PB 270 785 PC A02/MF A01
EPA-AA-FE 76-01
Technical Support Report for Regulatory Action
Methodology for Calculation of Diesel Fuel
to Gasoline Fuel Economy Equivalence Factors
January 1976
Notice
Technical support reports for regulatory action do not necessarily
represent the final EPA decision on regulatory issues. They are in-
tened to present a technical analysis of an issue and recommendations
resulting from the assumptions and constraints of that analysis. Agency
policy considerations or data received subsequent to the date of release
of this report may alter the recommendations reached. Readers are
cautioned to seek the latest analysis from EPA before using the in-
formation contained herein.
Emission Control Technology Division ,. -.
Office of Mobile Source Air Pollution Control
Office of Air and Waste Management
U.S. Environmental Protection Agency
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Summary
This study presents a methodology for calculating energy equiva-
lence conversion factors for fuel economy of diesel fueled passenger
vehicles relative to gasoline-fueled passenger vehicles. Both dif-
ferences in volumetric heating values for diesel and gasoline fuels as .
well as process energy savings attributable to refinery production
shifts to diesel fuels are treated in the methodology.
Three illustrative cases were computed utilizing the developed
methodology; a case representing the maximum process energy savings, a
case where the ratio of diesel to gasoline fuel production (for auto-
motive usage) becomes 20%/80%, and a case where diesel fuel consumption
increases by only 1% relative to gasoline consumption.
The three cases produce numerical differences of varying signi-
ficance. However, since diesel fuel consumption by automobiles is
miniscule at present and projections of diesel passenger car sales would
have to be speculative at best it is recommended that for the near-term
only differences in volumetric heating values for the two fuels be
considered for computing the conversion factor.
Approved - Division Dire<
or
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Statement of Problem
The objective of this study is to compute a fuel economy conversion
factor for the diesel-fueled vehicle to place it on an energy equivalent
basis with the gasoline-fueled vehicle. By energy equivalence it is
meant that adjustments to the fuel economy of the diesel fueled vehicle
should be made to account for net differences in heat content per unit
volume between gasoline and diesel fuels and process energy savings (or
penalties) resultant from refinery production shifts to diesel fuel. In
effect this approach results in a conversion factor that indirectly
adjusts fuel economy on a crude oil input basis although the computa-
tions are normalized to a gasoline volume unit, (i.e., miles per gallon
gasoline equivalent) and the savings are allocated completely to diesel
fuel.
Sources of Data
The conversion of diesel vehicle fuel economy to a gasoline equiva-
lent basis to account for the differences in the net heat content of
diesel and gasoline fuels is relatively straight-forward and requires
knowledge only of specific gravity and heat of combustion for the fuels.
These values were obtained from SAE paper 740522 and SAE Standard J1082.
The adjustment for differences in process energy consumption is not
straight-forward as there exists a wide divergence of opinion in industry
as to the magnitude of this difference; however, there is general agree-
ment that diesel fuel production would result in savings of energy at
the refinery at least up to the point where diesel fuel becomes 50% of
the automotive fuel production.
As part of its diesel vehicle evaluation program EPA contracted
with the Exxon Research and Engineering Company (Contract No. 68-01-
2112) to "study the effects of changing the proportions of automotive
distillate and gasoline produced by petroleum refining". This study was
completed in July 1974 and was selected as the principal source of data
for this study.
The main conclusions of the Exxon Study were:
(1) Maximum process energy savings were obtained when the amount
of automotive distillate (diesel fuel) produced was approximately half
the total automotive fuel output (on a BTU basis).*
(2) Beyond a 55/45 ratio of automotive distillate/gasoline, process
energy consumption, automotive fuel cost, and investment all increased.
(3) At about a 70/30 ratio, energy consumption and costs approxi-
mated the base case values (the base case assumed a 10/90 ratio).
*Exxon used the term "automotive" to include trucks, cars, buses. This
definition will also be utilized in this report. Exxon also used the
term "distillate" to mean diesel fuel. Likewise, this definition will
be used in this report.
