Energy & Fuels 2008, 22, 2207–2215
2207
Effects of Inlet Pressure and Octane Numbers on Combustion and
Emissions of a Homogeneous Charge Compression Ignition (HCCI)
Engine
Haifeng Liu, Mingfa Yao,* Bo Zhang, and Zunqing Zheng
State Key Laboratory of Engines, Tianjin UniVersity, Tianjin, 300072, China
ReceiVed August 25, 2007. ReVised Manuscript ReceiVed April 23, 2008
The influence of inlet pressure (Pin) and octane numbers on combustion and emissions of a homogeneous
charge compression ignition (HCCI) engine was experimentally investigated. The tests were carried out in a
modified four-cylinder direct injection diesel engine. Four fuels with different research octane number (RON)
were used during the experiments: 90-RON, 93-RON, and 97-RON primary reference fuel (PRF) blend and
a commercial gasoline, 94.1-RON(G). The inlet pressure conditions were set to give 0.1, 0.15, and 0.2 MPa
of absolute pressure. The results indicate that, with the increase of inlet pressure, the start of combustion
(SOC) advances and the cylinder pressure increases. The effects of the PRF octane number on SOC are weakened
as the inlet pressure increased. However, the difference of SOC between gasoline and PRF is enlarged with
the increase of the inlet pressure. The successful HCCI operating range is extended to the upper and lower
load as the inlet pressure increased. The maximum achievable load of gasoline is higher than that of PRF with
the cases of supercharging. The HC and NOx emissions of the HCCI engine decrease when supercharging is
employed, while CO emissions increase remarkably. The PRF octane number has little effect on HC, CO, and
NOx emissions when supercharging is employed. Nevertheless, the HC and CO emissions of gasoline are
higher than those of PRF with supercharging.
1. Introduction
Driven by its potential for high efficiency operation with
significantly lower NOx and particulate emissions than conventional spark-ignited (SI) and diesel engines, homogeneous charge
compression ignition (HCCI) has been received as one of the
most promising internal combustion engine concepts for the
future. In a HCCI engine, the lean locally homogeneous air-fuel
mixture is compressed in the cylinder and auto-ignites simultaneously at multiple locations within the cylinder. The heat
release from these regions compresses the remaining charge,
promoting further auto-ignition events, and rapidly, the mixture
combusts without any flame propagation.1–3
However, HCCI combustion still poses some challenges that
must be overcome before it can be integrated into practical
applications. The main limitations of HCCI combustion are the
narrow operating window that results from the lack of directignition control and the limited power density. Ignition timing
is mainly controlled by the chemical kinetics of the air-fuel
mixture.4 The power density is limited by an excessive energy
release rate and the associate noise, vibration, and harshness
* To whom correspondence should be addressed: State Key Laboratory
of Engines, Tianjin University, Tianjin 300072, China. Telephone: 86-2227406842 ext. 8014. Fax: 86-22-27383362. E-mail: y_mingfa@tju.edu.cn.
(1) Onishi, S.; Jo, S. H.; Shoda, K.; Jo, P. D.; Kato, S. Active thermoatmosphere combustion: A new combustion progress for internal combustion
engines. SAE Tech. Pap. Ser. 1979, 790501.
(2) Kim, D. S.; Lee, C. S. Effect of n-heptane premixing on combustion
characteristics of diesel engine. Energy Fuels 2005, 19, 2240–2246.
(3) Stanglmaier, R. H.; Roberts, C. E. Homogeneous charge compression
ignition (HCCI): Benefits, compromises, and future engine applications.
SAE Tech. Pap. Ser. 1999, 1999-01-3682.
(4) Shigeyuki, T.; Ferran, A.; James, C. K. A reduced chemical kinetic
model for HCCI combustion of primary reference fuels in a rapid
compression machine. Combust. Flame 2003, 133, 467–481.
effects. Under some operating conditions, an HCCI engine can
produce higher HC and CO emissions than a SI engine.
