The performance data of experimental Stirling engines are considered along with questions of exhaust-gas composition, engine noise, engine volume and weight, engine control, and the engine-starting process. The Stirling engine can use practically any liquid or gaseous fuel for its operation. It is found that technically a use of the Stirling engine in motor vehicles is feasible. Economic questions related to an introduction of the Stirling engine are discussed along with possible new developments which could improve the economic situation in favor of a use of Stirling engine

Kuhlman, P.

English

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•I`IASA TECIIN I CAI. MEMORANDUM
	
NASA TM-75442
TIIL S'I MANG LNGINI: FOR VI'HICLI: PROPIILSION
P. Kuhlm,inn
ASA	 75442) THL' STIRLI4G P4GINE FOR
VFIIIrLE PROP'ILSION (Natio[,al Aeronautics anA
- pace 4dministration)	 14 p HC A02 /P.F AOI
CSCL 10A.
translation of "Der Stirlinginotor a s
VDI-Berirhte, No. 224, 1974,
N79-12547
Unclas
G3/44 35010
 Fahrzeugantrieb."
pp. 87-91
IJ113 	 Wee
C', ^^^ Al
t
NXIiONAL AERONAtH ICS AND SPACE ADMINISTRATION
IVY
 I INGTON, D.C. 20546
	 NOVEMBER 197E
Till. STIRLING I:N(;I'^T FOR Vl:lllCl.li PROPULSION
1 1 . Ktihlm rtn
1. Introduction
The re,luirements for protection of the environment launched /^7
the di,CUSSrOrl, of alternate means of propulsion several years
ago; the energy crisis has added some new aspects to these dis-
Cussion,. One rf the most interesting alternate means of pro-
pulsion is the Stirling motor whose characteristics and present
state of development- - in particular with respect to its use for
vehicle propulsion--will be related helow.
2. Characteristics
The characteristics of the Stirling engine will be de-
scribed as they are found on present experimental motors, to
allow some judgment about the possibilities of its use. It
must be kept in mind, of course, that improved data are to he
expected by the time of its later application.
2.1 Power output data
Fig. 1 shows the torque of a 4-cylinder Stirling engine
with 85 kW (about 120 PS) against revolutions with the mean
Cycle I)ressur• e serving as parameter. The mean cycle pressure
is the average value per time unit of the working gas pressure
in the cylinder and is a measure for the engine load. A torque
increase at lower rpm is to be noted, but is not very high in
this case. A steeper torque increase could he achieved--for a
vehicle motor for instance--thrcugh changes in design.
1
In rig. ^, the efficiency of the same engine is plotted,
also as a function of rpm and average cycle pressure. Tht^
i ► ighest efficiency of shout 330 occurs at the highest load and
at average rpm.	 Particularly important for vehicular use are
the efticicncics, which arc t'avorahlc even for partial loads
over the entire rpm range, leading to lot: fuel consumption for
mixed load operations.
2.2 Composition of exhaust gases
One of the most important arguments for the use cl' the
Stirling engine in motor vehicles is its clean exhaust gas.
Digs. 3 and 5 X110%% test results from an experimental motor that
operated without recycling of the exhaust gas. Emission of CO
and HC is extremely low and far hencath all expected require-
nlcats.	 Because of the high combustion temperatures NO x co ► is- /88
lion values are not so favorable but are still at such a low
level that 5 g/l'SII for the sum of NU x . ► ml Ilt: are not excecdcd
Ill tilt I .i-step cyc I L'.	 I nvest igat ions by Phil ills have shomi
that even the previous limit of 0.4 g/mile ill 	 CVS -test can
he reduced by recycling of exhaust gases.
	
It is finally worth
noting that the exh;lust fain ► the Stirling engine is free of soot
and odors.
2.3 Noise
ill
	 to the clean exhaust gas the Stirling engi ne
also shuns allother characteristic favorable to the c nv i ronmcnt ,
very low noise generation. Fig. 6 shows the acoustic pressure
levels for various rpm aml loads, as measured oil 	 one-cylinder
engine with 22 kW (30 PS) , at I 111 d 1 Stalll• e .	 The value of 85
dh(l1) at rated load is very lot:, i.e., the Stirling engine is
riot only much kluieter than a Diesel ctlgine btlt also quieter than
ml tit t o cng i nc .
2.4	 1'olunre and we igfit
In compa ► risoil to internal combustion motors, tlie external
corr ► hust ion system and the heart exchangers oI* the Stirl ing err-
gine require more room and increase the weight. Oil the other
hand, mean effective pressure and power output her liter (out-
put/stroke volume) are decidedly higher for the Stirling; engine
than for internal combustion motors. The mean effective pres-
sure of Stirling engines is today at around 25 bar, with work
being; done for each engine turnover--as in the two-cycle motor.
Forecasts about the dimensions of futr ► re Stirling engines are
not immediately possible since the present experimental motors
are still pretty big; and not yet optimized in their volume and
weight. It can safely be assumed that with progressive improve-
ments in design and increase of power output through increase
of internal pressure, weight per MV will be cut in half for
future engines, reaching values below 4.; kh/PS, resp. 6 kg/kW.
'these data are for engines using 1101irrm as working gas while
concentration of output can he much more increased by switching
over to hydrogen.
2.5 Controllability
As with all engines requiring external g reat supply, more
control apparatus is req ► iired for the Stirling engine than for
an internal combustion engine. On the one hand, the output of /89
the Stirling engine must be controlled by intervention in the
thermodynamic process (output control); on the other hand, the
heat gene ratio ►► , i.e., fu01 and air supply, must he acla ► pted to
the engine output (temperature control).
Temperature control presents no basic difficulties. Duel
and air supply for tiro experimental motors are controlled by a
thermo - element and an amplifier so that the t0mp0rature of the
3
tM. .630"(:
 