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3
(4) Beyond the 70/30 automotive distillate/gasoline ratio, energy
consumption and costs increased sharply. That, is, a penalty in energy
consumption and cost was predicted for further increase in distillate
usage.
(5) The energy consumption was influenced by heavy fuel oil
production with slightly higher consumption of energy at the.refinery
for the case of low volumes of heavy oil production (this is the current
case as most heavy oils are imported the modern US refinery maximizes
refined product yields for economic reasons). However, for purpose of
this study the relatively small differences in energy consumption caused
by differences in heavy oil production is ignored and the maximum savings
in each case is used.
(6) The maximum savings in energy consumption (55/45 ratio of
distillate to gasoline) were about 2% relative to the base case. This
savings is equivalent to a 2% savings in crude oil. When related to the
total production of automotive fuel the percentage would be almost
doubled (because the total automotive fuel output was 60% to 51% of the
total refinery product output with the difference attributed to the
range in heavy oil production depending upon whether it is imported or
produced domestically). It is reasonable to attribute the energy savings
entirely to automotive fuel since the yields of all other products are
likely to remain constant.* (This assumption may be subject to argument
since the fuel economy standards will force reduced consumption of
gasoline and a change in the ratio of gasoline to diesel fuel production
of itself.)
(7) The actual process energy savings versus proportion of distillate
to total automotive fuel production is shown plotted in figure (1).
Computational Results
The normalization of diesel vehicle fuel economy to the gasoline
fueled vehicle fuel economy to adjust for the differences in the heating
values of the fuels is straight-forward and is described in appendix I.
On a btu-content-basis the conversion factors are:
Diesel fuel # 1
Mpg (Indolene HO III equivalent) = 0.905 x Mpg Diesel
Diesel fuel # 2
Mpg (Indolene HO III equivalent) = 0.873 x Mpg Diesel
*This was the conclusion of Exxon. In this study an arguement will be
made to attribute all savings to the diesel fuel account. However,
other philosophies of allocation of savings are possible and will be
discussed also.
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FIGURE 1
10
% SAVINGS RELATIVE TO BASE CASE
Low fuel oil yield
High fuel oil yield
Automotive Distillate as
% of Total Automotive Fuel*
19
28
37
46
55
64
73
Base 0.3 1.2 1.7 1.9 1.8 1.4 -0.6
Base 0.4 0.9 1.3 1.8 2.0 1.7 0.6
'+2
+1
.0.
--: W '
w
i <
.z u
3 w
w i
.
o ;
* H .'
-* H .
13 '
M
-« ei , 10
HIGH FUEL OIL
YIELD
LOW FUEL OIL
YIELD
I
_L
I
20 30
AUTOMOTIVE DISTILLATE AS
40.,._ _. 50 60
OF TOTAL AUTOMOTIVE FUEL (BTU BASIS)
70
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The conversion factors above consider only equivalence of the two
primary fuels on a product-out basis.* Since process energy consumption
at the refinery is influenced by the product yield (that is, the mix of
products refined), adjustments to the above conversion factors to
credit (or debit) the diesel vehicle fuel economy for process energy
savings (or penalties) should also be made. The savings in real energy
to the nation would vary directly with the actual ratio of diesel to
gasoline fuel production and indirectly with the sales or production mix
of diesel and gasoline fueled vehicles. Therefore, several options for
factoring energy process savings (or penalties) into the conversion
factor exist.
(1) iAdjust the conversion factor for process energy savings on an
annual basis to reflect actual shifts in diesel/gasoline consump-
tion caused by shifts in diesel/gasoline vehicle sales using calendar
year 1975 as the base year to calculate savings.
(2) Predict the ultimate or equilibrium saturation of the in-use
automotive fleet market by the diesel fueled vehicle and calculate
the process savings from the resultant saturation ratio of diesel
to gasoline fuels production using 1975 as the base year to calculate
the savings. For example, if diesel sales are projected to saturate
at 25% of annual automotive sales the equilibrium ratio of diesel/
gasoline fuel consumption can be derived and energy savings calculated.
