As part of the NASA funded portion of the SP-100 Advanced Technology Program the Space Power Research Engine (SPRE 1) was designed and built to serve as a research tool for evaluation and development of advanced Stirling engine concepts. The SPRE 1 is designed to produce 12.5 kW electrical power when operated with helium at 15 MPa and with an absolute temperature ratio of two. The engine is now under test in a new test facility which was designed and built at NASA Lewis specifically to test the SPRE 1. The SPRE 1, the NASA test facility, the initial SPRE 1 test results, and future SPRE 1 test plans are described

Cairelli, James E.

English

NASA Technical Reports Server

NASA Technical Memorandum 100241 
SPRE I Free-Piston Stirling Engine Testing 
at NASA Lewis Research Center k 
kr' f ' [NASIl-Tfl- m0241) SPRE 1 FREE-PISTON m a -  11747 
f 
;'e STIRLING ENGINE TESTING AT N A S A  LEWIS 
RESEARCH C E l T E B  ( N A S A )  15 p Avail: NTIS 
HC A03/HP 801 CSCL 10B U n c l a s  
G3/20 0 107322 
James E. Cairelli 
h i s  Research Center 
Cleveland, Ohio 
I 
Prepared for the 
1988 Solar Energy Conference 
sponsored by the American Society of Mechanical Engineers 
Golden, Colorado, April 10-14, 1988 
1 
https://ntrs.nasa.gov/search.jsp?R=19880002365 2020-03-20T08:43:31+00:00Z
SPRE I FREE-PISTON STIRLING ENGINE TESTING AT NASA LEWIS RESEARCH CENTER 
James E. Cairelli 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 
Abstract - As part of the NASA funded portion 
of the SP-100 Advanced Technology Program the 
Space Power Research Engine (SPRE I) was 
designed and built to serve as a research tool 
for evaluation and development of advanced 
Stirling engine concepts. The SPRE I is 
designed to produce 12.5 kW electrical power 
when operated with helium at 15 MPa and with 
an absolute temperature ratio of two. 
facility which was designed and built at NASA 
LeRC specifically to test the SPRE I. This 
paper describes the SPRE I, the NASA test 
facility, the initial SPRE I test results, and 
future SPRE I test plans. 
Introduction - As part of the NASA funded 
portion of the SP-100 Advanced Technology 
Program the Space Power Research Engine (SPRE 
I) was designed and built, on contract by 
Mechanical Technology Inc. (MTI), to serve as 
a research tool for the evaluation and devel- 
opment of advanced Stirling engine concepts. 
These include active dynamic balancing, 
hydrodynamic bearings, heat pipe heat exchang- 
ers and power control. The Stirling engine 
technologies developed in the SPRE I ultimate- 
ly will be used in nuclear and solar space 
power systems. The technologies developed in 
this project also apply directly to solar 
terrestrial power systems. 
In 
W 
m OD The engine is now under test in a new test 
&, 
A new test facility has been designed and 
built at NASA Lewis Research Center specifi- 
cally €or testing the SPRE I. The SPRE I has 
been installed in the NASA test facility and 
is now under test. The following describes 
the SPRE I engine. the NASA test facility, 
initial NASA test results, and future SPRE I 
test plans. 
SPRE I Enqine - The SPRE I is a free-piston 
Stirling engine coupled to a linear alternator 
to produce electrical power. Two SPRE 
engines were derived directly from the Space 
Power Demonstrator Engine (SPDE) which was 
designed, built, and tested by MTI as a two 
cylinder opposed engine having a common 
expansion space (hot end). The SPDE was cut 
in two and the hot end of the cylinders were 
modified to close off the cylinders; thus 
forming two separate engines. The SPRE I was 
delivered to NASA LeRC. The SPRE I1 remains 
at UTI. Additional description of the SPDE 
and SP-100 Stirling Program may be found in 
references 1, 2, 3, and 4. 
Figure 1 is a photo of the SPRE I as it 
arrived at NASA. Figure 2 is a later photo 
showing the SPRE I installed in the NASA test 
cell. 
