Performance HCCI Ethanol Fuelled Engine

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Predicting Fuel Performance for Future HCCI Engines
a b a a
Vi H. Rapp , William J. Cannella , J.-Y. Chen & Robert W. Dibble
a
Department of Mechanical Engineering, University of California–Berkeley, Berkeley,
California, USA
b
Chevron Energy Technology Company, Richmond, California, USA
Accepted author version posted online: 04 Dec 2012.

To cite this article: Vi H. Rapp , William J. Cannella , J.-Y. Chen & Robert W. Dibble (2012): Predicting Fuel Performance for
Future HCCI Engines, Combustion Science and Technology, DOI:10.1080/00102202.2012.750309

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Predicting Fuel Performance for Future HCCI Engines

Vi H. Rapp1,*, William J. Cannella2, J.-Y. Chen1, Robert W. Dibble1
1
Department of Mechanical Engineering, University of California–Berkeley, Berkeley,
California, USA, 2Chevron Energy Technology Company, Richmond, California, USA

*Corresponding author: E-mail: vhrapp@berkeley.edu
Downloaded by [Universiti Teknologi Malaysia] at 20:55 11 January 2013

Abstract

The purpose of this research is to investigate the impact of fuel composition on auto-

ignition in HCCI engines in order to develop a future metric for predicting fuel

performance in future HCCI engine technology. A single-cylinder, variable compression

ratio engine operating as an HCCI engine was used to test reference fuels and gasoline

blends with Octane numbers (ON) ranging from 60-88. Correlations between fuel

composition, ON, and two existing methods for predicting fuel auto-ignition in HCCI

engines (Kalghatgi’s Octane Index and Shibata and Urushihara’s HCCI Index) are

investigated. Results show that Octane Index and HCCI Index poorly predict the impact

of fuel composition on auto-ignition for fuels with the same ON. The effect of ethanol in

delaying auto-ignition depends on the composition of the original gasoline blend; the

same is true for the addition of naphthenes. Low temperature heat release (LTHR)

correlates well with auto-ignition for gasoline fuels exhibiting LTHR.

KEYWORDS: Auto-ignition, Homogenous charge compression-ignition (HCCI), Fuel

composition

INTRODUCTION

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Increasing concern with climate change has encouraged the development of alternative

fuels and advanced engine technologies that improve efficiency and reduce CO2

emissions. Homogeneous charge compression ignition (HCCI) engines offer the potential

for Diesel-like efficiencies and low nitrogen oxide emissions compared with conventional

gasoline and Diesel engines. HCCI engines also offer fuel flexibility as they can operate

using a wide variety of fuels such as Diesel, gasoline, and alternative fuels (Thring, 1989;

Fuhs, 2008). In the early twentieth century, Weiss and Mietz developed the first HCCI-

like combustion engine, called the hot-bulb engine (Erlandsson, 2002). The hot-bulb

engine offered a simple and durable design that had brake thermal efficiencies

comparable to contemporary Diesel engines. Later, in 1979, Onishi et al. (1979)

published the first research on a gasoline-fueled HCCI engine. The two-stroke gasoline

engine, using a process dubbed Active Thermo-Atmosphere Combustion (ATAC) by the

authors, increased fuel economy and decreased exhaust emissions at part-throttle

operation.

In 1983, Najt and Foster (1983) achieved compression ignition homogenous charge

(CIHC) combustion in a four-stroke gasoline engine. Using the same engine as Najt and

Foster, Thring (1989) studied the effects of exhaust gas recirculation, intake temperature,

and compression ratio; he was also the first to use the acronym HCCI. Seven years after

Thring, the first research burning Diesel fuel in an HCCI engine appeared (Gray and

Ryan, 1997) and led to research testing other fuels, such as alcohols (Oakley et al., 2001),

hydrogen (Shudo and Ono, 2002), natural gas (Christensen, Johansson and Einewall,

1997; Hiltner et al., 2000; Olsson et al., 2002; Stanglmaier, Ryan and Souder, 2001),

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propane(Au et al., 2001; Flowers et al., 2001), and many fuel blends with additives (Eng,