(3) Assume that the ratio of diesel fuel to gasoline production
that represents the maximum process energy savings is a desirable
national goal and calculate the conversion factor on basis of the
maximum process energy savings relative to the 1975 base year. In ..
this case, the Exxon study predicts that the maximum savings is 2%
with an optimum ratio of 55:45 diesel/gasoline production. ...-'.
This study is intended to treat only the technical issues but the
selection of the appropriate option from the above could be dictated by
the legal interpretation of the requirement of the Energy Policy and
Conservation Act - that is, should the conversion factor be based on
real savings from the actual sales of diesel vehicles or should it be
based upon potentially realizeable savings in the long term. In the
former case option 1 would be appropriate whereas in the latter case
options 2 and 3 might be appropriate.
Another important consideration in the treatment of the energy
savings is the issue of how to allocate the process energy savings among
the products of the refinery.
One obvious way is to allocate the savings among all of the products.
If S is the % savings in process energy the savings factor basically
becomes
*If not adjusted for the differences in heat content per unit volume
diesel mpg values will be inflated relative to gasoline. This is because
more gallons of gasoline can be produced from the same amount of crude
oil (neglecting process energy requirements for the moment); thus, the
need to adjust on a common Btu-basis.
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1 - S/100
However, it is reasonable to argue that since the automotive fuels
sector creates all of the savings it should receive all of the savings
as a credit exclusively to the automotive fuels account. The factor
would then be adjusted by the fraction f , of refinery product represented
by the automotive-fuels account. The savings factor would be
1 " S/100f
x
The manufacturers of diesel-fueled vehicles might argue further refine-
ments. If .the diesel fueled vehicle generates the process energy savings
then the entire savings should be allocated to the diesel fuel account.
If f is the fraction of diesel fuel production the savings becomes
Finally, the passenger diesel car manufacturer might argue that it
is the diesel car that generates the process savings so all of the
savings should be allocated to the passenger car diesel fuel account.
Again if F._ is the fraction of passenger car automotive fuel assumption
the savings becomes
1 - S -
100 FAD
The numerical differences between the methods of allocation can be
illustrated by considering the case from the Exxon study where the
maximum process energy savings is achieved. In this case, 5=2%, f -
0.50, fD = 0.50 X 0.55, and f^ = 0.50 X 0.55 X .4S/.55 since 2% maximum
process savings is achieved, automotive fuels accounts for 50% of total
refinery product, diesel fuel for 55% of the automotive fuel, and diesel
fuel for trucks accounts for 10% of the automotive fuel. Thus, the
factors become
Case Factor
1. Allocation amongst all products 1.02
2. Allocation to automotive fuels 1.04
3. Allocation to all diesel fuels . 1.08
4. Allocation to only passenger car diesel fuels * 1.10
In this study the method allocating the process savings to the
total diesel fuel account (truck and auto) was selected primarily
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because the actual savings accrues from both truck and passenger car
diesel usage.* .Further, diesel trucks also contribute to process energy
savings as they displace gasoline truck counterparts. Also it might be
difficult in the future to accurately separate diesel fuel sales between
trucks and cars as they would likely be fueled by the same fuel dispensers.
Currently, the bulk of truck fuel is dispensed by separate fuel pumps
which provides a check against statistical methods of predicting truck
fuel usage.
Unfortunately, from the standpoint of this study projections of
future diesel vehicle sales are non-existent or at least they were
unknown to the study, therefore meaningful computations for options one
and two were not possible. Therefore, illustrative computations were
performed for three cases, a scenario for option 3, a scenario where it
is arbitrarily assumed that in the mid-term (5-10 years) automotive
diesel fuel consumption would reach 20% of the total automotive fuel
consumption and a near-term (0-5 years) where automotive diesel fuel
consumption gains 1% at the expense of gasoline.