The SPRE is designed to produce 12-1/2 kW 
of electrical power with 50 kW thermal power 
input to the heater while operating at 100 Hz 
with helium at 15 MPa and an absolute heat 
exchanger wall temperature ratio, T(heater)/ 
T(cooler), of two. Heat is supplied to the 
engine by circulating molten salt through the 
engine heater and waste heat is rejected by 
circulating water through the engine cooler. 
For these tests the electrical power produced 
by the engine is absorbed by a water cooled 
resistance load. 
The SPRE engine is shown in cross section 
in figure 3. The engine has two moving parts, 
a power piston and a displacer plston con- 
tained in a single cylinder. The displacer 
separates the hot end of the cylinder from the 
cold end. The hot end of the cylinder is 
connected to the cold end through annular type 
heat exchangers 
cooler). 
In order to 
both the piston 
hydrostatic gas 
(heater, regenerator, and 
have potential for long life, 
and displacer are guided by 
bearings. This eliminates any 
1 
chance of wear due to contact between the 
moving parts and stationary parts. 
displacer gas spring is divided into a forward 
gas spring and an aft gas spring. The weights 
of the pistons and the spring rates are 
designed such that the resulting spring-mass 
systems operate with about an 0 5  degree phase 
angle between the piston motions. 
Each piston has its own gas spring. The 
The motion of the displacer causes the 
helium working gas contained in the cylinder 
to flow through the heat exchangers alter- 
nately between the hot end of the cylinder 
(expansion space) and the cold end of the 
cylinder (compression space). The working gas 
temperature alternates between hot and cold, 
as it flows back and forth through the regen- 
erator, causing a corresponding change in 
pressure. The power piston motion changes the 
total volume occupied by the working gas. 
The piston and displacer motions are 
synchronized so that the working gas is 
compressed by the power piston while the gas 
is in the cold end of the cylinder and expand- 
ed while the gas is in the hot end of the 
cylinder. The heat energy extracted from the 
gas during expansion is replenished by the 
heater and the energy transmitted to the gas 
during compression is extracted from the gas 
by the cooler. 
The net power produced by the power piston 
is converted to electrical power by a linear 
alternator which consists of an alternator 
plunger, with permanent magnets, attached t o  
the power piston (the armature); and station- 
ary electrical windings and core material 
attached to the engine case (the stator). 
Because the single cylinder engine is not 
inherently balanced, a large mass (about 2000 
kg.) is attached t o  the hot end of the engine 
to reduce the maximum case motion to less than 
0.1 mm. The engine and mass are separated by 
a thin walled stainless steel spool piece t o  
minimize axial thermal conduction losses. 
The engine instrumentation used €or these 
tests are shown in figure 4 .  The instrumenta- 
tion is used to measure steady state surface 
and gas temperatures; dynamic pressures within 
the cycle and in each of the gas springs; and 
piston and diSplaCer dynamic positions. These 
measurements are used to analyze the engine’s 
thermodynamic cycle and to evaluate thermal 
losses. 
- Salt Heatina Svstems - The SPRE salt heating 
system is shown schematically in figure 5. 
About 2500 kg. of a molten salt mixture of 
sodium nitrate, sodium nitrite, and potassium 
nitrate, are contained in an insulated storage 
tank heated by electric heaters. The salt is 
pumped through the engine heater and then 
returns to the tank where it is reheated. An 
automatic control maintains the salt tank 
temperature, over a range of 150C to 450C, by 
regulating the power to the electric heaters. 
The temperature range can be extended t o  540C 
by changing the salt mixture. The salt flow 
is maintained at about 1.7 L/sec. by a fixed 
orifice in the salt return line. 
The measurements made in the salt heating 
system are used to determine the thermal 
energy input to the engine heater; and to 
monitor and control the salt temperature. 
thermal shock, steam is used to heat the 
engine to match the salt temperature before 
the circulating pump is started, 
Coolinq Water System - The SPRE cooling water 
system Is shown in figure 6. The engine is 
cooled by circulating water through the engine 
cooler. The pressure vessel surrounding the 
alternator, and several electrical load 
components are also water cooled. Automatic 
controls are used to maintain constant total 
water flow and inlet temperature to the 
engine. The flow control range is from 0.5 to 
1.5 L/sec. The temperature control is 
adjustable from about 20 t o  BOC. 