Leppard and Sloane, 2003; Yao, Zheng and Liu, 2009)

Although HCCI engines offer fuel flexibility and a solution for meeting new, strict

pollution requirements, HCCI engines have a limited load range and cannot support high

load demands required by automobiles. Hybrid HCCI engines, such as SI-HCCI (Zhang,

Xie and Zhao, 2009; Koopmans et al., 2003), HCCI-DI (Canova et al., 2007; Helmantel

and Denbratt, 2004), or HCCI-electric(Wu and Zhang, 2012), offer a solution for

reducing emissions and increasing the load operating range. As HCCI engine technology

becomes more widely used in automotive technology, developing fuels to support hybrid

HCCI engines will become increasingly important.

Conventional methods for quantifying fuel auto-ignition, such as Research Octane

Number (RON), Motor Octane Number (MON), Octane Number (ON = ½RON +

½MON) and Cetane Number, poorly predict auto-ignition in HCCI engines (Kalghatgi,

2005; Shibata and Urushihara, 2007). Kalghatgi (2005) developed an Octane Index (OI)

for measuring the auto-ignition or anti-knock quality of a practical fuel at different

operating conditions. The OI, not to be confused with ON, is defined as,

OI = (1 K)RON + (K)MON, (1)

where K is a parameter specified by engine operating conditions. Although the OI may

be applicable for HCCI operation (Kalghatgi, 2005), the OI does not fully describe the

impact of fuel composition on auto-ignition in HCCI engines.

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Shibata and Urushihara (2007) investigated the impact of fuel composition on auto-

ignition in HCCI engines and introduced three HCCI Indices. While all three HCCI

Indices predict auto-ignition similarly, the relative HCCI Index (HIrel) predicts auto-

ignition of fuels using the fuel composition and MON (Shibata and Urushihara, 2007).

The HIrel is defined as,

HI rel = MON + α(nP) +β(iP)
(2)
(O) δ(A) + (OX),

where nP is the percent n-paraffins by volume, iP is the percentiso-paraffins by volume,

O is the percent olefins and cycloalkanes by volume, A is the percent aromatics by

volume, OX is the percent oxygenates by volume, and α, β, γ, δ, and ε are temperature

dependent parameters.

In this paper, the capability of two existing methods (the OI and the HIrel) for predicting

the impact of fuel composition on auto-ignition in HCCI engines is investigated and

correlations between fuel composition and auto-ignition of fuels in a HCCI engine are

explored. Fuels tested consist of following five different blends:

1. Primary reference fuels (PRF):blends of isooctane and n-heptane

2. Toluene reference fuels (TRF): PRF fuels blended with toluene

3. Ethanol reference fuels(E-PRF): PRF fuels blended with Ethanol

4. Gasoline blendstocks

5. Gasoline blendstocks with different pure compounds added (“additives”)

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Following this Introduction, the instrumentation and experimental design are described.

Next, results and discussions are presented. Last, future work is suggested and

conclusions are drawn.

MATERIALS AND METHODS

Engine and Fuel Specifications

Similar to previous metrics for predicting fuel auto-ignition, such as RON and MON

(ASTM RON Standard, 2011; ASTM MON Standard, 2011), experiments were

conducted using a variable compression ratio, single cylinder cooperative fuel research

(CFR) engine operating in HCCI mode. Engine specifications and operating conditions

are listed in Table 1. The engine was preheated by operating in spark-ignition mode

under stoichiometric conditions. Once the coolant temperature reached 80°C, the

equivalence ratio was decreased to =0.33 ( =3.0). Next, the compression ratio (CR)

was slowly increased until stable auto-ignition (no misfiring) occurred. The lowest CR

limit was determined by decreasing the CR until HCCI operation became unstable. The

highest CR limit for each fuel was determined by increasing the CR until the in-cylinder

pressure exceeded 50 bar (a limit to safe guard the mechanical integrity of CFR engine)

or the ringing intensity became too great (Eng, 2002). For each experiment, equivalence

ratio was held constant at φ=0.33 (λ=3.0). Data were taken at various compression ratios

between the lowest and the highest limits. For a fixed CR, 300 thermodynamic cycles

(each cycle with 720 CAD) of in-cylinder pressure data were collected along with

exhaust emissions before the catalytic converter.

