Tests were conducted at the altitude laboratory erected at the Bureau of Standards for the National Advisory Committee for Aeronautics to determine the changes in engine performance with changes in atmospheric temperature and pressure at various levels above the earth's surface, with special reference to (a) the variables affecting the functioning of the carburetor and (b) the changes in performance resulting from variables in the carburetor itself. This report constitutes a concise statement of the difficulties to be encountered in this branch of carburetion

Tice, Percival S

English

NASA Technical Reports Server

REPORT NO. 48.
CARWRETING CONDITIONS CHARACTERISTIC OF
By PmCIVAJ S. TICK
WWJME.
.——-—
MU2RAET ENGINES.l
Tests have been conducted at the altitude laboratory erected at the Bureau of Standards ‘
for the National Advisory Committee for Aeronautics to determine the changes in engine per-
formance with changes m atmospheric temperature and pressure at various Ievels above the
earth’s surface, tith special reference to (a) the variabks aflecting the functioning of the car-
buretor and (b) the changes in performance resulting from wuiabks in the carburetor itself.
This work has resuhed in the folhxving cunclusione:
()(1) Wtw ratio ~~1 shotid be constant at all pressure Ievels, for maximum power at
all leds.
(2) Ohange in ticosity of fuel with temperature change maybe an importmt
characteristic of the carburetor.
(3) Unwarranted waste of fuel is invariably involved in the use of carburetors
corrected for barometric changes.
metering
not ftiy
(4) Heating of the mixt& causes a loss in power output accompanied by au increase in
the specific consumption of fuel (lb. gasolinef brake horsepowerlhour), at least with such
fuek as are avdable for war purpose&
CARBTJRETINGCONDITIONS 0EAR4CI’ERISTICOF AIRCRAFT ENGINES
The following rew.dta am offered as characteristic of the conditions surrounding the car-
bureting system of an aircraft augine. The whole is indicative of the ohanges experienced
following those variations in barometric pressure awxmntared in the service operation of such
engines.
Briefly, the purpose of this report is merely to summmize certain of the performance
charackriatim of an aircraft engine, to be followed later by a detailed inmWigation and report
on the carburetion requirements ‘and means for satisfying them in this service.
All of the taste hwm which the following materkd is taken were made in tie altitude labo-
ratory at the Bureau of Standards, employing a 150-horsepower Hispano-%iza, type A engine,
having eight cylinders in blocks of four, set at 90°:
Borel.ZOmm.(4.7L?inchss).
stroke=130mm. (5244inched.
Ccmprawionratio(o,
TotalvoI&e
)earsncavdume 4’”
,
A constant speed of 1,500 revolutions per minute (the speed of maximum mean eflective pres-
surw) was mkint.ained throughout the runs.
It is known that the vahw.s of the air readings used in the foIIowing plottings are somwvhat
high; and, for this reason, it is pointed out that the results including air/fuel ratios for the
mixtures shouId be employed qualitatively rather than quantitatively, though the results are
in perfect agreement among themselves.
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OAEBUREiTINQUONDJ!I’IONSaHABA~TIO OF MROMJKCWWIIWS. 691
The @@.S used are described as fo~ows:
Plot I.-Curve a, altitude, in feet shove sea Ievel, verms barometric pressure; and
curve b, temperature (mean ammal) versus aItitude in feet above sea level.
Plot 2.—Engine pumping capacity in pounds of air per hour per 100 cubic feet of piston
displacement, with open throttle, versus barometric pressure. me points on this curve are
means of a great many measurement using a carburetor capabIe of giving maximum output
over the whole range of pressures; and while the readings are lmown to be high, as noted in
the introduction, the duiracteristio is quite d&niteIy established. It will be noted that the
curve quite pronouncedly turns upward as the pressure becomes less. This follows from the
faot that the pramre drop to cause air to flow at constant veloci@ is 1ssswith lesser density,
thereby permitting the aspiration of proportionately greater volumes of air as the density
value lowers. The grap~ plot 3, showing manifold pressure drop plotted against ba~metrio
pressure, in this case carburetor inlet pressure, is characteristic of the variation in pressure
drop to cause air to flow in the intake systam w-M changes in atmospheric density.
PIot 4.—Pounds of gasoline (0.7360 sp. gr.) per hour per 100 cubic feet piston displace-
ment, with open throttle, versus barometric pressure. The fuel used in th=e tds is desmibed
by the fractionation curve of plot 5. In plot 4 is Shown the manner in which con-mntional
carburetor, designed for compensation in the ordinary sense at ground level, cause enrichment
of the mixture with lowered atmospheric prcsmre. Curve a is from bta of a device having
a manual control reset to give greatast power output at each bsrometic le-iel. Curves b, c, d,