Because the homogeneous mixture auto-ignites, combustion
starts more or less simultaneously in the whole cylinder. To
limit the rate of combustion under these conditions, the mixture
must be highly diluted. Without sufficient mixture dilution,
problems associated with extremely rapid combustion and
knocking-like phenomena will occur, as well as excessive NOx
production. Thus, charge dilution is provided in the form of
excess air (very lean air/fuel ratios) or by exhaust gas recirculation (EGR). This dilution effectively slows down the rate of
combustion. However, the high dilution also limits the amount
of fuel that can be added for a given mass of charge; i.e., engine
power is low relative to the mass flow through the engine. The
necessity of running the engine with higher loads has prompted
the investigation of supercharged HCCI. Examples of other
studies that have applied boosting methods to increase the speedload window of HCCI include boosting with a turbocharger on
a PFI six-cylinder engine5 and boosting a single-cylinder engine
by a stand-alone compressor.6,7 In addition, the SI, naturally
aspirated(NA) HCCI, and supercharged HCCI are investigated
in the Gharahbaghi et al.8 paper, which results show that a
smaller supercharger with a moderate boost is preferable to avoid
an unacceptable penalty in fuel economy. All of these results
(5) Olsson, J.-O.; Tunestal, P.; Johansson, B. Boosting for high load
HCCI. SAE Tech. Pap. Ser. 2004, 2004-01-0940.
(6) Yap, D.; Megaritis, A. Applying forced induction to bioethanol HCCI
operation with residual gas trapping. Energy Fuels 2005, 19, 1812–1821.
(7) Christensen, M.; Johansson, B.; Amneus, P.; Mauss, F. Supercharged
homogeneous charge compression ignition. SAE Tech. Pap. Ser. 1998,
980787.
(8) Gharahbaghi, S.; Wilson, T. S.; Xu, H.; Cryan, S.; Richardson, S.;
Wyszynski, M. L.; Misztal, J. Modelling and experimental investigations
of supercharged HCCI engines. SAE Tech. Pap. Ser. 2006, 2006-01-0634.
10.1021/ef800197b CCC: $40.75 © 2008 American Chemical Society
Published on Web 06/17/2008
2208 Energy & Fuels, Vol. 22, No. 4, 2008
Liu et al.
Table 1. Engine Specifications
displacement
bore
stoke
compression ratio
inlet valve open
inlet valve close
exhaust valve open
exhaust valve close
1.3 L
112 mm
132 mm
17.5
13.5° BTDC
38.5° ABDC
56.5° BBDC
11.5° ATDC
are showing satisfactory improvement in terms of load and NOx
over aspirated HCCI.
On the other hand, fuel properties are an important factor to
determine the ignition timing. Many efforts have been taken to
find better fuel for HCCI engine operation to control the autoignition process and expand the operation window. Since 1990s,
a wide range of fuels, such as diesel, n-heptane, dimethyl ether
(DME), gasoline, isooctane, ethanol, and methanol, etc., have
been tried in HCCI combustion.9–13 Tanaka et al.9 investigated
the auto-ignition characteristics, according to the fuel components (such as paraffins, cyclic paraffins, olefins, cyclic olefins,
and aromatic hydrocarbon), using a rapid compression machine
(RCM) and, as a consequence, proposed a combustion control
method using various fuels and additives, according to the
running conditions of a HCCI engine. Hou et al.10 investigated
the effect of the addition of high-octane oxygenated fuel,
including methyl tertiary butyl ether (MTBE), ethanol, and
methanol, on combustion phasing and the combustion rate of
HCCI combustion fueled with n-heptane as a base fuel. The
results show that MTBE has more advantages over methanol
and ethanol in the potential of extending the operating range of
n-heptane-fueled HCCI combustion. A study on the controlling
strategies of HCCI combustion with using the fuel of dimethyl
ether and methanol has been carried out in the previous work.11
The results show that the maximum indicated mean effective
pressure (IMEP) of HCCI operation can reach 0.74 MPa using
DME/methanol dual fuel. However, the maximum IMEP of pure
DME can only reach 0.44 MPa. Moreover, numerous experimental and numerical studies have been investigated using the
PRF, n-heptane, and iso-octane.12–15 These investigations have
been very useful for probing the effect of the fuel octane
numbers on HCCI combustion.
In previous works, the effects of research octane number
(RON) of primary reference fuel (PRF) on HCCI combustion
have been investigated.13 The results have shown that the
maximum IMEP of high octane number is higher than that of
the low octane number. However, if the octane number is too
(9) Tanaka, S.; Ayala, F.; Keck, J. C.; Heywood, J. B. Two-stage ignition
in HCCI combustion and HCCI control by fuels and additives. Combust.
Flame 2003, 132, 219–239.
(10) Hou, Y.; Lu, X.; Zu, L.; Ji, Li.; Huang, Z. Effect of high-octane
oxygenated fuels on n-heptane-fueled HCCI combustion. Energy Fuels 2006,
20, 1425–1433.