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The effects of fuel composition on auto-ignition timing n HCCI engines, measured by

CA50 (the crank angle degree at which 50% of the cumulative heat has been

released),was explored by testing twenty different fuel blends at an engine intake

temperature of 150°C and one fuel, PRF70, at intake temperatures of 70°C, 115°C, and

150°C. Of the twenty fuels, eight fuels were reference fuel blends. The reference fuel

blends consisted of PRF60, PRF70, PRF75, PRF85, PRF88, TRF70, S70, and E-PRF70.

For primary reference fuels (PRF), the number following "PRF" is the RON, MON, and

percent isooctane by volume. TRF70 (46% n-heptane and 54% toluene, by volume) has a

calculated RON of 70.5and MON of 63.5, which were calculated using a linear-by-

volume blending equation created by Morgan et al.(2010). For S70 (64% isooctane, 31%

n-heptane, and 5% toluene by volume) the RON and MON were measured, by Chevron,

as 70.5and 69.6, respectively. A RON of 69.5 and MON of 68.6 were calculated for E-

PRF70 (64% isooctane, 31% n-heptane, and 5% ethanol by volume) using the blending

RON and blending MON values from Anderson et al.(2010). A summary of the reference

fuel blend compositions, RON, and MON are given in Table 2.

Two different base gasolines, typically used in U.S. gasoline blends, were provided by

Chevron, labeled G1 and G2. Hydrocarbon class information and the RON and MON of

the base gasolines are provided in Table 3. The base gasoline fuels were blended with

different “additives”: n-heptane, ethanol, cycloparaffins, and aromatics. RON and MON

for the gasoline fuel blends are listed in Table 4along with the type of additive. Fuels

with a calculated RON and MON were determined using the blending RON and blending

MON of the additive.

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Determining Octane Index, Relative HCCI Index, And Heat Release

The K factor for the OI, shown in Eq. (1), is a function of the in-cylinder temperature

when the in-cylinder pressure, during the compression stroke, reaches 15 bar (Tcomp,15bar)

(Kalghatgi, 2005). The K factor is computed using the following equations:

K = 0.0426 (Tcomp,15bar ) 35.2 if Tcomp,15bar 825 K , (3)

or

0.0056(Tcomp,15bar ) 4.68 if Tcomp,15bar 825K . (4)

For the UC Berkeley CFR engine operating at 600 RPM with an intake temperature of

150°C, Tcomp,15bar is estimated to be 780K, yielding K=-0.312 using Eq. (4) (Kalghatgi,

2005).

Because the OI was used to develop the HIrel, the temperature dependent constants for the

HIrel, shown in Eq.(2), are also functions of Tcomp,15bar. For Tcomp,15bar = 780K, Shibata and

Urushihara list values for the constants as follows (Shibata and Urushihara, 2007):α = -

0.487, β= -0.380, γ= -0.246, δ= -0.222, and ε=0.049. These constants were determined

using a similar method as the K factor in the Octane Index.

In addition to developing the relative HCCI Index, Shibata et al. (2005) suggested that

low temperature heat release (LTHR) might correlate with auto-ignition better than high

temperature heat release (HTHR). LTHR is defined as total heat release (Killingsworth,

2007) d from combustion at in-cylinder temperatures less than 1000K while HTHR is

total heat released from combustion at in-cylinder temperatures greater than 1000K. In

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their study, correlations between auto-ignition and LTHR were investigated by dividing