and e are results with several carburetors, some of which embody a measure of correction for
enrichment with lowered density. Curve L is cakndated from the equation:
- -.
in which Q= the quantity of fueI dischmged at sea leveI, Q1= the quantity of fd discharged
at any other level, corresponding ta the pressure Pi. This ecpation assumes a lxmstant value
for the cmfbient in the equation:
v-u-
for the fuel metering passage. Curve g, plot 4, is that for carburetor (a), but with ground
setting of the manual control at all barometric levels. The great waste of fuel resulting horn
the use of a carburetor uncompensated for wide barometric changes is obtioue from the diagram,
particuhdy whau it is considered together with that of plot 7, wherein are given the brake
mean etTective prassures corresponding with the two rates of fuel consumption prmented in
CUI’V~ a and d of plot 4.
The observed enrichments, expressed as per cd excess fuel m the mixture, for the several
cases of plot 4, are plotted in plot 6, against barometric pressure.
The considerable variatiom mnong the curves of plots 4 and 6 are Iargely the result of
variations in the extent to which chsage in viscosity of the fuel with temperature enters the
results. No two of the severa3 carburetors used in these teds have identically proportioned
fuel metering passagea; hence it is to be expectid that considerable differences will be apparent
among the results, considering them from this tiwpoint alone.
In plot 7—maximum brake m. e. p. versus barometric prwmre-it is seen that no justMca-
tion can be found for the fual consumption rate of curve d, plot 4. Not only is fuel wasted
at all levels and power lost above 14,OOOfeet altitude, but the mixtures are so rich at the greatest
altitudes as to cause fouling of spark plugs txndcombustion chamber walls.
In plot 8 are preseded the reIationshipa between brake m. e. p. and mixture ratio, with
the latter varied through wide limits at each pressure level. The important conclusion to be
reached from this diagram is that maximum power output at each pressure level (operating at
the rated speed of the engine), is secured with the same ratio of air b fuel in the mixture. Fur-
ther, the mean cylinder premwes decrease more rapidly with a given change in the air-fuel ratio
the lower the atmospheric pr~. This points to the need for much closer regulation of the
mixture with lowered pressures as compared with the higher. (Note that the mixture ratio
for maximum brake m. e. p.% should have a somewhat lower value than indicated in this dia-
gram, because of the too high air readings previously noted.)
.—
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694 ANNUALREPORTNATIONALADVISORYCOMMITTEEFORAERONAUZWS.
The effect of hot+ater jacketing of the intake manifold in the engine used (H.ispano 150-
horsepower) is shown for two sets. of teat rum in diagram plot 9, these two tcsta being fairly
representative of the results found in other tests that have been made with this engine. The
loss following jacketing of the manifold branching immediately above the carburetor is greatest
at the greatmt atmospheric pressure, which latter is accompmied by the highast atmospheric
temperature, and becomes of relatively less importance, up to 17,000 feet altitude (note curve
c, plot 9), as the atmospheric pressures and temperatures became lower.
In thase teate a constant intake manifold water jacket temperature of 37” Gl= 96.8° I’. was
maintained. In such a case the pumping loss in the engine following heating of the mixture
will vary directly with the temperature difference between mixture and jacket and in an inverse
manner with the mixture density.
This assumesthat the whole of the heat given up by the constant temperature water jacket
appears as sensible heat in the mixture. Of coume, this is not realized, since some of the heat
taken up by the mixture is used to evaporate the fuel and becomes latent. The proportion of
the total heat received which is so used depends upon so many variables, considering different
fuels and different carbureting methods, that it is impomible to stati the two rcaulti in general
terms. In any caee, thanet rcmdt only is of importance and ita characteristic, as found in these
tests, is shown clearly by the curves of plot 9.
It is noteworthy also that, in general, loss in output resulting from manifold hot-water
jacketing is accompanied by greater fuel consumption values, regardless of carburetor design
and method of control, and with fixed adjustment carburetor designs the economy loss following
heating in this way may attain quite serious proportions.
r.
OAESURETINQ OONDI!CIONS OEARA m’EET.STIO OF AIROBAVT ENGINES. 595
Plot 8.
I
,
ACHANGE /’M/X+jE /%ATfO(A4’7+GAS) L-1--l
&VERAL ATM05PHERICPRE55URESI
/2 M /6 20
lb. Airperflour+ lb. Gosufin;8perHkwr.
.—
*


Carbureting conditions characteristics of aircraft engines

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