(11) Yao, M.; Chen, Z.; Zheng, Z.; Zhang, B.; Xing, Y. Study on the
controlling strategies of homogenous charge compression ignition combustion with fuel of dimethyl ether and methanol. Fuel 2006, 85, 2046–2056.
(12) Lim, O. T.; Sendoh, N.; Iida, N. Experimental study on HCCI
combustion characteristics of n-heptane and iso-octane fuel/air mixture by
the use of a rapid compression machine. SAE Tech. Pap. Ser. 2004, 200401-1968.
(13) Yao, M.; Zhang, B.; Zheng, Z.; Chen, Z.; Xing, Y. Effects of
exhaust gas recirculation on combustion and emissions of a homogeneous
charge compression ignition engine fueled with primary reference fuels.
Proc. Inst. Mech. Eng., Part D: J. Automob. Eng. 2007, 221, 197–213.
(14) Zheng, Z.; Yao, M. Numerical study on the chemical reaction
kinetics of n-heptane for HCCI combustion process. Fuel 2006, 85, 2605–
2615.
(15) Jia, M.; Xie, M. A chemical kinetics model of iso-octane oxidation
for HCCI engines. Fuel 2006, 85, 2593–2604.
Figure 1. Experimental setup: 1, engine; 2, dynamometer; 3, air
compressor; 4, air tank; 5, air flow meter; 6, electric heater; 7, pressure
transducer; 8, encoder; 9, charge amplifier; 10, data acquisition system;
11, fuel tank; 12, fuel flow meter; 13, fuel pump; 14, fuel injector; 15,
electronic control unit; and 16, exhaust analyzer.
high, the engine cannot run smoothly at low IMEP and the HCCI
operating speed is also limited (e.g., RON 90). Therefore, to
improve the maximum IMEP, the research for high octane
number (RON > 90) is necessary.
In this work, the tests were carried out on a modified fourcylinder direct-injection diesel engine using high octane numbers
(RON > 90) PRFs and commercial gasoline in different inlet
pressures. The purpose of this research is to obtain a basic
understanding of the influence of fuel octane numbers on
combustion characteristics, engine performance, and emissions
of a HCCI engine at different inlet pressures.
2. Experimental Apparatus and Methods
A diesel engine (Yu Chai 4112 series, YC4112ZLQ) was
converted to run in HCCI mode. The four-cylinder engine was,
however, modified to operate in one cylinder only. This arrangement
gives a robust and inexpensive single-cylinder engine but at the
cost of the reliability of the brake-specific results. With a pressure
transducer, indicated results can be used instead. The most important
engine parameters are shown in Table 1.
The boost pressure was generated by an external air compressor
that could be used to control the inlet pressure up to 0.3 MPa. A
large tank was added as a pressure stabilizer in the inlet system to
reduce the effect of inlet air cycle variation on the measurement of
the air flow rate. The air flow meter was mounted in the rear of the
tank. Figure 1 illustrates the experimental setup. An electric heater
was installed in the inlet system upstream of the injector to keep
the stable combustion. According to the previous experiment
results,16 the gasoline and the 93-RON fuel can be fired until the
inlet temperature (Tin) reaches 363 K. Therefore, the Tin was set to
363 K during all experiments.
A pressure transducer (Kistler 6125A) was fitted flush with the
wall of the cylinder head, connected via a charge amplifier (Kistler
5011) to a data acquisition board (National Instruments) fitted in a
compatible PC. The cylinder pressure data were recorded in half
crank-angle increments, triggered by an optical shaft encoder.
An electronic port fuel injector, typical of those used in modern
SI engines, was also installed in the inlet pipe at a location
approximately 30 diameters upstream of the inlet valve. This injector
was used to inject fuel into the inlet air for operation of the engine
in HCCI mode. An injector controller was used to drive the injector,
controlling both the injection timing and the fuel quantity by the
pulse width of the injection event.
The concentrations of CO2, CO, O2, NOx, and THC in the exhaust
gas were measured by an exhaust analyzer (Horiba MEXA7100DEGR), which measures HC by a hydrogen flame ionization
(16) Zhang, B.; Yao, M.; Yang, D.; Zheng, Z.; Chen, Z.; Zhang, Q.;
Xing, Y. Experimental study on the effect of fuel properties on HCCI
combustion characteristics at various intake temperatures. Trans. CSICE
2008, 26, 1–10.
