Note: Descriptions are shown in the official language in which they were submitted.
HOMOGENEOUS CHARGE
COMPRESSION IGNITION ENGINE FUELS
This application is a PCT International application which claims the benefit
of and priority to
U.S. Provisional Application Ser. Nos. 61/914,607 filed December 11, 2013 and
61/914,614
filed December 11,2013 and U.S. Application Serial Nos. 14/551,319 filed
November 24, 2014
and 14/551,360 filed November 24, 2014, entitled "Homogeneous Charge
Compression Ignition
Engine Fuels".
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR
DEVELOPMENT
[0001] None.
BACKGROUND
[0002] More stringent engine emission standards regulated by the
Environmental Protection
Agency (EPA) provide incentive to study and to apply advanced combustion modes
in internal
combustion engines. Among recently proposed advanced combustion modes, the
Homogeneous
Charge Compression Ignition (HCCI) engine stands out as a very promising
technology with
better fuel economy and lower pollutant emissions compared to conventional
diesel and gasoline
engines. HCCI utilize a pre-mixed homogeneous charge of air/fuel, which is
auto-ignited by
engine compression and burns quickly to maximize combustion efficiency. As a
result, the fuel
chemistry plays a dominant role in combustion phasing and engine performance.
[0003] One of the major hurdles to creating commercially viable HCCI
engines is developing
technology to extend the engine high load limit. Because the chemical kinetics
control
combustion, the chemical properties of fuel components play a dominant role in
HCCI engine
performance. Combustion stability can also be a problem for HCCI engines under
low load
condition due to large cycle variations, i.e., during engine idling. This
requires a fuel that has
slow burning rate under high load but is still reactive under low load
condition for HCCI
engines. Accordingly, a need exists for improved hydrocarbon fuels that
provide increased power
and a broader operating range in HCCI engines, including increased load limit.
Date Recue/Date Received 2022-05-04
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BRIEF SUMMARY OF THE DISCLOSURE
[0004] The invention generally relates to fuels and processes for making
fuels that provide
improved performance when combusted in a homogeneous charge compression
ignition engine.
In certain embodiments, the invention relates to a fuel for a homogeneous
charge compression
ignition engine that comprises a mixture of hydrocarbons, each hydrocarbon in
the mixture
comprising from 4 to 14 carbon atoms, at least 20 wt. % n-paraffins, at least
20 wt. %
naphthenes, 20 wt. % or less of aromatic hydrocarbons and 5 wt. % or less of
olefins. Optionally,
at least 90 wt. % of the mixture of hydrocarbons may consist of hydrocarbons
comprising from 6
to 10 carbon atoms, or at least 75 wt. % of the mixture of hydrocarbons may
consist of
hydrocarbons comprising from 7 to 9 carbon atoms. In certain embodiments, 15
wt. % or less, or
optionally 10 wt. % or less of the hydrocarbons contain five or fewer carbon
atoms.
[0005] In certain alternative embodiments, the fuel may optionally comprise
at least 25 wt.
%, at least 30 wt %, or even at least 35 wt. % of n-paraffins. The fuel may
optionally comprise at
least 25 wt. % of naphthenes, at least 30 wt. % of naphthenes, or at least 35
wt. % of naphthenes.
In certain embodiments, the fuel may optionally comprise 15 wt. % or less of
aromatics, or even
wt. % or less of aromatics.
[0006] In certain alternative embodiments, the fuel may additionally
comprise 3 wt. % or
less, 2 wt. % or less, or even 1 wt. % or less of olefins. In various
embodiments, the fuel may
additionally possess a dry vapor pressure equivalent (as measured by method
ASTM-D5191) at
37.8 C of 10 psi (69 kPa) or less, 9 psi (62 kPa) or less, 8 psi (55 kPa) or
less, 7 psi (48 kPa) or
less, or even 6 psi (41 kPa) or less. In certain embodiments, the fuel
preferably contains a
quantity (wt. %) of naphthenes that is greater than the quantity (wt. %) of
normal paraffins.