LTHR by HTHR for each fuel, yielding a heat release ratio. In this paper, the net heat

release per crank angle degree (dQ/dθ) was determined using the first law of

thermodynamics (Heywood, 1988; Stone, 1999),

dQ γ dV 1 dp
= p + V (5)
dθ γ 1 dθ γ 1 dθ

where γ is the specific heat ratio, p is pressure, and V is volume. To avoid numerically

differentiating the discrete pressure measurements and amplifying signal noise, Eq. (6)

was be rewritten as,

dQ 1 d(pV) dV
= +p (6)
dθ γ 1 dθ dθ'

and the cumulative net heat release, Qi, was computed as a finite sum instead of a

continuous integral using (Killingsworth, 2007),

i
1
Qi [pi Vi p 0 V0 ] p j ( V ) j, (7)
1 j 0

where i and j imply a discrete measurement of pressure and volume at a given crank

angle degree. The specific heat ratio, γ, was assumed to be constant during compression

and expansion. A linear fit between the compression γ and expansionγ was used to

calculate dQ/dθ during combustion. Figure 1 shows the inflection points in the heat

release rate that were used to distinguish between LTHR and HTHR. We assumed that

LTHR began when the heat release rate was greater than zero and that HTHR ended

when the heat release rate dropped below zero.

Measurement Instrumentation

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In-cylinder pressure was measured using a 6052B Kistler piezoelectric pressure

transducer in conjunction with a 5044A Kistler charge amplifier and was recorded every

0.1 crank angle (CA) degree. The cylinder pressure transducer was mounted in the

cylinder head. Intake pressure was measured using a 4045A5 Kistler piezoresistive

pressure transducer in conjunction with a 4643 Kistler amplifier module. Crank angle

position was determined using an optical encoder, while an electric motor, controlled by

an ABB variable speed frequency drive, controlled the engine speed. A Motec M4 ECU

(Engine Control Unit) controlled injection timing, injection pulse width, and injection

duty cycle. Before collecting data for each experiment, the engine was run until the

coolant temperature reached 80°C and combustion became steady. A Horiba analyzer

was used for measuring exhaust gases (CO, CO2, O2, unburned hydrocarbons (UHC), and

nitrogen oxides (NOx)). For lean complete combustion, emissions measurements were

used to deduce the normalized air-fuel ratio using,

nc [O 2 ]
1 , (8)
nH no [CO 2 ]
nc
4 2

where nc is the number of carbon atoms in the fuel, nH is the number of hydrogen atoms

in the fuel, nO is the number of oxygen atoms in the fuel, [O2] is the percent oxygen

measured in the emissions, and [CO2] is the percent carbon dioxide measured in the

emissions. The number of carbon, hydrogen, and oxygen atoms were estimated using the

fuel composition. The uncertainty in the normalized air-fuel ratio is approximately± 0.05.

EFFECTS OF FUEL COMPOSITION ON AUTO-IGNITION

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The following results demonstrate the impact of fuel composition on auto-ignition in

HCCI engines. First, auto-ignition timing, measured by CA50, of each fuel at various

compression ratios is presented. Second, the Octane Index (OI) is compared with

experimental data. Third, the relative HCCI Index (HIrel) is compared with experimental

data. Fourth, correlations between fuel composition, auto-ignition and low temperature

heat release (LTHR) are investigated.

Auto-Ignition Timing (Ca50)

The effects of fuel composition on auto-ignition were first investigated by measuring the

auto-ignition timing (CA50) of each fuel at various compression ratios. Figure 2 plots

CA50 versus CR for the twenty fuels tested showing that fuel blends with similar ON do

not always auto-ignite at the same CR, which is consistent with previous research (Liu et

al., 2009). The uncertainty in CA50 and CR was calculated to be ±0.5 and ±0.1,

respectively (Taylor, 1997). For example, tested fuels with ON~70 are: PRF70, S70,

TRF70, E-PRF70, G2, and G1-H. As seen in Fig. 2, PRF70, S70, and G1-H auto-ignite

at similar CR values. However, TRF70 auto-ignites half a CR higher than PRF70, E-

PRF70 auto-ignites a full CR higher than PRF70, and G2 auto-ignites three CRs higher

than PRF70. The results also suggest that ethanol inhibits auto-ignition more than toluene

as S70 auto-ignites at the same CR as PRF70, while E-PRF70 auto-ignites a full CR

higher than PRF70 and half a CR higher than TRF70.