Inlet Pressure and Octane Numbers of a HCCI Engine
Table 2. Fuel Properties
molecular formula
boiling point (°C)
distillation (°C)
T10
T50
T90
density (kg/m3)
low heating value (MJ/kg)
RON
benzene (% v/v)
alkene (% v/v)
aromatics (% v/v)
n-heptane
iso-octane
C7H16
98.4
C8H18
99.2
688
44.93
0
692
44.65
100
gasoline (G94.1)
66.7
89.5
154.9
733
43.9-44.4
94.1
1.3
28.0
25.5
(FID) method, CO and CO2 by a nondispersive infrared (NDIR)
method, and NOx by a chemiluminescent NOx analyzer (CLA).
This system was used to start and warm up the engine in standard
diesel configuration until the lubricating oil temperature reached
85 °C and the cooling water temperature reached 80 °C, then it
was switched into HCCI mode. The engine speed was set to 1400
rpm during all experiments. Four fuels were used during the
experiments: 90-RON, 93-RON, and 97-RON primary reference
fuel blend and a commercial gasoline, 94.1-RON(G), which was
bought from the Chinese market. We use the code PRF90, PRF93,
PRF97, and G94.1 as the marker of four fuels in this paper, which
the numbers are the RON of the fuel. Table 2 shows brief properties
of the fuels. Although the grade of the gasoline was 93, the real
RON was 94.1 from the authoritative test by North Institute of China
Petrochemical Corporation. The RON of the gasoline was measured
in the standardized research method (see American Society for
Testing Materials Designation, D 2699-01a, 2001) tests with
cooperative fuel research (CFR) engines. Three different levels of
inlet pressure were used in the experiments presented. At first, the
engine was operated NA, and then a boost pressure of 0.5 and 1
bar was used, giving 0.1, 0.15, and 0.2 MPa of absolute pressure.
3. Results and Discussion
3.1. Effects of Inlet Pressure and Octane Numbers on
Combustion Characteristics. The cylinder pressure and heat
release rate trace is presented in Figure 2, which the fueling
rate investigated is 29 mg/cycle. Each pressure trace is the mean
pressure trace for 50 engine cycles. The cylinder pressure data
was analyzed using a single-zone heat-release model with an
assumption that the mixture of air and fuel and the temperature
is homogeneous in the whole cylinder volume. In addition, there
is no mass leakage from the cylinder. The heat-transfer
coefficient was obtained via Woschni’s correlation. The rate of
heat release (ROHR) and mean gas temperature are calculated
by this model, which has been used in previous research.11,13
In fact, the single-zone model is an simple effective method to
the combustion analysis. Heywood stated17 that, in comparison
to a single-zone model, the advantage of a two-zone analysis is
that the thermodynamic properties of the cylinder contents can
be quantified more accurately. However, the two-zone model
has its disadvantages that the unburned and burned zone heattransfer areas must both now be estimated and a model for the
composition of the gas flowing into the crevice region must be
developed. Therefore, we choose the single-zone model to
analyze the combustion process.
As can be seen from the Figure 2, the peak of cylinder
pressure is increased and the heat release rate is advanced with
increasing inlet pressure. The reason is that the collision
frequency among molecules increases with the increase of the
boost pressure, which leads to the increase of the combustion
(17) Heywood, J. B. Internal Combustion Engine Fundamentals;
McGraw-Hill Book Company: New York, 1988; pp 148-154, 388-389.
Energy & Fuels, Vol. 22, No. 4, 2008 2209
reaction velocity. Figure 2a shows that the fuel chemistry has
a remarkable effect on the HCCI combustion progress in
different boost pressures. In comparison to the PRF93, the SOC
of G94.1 is earlier without boosting, while it is later with
boosting. The research of Kalghatgi18,19 shows that the sensitive
fuel will become more resistant to auto-ignition by boosting
the inlet. On the other hand, if Tcomp15 (the pressure reaches 15
bar during the compression stroke) is increased, the sensitive
fuel will become more prone to auto-ignition compared to the
PRF. As can be seen from Table 2, the G94.1 is a sensitive fuel (RON > MON). It contains less paraffins and more
aromatics and olefins compared to PRF fuels. Therefore, as the
inlet pressure increases, the G94.1 becomes more resistant to
auto-ignition compared to PRF93, a nonsensitive fuel. On the
other hand, without boosting, the compression temperature is
increased (which will be discussed in the next section), which
lead to the earlier SOC of the G94.1. This illuminates that the
auto-ignition depends upon the fuel as well as the pressure and
temperature development in the unburnt mixture.