[0007] Certain embodiments of the present invention relate to a process for
making a fuel for
a homogeneous charge compression ignition engine. In certain embodiments, the
process
comprises blending hydrocarbons to form a fuel mixture, where the power index
of the fuel
mixture when combusted in a homogeneous charge compression engine is greater
than or equal
to 1.5. The power index is defined by the equation:
Power Index = (AREAx * MIMEPx) / (AREAy * MIMEPy)
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where MIMEP is the maximum indicated mean effective pressure achieved inside a
homogeneous charge compression ignition engine cylinder during combustion of
the fuel
mixture (x) or the reference fuel (y), respectively, and an equal mass of each
fuel is combusted.
The variable y is a reference fuel comprising 11 wt. % n-heptane, 37 wt. % iso-
octane, 32 wt. %
toluene, 11 wt. % methyl-cyclohexane and 9 wt. % 1-hexene., while AREAx and
AREAy
represent distinct areas on a graph of load (IMEP) versus engine revolutions
per minute (RPM)
for the fuel mixture (x) and the reference fuel (y), respectively, each
distinct area having an
upper bound at the IMEP during combustion of each fuel at an RPM ranging from
1500 RPM to
2500 RPM, and having a lower boundary at the IMEP below which combustion of
each fuel
becomes unstable at an engine RPM ranging from 1500 RPM to 2500 RPM. In
certain
embodiments, the power index of the fuel mixture is greater than or equal to
1.75, or even greater
than or equal to 2.
[0008] Certain embodiments of the process comprise blending hydrocarbons
into a fuel
mixture that comprises hydrocarbons containing from 4 to 14 carbon atoms, at
least 20 wt. % n-
paraffins, at least 20 wt. % napthenic hydrocarbons, 20 wt. % or less of
aromatic hydrocarbons
and 5 wt. % or less of olefins. In certain embodiments of the process, at
least 90 wt. % of the
fuel mixture consists of hydrocarbons containing from 6 to 10 carbon atoms. In
certain
embodiments, at least 75 wt. % of the fuel mixture consists of hydrocarbons
containing from 7 to
9 carbon atoms. In certain embodiments of the process, 15 wt. % or less, or
optionally 10 wt. %
or less of the hydrocarbons in the fuel mixture contain five or fewer carbon
atoms.
[0009] In certain embodiments of the process, the fuel mixture may comprise
at least 25 wt.
% or at least 30 wt. % of n-paraffins. Optionally, the fuel mixture may
comprise at least 25 wt.
%, at least 30 wt. %, or even at least 30 wt. % of naphthenic hydrocarbons. In
certain
embodiments of the process, the fuel mixture comprises 15 wt. % or less, or
even 10 wt. % or
less of aromatic hydrocarbons.
[0010] Optionally, the fuel mixture of the process may additionally
comprise 3 wt. % or less,
2 wt. % or less, or even 1% or less of olefins. Optionally, the fuel mixture
of the process
possesses a dry vapor pressure equivalent (as measured by method ASTM-D5191)
at 37.8 C of
psi (69 kPa) or less, 9 psi (62 kPa) or less, 8 psi (55 kPa) or less, 7 psi
(48 kPa) or less, or
even 6 psi (41 kPa) or less. In certain embodiments of the process, the
quantity (wt. %) of
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napthenic hydrocarbons in the fuel mixture is greater than the quantity (wt.
%) of normal
paraffins in the fuel mixture.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] A more complete understanding of the present invention and benefits
thereof may be
acquired by referring to the follow description taken in conjunction with the
accompanying
drawings in which:
[0012] FIG. I depicts a hypothetical speed versus load (IMEP) map of two
fuels, A and B,
over a range of engine speeds expressed in revolutions per minute (RPM).
[0013] FIG. 2 comprises several graphs, each depicting a correlation
between the power
index for several fuel blends and the quantity of n-paraffins, naphthenes,
olefins or aromatics
present in each fuel blend.
[0014] FIG. 3 is a graph that plots engine load (gross IMEP) versus
negative valve overlap
(NVO) as an indicator of operating range for several novel fuel blends versus
certification
gasoline (RD387).
[0015] FIG. 4 is a graph depicting performance of certification gasoline
(RD387) compared
to several novel fuel blends with respect to negative valve overlap (NVO)
during combustion in
an HCCI engine.
[0016] FIG. 5 is a graph depicting performance of several novel fuel blends
with respect to
combustion phasing (degrees after top dead center, or deg ATDC) during
combustion in an
HCCI engine.