To avoid exposing the engine to severe 
The cooling water system is instrumented 
to measure the heat rejected by the engine 
cooler, alternator cooler, and the electrical 
load; for flow and temperature control; and to 
monitor the system. 
Helium Svstem - Figure 7 is a simplified 
schematic o t  the helium system. The system 
consists of a helium bottle supply, mean 
pressure control with an operating range of 5 
to 15 MPa to charge the engine with working 
gas, a control to center the displacer stroke 
for starting, and circulating pumps for the 
displacer and power piston hydrostatic bear- 
ings. Displacer centering is accomplished by 
adding gas to or venting gas from the aft 
displacer spring. An emergency stop valve is 
also included to stop the displacer motion by 
connecting the displacer forward and aft gas 
spring volumes together. 
Instrumentation is provided to monitor the 
engine mean pressure and gas bearing circula- 
tion circuits. 
Electrical Load System - In order to absorb 
the electrical power produced by the 
alternator and to control the power piston 
stroke, an electrical load system is included 
in the test setup. Figure 0 is a simplified 
schematic drawing of the electrical load 
system. The load system contains an a.c. 
circuit comprised of the alternator in series 
with manually selected tuning capacitors and 
an untiltered d.c. circuit made up of a 
full-wave rectifier, 16 selectable fixed 
resistors, and two transistor regulated 
variable resistors. 
Since the alternator voltage and result.ant 
d.c. voltage is directly proportional to the 
piston stroke, the power piston stroke control 
is accomplished by using an automatic voltage 
control which selects the proper number of 
L 
fixed resistors and amount of variable resis- 
tance to maintain the desired d.c. voltage. 
The normal piston amplitude (half stroke) 
control range is from 5 mm to 10 nun. 
A variable transformer connected t o  the 60 
Hz a . c .  commercial line power and tied to the 
load system through a pair o f  relay contacts 
is used t o  start the engine. T h i s  arrangement 
provides a precise and convenient way t o  
' control piston stroke while the load system is 
being brought "on line". During starting t h e  
contacts are closed and the voltage adjusted 
to drive the alternator/piston. After the 
engine has started, the load control is 
adjusted. slowly, t o  absorb all of the alter- 
nator electrical output. At this point, the 
contacts are opened t o  isolate the starting 
power source f r o m  the load system. T h i s  
procedure provides a n  absolutely "bumpless" 
transfer from the starting system to the load 
control system. 
The load system instrumentation include 
alernator voltage and current, load voltage, 
and tuning capacitor voltage. These dynamic 
measurements are used to analyze the alterna- 
tor and load characteristics. In addition, 
alternator, load, and starting power are 
measured. 
Safety Systems - In order to protect the 
engine from damage due to hardware malfunction 
or operator error, several engine and facility 
operating parameters are continuously moni- 
tored. These include piston and displacer 
positions, bearing differential pressures, 
cooling water flow, and engine case accelera- 
tion. If any of these parameters are out of 
preset limits, engine shutdown is initiated 
automatically by opening the displacer spring 
short circuit valve and connecting all resis- 
tors into the alternator load (a pseudo 
electrical short circuit). This shutdown 
procedure has been demonstrated to be smooth , 
well controlled, and with no risk to the 
engine; even at maximum power condition. 
Data Acquisition - Steady state measurements 
(up to 256 data channels) are monitored and, 
upon request, are recorded using a central 
data system known as ESCORT. The dynamic 
measurements are recorded and processed by a 
dedicated personal computer (PC) using fast 
Fourier analysis. Processed data results are 
displayed on the PC's CRT display and are 
transmitted by computer link into ESCORT where 
they are merged with the steady state data. 
To aid in running the test, a limited number 
of "on line" calculations are made by ESCORT. 
The results of these calculations and all data 
input to ESCORT, including PC data, can be 
viewed on two ESCORT CRT displays. Upon 
command, hard copies of the ESCORT displays 
can be made on a local printer. Data recorded 
by ESCORT is transmitted to a data base file 
in the central computer for further batch 
processing, after the test. 
Test Results - At this time only about 35 
hours of SPRE I testing has been done, at 
NASA,; primarily to check out the facility 
systems and to verify the accuracy of the 
data. This process is still under way. The 
engine has been operated at temperature ratios 
of 1.6, 1.8, and 2.0, mean pressures of 5.0, 
7.5, 10.0, 12.5, and 15 MPa; and piston 
amplitudes of 5.0, 6.0, 7.0, 8.0, and 9.0 mm. 