The results also show adding the same amount of ethanol, 10% by volume, to G1

(RON=87) and G2 (RON=70) does not have the same effect on auto-ignition. As shown

in Fig. 2, G1-E2 (RON=90) auto-ignites about one CR higher than G1, while G2-E2

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(RON=78) auto-ignites three CRs higher than G2. Like ethanol, the naphthene, N1,

affects auto-ignition of G2 more than G1; G2-N1 (RON=N/A) auto-ignites about half a

CR higher than G2 and G1-N1 (RON=86) auto-ignites at about the same CR as G1.

The Octane Index (Oi)

Although the OI was developed for predicting anti-knock qualities of practical fuels in

spark ignited engines, Kalghatgi (Kalghatgi, 2005) suggests that the OI can be used for

predicting auto-ignition of fuels in an HCCI engine. For the CFR engine running at the

same inlet temperature, inlet pressure, and RPM, the compression ratio when CA50=6

deg ATDC is used for quantifying the fuel’s propensity of autoignition. This operating

condition was chosen because data was successfully collected for all fuels when CA50=6

deg ATDC.Figure3 shows the relationship between OI and compression ratio when a

CA50=6 deg ATDC for nineteen of the twenty fuels tested. The OI could not be

calculated for G2-N1 because RON and MON were unavailable. The OI accurately

predicts auto-ignition of the primary reference fuel (PRF) blends, agreeing well with

previous results (Liu et al., 2009; Yao, Zheng and Liu, 2009). These results were

expected because for PRF blends OI = ON. The OI poorly predicts auto-ignition of some

fuels with the same ON. For example, the OI predicts S70, E-PRF70, and G2 will have

similar auto-ignition characteristics; however, E-PRF70 auto-ignites almost one CR

higher than S70 and G2 auto-ignites almost two CR higher than S70.

Additionally, the OI poorly predicts auto-ignition of fuels containing naphthenes. The OI

predicts G1-N1 (RON=86, MON=79) and G1-N2 (RON=87, MON=81) having similar

auto-ignition characteristics, but the experimental results show G1-N2 auto-igniting one

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CR higher than G1-N1. The OI also predicts G1-A2 (RON=91, MON=82) auto-igniting

after G1-N2, but the experimental results show G1-N2 auto-igniting one CR higher than

G1-A2. Although the OI correlates with the RON and MON, it is not sufficient for

predicting auto-ignition of fuels with similar ON.

The Relative HCCI Index (Hirel)

The HIrel, introduced by Shibata and Urushihara (2007), is the first published research for

predicting auto-ignition of fuels in an HCCI engine using the fuel composition and MON.

Like the OI, the HIrel predicts auto-ignition order of fuels; fuels with higher HIrel require

higher CR for auto-ignition (i.e. more difficult to auto-ignite). Figure 4 shows the

relationship between HIrel and CR when CA50=6 deg ATDC for nineteen of the twenty

fuels tested. The HIrel could not be calculated for G2-N1 because MON was unavailable.

The HIrel accurately predicts auto-ignition of some PRF blends, and some gasoline fuels

blended with ethanol. However, the HIrel does not accurately predict ignition order of

some fuels with similar ON. For example, PRF70, S70, G1-H, TRF70, E-PRF70, and G2

have the same HIrel, but experimental results show TRF70, E-PRF70, and G2 auto-

igniting at different CRs than PRF70, S70, and G1-H and G2-E2 auto-igniting at different

CR’s than PRF 75.

The HIrel also poorly predicts auto-ignition of fuel blends containing different aromatics.

The HIrel assumes that fuels containing different aromatic compounds at the same

concentration and approximately same MON will have similar effects on auto-ignition, as

all aromatics are grouped together in Eq. (2). Fig. 4 shows that different aromatics at the

same concentration and approximately same MON can have different effects on auto-

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ignition. For example, G1-A1 has a lower HIrel than G1-A2, but the experimental results

show G1-A1 auto-igniting half a CR higher than G1-A2. Since naphthenes are not

included in Eq. (2), and if their effects are assumed to be the same, the HIrel relation

would predict G1-N1 and G1-N2 having similar effects on auto-ignition (Shibata and

Urushihara, 2007). However, Fig. 4 shows G1-N1 auto-igniting one CR lower than G1-

N2. It should be noted that temperature dependent constants used for computing HIrel

were developed using the K factor in the OI. Therefore, the results from the HIrel were

expected to be similar to the results from the OI.