[0017] Figure 6 is a graph depicting the emissions index of total
hydrocarbons (THC) versus
the emissions index of NO (g/kg-fuel) for several novel fuel blends during
combustion in an
HCCI engine.
[0018] Figure 7 is a graph depicting the emissions index of CO versus the
emissions index of
NO (g/kg-fuel) for several novel fuels blends during combustion in an HCCI
engine.
[0019] The invention is susceptible to various modifications and
alternative faults, specific
embodiments thereof are shown by way of example in the drawings. The drawings
may not be to
scale. It should be understood that the drawings and their accompanying
detailed descriptions are
not intended to limit the scope of the invention to the particular form
disclosed, but rather, the
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intention is to cover all modifications, equivalents and alternatives falling
within the spirit and
scope of the present invention as defined by the appended claims.
DETAILED DESCRIPTION
[0020] The present disclosure pertains to the properties and chemical
components that make
a novel fuel that improves the performance of HCCI engines. The properties
include improved
engine operating limits as well as increased power and efficiency. We assessed
the performance
of seven low-octane fuel blends and nine high-octane fuel blends on an HCCI
engine utilizing a
high-fidelity computer simulation tool. Each simulated fuel blend was
constructed to comprise
different combinations of eight chemical compounds, with each compound
representing a
distinct chemical genus, including n-paraffins, iso-paraffins, aromatics,
naphthenes and olefins
(hereinafter referred to as PIANO compounds). From this work, we derived an
equation that
correlates the presence of each PIANO hydrocarbon component in a fuel blend to
the relative
performance of that fuel blend for HCCI combustion. We then tested several
hydrocarbon fuel
blends in an actual HCCI engine to confirm the results of our modeling work
and demonstrate
that the presence of certain chemical species strongly correlates with
increased fuel performance,
while the presence of other chemical species inversely correlates with
increased fuel
performance.
[0021] The examples are intended to be illustrative of specific embodiments
in order to teach
one of ordinary skill in the art how to make and use the invention. These
examples should not be
interpreted to limit, or define, the scope of the invention to less than is
fully encompassed by the
full disclosure of the invention herein and its legal equivalents.
EXAMPLE 1
[0022] HCCI engine combustion was modeled using the Chemkin software and
the detailed
gasoline mechanisms developed by Reaction Design of San Diego, CA. A single
zone HCCI
engine model was used to simulate HCCI engine combustion. The modeled test
engine had the
specifications listed in Table 1.
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Table 1: Test Engine Specifications
Bore ;tameter (cm) 8.6
D s-pce men t volume (cm-) 500
IVC (aTDC) -136
EVO (aTDC) 138
Engine connectino rod to crank ratkus ra6o 3.326
[0023] Test fuels were formulated to have either a high octane number (HON)
corresponding
to about 91 RON, or low octane number (LON) corresponding to about 60 RON.
Fuels were
formulated by blending eight fuel components, including n-heptane, n-hexane,
iso-octane,
methyl-pentane (iso-hexane), toluene, ethyl-benzene, methyl-cyclohexane (mch),
and 1-hexene.
The percentage of the individual chemical component was varied in the
different test fuels to
assist in determining the relative impact of each chemical component on HCCI
engine
combustion while still maintaining the targeted 91 RON and 60 RON of the
surrogate fuels. The
RONs of the surrogates were calculated based on the mass percentage of each
fuel component
and its corresponding RON. Table 2 provides the fuel composition, RON, and MON
(based on
linear expressions using fuel mass fractions) for the 16 gasoline surrogates
studied. HON1 to
HON9 represent the nine HON gasoline fuels, and LON1 to LON7 represent the
seven LON
fuels.