Parts b-d of Figure 2 show the effect of RON on the
combustion process at a given inlet pressure. The PRF97 is not
shown in Figure 2b, because it can not reach stable combustion
at this operating condition. It is either a misfire or knock. As
parts b-d of Figure 2 show, with the increase of the inlet
pressure, the influence of the RON gap on the SOC is less and
less. This suggests that a small RON gap has a different effect
on the auto-ignition at different boost pressures. Without
boosting, the gap of SOC is nearly 5 CAD between PRF90 and
PRF93, while it is nearly the same at 1 bar boost pressure. On
the one hand, the SOC is advanced too early at the case of higher
boost pressure, which weakens the gap of SOC between three
PRF. On the other hand, with the increase of RON, the peak of
ROHR increases after supercharging. It suggests that the PRF
with a higher RON occurs in the more precombustion reaction,
resulting in the increase of the combustion rate. Finally, the
SOC is advanced, and the peak of ROHR is increased. This
may be another reason that the gap of SOC between three PRF
changes is small, except the too early SOC.
The mean gas temperature trace is presented in Figure 3,
which the fueling rate investigated is also 29 mg/cycle. The
in-cylinder mean gas temperature was calculated from the
cylinder pressure using the single-zone mode, and it is an
average value. Parts a and b of Figure 3 show the mean gas
temperature of PRF93 and G94.1 at different inlet pressures.
As can be seen from the figure, the peak of the mean gas
temperature is diminished with an increasing inlet pressure. The
reason is that the excess air from the supercharger reduces the
peak combustion temperature within the cylinder and thus
decreases the knock occurrence. A low combustion temperature
is preferable to allow for heat release without reaching the NOx
critical temperature. However, Sjöberg20 suggested that reaching
a peak in-cylinder temperature of 1500 K is necessary to have
a sufficient OH concentration for the CO oxidation. The low
cylinder temperature in higher boosting will lead to increased
amounts of CO emissions. In addition, parts a and b of Figure
3 also show that the mean gas temperature with starting HCCI
combustion for PRF93 and G94.1 are all between 1050 and
(18) Kalghatgi, G. T. Auto-ignition quality of practical fuels and
implications for fuel requirements of future SI and HCCI engines. SAE
Tech. Pap. Ser. 2005, 2005-01-0239.
(19) Kalghatgi, G. T.; Head, R. A. The available and required autoignition quality of gasoline-like fuels in HCCI engines at high temperatures.
SAE Tech. Pap. Ser. 2004, 2004-01-1969.
(20) Sjöberg, M.; Dec, J. E. An investigation into lowest acceptable
combustion temperatures for hydrocarbon fuels in HCCI engines. Proc.
Combust. Inst. 2005, 30, 2719–2726.
2210 Energy & Fuels, Vol. 22, No. 4, 2008
Liu et al.
Figure 2. Cylinder pressure and rate of heat-release traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of RON without
boosting fueling PRF90 and PRF93, (c) effects of RON at 0.5 bar boost pressure fueling PRF90, PRF93, and PRF97, and (d) effects of RON at 1
bar boost pressure fueling PRF90, PRF93, and PRF97.
Figure 3. Mean gas temperature traces: (a) effects of inlet pressure fueling PRF93, (b) effects of inlet pressure fueling G94.1, and (c) effects of
RON at 1 bar boost pressure.
1100 K. The temperature of SOC drops somewhat with the
increase of the boost pressure. The chemical kinetics modeling
of HCCI combustion has concluded that HCCI ignition is
controlled by hydrogen peroxide (H2O2) decomposition.21 The
main ignition needs the mixture up to 1050-1100 K, necessary
for H2O2. Figure 3c shows the mean gas temperature of three
PRFs at 0.2 MPa inlet pressure. It indicates that, with the
increase of RON, the peak in-cylinder temperature increases a
little. The main reason may be that the ignition delay will be
longer with the increase of RON, resulting in the more
precombustion reaction and increased combustion rates and the
peak temperature.
(21) U.S. Department of Energy Efficiency and Renewable Energy Office
of Transportation Technologies. Homogeneous charge compression ignition
(HCCI) technology. A Report to the U.S. Congress, April 2001; p 19.
Inlet Pressure and Octane Numbers of a HCCI Engine
Energy & Fuels, Vol. 22, No. 4, 2008 2211
Figure 4. Combustion efficiency traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure,
and (c) effects of fuel RON at 1 bar boost pressure.
3.2. Effects of Inlet Pressure and Octane Numbers on
Combustion Efficiency and Indicated Efficiency. The combustion efficiency was evaluated from the exhaust gas composition, and it is a measure of how complete the combustion is.