14
1Nnl)
1600
1500
.400
• 700
. f'Ac:S IS
Uk A:%A ► fr1WA
70
6
Y 50
140
 30
E
	
n 20	 - 	 — I I i0o
i
	
10	
— - —	
2s	 itoo
	
o	 _	 to
	
Soo	 1000	 1500
Molordrehtahl n IU/minl rpm
Fib. 1 Torque curves of a 4-
cylinder Stirling engine for
various mean cycle pressures.
	
ITwtl
	 --- pT IUD+^'	 ~"n0^	 M('0 IDDT I	 _^	 IMmI
	
I	 '
	
10-	 loo
	
e	
- 75-
	
,OJ	 I
	
6	 O"
	
As	
50
/ 2S
"T15-
	
500	 750	 1000	 n I	 I	 0
rpm
Fig. 3 ('0-emission of a one-
cylinder Stirling engine.
16
Me	
1
	 —	 —	 150
	
I'*Pj 14 4 ___ Dm IYp/c,
	 _	 -	 I	 M
	
IDDmI	 I	 `t	 IMTI
125
10	 -	 100
>S
e-- -	 --- - - --	 ...- --	 75
3s0 ^^^	 50
-- -	 --	 )Opi
	
]SO	 7`'
0
500	 750	 1000	 n Imn'I	 1500Fig. 5 ND-3r,issi,)n ,f a one-
cyllnd^r Stirlin - engine.
.1
DIUMNAL PAGF. 1.41
^QB Iii 4^^IY
i" 351
	
I !	 i	
^•bp
30
t--
E'
x
25
C
1He n 6)0 •C
t
	 to'WA : 42 9C
t201
151 1	 !
500	 1000	 150
Molordrehz0hl nfWminl rpr"
Fig. 2 Efficiency of a 4-
cylinder Stirling engine for
various mean cycle pressures.
	
M" 	 t	 _	 r—
	
a 
	 150i
	
Im.pl	 -^^ Dm IMD /cmrl 	 'IO-	 M
•-^
	
NC IDD'n C ' ) I	 ^	 IHmI
	
'^-	 +t5
u	 I----
i
1100
+0
o
500	 750	 1090	 n (mn''I	 ISM
rpm
Fig. 4 HC-emission of a one-
cylinder Stirling; engine.90J.= --
e11	
-	
.^
70	
—
last
bI l	 - O	 100 % _-
M -^-- 5O%
O —
I
• - 0%
40
500	 750	 K100	 Q50 mM 1500 t^m
Fig. 6 Acoustic pressure
levels measured at 1 in dis-
tance from a one-cylinder
Stirling engine.
working; gas in the heater of the Stirling engine remains equally
high for all load conditions. 	 In princil)le it is possible to
use simpler temperature controls but tliey were not developed.
Various systems are presently used for output control, each
showing; advantages :ind disadvantages.
When using; pressure control the mean pressure of the work-
ing; gas in the cylinder is varied by the introduction or• the
pumping; out of working gas (helium).	 This type of control
shows the hest efliciencies for partial loads, but the pumping;
out requires a certain amount of time on the order of several
seconds, depending; on the size of the compressor.
Short-circuit control, where top and bottom faces of the
working cylinder are connected with each other thror. , gh a valve,
reacts without loss of time but causes high losses, i.e., low
efficiencies at partial loads.
For damaged-space control, additional spaces, also filled
With working gas, are connected to the cylinder volume through
valves, which causes a drop in the output. This control also
works practically without time delay, efficiencies for partial
loads are not as good as with pressure control but better than
for short-circuit control.
According to the application, the above methods can be
combined for output control. The toughest requirements are
for the operation of vehicles in city traffic where there are
very frequent load changes and a requirement for hrief response
times.	 It is possible to meet these requirements today but a
lot of' development work mast still be done for optimi:'ation of'
the output control towards lower fuel consumption.
9
S
.0 Starting pi
TO'1'0 .tart the Stirling vngine a fu. • I IIt ► III l) : ► 11.1 h ►► r11er : ► ir -
cortthressor are actuated by a smaI I . • lectri. motor.
	