Table 2: Hypothetical Fuel Blends Modeled
1 ethyl- 1-
toluene MCH RON MON
hcptanc hcxanc octane hexane bcnzcnc hcxcnc
HON1 11.00/o 0.0% 37.0% 0.0% 32.0% 0.0% 11.0% 9.0% 90.3 85.7
HON2 0.0% 15.0% 33.0% 0.0% 34.0% 0.0% 10.0% 8.0% 90.9 86.4
HON3 10.0 ,/0 0.0% 43.0% 0.0% 29.0% 0.0% 10.0% 8.0% 91.2 87.1
HON4 0.0% 10.0% 33.0% 0.0% 29.0% 0.0% 20.0% 8.0% 91.1 87.1
HON5 0.0% 10.0% 33.0% 0.0% 29.0% 0.0% 10.0% 18.0% 91.3 86.0
HON6 10.0% 0.0% 25.0% 8.0% 39.0% 0.0% 10.0% 8.0% 91.0 85.9
HON7 0.0% 12.0% 41.0% 0.0% 25.0% 0.0% 12.0% 10.0% 90.4 86.6
HON8 10.0% 0.0% 33.0% 0.0% 8.0% 30.0% 10.0% 9.0% 90.2 84.2
HON9 0.0% 12.0% 41.0% 0.0% 26.0% 0.0% 12.0% 9.0% 90.8 87.0
LON1 32.0% 0.0% 27.0% 0.0% 8.0% 0.0% 33.0% 0.0% 60.9 60.1
LON2 0.0% 39.0% 9.0% 15.0% 7.0% 0.0% 30.0% 0.0% 60.2 59.9
LON3 10.0% 19.0% 0.0% 34.0% 7.0% 0.0% 30.0% 0.0% 60.2 59.7
LON4 29.0% 0.0% 17.0% 7.0% 7.0% 0.0% 40.0% 0.0% 60.0 59.3
LON5 29.0% 0.0% 16.0% 8.0% 7.0% 0.0% 30.0% 10.0% 60.0 58.0
LON6 29.0% 0.0% 0.0% 24.0% 17.0% 0.0% 30.0% 0.0% 60.1 58.3
LON7 20.0% 14.0% 30.0% 0.0% 0.0% 0.0% 36.0% 0.0% 60.0 60.2
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[0024] In order to examine the fuel performance under a variety of
operating conditions, six
engine parameters were varied, including engine compression ratio, speed,
initial temperature,
initial pressure, EGR (Exhaust Gas Recirculation) rate, and equivalence ratio.
Table 3
summarizes the variables and their values studied in this work, although not
all data obtained are
reproduced here or are necessary for a full understanding of the invention
disclosed herein.
Table 3: Computer Modeled Engine Parameters
Compression ratio 12, 13, 14
Engine speed (rpm) 1000, 1500, 2000
Initial temperature (K) 375, 425, 475
Initial pressure (atm) 1, 1.5, 2.0
EGR (%) 30, 40, 50, 60
Equivalence ratio 0.5, 0.6, 0.7, 0.8
[0025] Practical HCCI engine operation is subject to different constraints.
Specifically, the
high load limit of engine operation is usually restricted by the maximum
pressure rise rate during
the combustion. A high pressure rise rate is the direct cause of engine
knocking and may lead to
severe mechanical damage. The Ringing Intensity Index (RII) was defined in the
literature by
J.A. Eng' to quantify the knocking intensity in HCCI engines. RII is
calculated using the
equation:
tdP)
13 RII ¨ 1 kµ. \MIA, jrna
' _____________________________________ VyRTõ,.
2y ax
where
'dip`
is maximum pressure rise rate in the cylinder, kPa/msec
13 = 0.05 is a scaling factor msec2
is maximum pressure in the cylinder, Pa
Tax is maximum temperature in the cylinder, K
is the gas constant, J/kg K
is the ratio of specific heats C /C
p v
[0026] In our computer modeling, the limit of RII was set to 5 MW/m2 and
defined the
highest possible engine load condition when running with different fuel
blends. The lower limit
of the accessible engine load is determined by either misfire or an unstable
combustion
condition. In our modeling, a maximum in-cylinder temperature below 1300 K was
deemed to
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result in a misfire. Unstable combustion was defined as that occurring later
than 19 aTDC (after
Top Dead Center) and is a typical constraint on the operability of an HCCI
engine.
