Evap. 75-4
Evaporative Emissions Regulations Development
In-House Test Program
Report No. 3
Effect of Fuel Composition
by
Eric Ellsworth
Notice
Technical support reports for regulatory action do not necessarily
represent the final EPA decision on regulatory issues. They are intended
to present a technical analysis of an issue and recommendations resulting
from the assumptions and constraints of that analysis. Agency policy
constraints or data received subsequent to the date of release of this
report may alter the conclusions reached. Readers are- cautioned to seek
the latest analysis from EPA before using the information contained herein.
Reprinted 7-76
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Contents
Page
1. Introduction 1
2. Summary and Conclusion 1
\
3. Technical Discussion 1
3.1 Volatility 1
3.2 Distillation 2
3.3 Weathering 3
4. Sample Calculation 3
5. Closure 4
6. Figures 5,6
7. References 7
8. Appendix A 8
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1. Introduction; This report will deal with the interrelationship of
fuel characteristics and evaporative losses. The characteristics which
we will try to correlate to evaporative losses are Reid Vapor Pressure
(RVP), the shape of a fuel's distillation curve and weathering.
2. Summary and Conclusion; Evaluation of the effect on evaporative
emission of RVP, distillation temperatures,and weathering has indicated
that some general trends are present. As the RVP decreases, the evap-
orative losses decrease. The slope of a fuel's distillation curve will
give an indication of how much fuel vapor will be generated during the
hot soak mode of an evaporative emission test. The effect of weathering
is largely dependent on the temperatures which a fuel encounters.
Temperatures above the initial boiling point of a fuel weathers that
fuel more severely because the mechanism by which the fuel weathers
changes from diffusion to distillation.
Correlations and engineering assumptions can be drawn, with reasonable
accuracy, from fuel composition indicators such as RVP and distillation
curves. Although weathering does not indicate a particular fuel char-
acteristic it is an indication of the change in fuel characteristics
due to evaporation.
3. Technical Discussion:
3.1 Volatility; Reid Vapor Pressure (RVP) is the primary indicator
of fuel volatility. The RVP for a fuel is arrived at by placing a
measured amount of fuel in a special bomb (sealed container),
heating the bomb to 100°F, and recording the resultant pressure. A
mixture of the base fuel blend and the pressurizing agents results
in a final fuel RVP. The base fuel's RVP is fixed by the type of
crude oil used and the way in which it was refined. As a rule, the
base fuel blend has too low a RVP to provide acceptable vehicle
operation. To increase the fuel's RVP, a hydrocarbon with a high
RVP, the pressurizing agent, is added to the base fuel. Materials
such as butane, propane and/or pentane could be used as pressurizing
agents (1).
Studies indicate that there is a relationship between RVP and
total vehicle evaporative emissions (Fig. 3-la). As the RVP of a
fuel increases, the evaporative losses from a vehicle with that fuel
increase (Fig. 3-lb). An analysis of the loss composition indicates
more than 50% of the components are butane and pentane (1).
Also, there exists a relationship between RVP and engine perform-
ance, specifically the cold starting of an engine. As the volatility
of a fuel decreases (lower RVP) the cranking time required to start
the engine increases. This condition is most pronounced at low
ambient temperatures, 0°F (2).
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3.2 Distillation Temperatures; A typical distillation curve indicates
the temperature at which various percentages of a fuel are evaporated
(Fig. 3-2a). There are several methods by which a distillation curve
may be arrived at such as the ASTM method or the single plate distil-
lation method. The significant difference between these two methods
is that the ASTM method allows for refluxing because equilibrium between
the fuel liquid and vapor does not exist (1) (Fig. 3-2b). The shape
of the distillation curve will indicate the amount of fuel evaporated
from a fuel bowl during a hot soak, specifically the shape of the
curve up to and Including the peak carburetor bowl temperature (2).
The high temperature, approximately 150°F, encountered by the
carburetor during a hot soak dictates that the mechanism for generating
fuel vapor will be distillation. A model utilizing the distillation
curve can predict carburetor hot soak losses (Fig. 3-2c).
DT - D
2 1
w = em VF *
T - i _ n (3)
•1
TI = temperature of fuel in the carburetor at the beginning
of the hot soak period.
W = weight of fuel evaporated
= temperature of fuel in the
of the hot soak period.