'file I) if
is ignited Ily a spark plug front 1 briefly turned on ignition
system. hringing the heater of the tit irl ing engine subsequent l+'
to tilt•
 operating temperature. The engine is then turned over
hV :1 st: ► rter :111,1 :unt roue: ► to run innnediatel y on its .) ►,n.	 As
soon as tile hreak in the heater temperature, ►;hiih occut's during
startup, is componsate.l for the engine can accept fu11 load.
The . • lit ire start ink; proce,lure tales 30 -00 secs.
'.7	 Multifuel capahility
I he tit i r l i ng motor is known for its Character istic to oper-
ate hI111 I)1'3CtiC:tl IN' :1111	 1 i till i11 ;111,1 g;lseous	 fuels.	 '1'hiS 111111t i-
Cue l Capab i l i t y %%a s .1 1 so kit-111011.4t r,l t ed ►, i t 11 :11 ►;1101	 .• r i es of
fuels on experinle11t.11 motors.
	
I;ec, ► LISe o1 the continuous, ex -
t erna I , comhus t i on emp l oyed 111 t he St i r 1 i 11g ertg i ne 110 reciu i re -
me11ts as to octane number or cet:lne number are 1111,10 of the fuel.
Rey'on.l that tC1L - hIl iCa l combust ion ,la t,1 1 i le HI.' heat 1110 InO ►;er of
the fuel or the air ratio have no influ.'nCe on the engine outl)ttt
i. ^Required I^urthe ► ' It^t'^,1^ ► nlent
'I'h. , :IIJI'alCtet'iSt iCS of th.• tit irl in0 engine l iste.l in sec
tion 2 S 1o ►; ti1,1t	 it is ent ire l)' s III t:thl. • for 1! e11iC10 1)1'o1 , 1, 11si011
l':11'ticU1;11'1 ►' I)ositive are its enviro11ntentaI characteristics 111,1
its modest re(luirentetlts t'e0:tr.1in0 fuel ch, ► r;lcteriStics, ^,hile
►;ei011t x11.1 ► p l Hill e Ill u'; t still h. re.ltice.l through fut'ther .levrlop-
ill .' n t .
1.:11111:111v, t 1. • use of this Stir11110 en0i11. • for vrhii Ies is
Etas i h 1 C	 It hough a fe ►; I+1-011 1 ams S  i I l h, ► ve to he SO l VOLT .	 H I
s
0
cost effectiveness of a vehicle driven by a Stirling engine, as
compared to our present drives, will be decisive.
Operating costs are mainly determined by the efficiency of
the propelling machine.
	 For that reason costs for the Stirling /30
engine ►sill he lover than for the Otto engine, resp. higher than
for the Diesel engine. The Stirling engine, whose drive mech-
anism does not come into contact with the combustion gases and
which gets along without valves, will not require much effort
for maintenance so that acceptable operating costs can he ex-
pected on that account, too.
The purchase price looks somewhat more problematical. Most
of the Stirling engines with rhombic drive that were built and
tested in the past are complicated and expensively built and
could hardly be produced cost effectively even in large series
taking advantage of all possibilities for economi- ing. So, in
addition to research efforts for the improvement of technical
data of the Stirling engine, development work mint be directed
towards simple engines that can he m.intil - actured in production
and economically.
So ►rne progress has been made in this direction in the "11e-
velopment Group .-Stirling, linginc l M.A.N.-A1101." As an example,
the new concept for heater heads should he mentioned: identi -
cal straight pieces of pipe, hends and housing parts are
soldered together in a one-step process to form the completed
heater head, Fig. 7. The first experiments already showed how
free of problems this construction is and how it promises con-
siderable savings in production costs. Just like the heater
head, a whole series of Subassemblies of the Stirling engine
must be further developed, like the air preheater, the combus-
tion chamber, the piston rod gasket, the control and the drive.
Some partial successes have already been achieved in these areas
7
OX ;NAI. 
rAIP rr
r),F P00110	
' '..i r V
as well	 Part icularIv noteworthy is the simplification of the
transmission, when changing from rhombic drive to the cross-
head drive of a double-acting Stirling; engine. 	 In the double-
acting engine the top of the piston in each cylinder works
together with the bottom of another cylinder, with which it is
connected via the heater, regenerator and radiator, Fig. 8.
This cuts the number of moving parts to half, which expresses
itself in considerable simplification and cost reduction of the
engi tic . A double-acting Stirling engine, with suhassemblies of
new design, is now being constructed. hata for the approximate
costs of production of this engine are not yet available, how-
ever.
Not only in Germany,hut ahroad as well does the further
development of the Stirling engine go on at an intensive pace.
The effort invested in this development permits expectations
that the Stirling engine will approach the goal, of being used
for vehicle propulsion, much closer during the next few years.
1	 2
Fig. 7 New heater head for a
one-cylinder Stirling engine.
1 He N^ r Raum t{W V00%" r R"nemtor
2 Kaltman rimop	 5 KuNar I+Adlator
3 EMJtur Heotfr	 6 Koite, Raum (oLd eZOGVA
Fig. 8 Schematic of a double-
acting Stirling engine.
8
REFLRENCES
1 .	 lei jer, R. J.:	 Der Ph i 1 ips - St irl iiigmotor, MTZ 29 (1968)
No. 7, ply. 284-298.
2. Zacharias, F. 	 lietrachtunge:. zum ausseren Verbrennungs-
system Iles Stirling-IleisSgasmotors.
	 "Considerations
of the external combustion system of the Stirling-hot
gas engine."	 MTZ 32 (1971), ho. 1, pp. 1-5.
3. Alm, C. B. S., et al.:	 Environmental characteristics of
Stirling l.ngines and Their Present State of Development
in Germany anal Sweden.	 Paper 28, CIMAC, Washingtorn
(1973).
4. Kuhlmami, 1 1 .: Das hennfeld pies Stirlingmotors. "The per-
formance graph of the Stirling engine." MT_ 34 (1973),
No. 5, pp. 135-139.
9
tIDiscussion	
/41
Dr. Th. Lin:
Thy •
 Stirling engine delivers lower values of CO. That is
remarkable, since the pressure in the combustion cha ►nber is lower
than in the Otto engine.
	