The following is a sampling of the data taken 
to date at a temperature ratio of two. 
Figure 9 shows the measured piston P-V power 
versus the piston stroke at the various 
pressures. The maximum P-V power measured was 
11.60 kW. The P-V power tends to vary linear- 
ly with both the piston amplitude and the mean 
pressure. 
figure 10. The maximum alternator power 
measured was 7.47 kW. The trends of 
alternator power are similar to those of the 
P-V power. The alternator power varies 
linearly with piston amplitude. However, the 
relation to mean pressure appears to be more 
complex. 
The measured alternator power is shown in 
Figure 11 shows the relation of displacer 
amplitude to piston amplitude at various 
pressures. The displacer amplitude tends to 
vary linearly with piston amplitude and 
appears to increase with increasing pressure 
by a constant which is a function of the mean 
pressure. 
The phase angle of the displacer motion 
relative to to piston motion is shown in 
figure 12. The displacer phase angle varies 
from about 84 degrees to 09 degrees. The 
angle appears to decrease slightly (about two 
degrees) with increasing piston amplitude for 
a constant pressure. For each different 
pressure, the phase angle appears to shift by 
a constant angle which is a function of the 
pressure. 
Figure 13 shows the relation of engine 
frequency to mean pressure. The predicted 
frequency is also shown for reference. The 
measured frequency appears to be about 2 to 3 
Hz higher than the predicted value. 
SPRE Test Plans - SPRE baseline performance 
will be measured following completion of 
facility check out. Baseline performance 
tests will be repeated with a dynamic balancer 
installed to replace the 2000 kg mass. 
Testing will then turn to component develop- 
ment primarily to evaluate the use of hydrody- 
namic displacer bearings and alternate regen- 
erator materials. Throughout the SPRE testing 
there will be an effort made to gather appro- 
priate engine data for comparison to the NASA 
computer code predictions and to improve the 
code to better predict engine performance. We 
plan to present the results of these activi- 
ties in future NASA reports. 
References - 
1. Dochat. G. R., "Free-Piston Stirling . 
Engines for Space Power", Proceedings of 
---
the Twenty-Second Automotive Technoiogy 
Develoment Contractor's Coordination 
Meeting, Society of Automotive Engineers, 
Warrendale PA, 1984, pp. 209-213. 
3 
2. Slaby, J. G., "Overview of the 1985 NASA 4. Slaby, J. G., "Overview of Free-Piston 
Lewis Research Center SP-100 Free-Piston Stirling SP-100 Activities at the NASA 
Stirling Engine Activities", NASA Lewis Research Center", NASA TM-87224, 
TM-87028, 1985. 1986. 
3. Slaby, J. G., "Overview of Free-Piston 
Stirling SP-100 Activities at the NASA 
Lewis Research Center", NASA TM-87156, 
1985. 
FIGURE 1, - SPRE I AS RECEIVED AT NASA. 
4 
ORIGINAL PAGE IS 
DE POOR QUALITY 
t 
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FIGURE 2. - SPRE I INSTALLED I N  THE NASA FACIL ITY .  
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FM FLOWMETER 
P PRESSURE TRANSDUCER 
PR PRESSURE REGULATOR 
T/C THERMOCOUPLE 
AP PRESSURE TRANSDUCER DIFFERENTIAL 
AT DIFFERENTIAL TEMPERATURE 
TEMPERATURE 
SET POI NT 
Y 
@ SALT RESERVOIR 
FIGURE 5 .  - SALT HEATING SYSTEM. 
FM FLOWMETER 
P PRESSURE TRANSDUCER 
PR PRESSURE REGULATOR 
T/C THERMOCOUPLE 
A P  PRESSURE TRANSDUCER DIFFERENTIAL 
A T  DIFFERENTIAL TEMPERATURE 
GAS #VENT 
TOWER 
WATER 
HEAT EXCHANGER STRAINER 
FIGURE 6 .  - COOLING WATER SYSTEM. 
9 
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ALTER- 
NATOR 
208 =: VAC 
WATER-COOLED 
LOAD ( 2 0  
OHMS EACH) i 
LOAD VOLTAGE CONTROL 
.. 