[0027] It is common for HCCI fuel performance to be measured by maximum
Indicated
Mean Effective Pressure (IMEP) in the cylinder without violating the RH
constraint (defined
above). IMEP can be used as a fuel performance metric applicable to engines
running with a
constant speed and when the high load limit is of primary concern. However, in
addition to the
high load limit, the low load boundary (either misfire or unstable combustion)
is also of concern
when assessing the fuel performance. IMEP also does not give a full assessment
of fuel
performance over a range of HCCI engine speeds. Therefore, we developed a new
metric, termed
Power Index (PI), that can quantify HCCI fuel performance over the entire
engine operating
range. The engine operating range is constrained by Rh, maximum in-cylinder
temperature, and
the maximum crank angle (expressed as degrees after top dead center) at which
stable
combustion can be maintained (i.e., without excessive misfire or cyclical
variation). The
performance of a given HCCI fuel blend typically falls within these engine
operating constraints,
and can be established on an engine speed-load map. Figure 1 depicts a
hypothetical speed-load
map of two fuels, A and B, over a range of engine speeds (expressed in
revolutions per minute,
or RPM). The maximum IMEP at each speed provides the upper limit, while the
minimum limit
at each speed is represented by the IMEP at which combustion becomes unstable
due to misfire
or unacceptable cyclic variation of the engine that may be caused by a lean
condition or
adjustments to negative valve overlap. Accordingly, we define the Power Index
(PI) for a fuel
as:
Power Index = (Area test fuel * Max IMEP test fuel) /I (Area basereiereitiee
wet * Max IMEP reference reel)
where the Area represents the operating range of either the test fuel or the
base fuel on the speed-
load map. The high octane number fuel surrogate HON1 in Table 2 was selected
as the base fuel
for this modeling work, and was arbitrarily assigned a PI of 1. In alternative
embodiments, an
alternative base (i.e., reference) fuel may be chosen. For example, in certain
embodiments, the
base (reference) fuel may comprise 50 wt. % n-heptane and 50 wt. % iso-octane.
In the
modeling work, a larger calculated PI value was indicative of a fuel blend
having better overall
performance for HCCI engine combustion.
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[0028] Parametric modeling studies were conducted for each surrogate fuel
blend on HCCI
engine combustion based on the variables provided in Tables 1 - 3. The area of
the engine speed-
load map for each fuel blend was calculated and used to calculate Power Index
according to the
above equation. The results are provided in Table 4, and show that the
calculated PI for the LON
fuels were larger than the PI of the HON fuels. Indeed, the calculated PI for
all HON fuels was
close to the base fuel HON1, with a PI of 1. Thus, LON fuels (target RON 60)
were deemed
preferable as a HCCI fuel versus the HON fuels (target RON of 91).
[0029] Table 4 summarizes the actual values of the PI and maximum engine
load (IMEP ) at
both 1000 and 1500 RPM for each modeled fuel blend. Results were also obtained
at 2000 RPM
for each fuel, but are not shown.
Table 4: Calculated Power Index and Maximum Achievable Load at 1000 RPM and
1500 RPM
Maximum load Maximum load
Fuel Name Power Index
(bar) (1000 rpm) (bar) (at 1500 rpm)
HON1 1 6.81 6.00
HON2 1.07 6.95 5.68
HON3 0.97 6.75 5.98
HON4 1.25 6.71 5.95
HON5 0.95 6.93 5.70
HON6 0.91 6.83 6.02
HON7 1.10 6.90 5.67
HON8 0.91 6.98 5.91
HON9 1.09 6.89 6.01
HON fuels avg. 1.03 6.86 5.88
LON1 2.21 8.98 9.15
LON2 2.29 8.97 9.13
LON3 2.04 8.89 9.29
LON4 2.34 9.73 9.27
LON5 2.04 9.02 7.76
LON6 2.02 9.31 6.96
LON7 2.37 9.07 9.16
LON fuels avg. 2.19 9.14 8.67
The LON5 fuel was the only LON fuel containing 1-hexene, indicating that this
species,
and perhaps olefins in general, may have adversely affected the performance of
this LON fuel
blend. Meanwhile, the performance of the HON fuels appeared to be relatively
insensitive to the
presence of olefins.
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[0030] LON6 had the largest toluene content among all LON fuels. While not
wishing to be
bound by theory, this may have resulted in a lower reactivity of LON6 relative
to other LON fuel
blends tested, leading to a lower PI because higher intake temperature
(resulting in less air/fuel
charge to the cylinder according to the ideal gas law) was required to ignite
the fuel blend. The
LON fuel with the highest PI was LON4. This fuel blend had the highest MCH
content, which is
consistent with naphthenic content benefitting HCCI engine combustion.