T~'= Peak temperature of fuel during the hot soak period.
TI = Density of fuel at T .
V = Volume of fuel in the carburetor bowl at the start of
the hot soak period.
D = Weight fraction of fuel evaporated in proceeding from
the initial boiling point to the temperature T during a
single plate equilibrium distillation.
F = Experimentally determined diffusion loss factor, dependent
primarily on total time for soak.
In this model, as the slope of the distillation curve increases be-
tween TI and T2 the weight fraction of fuel evaporated increases.
With the increase of the DT~ terms the difference between
increases, thus increasing "the weight of fuel evaporated. It should
be noted that the D-p, and 1-D-j, terms account for fuel weathering* (7)
which occurs prior to the hot soak and is a constituent of running
losses (Fig. 3-2c).
There is an interrelationship between RVP.and fuel distillation.
By plotting RVP and percent evaporated at a specific temperature a
map showing equal weight of fuel evaporated can be drawn. This in-
dicates that a trade off can be made between RVP and the shape of
the fuel distillation curve which in turn could allow changes in fuel
composition without changing overall vehicle losses (Fig. 3-2e). This
could allow the shifting of diurnal and hot soak losses depending on
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which mode of loss would be more controllable.
The distillation process is also responsible for the different
hydrogen-carbon ratios used when calculating the mass of evaporative
emissions. An inverse relationship exists between the hydrogen-carbon
ratio and the nominal molecular weight of the condensed evaporative
emissions. Both the fuel tank and carburetor undergo temperature rises
with the carburetor experiencing the higher peak temperatures. Because
of these higher peak temperatures more of the heavier hydrocarbons,
hydrocarbons with higher molecular weights, are evaporated thus the
hydrogen-carbon ratio ends up being 2.20 for the carburetor losses.
The hydrogen-carbon ratio for the fuel tank losses is 2.33, which is
a result of the tank losses being made up of materials with low
molecular weights, such as butane, pentane and hexane. Indolene
test fuel has a hydrogen-carbon ratio of 1.95. The above mentioned
hydrogen-carbon ratios will remain unchanged for evaporative losses
as long as the Indolene test fuel composition specifications remain
unchanged. (see Appendix A).
The interrelation of the shape of distillation curve and engine
performance is quite complex (Fig. 3-2d). The portion of the curve
which affects evaporative loss is the portion below the 50% evaporated
point. This portion of fuel also affects cold and hot starting, vapor
lock, engine warm-up, carburetor icing and hot stalling (6).
3.3 Weathering: The increase in specific gravity of a fuel, in an
open system, over a period of time is weathering. Evaporation is the
mechanism by which a fuel weathers therefore the relationship between
fuel characteristics and evaporative losses also holds true for
weathering. A high RVP fuel will weather at a higher rate than a low
RVP fuel. Also, there is a direct relationship between the severity
of weathering and time and/or temperature. As a result of the
weathering a fuel will no longer have the characteristics of its
initial composition, the fuel's RVP will be lower, its initial boiling
point will be higher, and the front end of the distillation curve will
be flatter.
Weathering will occur during soaks by diffusion and if the
temperature is high enough by distillation. If repeated tests are
run on a vehicle, without changing fuel, each successive test will
show a lower loss due to the decreasing quantity of light hydrocarbons
in that test fuel (Fig. 3-3a). It should be pointed out that diffusion
is the slower of the two processes and weathering due to this mechanism
is insignificant provided the vehicle does not undergo an extended soak.
4. Sample Calculation; For the purpose of illustrating the loss model the
following assumptions were used:
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Fuel - Mobil 1 (3)
GT = .765 g/cc
DT = .5%
1
DT = 20%
2
F = .75g loss in one hour
T2 - D"
W =
V = lOOcc
T = 95°F
T2 = 150°F
VF
= (.765) (100) (.75)
11.24 g.
r.2o -
I 1 -
.005]
.005J
5. Closure; The primary purpose of this report is to provide an insight
into the effects of fuel on evaporative emissions. Evaporative emission
from vehicles can be reduced by a change in fuel composition which would
supplement existing vehicle evaporative emission control systems. It
should be pointed out that a major modification in fuel composition to
reduce evaporative emissions could have an effect on engine operation
which may require engine nodifications, thus limiting the use of this
fuel to new vehicles with modified engines. In addition, service stations
would have to carry the modified fuel along with the standard fuel.