Referring to the literature,17 the combustion efficiency equation
is presented as follows:
(
ηc ) 1 -
∑x Q
1
1
[qf/(qa + qf)]Qf
)
× 100%
(1)
where xi values are the mass fractions of HC, CO, and H2,
respectively, the Qi values are the lower heating values of these
species, Qf is the lower heating value of the fuel, and qf and qa
are the mass flow rate of fuel and air, respectively. The heating
value of CO and H2 is 10.1 and 120 MJ/kg, respectively,
according to ref 17. The composition of the unburned HC is
not usually known. However, the heating values of hydrocarbons
are closely comparable; therefore, the fuel heating value is used.
In addition, the particulate emission can be ignored here because
it is very low in the HCCI combustion mode. The calculated
combustion efficiencies are presented in Figure 4. The main
trend is that the combustion efficiency increases with an increase
of the cycle fueling rate. This is due to the use of a richer
mixture and hence higher temperature.
Figure 4a shows that, with the increase of the inlet pressure,
combustion efficiency increases at the 0.15 MPa case and then
decreases at the 0.20 MPa case for a given fueling rate. The
reason is that the collision frequency among molecules increases
with the increase of the boost pressure, which leads to the
increase of the combustion reaction velocity. Therefore, more
fuel can be combusted at 1.5 bar inlet pressure. However, on
the other hand, the higher inlet pressure leads to the lower
mixture concentration for a given fueling rate, which results in
the decline of the cylinder temperature and hence the combustion
efficiency. The influence of the cylinder temperature on the
combustion efficiency is dominant at 0.2 MPa inlet pressure.
Figure 4a also shows that the combustion efficiency of G94.1
and PRF93 are nearly identical without boosting. However, the
combustion efficiency of G94.1 is lower than that of PRF93
with boosting. Moreover, the higher the inlet pressure, the larger
the difference of combustion efficiency. These result from the
lower cylinder temperature of G94.1.
Parts b and c of Figure 4 show that the RON of PRF has
little effect on combustion efficiency with boosting. Nevertheless, in comparison to the PRF, the combustion efficiency of
G94.1 is lower after boosting. The G94.1, as a sensitive fuel, is
harder to auto-ignite than that of PRF after boosting. The peak
of ROHR of G94.1 is lower than that of PRF93, which results
in the lower cylinder temperature and higher unburned HC and
CO emissions. Therefore, the combustion efficiency of the G94.1
is the lowest. In addition, Figure 4b also shows that, with the
increase of RON, the combustion efficiency of PRF decreases
a little especially at a lower fueling rate. However, the Figure
4c shows that the combustion efficiency increases a little as
the RON increases. This also indicates that, with the increase
of the inlet pressure, the more complete combustion reaction
occurs for higher RON, resulting in a higher peak temperature
and thus a higher combustion efficiency.
The gross indicated efficiency was evaluated by measuring
the fuel flow and the indicated mean effective pressure during the compression and expansion strokes only. This means
that the effect of supercharging on the gas exchange process is
absent. Figure 5 shows the gross indicated efficiency for the
different cases. As shown in Figure 5a, with an increased inlet
pressure, the efficiency obtained for PRF93 and G94.1 are
increased at the 0.15 MPa case and then reduced at the 0.2 MPa
case. There are two dominated factors, combustion efficiency
and SOC, that affect the indicated efficiency. With the increase
of the inlet pressure, the SOC advances. However, the SOC is
too early at the 0.2 MPa case, which can cause the decrease of
the indicated efficiency. On the other hand, with the increase
of the inlet pressure, combustion efficiency increases at the 0.15
2212 Energy & Fuels, Vol. 22, No. 4, 2008
Liu et al.
Figure 5. Gross indicated efficiency traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure,
and (c) effects of fuel RON at 1 bar boost pressure.
Figure 6. Operating range traces: (a) effects of inlet pressure fueling PRF93 and G94.1 and (b) effects of inlet pressure fueling PRF90, PRF93, and
PRF97.
MPa case and then decreases at the 0.20 MPa case for a given
fueling rate. In comparison to the PRF93, the maximum
achievable gross indicated efficiency of G94.1 is higher. That
is the result of the more favorable combustion timing for the
G94.1 fuel. Parts b and c of Figure 5 show that the maximum
gross indicated efficiency is increased with increasing of RON.
The maximum gross indicated efficiency of G94.1 is higher than
that of PRF. In addition, they also show that the octane numbers
have less effect on the gross indicated efficiency at the higher
inlet pressure.
