Paper # 070IC-0264 Topic: Internal Combustion and Gas Turbine Engines 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013 Autoignition Characterization of Primary Reference Fuels and n- Heptane/n-Butanol mixtures in a Constant Volume Combustion Device and Homogeneous Charge Compression Ignition Engine Marc E. Baumgardner 1 Anthony J. Marchese 1 S. Mani Sarathy 2 1 Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 2 Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia Premixed or partially premixed compression ignition modes, such as homogeneous charge compression ignition (HCCI), have been a particular focus among researchers because of their potential to deliver enhanced fuel efficiency and meet exhaust emissions mandates without the addition of costly after-treatment technologies as currently required with traditional spark ignition (SI) and direct injection compression ignition (DICI) engines. These advanced combustion strategies thus seek to combine the advantages of SI and DICI engines while avoiding their disadvantages. Previous studies have suggested that future fuels with desired properties for optimal performance in these advanced combustion modes might have properties between that of traditional gasoline and diesel fuels and will most likely consist of mixtures of petroleum derived products (aromatics, straight and branched alkanes), alcohols, synthetic alkanes, and fatty acid methyl esters. Existing ignition quality metrics such as Octane or Cetane Number are reasonably consistent for petroleum-derived fuels in SI and CI engines but can fail to adequately characterize autoignition in advanced combustion modes, particularly when higher concentrations of alcohols are included in the blend. Accordingly, in this study, the autoignition behavior of primary reference fuels (PRF) and blends of n-heptane/n-butanol were examined in a Waukesha Fuel Ignition Tester (FIT) and a Homogeneous Charge Compression Engine (HCCI). Fourteen different blends of iso-octane, n-heptane, and n-butanol were tested in the FIT – 28 test runs with 25 ignition measurements for each test run, totaling 350 individual tests were done in all. These experimental results supported previous findings that fuel blends with similar octane numbers can exhibit very different ignition delay periods. The present experiments further showed that n-butanol blends behaved unlike PRF blends when comparing the autoignition behavior as a function of the percentage of low reactivity component. These same fuel blends were also tested in a John Deere 4024T diesel engine, which was modified to operate in HCCI mode. The HCCI and FIT experimental results were compared against multi-zone models with detailed chemical kinetic mechanisms for PRF’s and n-butanol. For both the FIT and HCCI engine data, a new n-butanol/n-heptane kinetic model is developed that exhibits good agreement with the experimental data. The results also suggest that that the FIT instrument is a valuable tool for analysis of high pressure, low temperature chemistry and autoignition for future fuels in advanced combustion engines. 1. Introduction Depletion of fossil fuels and climate change from anthropogenic greenhouse gas emissions arguably represent the first civilization-scale challenges ever faced by the human race (Hirsch, et al., 2005) (McKibben, 2006). While there is no single technological solution for these global challenges, an effective means of rapidly addressing these concerns is to increase the thermodynamic efficiency of energy conversion devices that consume fossil fuels. Because of its widespread use as a mobile energy source, the internal combustion engine will continue to be a principal source of greenhouse gas emissions and the principal consumer of liquid fossil fuels. As detailed in the 2007 IPCC report on climate change (Pachauri, et al., 2007), the global transportation sector accounts for 13.1% of the total anthropogenic contribution to greenhouse gasses (GHG). In the United States, the EPA estimates that 27% of the over 6800 Tg of CO2 Eq emitted in 2010 were from the transportation sector (EPA, 2012), and 62% of these emissions were from (LDV) light duty vehicles (i.e. passenger cars and light duty trucks, SUV’s, and minivans). Therefore, increasing the efficiency of internal combustion engines, presents an immediate and major opportunity for greenhouse gas reduction and extension of fossil fuel reserves. Spark Ignited (SI) engines, despite having increasingly lower emissions due to the advancements of 3-way catalysts are limited by the narrow range of air/fuel ratios required for 3-way catalyst operation and low compression ratios
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Paper # 070IC-0264 Topic: Internal Combustion and Gas Turbine Engines
8th
U. S. National Combustion Meeting
Organized by the Western States Section of the Combustion Institute
and hosted by the University of Utah
May 19-22, 2013
Autoignition Characterization of Primary Reference Fuels and n-
Heptane/n-Butanol mixtures in a Constant Volume Combustion
Device and Homogeneous Charge Compression Ignition Engine
Marc E. Baumgardner1
Anthony J. Marchese1 S. Mani Sarathy
2
1Department of Mechanical Engineering, Colorado State University, Fort Collins, CO
2 Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal,
Kingdom of Saudi Arabia
Premixed or partially premixed compression ignition modes, such as homogeneous charge compression ignition (HCCI), have been a
