initial engine start-up, a 
compliant wear couple is implemented on the bearing and seal surfaces to tolerate transitory contact.  The inside 
diameters of the cylinders are plated using Poly-Ond® and the outside diameters of the rod and pistons are coated 
with Xylan® 1054. 
Cold End Assembly 
The cold end assembly consists of the alternator assembly, pressure vessel, and joining ring (see Figure 6).  The 
alternator assembly includes the linear alternator, power piston and cylinder, and the suspension system.  A flexure 
support is used instead of a gas bearing to support the piston and plunger.  The flexure provides radial and 
circumferential stiffness which resists the forces between the magnets and stator iron.  The entire alternator 
assembly, including power piston and cylinder, will be supplied as a single assembled unit by Qdrive, Inc. (Troy, 
NY).  The alternator is designed to deliver 5 kWe at a nominal stroke of 22 mm and 6 kWe at an overstroke of 
24 mm, both at 85 Hz.  To reduce lead time and expense, an off-the-shelf size was customized to obtain the power 
output, voltage level, and temperature capability of the design requirements.  These modifications included slightly 
larger magnets, Hiperco laminations in the stator, samarium cobalt magnets, and custom windings.  A performance 
penalty was accepted in lieu of an optimized custom alternator design.  The current design is calculated to provide 
88% efficiency at 5 kWe, versus approximately 93% efficiency with an optimized design. 
Hot End 
Assembly
Displacer Drive 
Assembly
Cold End 
Assembly
330 mm
560 mm
 
FIGURE 1. Demonstrator Engine. 
Weld Joints
Weld Joint
 
Displacer Cylinder Liner
Regenerator Matrix
Outer Liner
 
 FIGURE 2. Heater Head Assembly. FIGURE 3. Regenerator Assembly. 
Weld Joints
 
FIGURE 4. Cooler Assembly. 
Displacer 
Cylinder
Instrumentation 
Ring
Post and 
Flange
Alternator 
Adapter
Gas Spring   
Cylinder
Displacer 
Assembly
Displacer 
Dome
Displacer 
Support
Displacer 
Rod
Gas Spring 
Piston
Tuning 
Weight
Radiation 
Shields
 
FIGURE 5. Displacer Drive Assembly (left) and Displacer Assembly (right). 
Joining Ring
Alternator
Pressure Vessel
with Cooling Jacket
 
FIGURE 6. Cold End Assembly. 
Bearings and Porting 
The displacer is supported with a hydrostatic gas bearing located on the displacer rod.  The bearing is internally 
pumped, pulling gas from the aft gas spring when its amplitude is high, charging a volume internal to the rod when a 
port opens at roughly 50% stroke.  Two rows of 12 holes, each 0.30 mm in diameter, feed the bearing from this 
volume.  The gas then flows along the rod and drains to the forward gas spring.  A centering port located 
approximately at the mid-stroke position returns the bearing gas flow back to the aft gas spring, along with any net 
flow occurring due to seal leakage.  To correct for seal leakage along the power piston seal, a centering port 
connects the compression space to the piston gas spring at approximately the mid-stroke position of the power 
piston.   
Convertor Operation and Data Monitoring 
The output power of the SCA is a function of the hot end temperature, cold end temperature, mean pressure, 
frequency, displacer stroke, piston stroke, and phase angle.  Heat will be provided via an electrically heated pumped 
loop using Therminol® 72 heat transfer fluid.  The heating loop will have a separate controller to maintain a set 
temperature value up to 617 K.  Heat will be rejected through a water/glycol pumped loop connected to an air-
cooled radiator.  A separate controller on the cooling loop will maintain a set temperature by operating a mixing 
valve.  A pair of capacitance gap sensors monitors both displacer and power piston positions.  The sensors are 
located 180° apart and their signals averaged to eliminate variations due to eccentricities, thermal growth, and so 
forth.  Since the sensors have a small range (~2 mm) compared to the stroke (22 mm), a taper is machined into the 
target to turn down the displacement.  The controller monitors both displacement signals to safeguard against an 
overstroke condition.  It also uses the power piston stroke as the process variable in the control scheme.  A separate 
control load is used in parallel with the user load, and by pulse width modulation (PWM) switching of the control 
load, a change in total load resistance is induced to maintain the piston stroke set point (Kirby and Vitale, 2008).  
Alternator voltage and current will be measured to determine the output power.  Flow rate and temperature drop will 
be measured across the heat exchangers to determine total heat in and rejected heat.  Overall engine mean pressure 
and dynamic pressures of the cycle and displacer and piston gas springs will be monitored to help evaluate engine 
performance.  All measurements will be displayed and recorded using LabVIEW (National Instruments).  Figure 7 
presents the instrumentation and data acquisition display screen. 
 