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Figure 3-la
Daily Losses vs. Temperature
Figure 3-lb
RVP vs. Tank Losses
CO
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Figure 3-2c
Measured vs. Calculated Losses
Figure 3-2d
Distillation and Engine Performance
CO
3
CO
CO
•s
3
u
20
15
10
cu
n
u
*
M
a)
cu
H
400
300
200
100
Carburetor Icing
Hot Stalling
Acceleration
and Power
15
20
20
80
100
Measured losses, grains
Reference (3)
Percent Recovered (wt)
Reference (2)
Figure 3-2e
RVP vs. Distillation
°0
vO
0)
cu
CO
•rl
13
c
CU
o
Figure 3-3a
7300 Specfic Gravity vs. Test Cyclj
7800.
7260
O
CU
3
•/
4
RVP
Reference (3)
Test Cycle
Reference (8)
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Effect of Fuel Composition
References:
1. Wade, D.T., "Factors Influencing Vehicle Evaporative Emissions", SAE
Transaction, 670126, 1967, p. 743.
2. Paterson, D.J. and Henein, N.H., "Emissions from Combustion Engines
and Their Control", Ann Arbor Science Publishers, Inc., Ann Arbor, MI
1972.
3. Biller, William F.; Manoff, Michael; Sacholev, Jyatin; Zegal, William C.,
"Mathematical Expressions Relating Evaporative Emissions from Motor
Vehicles without Evaporative Loss-Control Devices to Gasoline Vol-
atility", SAE Transactions, 720700, 1972.
4. Koehl, W.J. Jr., "Mathematical Models for Prediction of Fuel Tank
and Carburetor Evaporative Emissions", SAE Preprint 690506, May, 1969.
5. Melson, E.E., "HC Control for Los Angeles by Reducing Gasoline Vol-
atility", SAE Transactions, 690087, 1969, p. 775.
6. Jackson, M.W.; and Everett, R.L., "Effect of Fuel Composition on Amount
and Reactivity of Evaporative Emissions."'; SAE Transactions, 690088, 1969,
p. 802.
7. Muller, H.L.; Kay, R.E.; and Wagner T.O., "Determining the Amount and
Composition of Evaporative Losses from Automotive Fuel Systems"* SAE
Transactions, Vol. 75, 1967.
8. Ecclestion, B.H.; Noble, B.F.; Hurn, R.W.,"Influence of Volatile Fuel
Components on Vehicle Emissions!', United States Department of the Interior
Bureau of Mines, RI, 7291, 1970.
9. Ecclestion, B.H.; Hurn, R.W., "Effect of Fuel Front - End and Midrange
Volatility on Automobile Emissions", United States Department of the
Interior Bureau of Mines, RI, 7707, 1972.
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Appendix A
Amoco Oil Company
2500 New York Avenue
Whiting. Indiana 46394
Research and Development Department
219-659-2700
July 16, 1975
Mr. Gary Wolf son
EPA Mobile Sources
2565 Plymouth Road
Ann Arbor, Michigan 48105
Dear Mr. Wolf son:
During June, we discussed the hydrogen-carbon (H/C) ratio for hydrocarbon
vapors from the Indolene fuel now used for evaporative emission certifica-
tion tests. SAE Recommended Practice J 171a, "Measurement of Fuel Evap-
orative Emissions from Gasoline Powered Cars and Light Trucks Using the
Enclosure Technique," shows a H/C ratio of 2.33 for fuel vapors from fuel
tanks and 2.20 for vapors from carburetor bowls. You wanted to determine
whether the H/C ratio of vapors from Indolene certification gasoline had
changed since the original estimates were developed.
I do not have any recent measurements or calculations of H/C ratios of
Indolene vapors. Nevertheless, we do not believe there has been any sig-
nificant change in the overall average H/C ratio of these vapors, but
individual batches will vary slightly. Indolene certification fuels are
still manufactured against the same strict specifications, and surveillance
of recent and past gasoline liquid samples does not show any trend in H/C
ratios .
If you need any additional information, please let me know.
Very truly yours ,
D. K. LAWRENCE
DKL/vm
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