Low Temperature Heat Release

Previous research (Shibata et al., 2005) suggests low temperature heat release (LTHR)

may correlate with auto-ignition better than high temperature heat release (HTHR).

Figures 5 and 6 show the dependence of the heat release ratio (the ratio of average LTHR

to HTHR) on CR. For gasoline fuels, the heat release ratio decreases almost linearly as

CR (when CA50=6 deg ATDC) increases from 9 to 15 (see Fig. 5). Gasoline fuels with

the same ON show different heat release ratios and correlate well with CR. For example,

G1-H auto-ignites almost 2 CRs lower than G2 and the heat release ratio predicts G1-H

has about 3% less LTHR than G2. The heat release ratio also suggests gasoline fuels with

more LTHR will auto-ignite at lower CR, agreeing with previous research (Shibata et al.,

2005). For gasoline fuels auto-igniting at CRs greater than 15, no LTHR was detectable

using our instrumentation.

Figure 6 shows reference fuels with similar ON (PRF70, S70, TRF70, and E-PRF70)

have similar amounts of LTHR, suggesting the reference fuels should auto-ignite at

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similar CRs. However, these reference fuels do not auto-ignite at the same CR. For

example, ethanol in E-PRF70 was expected to suppress LTHR. Instead, E-PRF70 shows

similar amounts of LTHR as PRF70 while auto-igniting at the same CR as PRF75. The

results suggest that the addition of Ethanol makes auto-ignition more difficult, but

ethanol does not necessarily suppress LTHR. One possible explanation for E-PRF70

exhibiting the same LTHR as PRF70 is that the 31% n-heptane in E-PRF70 may promote

more LTHR than ethanol suppresses. For fuels exhibiting decreasingly low amounts of

LTHR, a water-cooled pressure transducers may provide better resolution (Sjoberg and

Dec, 2003; Stone, 1999).

Overall, the results show that LTHR correlates well with auto-ignition of gasoline blends

exhibiting LTHR, but does not correlate well with reference fuel blends. This suggests

that reference fuels for spark-ignited engines may not be appropriate reference fuels for

HCCI engines.

CONCLUSIONS

In this paper, we investigated the impact of fuel composition on auto-ignition in HCCI

engines in order to develop a future metric for predicting fuel performance in future

HCCI engine technology. The following conclusions were derived:

• For a fixed intake temperature, intake pressure, and equivalence ratio, fuels with

the same Octane Number (ON) do not auto-ignite at similar compression ratios (CR).

Additionally, the effect of ethanol (and naphthene) in delaying auto-ignition is dependent

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on the gasoline blendstocks.

The Octane Index and relative HCCI Index correlate well with auto-ignition of primary

reference fuel but poorly predict auto-ignition of gasoline fuel blends containing

naphthenes, aromatics, and ethanol.

Low temperature heat release (LTHR) correlates well with auto-ignition for gasoline

fuels with measurable LTHR but does not correlate well for reference fuels. The results

suggest that reference fuels may not be appropriate for describing fuel performance in

HCCI engines. For fuels auto-igniting at CRs greater than 15, LTHR could not be

detected.

More than one metric may be required for predicting auto-ignition. For gasoline fuels that

exhibit LTHR, LTHR better predicts auto-ignition order than Octane Index and the

Relative HCCI Index. For fuels that do not exhibit LTHR, a different metric is needed.

To further advance development of a future metric for predicting fuel performance in

future HCCI engine technology, we recommend that more fuel blends containing linear

amounts of toluene, ethanol, and various aromatics, by volume, should be explored to

help identify reference fuels for a standard HCCI number. Additionally, different test

conditions, such higher RPM and lower intake temperatures, should be explored further

for the fuels used in this research. Trends established at different operating conditions

could be used with trends found in this paper to establish a standard HCCI metric.