The methodology and computations are described in appendix II. For.
the cases the calculations produced the following conversion factors:
Case I - Maximum process energy savings
Mpg (Indolene HO III equivalent) = 0.976 x Mph Diesel #1
Mpg (Indolene HO III equivalent) = 0.942 x Mph Diesel #2
Case II - Automotive Diesel Equal to 20% of total
automotive fuel
Mpg (Indolene HO III equivalent) = 0.943 x Mpg Diesel #1
Mpg (Indolene HO III equivalent) = 0.909 x Mpg Diesel #2
Case III - Automotive Diesel Increases
Market Share by 1%
Mpg (Indolene HO III equivalent) = 0.912 x Mpg (Diesel #1)
Hpg (Indolene HO III equivalent) = 0.880 x Mpg (Diesel #2)
Discussion
Table one summarizes all of the computations. For the first two
cases computed above significant differences in the conversion factors
result when energy savings are considered. However, these cases also
represent significant numbers of diesel vehicles in the automotive
vehicle population. The savings in Case III produces only a marginal
change in conversion factor. At the present time it. is estimated that
*In this study consumption of heating oils and Jet fuels was assumed
to remain constant. In reality significant shifts in either of these
;tnarkets could significantly impact on the energy savings. A point could
be reached where the combined usage of middle distillate fuels could
produce a penalty in process energy for further increases in diesel
vehicle population.
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Table I. Summary of Computations
Fuel
Diesel //I
Case
Factor
Factor
Total Factor
Diesel #2
I
II
III
I
t
II
III
Heating Value Diff
Only
0.905
0.905
0.905
0.973
0.873
0.873
Energy Savings
Only
1.079
1.042
1.008
1.079
1.042
1.008
0.976
0.943
0.912
0.942
0.909
0.880
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there are about 100,000 diesel-powered automobiles compared to about
110,000,000 gasoline-powered automobiles. Thus, to reach the ratios of
diesel fuel to gasoline fuel consumption for the first two cases con-
sidered would require replacement of greater than 55,000,000 gasoline
vehicles in the current fleet for the maximum energy savings and about
10 -15,000,000 gasoline automobiles for Case II. Clearly, while diesel-
powered vehicle sales are expected to escalate in 1977 - 1978 (if for
example VW and GM introduce diesel vehicles) the proportions of diesel/
gasoline fuel usage used in the Cases I, II will not be reached in the
next five years. Case III while still optimistic shows that energy
savings will be marginal in the foreseeable future.
Also it should be noted that the conversion factors will be utilized
only to calculate a manufacturer's average fuel economy but consumer
labels for the diesel-fueled vehicles will bear actual mileage in terms
of diesel fuel.
Recommendation
Until a legal clarification is obtained it is assumed that the
Energy Policy and Conservation Act requires that real (as opposed to
projected) energy conversion factors be utilized. Since the conversion
factors will be used only by DOT/EPA to compute a manufacturer's average
fuel economy and since the current diesel population is too small to
calculate meaningful process energy savings, it is recommended that a
conversion factor based only on differences in heating values be utilized
in the near-term.
In the longer term, the actual production records together with
certification applications will permit more accurate projections of
diesel sales and it is recommended that the conversion factors be then
revised annually reflecting both heating value differences and process
energy savings.
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Appendix I
Computation of Diesel Fuel Equivalence to Gasoline Reflecting
Only Differences in Heat Content
If the fuel economies (mpg) of automobiles powered by gasoline or
diesel engines are considered proportional to the Btu's/gal of the
corresponding fuels, the following expression could be written:
mpg(gasoline) _ Btu/gal(gasoline)
mpg(diesel) " Btu/gal(diesel)
For the average specifications of the gasoline and diesel fuels used
during the FTP for automobiles, this ratio has the following values:
mpg(Indolene HO III) _ 114,107
"^(Diesel D-l) = 126»10°
mpg(Indolene HO III) = 114,107
mpg(Diesel D-2) " 13°'65°
The net heat contents of the fuels .have been determined as indicated
below.