FIGURE 7. Instrumentation and Data Acquisition Display Screen. 
DESIGN ANALYSIS 
The design process for the SCA included thermodynamic analysis of the Stirling cycle, dynamical analysis of the 
complete mechanical system, and structural analysis of the pressure vessel.   
Thermodynamic Analysis 
Thermodynamic analysis of the Stirling cycle was performed using the HFAST (version 2.00) computer code 
developed by MTI under NAS3-25330.  Optimization studies were performed on several engine parameters to 
determine the best design when considering output power, overall efficiency, and specific mass.  These parameters 
included: heat exchanger length, tube diameter, and number; regenerator length, wire size, and porosity; engine 
frequency and phase angle.  Figure 8 presents the power balance diagram for the SCA at the design operating 
temperature. 
Dynamical Analysis 
Another code developed by MTI called STIRDYN was used to model the dynamical behavior (interaction between 
the displacer, power piston, and various volumes within the engine) of the SCA.  This code calculates the gas 
pressure variations in the volumes and the resulting forces on the displacer and power piston and can extract data 
from the HFAST output.  Five dynamical forces act on the displacer: compression space pressure force, expansion 
space pressure force, forward gas spring pressure force, aft gas spring pressure force, and inertial force associated 
with displacer mass.  Six dynamical forces act on the power piston: compression space pressure force, piston gas 
spring force, damping force representing compression space leakage losses, alternator magnet spring, alternator 
shaft force, and inertial force associated with power piston mass.   
Hysteresis Leakage
Engine Volumes
33
96
85
TotalVolume
Compression space
Piston gas spring
Aft gas spring
Forward gas spring
Heat in = 24863 W
17455W = Heat in
7408 W = pneumatic power
hcyc=0.298
hmech=0.784
Shaft power = 5810 W
Electric out = 5084 W
halt=0.875
hsystem= 0.204
53
0 759759
149
118
177316 494
1598 W
E
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E
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Porting and Bearings 78
726 W
(Frequency = 85 Hz, Mpiston = 5.58 kg; Mdisplacer = 1.857 kg)
(Theater = 830 K; Tcooler = 415 K)
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FIGURE 8. Power Balance Diagram of the 5-kWe SCA at Design Conditions. 
Structural Analysis 
Because the internal pressure of the vessel is high, a complete structural analysis was performed.  Since the cold end 
of the vessel is essentially isothermal, the analysis of that section is straightforward.  The heater head, however, is 
more challenging due to the somewhat unique combination of pressure and thermal stresses, both static and cyclic in 
nature.  The hot section is initially sized to provide ample creep life at the design temperature.  The length of the 
outer wall, which carries the temperature gradient, is then sized to minimize the thermal stresses.  The optimum 
regenerator length tends to be short in order to keep the pumping losses down, which improves the pressure stresses 
but tends to increase the thermal stresses.  To allow use of a shorter regenerator without negatively impacting the 
outer heater head wall, a separate inner wall is implemented.  The cavity between the inner and outer walls is 
pressurized to engine mean pressure.  Since the inner wall does not see the absolute pressure, higher thermal stresses 
can be accommodated.   
Table 1 summarizes stresses for critical locations around the heater head.  Figure 9 shows the contour plot of the 
mean stress component for the combined pressure and temperature stresses.  Figure 10 shows the contour plot of the 
alternating stress component for the combined pressure and temperature stresses.   
TABLE 1. Heater Head Stress Summary.  Stress Values are in MPa. 
Location Mean Stress Alt. Stress Stress Ratio Fatigue Safety Factor 
Upper Heater Head Wall (H1) 211 25.8 0.123 4.00 
Middle Heater Head Wall (H2) 383 13.2 0.0344 2.71 
Lower Heater Head Wall (H3) 475 8.8 0.0185 2.33 
Closure Plate Wall (H4) 427 57.9 0.136 1.89 
Upper Regenerator Wall (H5) 283 112 0.396 1.92 
Middle Regenerator Wall (H6) 44.3 144 3.248 2.70 
 
0 54 107 161 214 268 321 375 428 482 MPa
Equivalent Stress
H4
H2
H3
H1
 
FIGURE 9. Heater Head Mean Stress due to Pressure and Temperature. 
0 14 29 43 57 72 86 100 115 129 MPa
Equivalent Stress
H5
H6
 
FIGURE 10. Heater Head Alternating Stress due to Pressure and Temperature. 
CONCLUSION 
The efforts conducted continue to support the concept that a Stirling cycle convertor is viable for nuclear space 
power generation.  It offers high efficiency, low specific mass, and long life at the power levels and temperatures 
identified in the system studies.  The technology has been demonstrated at lower power levels (100 W per cylinder) 
as well as higher power levels (12.5 kWe per cylinder).  This design suggests that these goals are still achievable at 
the 5-kWe power level.  Testing in the near future will validate this. 
ACKNOWLEDGMENTS 
This work is supported by Auburn University Space Research Institute under Subagreement 07-SRI-208447-FMI 
“Free-Piston Converters.”  Any opinions expressed are those of the authors and do not reflect the views of Auburn 
University. 
REFERENCES 
Dhar, M., Stirling Space Engine Program, Volume 1—Final Report, NASA/CR—1999-209164/VOL1, Mechanical Technology 
Incorporated, Latham, NY, 1999. 
Kirby, R.L., and Vitale, N., “The Development of a Control System for a 5 Kilowatt Free Piston Stirling Space Convertor,” to be 
published in Proceedings of STAIF-2008, February 10-14, 2008, Albuquerque, NM. 
Mason L.S., “A Comparison of Fission Power System Options for Lunar and Mars Surface Applications,” CP813, Space 
Technology and Applications International Forum – STAIF 2006, American Institute of Physics, (2006) p 270-280.  
 


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