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ACKNOWLEDGEMENTS

Research conducted at the University of California, Berkeley was supported by the

Chevron Corporation. The authors also wish to acknowledge the assistance of T.

Dillstrom, A. Van Blarigan, and M. Wissink in conducting experimental measurements.

ABBREVIATIONS

ATAC Active Thermo-Atmosphere Combustion

ATDC After top dead center

CA50 Crank angle at which 50% of heat has been released

CAD Crank angle degree

CFR Cooperative Fuel Research

CIHC Compression ignition homogenous charge

CR Compression ratio

DI Direct Injection

E-PRF Ethanol Reference Fuel

ECU Engine control unit

HCCI Homogenous Charge Compression Ignition

HIrel, Relative HCCI Index

HTHR High temperature heat release

LTHR Low temperature heat release

MON Motor Octane Number

OI Octane Index

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ON Octane Number

PRF Primary Reference Fuel

RON Research Octane Number

SI Spark Ignition

TRF Toluene Reference Fuel

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Table 1. CFR engine specifications

Displacement 0.616 L

Stroke 114.3 mm

Bore 82.8 mm

Connecting Rod 254 mm

Engine Speed 600 RPM

Coolant Temperature 80°C ±1°C

Intake Pressure 1.035 bar

Intake Temperature 150°C ±1°C

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Table 2. Reference fuel blend compositions by volume percent with RON and MON

Fuel Name % iso-ocatne % n-heptane % Toluene % Ethanol RON MON

PRF 60 60 40 0 0 60 60

PRF 70 70 30 0 0 70 70

PRF 75 75 25 0 0 75 75

PRF 85 85 15 0 0 85 85

PRF 88 88 12 0 0 88 88

TRF 70* 0 46 54 0 70.5 63.5

S 70 64 31 5 0 70.5 69.6

E-PRF 70+ 64 31 0 5 69.5 68.6

n-heptane 0 100 0 0 0 0
*
RON and MON were calculated using method described by Morgan et al. (2010).
+
RON and MON were calculated using bRON and bMON values from Anderson et al.

(2010).

RON and MON of remaining fuels were assumed.

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Table 3. Base gasoline hydrocarbon class information (percent by volume)

Fuel %N- %Iso- %Ole %Cycloparaffins %Arom RO MON

Name Paraffins Paraffins fins atics N

G1 14.2 44.9 5.2 9.8 25.9 87 80

G2 4.7 48.2 0.3 34.4 12.4 70 65

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Table 4. Gasoline fuel blend information

Fuel Name Additive RON MON

G1-H N-heptane 70b 65b

G2-H N-heptane 60a 55a

G1-E1 Ethanol 93a 84a



G1-A1 Toluene 91a 82a

G1-A2 O-Xylene 91b 82b

G1-N1 Methylcyclohexane 86b 79b

G2-N1 Methylcyclohexane N/A N/A

G1-N2 Cyclohexane 87b 81b

“N/A” implies not available
a
RON or MON was estimated
b
RON or MON was provided by Chevron

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Figure 1. Distinction between low temperature heat release (LTHR) from low
temperature combustion and high temperature heat release (HTHR) from high
temperature combustion.

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Figure 2. Auto-ignition in HCCI engines varies almost linearly with compression ratio.
Fuels with the same octane number (ON), such as G2 and G1, do not auto-ignite at the
same compression ratios. Error bars are suppressed for visibility. Error in CA50 is
typically ± 0.5 of the shown value.

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Figure 3. The Octane Index poorly predicts auto-ignition fuels with similar Octane
Number (ON) but shows an almost linear relationship (R2=0.90) with compression ratio
for all fuel blends.

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Figure 4. HCCI index correlates well with primary reference fuel blends but poorly
predicts auto-ignition of fuel blends containing ethanol, aromatics, or naphthenes.

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Figure 5. For gasoline fuels, the ratio of LTHR to HTHR shows an almost linear decrease
with the compression ratio at a CA50=6 deg ATDC.

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Figure 6. Reference fuels with the same ON have similar amounts of LTHR even though
they auto-ignite at different compression ratios. PRF blends decrease with compression
ratio at a CA50=6 deg ATDC.

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