EXAMPLE 2
[0031] We also modeled the effect of increased intake pressure on the
properties of the
different fuel blends, and the results are provided in Table 5.
Table 5: Power Index and Maximum Achievable Load at Different Intake Pressures
Max. Load (bar) Max. Load (bar) (at Max. Load (bar)
Fuel Name Power Index (at P
- intake - 1 atm) Pintake = 1.5 atm) (at Pintake = 2 atm)
HON1 1 3.43 5.15 6.81
HON2 1.07 3.39 4.28 6.95
HON3 0.97 3.33 4.73 6.75
HON4 1.25 3.09 5.58 6.71
HON5 0.95 3.13 4.20 6.93
HON6 0.91 3.36 5.88 6.83
HON7 1.10 3.11 4.49 6.90
HON8 0.91 3.15 4.49 6.98
HON9 1.09 3.43 5.15 6.81
HON fuels avg. 1.03 3.26 4.78 6.86
LON1 2.21 3.36 5.62 9.15
LON2 2.29 3.41 7.45 9.13
LON3 2.04 2.99 6.71 9.29
LON4 2.34 3.11 5.83 9.73
LON5 2.04 3.36 6.92 9.02
LON6 2.02 3.41 6.79 9.31
LON7 2.37 3.09 4.49 9.16
LON fuels avg. 2.19 3.25 6.26 9.26
[0032] Both HON and LON fuel blends achieved similar maximum load
(indicated in bar) at
an intake pressure of 1 atm, suggesting that HCCI engine performance was not
particularly
sensitive to the fuel composition at atmospheric pressure. However, at 1.5 atm
intake pressure,
the LON fuel blends had significantly higher maximum load (IMEP) that the HON
blends (Table
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4). There was also significant variation in the IMEP achieved among the
different LON fuel
blends, indicating that the chemical composition of each fuel blend had a
significant impact on
its suitability for HCCI combustion. For example, at 1.5 atm intake pressure,
the LON2 fuel
reached 7.45 bar maximum load, while the LON7 fuel only achieved 4.49 bars.
EXAMPLE 3
100331 We examined the combustion rate for each component in a given fuel
blend and
plotted the results as mole fraction combusted versus engine crank angle. All
fuel components
except toluene were observed to rapidly decompose during an initial low
temperature heat
release stage, then gradually oxidize as combustion proceeded. We observed
that toluene was
consumed much slower than the other components, especially between the first
and the second
heat release stages. While not wishing to be bound by theory, the slow burning
rate of toluene is
theorized to help extend the combustion duration and reduce the engine ringing
intensity. Thus, a
small amount of aromatic species may be beneficial to HCCI engine combustion
when blended
into a low RON fuel.
1003411 The results for all fuel blends were analyzed statistically to
derive correlation and
covariance coefficients between the power index and the quantity of various
PIANO groups
present in each fuel. The correlations are shown in Table 6, and graphed in
Figure 2.
Table 6: Correlation and Covariance Between Power Index and PIANO Groups
Power index Paraffin Iso-paraffin Aromatics Naphthene
Olefin
Correlation
0.949 -0.743 -0.791 0.957 -0.932
coefficient
Covariance
0.062 -0.029 -0.026 0.066 -0.074
coefficient
100351 The correlation coefficients between the power index and the
presence of normal
paraffins and naphthenes were found to be close to 1, suggesting that normal
paraffinic and
naphthenic compounds were significant contributors to the Power Index of the
LON fuel blends.
The correlation coefficient for certain components (i.e., iso-paraffins,
aromatic hydrocarbons,
and olefins) was negative, suggesting that these groups were not beneficial.
However, this does
not necessarily indicate that these compounds must be completely removed to
create an optimal
HCCI fuel. For example, as mentioned above, a small amount of toluene in the
LON fuels helped
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to extend the combustion duration, and thus, lower the ringing intensity.
Also, the correlation and
covariance coefficients between the PI and the individual fuel component
unexpectedly indicated
that iso-hexane (as opposed to iso-octane) actually had a positive influence
on PI, as is
summarized in Table 7. MCH indicates methyl cyclo-hexane.