Lower pressure favors the formation of
k:0.	 It leads to the question whether the relative high CO-content
for the Otto engine is perhaps caused by disturbance factors and
not by the law of' combustion halance.
Ur.-Ing. 1 1 . Kuhlmann:
To the question about the CO-exhaust gas values it must he
said that they are caused less by the pressure in the combustion
system than by the air ratio durint; combustion. Since adequate
surplus air was always available during the driving tests favor-
able CO-test values resulted.
Dipl. Ing. B. Herrmann:
1. The burner system is only indirectly connected with the
engine characteristic. The exhaust gas values should therefore
be presented as functions of the combustion chamber parameters.
2. the heater head experiences high thermal stress, hot
gas (-2000'C) on the outside, hot hel itun oil 	 inside (600%) .
The service life of this component will be crucial for the cost
effectiveness. Corrosion will present a problem similar to that
for gas turbines.
lU
Dr.-1ng. 1'. Kuhlmann:
The question of .-nether the exhaust gas values measured
should not rather have been presented as function of the com-
bustion chamber parameters is justified. The presentation in
the engine performance graph was chosen to make comparison with
other engines and with legal requirements easier.
It was correctly remarked that the heater head is exposed
to very high thermal (hut also mechanical) stress. Experiments
on the Stirling engine have so far shown that the service life
of the heater head is influenced more by the creep characteris-
tic- of the material than by corrosion. These experiences relate
to operation with gas oil.
Uipl. Ing. K. Maly:
1. slow does one accelerate with the Stirling engine when
it is built into a vehicle, resp. how is mechanical work con-
t rolled?
2. What effect does this control have en the overall effi-
ciency (travel in town, for instance)?
Ur.-ing. P. KU1111Iiilnn:
The output control of the Stirling engine could not he di.s-
cussed during the lecture because of limited time. But ques-
tions resulting from the output control will he dcalt with in
the printed manuscript.
11
Dip l. "1 1 hys. G. I few ig:
What are the lowest teml ►eratures for which Stirling engines
Can he used, assuming one is satisfied with 5-10% efficiency?
I am thinking of the use of Stirling engines for technical
exploitation of solar energy.
Dr. - Ing. 1'. Kuhlmann:
The question about data for the Stirling; engine at reduced
heater temperature can he answered like this: measurements
showed a decrease in efficiency to 15 9, (resp. 7%) for reduction
of the heater temperature to 400°C (resp. 300°C). The engine
was not desis,, ned for such low heater temperatures, however,
which is why more favorable values may, he expectckl for .in engine
optimized for such operation.
Dr. F. ' oss:
[low high was the gas temperature in the comhustion chamber
during your experiments?
hr.-1ng. P. Kuhlmann:
Gas temperature was not measured, but according to calcula-
tion it lies between 1800 0 0 and 1900°C in the middle of the
combustion chamber.
12
i
r
1


The stirling engine for vehicle propulsion

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