Foster, Hampton H

Johash, Edmund R

English

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RM E53I02 
RESEARCH MEMORANDUM 
EVALUATION OF ETHYL ETHER AS AN IGNITION AID 
FOR TURBOJET ENGINE FUELS 
By Edmund R. Jonash and Hampton H. Foster 
Lewis Flight Propulsion Laboratory 
Cleveland, Ohio 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
WASHI NGTON 
October 23J 1953 
Declassified November 29J 1956 
https://ntrs.nasa.gov/search.jsp?R=19930088054 2020-06-17T11:57:30+00:00Z
lY 
, 
NACA RM E53I02 
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
RESEARCH MEMORANDUM 
EVALUATION OF ETHYL ETHER AS AN IGNITION AID FOR TURBOJET ENGINE FUELS 
By Edmund ~. Jonash and Hampton H. Foster 
SUMMARY 
An investigation was conducted to determine the effectiveness of 
ethyl ether as an ignition aid for low-volatility turbojet-engine fuels. 
The minimum spark energies re~uired to ignite a single J33 combustor over 
a range of altitude inlet-air pressures and flow rates were obtained with 
a low-volatility reference fuel and with reference fuel blends contain-
ing (1) 10 percent ethyl ether, and (2) 10 percent normal pentane. Re-
sults indicated that the addition of ethyl ether improved the ignition 
characteristics of the low-volatility (l-lb Reid vapor pressure) fuel sig-
nificantly; however, similar improvements in ignition were obtained with 
the addition of normal pentane in the same concentration. The improve-
ment in ignition characteristics was, therefore, attributed solely to the 
increase in the front-end volatility characteristics of the reference 
fuel. The observed trend substantiates the results of previous investi-
gations of the effect of fuel volatility on ignition characteristics. 
INTRODUCTION 
The ignition of turbojet engines at adverse conditions of high 
altitude and low temperature is of major concern to the operators of 
turbojet-powered aircraft. The variables that may affect ignition cha~­
acteristics at any given operating condition include combustor deSign, 
ignition plug design, ignition energy, and fuel characteristics. The 
extent to which combustor design and ignition energy may be used to 
alleviate ignition difficulties is limited by the design re~uirements of 
other performance characteristics, and by weight and plug durability 
conSiderations, respectively. An increase in the front-end volatility 
characteristics of the fuel, which has been found to improve ignition 
characteristics, affects, detrimentally, altitude vapor loss and fuel-
pumping characteristics. Therefore, other means of improving ignition 
characteristics of the turbojet engine are under consideration. Methods 
that may alleviate the problem in current operating aircraft without 
re~uiring redesign or modification of engine assemblies are of particu-
lar interest. 
The data of reference 1 indicate that certain accelerator-type 
additives may improve the ignition characteristics of turbojet-engine 
2 NACA RM E53I02 
fuels. More recently ethyl ether has been suggested as a fuel additive 
for a similar purpose. The objective of the investigation reported 
herein was to evaluate the effect of ethyl ether, added in a concentra-
tion of 10 percent to a low-volatility turbojet engine fuel, on the 
ignition characteristics of a ~ingle J33 turbojet combustor. The addi-
tion of ethyl ether reduced the front-end distillation temperature, 
which was found in reference 2 to improve ignition; therefore, a similar 
blend with the ethyl ether replaced by normal pentane was tested in 
order tc ascertain the separate effect of additive composition. Igni-
tion characteristics of the two blends and of the base fuel were eval-
uated in termE of the minimum spark energy required to ignite the com-
bustor over a range of inlet total pressures and at two air-flow rates. 
Combustor-inlet fuel and air temperature was maintained constant at 
100 F. 
Fuels 
The reference fuel used for this investigation was a 1 pound Reid 
vapor pressure blend (NACA fuel 50-197) obtained by cutting volatile 
components from a JP-3 fuel stock. Physical properties of this fuel are 
presented in table I; the A.S.T.M. distillation temperature curve is 
presented in figure 1. The two test blends evaluated contained 10 per-
cent (by volume) ethyl ether and 10 percent (by volume) normal pentane, 
respectively. Both additives were at least 98 percent pure. The 
A.S.T.M. distillation curves for these test blends are included in 
figure 1. 
Apparatus and Test Procedure 
The ignition characteristics of the fuel blends were evaluated in 
terms of the minimum spark energy required to ignite the combustor over 
a range of combustor inlet-air pressures and air-flow rates. A single 
tube J33 combustor (fig. 2), connected directly to the laboratory air 