3.3. Effects of Inlet Pressure and Octane Numbers on
the HCCI Operating Range. The operating range in terms of
load is tested. The limits for the operating range are defined by
some chosen variables. The criterion for the maximum achievable load is the maximum pressure rise rate of 1.0 MPa/CA
degree, beyond which combustion tends to become “knocky”.
The limits for the minimum load are misfiring, defined as
COV(IMEPgross) exceeding 10% for the engine. The minimum
and maximum engine loads that could be achieved for each inlet
pressure under HCCI conditions are presented in Figure 6. The
operating range of gross IMEP can become broad as the inlet
pressure increases. On the one hand, the use of a higher inlet
pressure can bring in more air, which leads to an increase of
the amount of fuel that can be injected and an increase of the
maximum load that can be achieved. On the other hand, the
engine can operate stably at a leaner air fuel ratio with an
increasing inlet pressure, which leads to lower values of load.
However, the benefits of intake boost would have been much
greater if the combustion phasing had been controlled independently using different intake temperature, EGR, or other
methods.
Figure 6a shows that the achievable minimum load is nearly
identical to the G94.1 and PRF93, but the achievable maximum
load of G94.1 is higher than that of PRF93. The reason is that
the main combustion of G94.1 is approximately at TDC after
boosting, which benefits to increase the gross IMEP. Figure 6b
shows that the achievable maximum load increases as the RON
increased for a given boost pressure. At higher inlet pressure,
the RON has less influence on the achievable maximum load.
The achievable minimum load is nearly identical to three PRFs.
3.4. Effects of Inlet Pressure and Octane Numbers on
Emissions. NOx formation is very sensitive to the temperature
history during the cycle. At temperatures over 1800 K, the NOx
formation rate increases rapidly with increased temperature.
With a homogeneous combustion of a premixed mixture, the
temperature is expected to be the same in the entire combustion
Inlet Pressure and Octane Numbers of a HCCI Engine
Energy & Fuels, Vol. 22, No. 4, 2008 2213
Figure 7. NOx emissions traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure, (c) effects
of fuel RON at 1 bar boost pressure, and (d) effects of maximum cylinder temperature fueling PRF93 and G94.1.
chamber, except near the walls. This, in combination with very
lean mixtures, gives a low maximum temperature during the
cycle. Therefore, Figure 7 shows that the NOx emissions are
overall very low in these tests.
The trend observable from Figure 7a is a remarkable increase
in NOx for the richest mixtures when running on the cases of
without supercharging. However, there is only a little increase
or no increase in NOx for the richest mixtures at 0.15 or 0.2
MPa inlet pressure. The reason is that, with the increase of inlet
pressure, more air can be drawn into the cylinder. When the
fueling rate is constant, the engine will run in leaner operation
to help reduce the high-temperature regions. For the cases of
supercharging, the reduction of mean gas temperature makes
the NOx emissions very low, even at the richest mixtures. Figure
7d shows that, because cylinder temperature is not beyond 1800
K, the temperature almost has no influence on NOx emissions.
The low homogeneous combustion temperature related to
HCCI can reduce the NOx emissions, but the combustion
temperature becomes too low to fully oxidize the fuel completely. The low combustion temperature leads to incomplete
combustion and high emissions of unburned hydrocarbons.
Figure 8 shows that the major trend is that the HC emissions
decrease with an increased cycle fueling rate. Keeping the boost
pressure constant, an increased fueling rate means a less diluted
mixture and higher combustion temperature and therefore higher
combustion quality. It can also be noted that the emissions of
HC are reduced after supercharging. The reason may be that
the collision frequency among molecules increases after supercharging, which leads that the fuel in the crevices and walls
quenching can be combusted better than without supercharging.
Figure 8a shows that the HC emissions of G94.1 are higher
than those of PRF93 when supercharging is employed. The
reason is that the mean gas temperature of G94.1 is lower than
that of PRF93. Parts b and c of Figure 8 show that the RON
for PRF has little effect on HC emissions with the cases of
supercharging, especially at the 0.2 MPa inlet pressure. Nevertheless, the HC emissions of G94.1 are higher than those of
PRF when supercharging is employed. Figure 8d shows that,
although the HC emissions decrease with the increase of the
maximum cylinder temperature, HC emissions strongly depend
upon fuel chemistry and inlet pressure. Therefore, the supercharging is beneficial to reduce the HC emissions.