Net heat content of Indolene HO III
The EPA Certification Division uses a, gasoline density, = 6.167
Ib/gal for exhaust emission computations.
Accordingly,, specific gravity, S.G. = 6.167/8.330 = 0.740
141 S
and API gravity = p _ n ,,.n - 131.5 = 59.7.
D \y Vj
The SAE paper 740522 indicates that "The following correlation for heat
content is quite accurate because it is based on a large volume of data
obtained by the CRC, the Bureau of Mines, and the API:
Low Heating Value, LHV = 16.24G - 3.007A + (I)
0.01714GV - 0.2983AG + 0.00053GAV + 17,685
where:
G = API gravity
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V = Average of 10,50 and 90% distillation points
A = Aromatics content, %."
Now, the average values for Indolene HO III are:
G = 59.7
V = (124.5 + 218.5 + 312.5)/3 = 218.5°F
A = 27% aromatics.
Therefore, substituting these values in equation (I) we get:
LHV = 18,503 Btu/lb.,
or for a = 6.167 Ib/gal.
LHV = 114,197 Btu/gal.
Net heat content of diesel fuel D-l
According to the EPA specifications, this fuel has on the average:
API gravity: 42
50% distillation: 445°F
Then, from the chart in Figure 4 of the SAE Standard J1082, we get
LHV = 126,100 Btu/gal.
Net heat content of diesel fuel D-2
According to the EPA specifications, this fuel has on the average:
API gravity: 35
50% distillation: 505°F
Then, from the chart in Figure 4 of the SAE Standard J1082, we get
'LHV = 130,650 Btu/gal.
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Appendix II
Methodology and Computation of Diesel Fuel Equivalence to
Gasoline Reflecting Heat Content Differences and Process. Energy Savings
Nomenclature
Let E = Total btu's of diesel fuel product,
E = Total btu's of gasoline fuel product
IL = heat of combustion of diesel fuel, Btu/gal
tL, = heat of combustion of gasoline fuel, Btu/gal
s = process energy savings, %
S = process energy savings, Btu
f = fraction of automotive fuel product produced by refinery
f- = fraction of automotive diesel fuel produced from refinery.
Superscript = baseline value.
Assumptions
(1) From Exxon Study process energy savings, s, is a function of
the ratio of diesel fuel production to total automotive fuel
production, ED//J. + g )' and is 6iven in figure 1.
(2) All process energy savings are credited to automotive fuel
production and specifically to the diesel fuel account.
(3) The fraction of automotive fuel consumption, f, is assumed to
be 0.50 (actually it is currently 0.60 because domestic production
of heavy heating oils is negligible, but if imports are ceased the
fraction would be closer to 0.50).
(4) For the baseline, the ratio of diesel fuel production to
gasoline production is assumed to be 10/90. Almost all of this
diesel fuel is currently consumed by truck diesels. For the meth-
odology truck consumption is assumed to remain a constant % of
distillate fuels.
Methodology
Process Energy Savings, S, = -~^ (E'D + E'G) .
Diesel to Gasoline Equivalence Factor = F
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r
HD (1 - S/V
itof(ED + EW
ED
If it is assumed that (E* + E'_) = (£_. + £) the equation sim-
D
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F = 0.-942
Case II
s =0.4% from Exxon Study, fig. 1
f = 0.50
EL/E = 0.20/.80
= 0.873 for diesel #2 from appendix 1.
1 - s
100£D
.873
1 - 0.004
Case III
873 =
0.960 -909
0.044% from figure 1, Exxon Study
E /E = 0.11/.89
-u'-° = 0.873 for Diesel #2 from Appendix 1.
1 - s
100£D
0.873
(.00044)/(.5)(0.11)
= 0-873 =
0.992 u-°°u
Similarly the calculations for diesel #1 are
Case I
17 - -905
.927
Q,,
= '976
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Case II
Case III
F = = 0 912
. * .992 U'yi/
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