supply and exhaust system, was used. The combustor was equipped with a 
small (10.5 gal/hr 800 spray-cone angJe) fuel nozzle in order to main-
tain. fuel atomization as nearly constant as possible. A production type 
F-99 ignition plug was used in conjunction with a variable-energy, 
constant-spark-rate (8 sparks/sec), capacitance-type ignition system 
(fig. 3). The spark-ignition system used in this investigation was, 
generally, similar to that used in reference 2. Spark energy was sup-
plied at a voltage varying from 350 to 1400 volts, depending on the 
required energy. A high-voltage "trigger" circuit ionized the electrode 
gap for passage of the low-voltage, high-energy spark. 
NACA RM ES3I02 3 
The minimum spark energy required for ign~tion was determined at 
pressures from atmospheric· to the minimum at which ignition could be 
obtained. The investigation included two air-flow rates, 1 . 87 and 3.75 
pounds per second per square foot (based on a combustor maximum cr09s-
sectional area of 0.267 sq ft) , which were representative of actual 
engine starting conditions. The inlet-air temperature was held constant 
at 100 F. The fuel was supplied to the combustor through a long copper 
coil immersed in the inlet-air ducting (fig. 2); inlet fuel and air 
temperatures were therefore similar. 
At prescribed combustor inlet air conditions, the ignition system 
was energized and adjusted to a desired spark-energy level. Fuel was 
admitted to the combustor by opening a fuel throttle slowly until igni-
tion occurred. Ignition was considered satisfactory if the flame filled 
the combustor and combustion continued after the spark was de-energized. 
By successively increasing or decreasing the selected value of spark 
energy, a minimum energy that would give repeated satisfactory ignition 
was determined. 
A more complete description of the apparatus and procedure used in 
this investigation may be found in reference 2. 
RESULTS 
The minimum-ignition-energy requirements of the reference fuel and 
of the two reference fuel blends are presented in figure 4 as functions 
of combustor-inlet total pressure. Data are presented for the two air-
flow rates representative of actual altitude starting conditions for 
turbojet engines. It is noted that as combustor-inlet pressure is 
reduced, or as the weight flow of air is increased, the required spark 
energy is increased. The data indicate that further increases in spark 
energy above about 3 joules per spark will not affect appreciably the 
low-pressure ignition limits of this combustor. 
The ignition-energy requirements for the reference fuel blends 
containing either ethyl ether or normal pentane were identical, indicat-
ing no effect of the additive composition. The addition of either the 
paraffin or the ether did, however, reduce the ignition energy reqUire-
ments of the reference fuel. Data of reference 2 indicate that minimum 
spark ignition energy is related to the A.S.T.M. lS-percent-evaporation 
temperature . It is noted in figure 1 that the two test blends had 
approximately equal lS-percent A.S.T.M. evaporation temperatures, which 
were below that of the reference fuel. The indications from reference 2 
that Reid vapor pressures of fuels do not reflect their ignition char-
acteristics is also substantiated by the data presented herein, since 
the two test blends having identical ignition characteristics had sig-
nificantly different Reid vapor pressures, as noted in figure 1. 
J 
NACA RM E53I02 
Figure 5 presents the relation between spark energy and the 
A.S.T.M. 15-percent-evaporation temperature developed in reference 2. 
The data of reference 2 are shown in figure 5 in addition to the data 
obtained in the present investigation. The curves represent different 
levels of severity of operating conditions, obtained from a correlation 
between spark energy and V~, where V is the combustor reference 
velocity, feet per second (based on combustor-inlet air denSity and 
combustor maximum cross-sectional area) and P is the combustor-inlet 
total pressure, inches of mercury absolute. It is noted that the slopes 
of the curves representing the present data agree with those from ref-
erence 2. The reduction in the 15-percent A.S.T.M. evaporation temper-
ature resulting from the addition of 10 percent ethyl ether, or 10 percent 
normal pentane, decreased the ignition energy required by approximately 
50 percent at the most severe operating conditions (~~ = 21). When 
the data obtained in the present investigation are compared with similar 
data from reference 2, it is observed that significantly lower ignition 
energies were required in the present investigation. The decrease in 
required energy is attributed to the elimination of a 1/16-inch annular 
opening between the spark plug and the combustor liner. Reduced air 