particular focus among researchers because of their potential to deliver enhanced fuel efficiency and meet exhaust emissions mandates
without the addition of costly after-treatment technologies as currently required with traditional spark ignition (SI) and direct injection
compression ignition (DICI) engines. These advanced combustion strategies thus seek to combine the advantages of SI and DICI
engines while avoiding their disadvantages. Previous studies have suggested that future fuels with desired properties for optimal
performance in these advanced combustion modes might have properties between that of traditional gasoline and diesel fuels and will
most likely consist of mixtures of petroleum derived products (aromatics, straight and branched alkanes), alcohols, synthetic alkanes,
and fatty acid methyl esters. Existing ignition quality metrics such as Octane or Cetane Number are reasonably consistent for
petroleum-derived fuels in SI and CI engines but can fail to adequately characterize autoignition in advanced combustion modes,
particularly when higher concentrations of alcohols are included in the blend. Accordingly, in this study, the autoignition behavior of
primary reference fuels (PRF) and blends of n-heptane/n-butanol were examined in a Waukesha Fuel Ignition Tester (FIT) and a
Homogeneous Charge Compression Engine (HCCI). Fourteen different blends of iso-octane, n-heptane, and n-butanol were tested in
the FIT – 28 test runs with 25 ignition measurements for each test run, totaling 350 individual tests were done in all. These
experimental results supported previous findings that fuel blends with similar octane numbers can exhibit very different ignition delay
periods. The present experiments further showed that n-butanol blends behaved unlike PRF blends when comparing the autoignition
behavior as a function of the percentage of low reactivity component. These same fuel blends were also tested in a John Deere
4024T diesel engine, which was modified to operate in HCCI mode. The HCCI and FIT experimental results were compared against
multi-zone models with detailed chemical kinetic mechanisms for PRF’s and n-butanol. For both the FIT and HCCI engine data, a
new n-butanol/n-heptane kinetic model is developed that exhibits good agreement with the experimental data. The results also suggest
that that the FIT instrument is a valuable tool for analysis of high pressure, low temperature chemistry and autoignition for future fuels
in advanced combustion engines.
1. Introduction
Depletion of fossil fuels and climate change from anthropogenic greenhouse gas emissions arguably represent the first
civilization-scale challenges ever faced by the human race (Hirsch, et al., 2005) (McKibben, 2006). While there is no
single technological solution for these global challenges, an effective means of rapidly addressing these concerns is to
increase the thermodynamic efficiency of energy conversion devices that consume fossil fuels. Because of its
widespread use as a mobile energy source, the internal combustion engine will continue to be a principal source of
greenhouse gas emissions and the principal consumer of liquid fossil fuels. As detailed in the 2007 IPCC report on
climate change (Pachauri, et al., 2007), the global transportation sector accounts for 13.1% of the total anthropogenic
contribution to greenhouse gasses (GHG). In the United States, the EPA estimates that 27% of the over 6800 Tg of CO2
Eq emitted in 2010 were from the transportation sector (EPA, 2012), and 62% of these emissions were from (LDV) light
duty vehicles (i.e. passenger cars and light duty trucks, SUV’s, and minivans). Therefore, increasing the efficiency of
internal combustion engines, presents an immediate and major opportunity for greenhouse gas reduction and extension of
fossil fuel reserves.
Spark Ignited (SI) engines, despite having increasingly lower emissions due to the advancements of 3-way catalysts
are limited by the narrow range of air/fuel ratios required for 3-way catalyst operation and low compression ratios
2
required to prevent engine knock. The ultimate efficiency of an SI engine is thus restricted by this operationally low
compression ratio and equivalence ratio range. Alternatively, compression ignition (CI) engines (i.e. diesel engines)
operate at higher compression ratios and have lower pumping losses due to fuel injection near top dead center (TDC),
which results in higher efficiency. However, the shortened fuel/air mixing time experienced in CI engines results in a
more heterogeneous fuel/air mixture and higher local flame temperatures, which can result in high concentrations of
oxides of nitrogen (NOx) and particulate matter (PM). As a result, despite the benefits of high efficiency, CI engines have
historically been the heaviest polluters of NOx and PM, both of which are highly problematic in terms of atmospheric
pollution and associated public health issues.
The urgent need to increase efficiency and reduce exhaust
emissions from internal combustion engines has resulted in an
increased interest in High Efficiency Clean Combustion
(HECC) engines. Premixed or partially premixed compression
ignition modes, such as homogeneous charge compression
Equation 11 is consistent with the results of Fig. 9, which showed that numerical predictions of CA50 varied
quadratically with fLTHR.