Table 7: Correlation and Covariance Between Power Index and Specific Fuel
Component
Power 11- ethyl-
n-hexane i-octane i-hexane toluene mch 1-
hexene
index heptane benzene
Correlation
0.656 0.216 -0.742 0.550 -0.844 -0.208 0.957 -0.791
coefficient
Covariance
0.048 0.014 -0.063 0.034 -0.064 -0.009 0.066 -0.026
coefficient
[0036] The correlation indicates that the diversity of the species in fuel
blends is also a very
important factor to Power Index. Both chemical composition and octane number
are important to
formulation of superior HCCI engine fuels, and the results provide support for
several
embodiments of the present invention.
[0037] Statistical analysis of all computer modeling performed yielded a
correlation between
the Power Index and presence of four of the five PIANO groups, including n-
paraffin, iso-
paraffin, aromatics, and naphthene (olefin is excluded because of the
permutations and
combinations rule in statistical analysis). This can be expressed as:
Power Index (PI) = 0.46 + 2.39 Paraffin + 0.12 Iso-paraffin + 2.76 Naphthene
¨0.32 Aromatics
where the overall fit has a coefficient of determination (R2) = 0.951.
EXAMPLE 4
[0038] Empirical testing was performed in an actual HCCI engine to confirm
the computer-
modeled results discussed in the Examples 1 ¨ 3. The University of Michigan
Auto Lab
performed tests utilizing a single cylinder HCCI engine having the
specifications listed in Table
8. The experimental conditions under which testing was performed are listed in
Table 9, while
the general properties and composition of the fuel blends tested are listed in
Table 10. RD387
was a commercial certified gasoline and was used as control for this work. NH-
20 and NH-40
are simple control blends of 20 wt. A and 40 wt. % n-heptane mixed with RD387
certification
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gasoline. R9, IS5 and IS6A were non-commercial hydrocarbon test blends
produced by Phillips
66 Company, Houston, Texas for these tests. Table 11 shows a detailed analysis
of the
composition of each test fuel by carbon number.
Table 8: Test Engine Specifications:
Displacement volume 550
(cm^3)
Cylinders 1
Stroke (mm) 94.6
Bore (mm) 86.0
Connecting rod length (mm) 152.2
Compression ratio 12.5
Number of valves 4
Piston shape Shallow bowl
Head design Pent-roof
Fuel delivery Direct injection
Table 9: Experimental conditions
Engine speed (rpm) 2000
Intake temperature ( C) 45
Intake pressure (bar) 1.0
Exhaust pressure (bar) 1.05
Coolant temperature ( C) 90
Oil temperature ( C) 90
Ringing index limit (MW/m2) 5
COV of IMEP limit (%) 5
El-Nox limit (g/kg-fuel) 1
Table 10: Test Fuel Specifications
RD387 NH20 NH40 R9 1S5 1S6A
LHV (kJ/kg) 43032 43445 43649 43665 44218 45202
Density (g/m1) 0.746 0.733 0.721 0.748 0.679 0.623
RVP (psi) 6.400 - 1.58 11.05 21.64
MW (g/mol) 93.039 - 95.776 108.1 82.6 71.4
Carbon (wt%) 86.4 - 85.6 84.2 83.2
Hydrogen (wt%) 13.6 - 14.4 15.8 16.8
H/C (molar) 1.897 1.959 2.039 2.002 2.235 2.403
RON 90.5 75 58 50 71 95
MON 82.6 71 56 50 69 88
AKI ((R+M)/2) 87 73 57 50 70 92
Aromatics (wt%) 32.3 25.8 19.4 13.5 2.5 0.0
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Paraffins (wt%) 8.1 26.5 44.9 22.6 34.2 10.4
I so-paraffin s (wt%) 37.5 30.0 22.5 31.0 40.0 89.3
Naphthenes (wt%) 16.9 15.1 11.3 30.4 22.2 0.0
Olefins (wt%) 4.5 3.6 2.7 0 0 0.3
Table 11: Composition of Each Test Fuel by Carbon Number (in wt. %)
Carbon RD387 (wt.%) IS5 (wt.%) IS6a (wt.%) R9 (wt.%)
Number
C4 1.7 1.8 4.1
C5 17.7 34.6 95.8 0.5
C6 12.9 38.1 9.9
C7 32.3 21.3 21.6
C8 20.6 3.9 25.2
C9 8.5 0.2 8.6
C10 3.8 0.2
C11 1.0
[0039] We examined the maximum engine load (MIMEP) achievable for each test
fuel by
"mapping" the operating range of each test fuel over a range of "negative
valve overlap" (NVO)
settings. NVO is the duration (measured in degrees) where the exhaust valve is
closed prior to
opening of the intake valve. The greater the NVO duration, the more exhaust,
or residual, that is
retained in the cylinder to a) dilute and b) preheat the incoming air/fuel
mixture. For each fuel