The formation of CO is much more complex. CO is believed
to be formed close to the walls where the temperature is high
enough for the oxidation of HC to start, but the cooling from
the walls prevents complete oxidation to CO2.17 On the basis
of this mechanism, wall temperature history and bulk temperature history have a strong impact on CO emissions. The higher
combustion temperature results in less CO. It can be seen from
Figure 9 that the CO emissions decrease with an increased cycle
fueling rate. Figure 9a shows that the CO emissions increase
with the increase of the inlet pressure. The CO emissions are
very high at the lower cycle fueling rate. The reason is that
peak in-cylinder temperature can not reach 1500 K, which is
too low to have a sufficient OH concentration for the CO
oxidation. Parts b and c of Figure 9 show that the PRF octane
number has little effect on CO with the cases of supercharging.
The CO emissions of G94.1 are higher than those of PRF for a
given inlet pressure when supercharging is employed. In
addition, Figure 9b shows that, with the increase of RON, the
CO emissions increase a little, especially at a lower fueling rate.
However, Figure 9c shows that the CO emissions decrease a
little as the RON increases. This also indicates that the more
complete combustion reaction occurs for higher RON, resulting
in the higher peak temperature and thus less CO emissions.
Figure 9d shows that the CO emissions decrease with the
increase of the maximum cylinder temperature. It indicates that
CO emissions strongly depend upon the cylinder temperature.
2214 Energy & Fuels, Vol. 22, No. 4, 2008
Liu et al.
Figure 8. HC emissions traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure, (c) effects
of fuel RON at 1 bar boost pressure, and (d) effects of maximum cylinder temperature fueling PRF93 and G94.1.
Figure 9. CO emissions traces: (a) effects of inlet pressure fueling PRF93 and G94.1, (b) effects of fuel RON at 0.5 bar boost pressure, (c) effects
of fuel RON at 1 bar boost pressure, and (d) effects of maximum cylinder temperature fueling PRF93 and G94.1.
It exhibits a good agreement with the CO formation mechanism.
That is to say, CO emissions are chemical kinetics products.
4. Summary and Conclusions
The effects of inlet pressure and octane numbers on combustion
and emissions of a HCCI engine were experimentally investigated.
Four fuels were used during the experiments: 90-RON, 93-RON,
and 97-RON PRF blend and a commercial gasoline, 94.1-RON(G).
The inlet pressure conditions were set to give 0.1, 0.15, and 0.2
MPa of absolute pressure. The most important results presented
in this paper can be summarized as follows: (1) The octane number
has different effects on the combustion process at different inlet
pressures. In comparison to the PRF93, the start of combustion
(SOC) of 94.1-RON(G) occurs earlier without boosting, while
occurring later with boosting. With the increase of the inlet pressure,
the effects of PRF octane number on SOC are weakened and a
more precombustion reaction occurs for PRF with a higher RON.
(2) The successful HCCI operating region is extended to the upper
Inlet Pressure and Octane Numbers of a HCCI Engine
and lower load with the increase of the inlet pressure. The octane
numbers have less influence on the achievable maximum load at
the higher inlet pressure, because of the too early SOC. Therefore,
the SOC should be controlled independently, unless it becomes
too advanced with the increase of the inlet pressure. (3) With the
increase of the inlet pressure, the combustion efficiency and gross
indicated efficiency increases and then decreases for a given fueling
rate. The combustion efficiency of 94.1-RON(G) is lower than those
PRFs when supercharging is employed. (4) The HC and NOx
emissions of a HCCI engine decrease with supercharging, while
CO emissions increase remarkably. The PRF octane numbers have
little effect on HC, CO, and NOx emissions with supercharging.
However, the HC and CO emissions of 94.1-RON(G) are higher
than those of PRF with supercharging for a given fueling rate.
Acknowledgment. The authors gratefully acknowledge the
Minister of Science and Technology (MOST) of China through its
project 2007CB210002 and the National Natural Science Found
of China (NSFC) through its project 50676066.
Energy & Fuels, Vol. 22, No. 4, 2008 2215
Nomenclature
ABDC ) after bottom dead center
ATDC ) after top dead center
BBDC ) before bottom dead center
BTDC ) before top dead center
COV(IMEP) ) cycle-to-cycle variation of indicated mean effective
pressure
EGR ) exhaust gas recirculation
HCCI ) homogeneous charge compression ignition
IMEP ) indicated mean effective pressure
NA ) naturally aspirated
Pin ) inlet pressure
PRF ) primary reference fuel
RON ) research octane number
SI ) spark ignition
SOC ) start of combustion
Tin ) inlet temperature
TDC ) top dead center
EF800197B