velocities in the vicinity of the spark gap favored ignition. This 
result illustrates the fact that minor design features can have important 
effects on ignition characteristics in a turbojet combustor. 
CONCLUDING REMARKS 
An investigation was conducted to evaluate the effectiveness of 
ethyl ether as an ignition aid for low-volatility turbojet-engine fuels. 
The limited data obtained in one combustor indicated that the addition 
of 10 percent (by volume) of ethyl ether to a l-pound Reid vapor pressure 
fuel decreased the ignition-energy requirements of the combustor sig-
nificantly. However, similar results were obtained with the addition of 
normal pentane in the same concentration; thus, the beneficial effect of 
ethyl ether on ignition must be attributed to its high volatility rather 
than to its composition. The results obtained substantiate previous 
data relating spark ignition energy requirements t o the A.S.T.M. 15 
percent-evaporation temperature. 
Lewis Flight Propulsion Laboratory 
National Advisory Committee for Aeronautics 
Cleveland, OhiO, August 28, 1953 
NACA RM E53I02 5 
REFERENCES 
1. Marshall, E. F.: Jet Propulsion Fuels. Prog. Rep. No. 29, Jan. 1 
to Feb. 28, 1950, Res. and Dev. Dept., Sun Oil Co., Mar. 15, 1950. 
(USAF Contract No. W33-038-ac-9086.) 
2. Foster, Hampton H., and Straight, David M.: Effect of Fuel Volatility 
Characteristics on Ignition-Energy Requirements in a Turbojet Com-
bustor. NACA RM E52J21, 1953. 
6 NACA RM E53I02 
TABLE I. - PHYSICAL PROPERTIES OF LOW-VOLATILITY REFERENCE FUEL 
Fuel properties NACA 50 - 197 
A. S . T.M. distillation 
D86 - 46) OF 
Initial boi ling poi nt 201 
Percentage evaporated 
5 242 
10 271 
20 297 
30 315 
40 333 
50 349 
60 363 
70 381 
80 404 
90 443 
Final boiling point 510 
Speci fic gravity 0 . 781 
Viscos i ty) cent i stokes 1.05 
at 1000 F 
Reid vapor pressure ) 1.0 
lb/ sq in . 
Aromatics ) percent by 10.7 
volumea 
aDeter mined by s i l i ca gel method. 
• 
• 
• 
NACA RM E53I02 
20 40 60 80 100 
Percent evaporated 
Figure 1. - Variation of distillation temperature with percentage 
evaporated for three fuel blends . 
7 
- - - - --
COOling coil 
Thermoc oUple 
(fuel temperature) 
Simplex fuel 
Spray nozzle 
Combustion-Chamber w
all 
Combustion-Chamber 
inner liner 
~ 
Figure 2. - Dia~tic cross section of Single tubular combusto
r. 
• 
• 
~ 
r;: 
~ 
/:>:j 
CJl 
CJ.I 
H 
o 
C\.) 
OJ 
• 
NACA RM E53I02 
Tc(fc{Tc"1 ¥ l 
l OOOVo (max.) 
high-energy supply 
(varia ble) 
Frequency 
control gap 
V
SpeCial high-
voltage low-
energy pulse 
transformer 
~ ....... ~ ........ ~"'\ 
T 
~O 
Calibrated d- c. oscilloscope to 
indicate maximum and minimum 
condenser voltage during firing 
Transformer T injects very high voltage, very 
low energy pulses with a variable frequency 
determined by setting of T2' Each high voltage 
pulse superimposed on the low voltage of Cl 
ionizes the combustor spark plug gap and permits 
the relatively high energy of Cl to be dis-
charged through the combustor spark plug. Cl con-
sists of a bank of condensers anyone of which can 
be selected to provide a specific known energy per 
spark . 
Combustor 
l spark plug 
Figure 3 . - Simplified circuit diagram of spark ignition system. 
9 
10 
CD 
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rl 
5 
. ., r 
! Ii 
, . , 
NACA RM E53I02 
, != I, -
-± 
f--t:.w: c- ' 
::+ftt =-, ~ ~ 
-,--
-t-+-t-+- -l-I-n-rEtt't 
" 'tt±: 
-
. 81~- ~-+----~--4-~r ~: -o -_t-~~CE~t~.-1~~~r-~y o 50 -197 
50 -197 + 10 
50 -197 + 10 
~ 
percent ethyl ether ~ 
percent ~-pentane ~ 
o 1 -: ++'- H=- c...E-
.f- _\+- n :: ... _==' f=-; ,- ,.:. .t-~..!.. 
:- ,1; .6~ 
:~ 
1/,\ ' 
~ -+ -j-+-.. , :r -!- =r: ., -1:.-'Ft-i=f '';- . , '-' cr- . 
. 002 ~~: +-:to No ignition .i.T"-t -::Ei::=9::t! '¢' - cT~' --E"c . I 
!EI'~=Fi!l:~~i~I-III1!1H+~!'J!I=~~~'~+~"~;~ ~!lIIII!E~-~~~--!!~'~'~! :j~~' ~- ~*~' ~g' rul~~'J H-I ,ii' H:P:: -r + : ·.t;'~~r1I-t-tt. Jttt IT ±tttFl=1=h=l+H+~ .001 r-r-
6 10 14 18 22 26 30 34 
Combustor - inlet total pressure, in . Hg abs 
Figure 4. - Effect of combustor - inlet total pressure on· minimum spark energy required for 
ignition of three fuel blends. Combustor - inlet air and fuel temperature, 100 F; 
J 33 combust or. 
f 
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NACA RM E53I02 
In 
Q) 
rl 
5 
..--;, 
8 
6 0 
o 
4 A 
Fuel 
50-197 
50-197 + 10 percent 
50-197 + 10 percent 
A.S.T.M. 15-percent evaporated fuel temperature,~ 
Figure 5. - Minimum spark energy required for ignition of three 
fuel blends as function of 15-percent evaporated fuel temper-
ature at several values of vi ¢P. Combustor-inlet air and 
fuel temperature, 100 F. 
NACA-Langley 
11 


Evaluation of Ethyl Ether as an Ignition Aid for Turbojet Engine Fuels

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