START I NG 
POWER 
FIGURE 8. - ELECTRICAL LOAD SYSTEM. 
MEAN 
PRESSURE 
MPA 
n/ 
4 6 8 10 12 
PISTON AMPLITUDE, MM 
FIGURE 9. - PISTON P-V POWER VERSUS PISTON AMPLITUDE. 
0 . 2 5  OHM 
11 
15 
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n g 10 
& 
0 n 
Y 
8 
s I - 5  
& 
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d 
n 
MEAN 
PRESSURE, 
MPA 
0 7 . 5  
- * 12.5  
A 10.0 
- 
0 1 5 . 0  
- 
I 
" 4  6 8 10 12 
PISTON AMPLITUDE, MM 
FIGURE 10. - ALTERNATOR POWER VERSUS PISTON AMPLITUDE. 
12 
I 
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t, 
. 10 
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=l 
-I 
4 8  
er 
W 
V 
5 
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n 
MEAN 
PRESSURE, 
MPA 
- 
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MEAN 
PRESSURE 
MPA 
0 7.5 
X 12.5 
A 10.0 
15.0 
70 
4 6 8 10 12 
PISTON AMPLITUDE, MM 
FIGURE 12. - DISPLACER PHASE ANGLE VERSUS PISTON AMPLITUDE. 
120 
N 
w 
W 
40 
20 
 PRESSURE 
MPA 
o 7.5 
x 12.5 
A 10.0 
0 15.0 
13 
Report Documentation Page 
I .  Report No. 2. Government Accession . No. 
NASA TM-100241 
0. Title and Subtitle 
3. Recipient's Catalog No. 
5. Report Date 
SPRE I Free-Piston S t i r l i n g  Engine Tes t ing  a t  
NASA Lewis Research Center 
7. Author@) 
James E. C a l r e l l i  
6. Performing Organization Code I 
8. performing Organization Report No. 
E-3865 
2. Sponsoring Agency Name and Address 
Nat ional  Aeronautics and Space Admin is t ra t ion  
Washington, D.C. 20546-0001 
10. Work Unit No. 
506-41-31 9. Performing Organization Name and Address 
11. Contract or Grant No. 
Nat ional  Aeronautics and Space Admin i s t ra t i on  
Technical Memorandum 
14. Sponsoring Agency Code 
Lewis Research Center 
C1 eve1 and, Oh1 o 441 35-31 91 
17. Key Words (Suggested by Authoqs)) 
Free-pi ston S t i  r l  1 ng 
T e s t  data 
T e s t  f a c i  1 i t y  
Space power 
13. Type of Report and Period Covered 
18. Distribution Statement 
U n c l a s s i f i e d  - U n l i m i t e d  
Subject  Category 20 
19. Security Classif. (of this report) 20. Security Classif. (of this page) 
Unc 1 ass i f i ed U n c l a s s i f i e d  1 4  
21. No of pages 
Prepared for  the 1988 Solar  Energy Conference sponsored by the American Society  
o f  Mechanical Engineers, Golden, Colorado, A p r i l  10-14, 1988. 
22. Price' 
A02 
6 ' r $ a r t  of  the NASA funded p o r t i o n  of  the  SP-100 Advanced Technology Program the 
The SPRE I i s  designed to produce 12.5 kW e l e c t r i c a l  power when operated 
The engine 
This  paper descr ibes the SPRE I, the NASA 
Space Power Research Engine (SPRE I >  was designed and b u i l t  to  serve as a 
research tool for eva lua t ion  and development o f  advanced S t i r l i n g  engine con- 
cepts. 
w i t h  hel ium a t  15 MPa and w i t h  an absolute temperature r a t i o  o f  two. 
i s  now under t e s t  i n  a new t e s t  f a c i l i t y  which was designed and b u i l t  a t  NASA 
Lewis s p e c i f i c a l l y  to  t e s t  the SPRE I .  
t e s t  f a c i l i t y ,  the i n i t i a l  SPRE I t e s t  r e s u l t s ,  and f u t u r e  SPRE I t e s t  p lans.  


SPRE 1 free-piston Stirling engine testing at NASA Lewis Research Center

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