Since the low temperature heat release occurs at lower pressure it is reasonable to examine the fLTHR as measured
at low pressure in the FIT (fLTHRFIT) to see how well it relates to that seen in the engine. To first examine the correlation
between the low pressure ignition delay data and the HCCI engine data, the Sarathy* mechanism is again used since
model results for FIT and engine conditions can be easily obtained for all fuels. Figure 11(a) is a plot of the predicted
CA50 for HCCI engine model using the Sarathy* mechanism against the fractional low temperature heat release
predictions by the same mechanism under FIT conditions (fLTHR24bar-model). A 2nd
order polynomial in the form of
Equation 11 was fit to the data in Figure 11(a) with an R2 of 0.998.
24bar Sarathy* FIT model fLTHR
0.04 0.05 0.06 0.07 0.08 0.09
Sara
thy*
Mo
de
l C
A50
-10
-8
-6
-4
-2
0
2
4
6
8
Ratio: FIT Ig Dly / dPmax
0.5 0.6 0.7 0.8 0.9 1.0
24b
ar
Sa
rath
y*
FIT
mo
de
l fL
TH
R
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Figure 11. Simple model fits relating FIT to the observed HCCI engine data (a) 24bar FIT model vs. model predictions for CA50 and
(b) actual ratio of FIT ignition delay to ignition delay found from max dP/dt curve.
In its current configuration, measuring fLTHR directly from the FIT data is difficult because the sample rate at which
the pressure data is recorded by the FIT is not sufficient to accurately resolve the LTHR. However, a proxy for this
method is proposed. As shown in Figure 4, the FIT-reported ignition delay can be much earlier than the actual ignition
delay (as defined by maximum pressure rise). As explained above, the FIT measures ignition delay as the time difference
between injection and the chamber pressure rising above a set trigger point. If one makes the simplifying assumption that
the FIT reported ignition delay is proportional to the LTHR then the ratio of FIT reported ignition delay to the ignition
delay found from dPmax (of FIT pressure trace) should have a linear correlation against the fLTHR – this is indeed the
case as is shown in Figure 11(b), which as an R2 of 0.941. Also, the error bars for the data in Figure 11 are large due to
the compounding effect of a ratio of small errors (see Figure 4 for the magnitude of the individual FIT errors, which are
quite small) – this same effect is also seen in Figure 12, which uses the trends found in Figure 11 to predict a simple
model of CA50 based on the FIT data; note that an excellent correlation is found with experimental data.
14
Ratio: FIT Ig Dly / dPmax
0.5 0.6 0.7 0.8 0.9 1.0
CA
50 [de
g A
TD
C]
-10
-5
0
5
10
15
20
Figure 12. Simple model showing the relation between the ratio of FIT ignition delay to ignition delay found from max dP/dt curve
vs. the CA50 found in an HCCI engine.
One caveat to the proposed relationships in Figure 12 is that they are based on the ignition delay data for a n-
heptane/n-butanol blend. A natural outcome of the derivation of Equation 11 is that only fuels with similar ignition delay
behaviors can be regressed using the same general equations. While there will certainly be a fLTHR:CA50 relationship
for every fuel (or fuel blend), the exact equation will depend on the blended fuels such that a given fuel blend may have a
different relation (i.e. the curve in Figure 9 will shift based on the fuels in the blend) but it is possible that these relations
may be assumed to be rooted in the most and least reactive fuels in the blend. In other words, two different fuel blends
may fall on the same blend line (i.e. Figure 9) if the two fuel blends share the same least and most reactive fuels as these
fuels will control the extreme bounds of reactivity.
4. Conclusions
In this study two sets of fuel blends were tested in varying amounts of blend ratios in both a FIT and an HCCI engine.
Two chemical models were also examined and tested against the experimental data; the Sarathy et al. mechanism
updated to include a current n-heptane sub-mechanism by Mehl et al. (dubbed Sarathy*) showed the best ability to model
the results obtained from both sets of experiments. The FIT seemed to accurately predict relative fuel ignition rankings,
though the magnitude of the differences was greater than that seen in an HCCI engine. However, the FIT experimental
data proved quite useful, serving as further validation of the examined models at low temperatures. Lastly, the fractional
LTHR showed great promise of being an indicator of CA50 location based on the derivation and data comparison herein.
Investigating the influence of fLTHR and the possibility of obtaining this information from an FIT will be the topic of
future research.
Acknowledgements
This research was funded in part by the Colorado Center for Biofuels and Biorefining (C2B2). The authors would also
like to thank Harrison Bucy and Nathan Pekoc for their technical assistance with the FIT as well as Aaron Gaylord,
Michael Herder, Scott Salisbury, Ryan Farah, Ryan Peters, and Nathan Zeleski for their assistance in converting the John
Deere 4024T engine to operate in HCCI mode. Co-author SMS acknowledges funding from the Clean Combustion
Research Center (CCRC) at KAUST, and from Saudi Aramco under the FUELCOM research program.
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