blend tested, the energy addition per engine cycle was held constant
regardless of the NVO
setting for each fuel by compensating the mass flow based on lower heating
value (LHV) such
that J/cycle was held constant. For this work, the energy addition per cycle
was made
independent of engine size by dividing by cylinder displacement volume. This
resulted in a new
metric, termed Energy Mean Effective Pressure (EMEP):
EMEP [bar] - EnergyAddition [J/cycle]
Displacement Volume [L]x100
[0040] The operating range of the R9 test fuel was compared to RD387
gasoline and NH40,
as shown in Figure 3, which plots gross IMEP versus the duration of NVO. The
graph
demonstrates that the amount of NVO required for stable HCCI combustion for R9
fuel was
significantly less than that of NH40 fuel, even though they had similar
calculated RON (refer to
Table 10). This indicates that the superior properties of R9 as an HCC1 fuel
were not simply a
consequence of its low octane number, but other physical and/or chemical
properties.
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[0041] The performance of the Phillips 66 Company test fuels with respect
to their NVO
range is compared in Figure 4 and with respect to combustion phasing (timing
advance) in
Figure 5. Although Figure 4 shows test fuels IS5 and IS6A achieved slightly
higher engine loads
than R9 (IS6A achieved about 3% higher maximum IMEP than R9), R9 was able to
sustain load
(IMEP) at significantly less NVO, indicating that R9 is a more favorable HCCI
fuel. Figure 5
shows that the R9 fuel blend maintained load (IMEP) at significantly later
combustion phasing
(defmed as degrees after top dead center at which 50 wt. % of the fuel charge
burns), which also
favored the R9 blend as an HCCI fuel versus RD387 certification gasoline and
the IS5 or IS6
blends. Overall R9 was determined to be an improved blend versus the other
test fuels over a the
range of operating conditions utilized.
[0042] For each test fuel, we also measured the emissions of total
hydrocarbons (THC),
nitric oxide (NO), and carbon monoxide (CO) in units of g/kg-fuel during
combustion in the
HCCI engine, where engine load was fixed at 9 bar. Figure 6 plots the
emissions index of THC
versus the emissions index of NO, while Figure 7 plots the emissions index of
CO versus the
emissions index of NO. Both figures demonstrate the R9 fuel blend to result in
the lowest
emissions of NO which is important due to engine NO emissions are highly
regulated in most
countries. In general, the lower octane number fuels can operate with less NO
emissions because
of less NVO required to enable auto-ignition, but they suffer higher CO and
THC emissions.
[0043] In closing, it should be noted that the discussion of any reference
is not an admission
that it is prior art to the present disclosure, in particular, any reference
that may have a
publication date after the priority date of this application. At the same
time, each and every
claim below is hereby incorporated into this detailed description or
specification as a additional
embodiments of the present invention.
[0044] Although the systems and processes described herein have been
described in detail, it
should be understood that various changes, substitutions, and alterations can
be made without
departing from the spirit and scope of the invention as defined by the
following claims. Those
skilled in the art may be able to study the preferred embodiments and identify
other ways to
practice the invention that are not exactly as described herein. It is the
intent of the inventors
that variations and equivalents of the invention are within the scope of the
claims while the
description, abstract and drawings are not to be used to limit the scope of
the invention. The
invention is specifically intended to be as broad as the claims below and
their equivalents.
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References:
1. Eng, J.
A., Characterization of pressure waves in HCCI combustion, SAE Paper 2002-01-
2859, (2002).
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