84
Energy & Fuels 2006, 20, 84-90
Effect of Methanol Addition into Gasoline on the Combustion
Characteristics at Relatively Low Temperatures
S. Y. Liao,*,†,‡ D. M. Jiang,† Q. Cheng,‡ Z. H. Huang,† and K. Zeng†
School of Energy and Power Engineering, Xi’an Jiaotong UniVersity, Xi’an 710049, People’s Republic of
China, and College of Chongqing Communication, Chongqing 400035, People’s Republic of China
ReceiVed July 27, 2005. ReVised Manuscript ReceiVed NoVember 9, 2005
A reliable and rapid cold start of the engine is related with the unburned hydrocarbon (HC) emissions as
well as energy efficiency. Because combustion characteristics are relevant parameters for a reliable combustion
process, an experimental investigation on the combustion characteristics has thus been made in a constant
volume combustion bomb for methanol-gasoline blended fuel at relatively low temperatures. The effects of
the equivalence ratio of the combustible mixture on the combustion pressure, ignition delay time, mass burning
rate, and the flame propagation speed are mainly studied. It is shown that moderate methanol addition can
slightly improve the combustion performance at low temperatures, compared to that of pure gasoline fuel,
because methanol addition into gasoline results in the improvement of blend evaporation. The exhaust emissions
are measured in terms of unburned HC, carbon monoxide (CO), and oxides of nitrogen (NOx) emissions. It is
reported that the emissions of HC during the rich fuel/air mixture combustion at relatively low temperatures
increase with the increasing addition of methanol into gasoline, because of the enhanced evaporation of blended
fuel, compared to gasoline. However, in view of the separated optimization equivalence ratio for low temperature
combustion, HC and CO emissions can be obviously reduced when moderate addition is used, because a
leaner mixture has been supplied to realize rapid combustion for blended fuels. The flame speeds are investigated
as well. It is shown that, at temperature ranging from 358 to 400 K, the suitable fuel/air ratio of blended fuels
to realize fast flame propagation is about 1.3.
Introduction
With rising oil prices and global warming being a dominant
environmental issue, it seems that the uses of alternative fuels
in the future are inevitable. These leading goals for both energy
security and the clean air project have resulted in heightened
interests in the worldwide utilizations of alternative fuels in
burners and engines. Currently, various alternative fuels have
been investigated for spark ignition engines to reduce the
consumption of gasoline and NOx, CO, and unburned hydrocarbon (HC) emissions. In alcohols, methanol and ethanol are
used most often as fuels and fuel additives, because their
potential to improve air quality when used to replace conventional gasoline in engines because of its good anti-knock
characteristics and the reduction of CO and unburned HC
emissions in engines.1 It has been reviewed that the blending
unleaded gasoline with ethanol increased the engine brake
power, torque, volumetric and brake thermal efficiencies, and
fuel consumption, while decreasing the brake specific fuel
consumption and the CO and HC emissions.2-5 Although the
* To whom correspondence should be addressed. E-mail: shyliao@
yahoo.com.cn.
† Xi’an Jiaotong University.
‡ College of Chongqing Communication.
(1) Moreira, J. R.; Goldemberg, J. The alcohol program. Energy Policy
1999, 27, 229-245.
(2) Thomas, V.; Kwong, A. Ethanol as a lead replacement: Phasing out
leaded gasoline in Africa. Energy Policy 2001, 29, 1133-1143.
(3) Nadim, F.; Zack, P.; Hoag, G. E., Liu, S. L. United States experience
with gasoline additives. Energy Policy 2001, 29, 1-5
(4) Yuksel, F.; Yuksel, B. The use of ethanol-gasoline blend as a fuel
in an SI engine. Renewable Energy 2004, 29, 1181-1191.
(5) Ulmera, J. D.; Huhnkeb, R. L.; Bellmerc, D. D.; Cartmelld, D. D.
Acceptance of ethanol-blended gasoline in Oklahoma. Biomass Bioenergy
2004, 27, 437-444.
use of ethanol is also accompanied with a disadvantage of an
increase in aldehyde and unburned ethanol emissions, ethanol
proponents have also typically focused on the renewable
characteristics, presenting the benefits of ethanol in terms of
local energy security and stabilization of the agricultural sector.
Thereby, ethanol is being regarded as one of the promising
alternative fuels for engines. Similarly, methanol, known as
methyl alcohol, has also been used as an alternative fuel for
automotive engines in many countries. Recently, its excellent
combustion properties have made it the strongest choice of the
automotive industry as well. The other most important characteristic of methanol is that it is undoubtedly the cheapest liquid
alternative fuel per calorific unit, which can be produced from
the widely available fossil raw materials including coal, natural
gas, and biosubstances.1 This essentially means that many countries can solve their energy imbalance problems because of the
petroleum shortage by using methanol as a source of energy.
It is known that methanol has a greater latent heat of
vaporization than gasoline; more methanol addition into gasoline
can make the cold start worse because of the lowering
in-cylinder temperature of the engine. Thereby, fuels with low
methanol content are mainly promoted in practical engine
utilization.6-11 Abu-Zaid et al. found that methanol has a
(6) Abu-Zaid, M.; Badran, O.; Yamin, J. Effect of methanol additive on
the performance of spark ignition engines. Energy Fuels 2004, 18, 312-315.
(7) Kelkar, A. D.; Hooks, L. E.; Knofzynski, C. Comparative study of
methanol, ethanol, 2-propanol, and butanol as motor fuels, either pure or
blended with gasoline. Proc. Intersoc. Energy ConVers. Eng. Conf., 23rd
1988, 4, 381-386.
(8) Lin, T. C.; Chao, M. R. Assessing the influence of methanolcontaining additive on biological characteristics of diesel exhaust emissions
using microtox and mutatox assays. Sci. Total EnViron. 2004, 284, 61-74.
(9) Gorse, R. A., Jr. The effects of methanol/gasoline blends on
automobile emissions. SAE paper number 920327, 1992.
10.1021/ef0502352 CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/02/2005
Effect of Methanol Addition into Gasoline
Energy & Fuels, Vol. 20, No. 1, 2006 85
preferably slightly less of stoichiometric, where the HC emissions are at a minimum.12 However, there are some significant
difficulties in achieving this goal for cold-start engines. One of
the most difficult problems is that the engine must be overfueled
to compensate for liquid fuel accumulation in the intake port
walls and cylinder such that the fuel/air ratio in the gas phase
is near stoichiometric. Thus, to obtain a lower HC emission
level and realize a reliable operation, the cold-start combustion
process should be well-addressed.
Often, methanol fuel is usually designated M-100 to identify
it as essentially 100% pure methanol, and a popularly used
methanol blend composed of 15% methanol and 85% unleaded
gasoline by volume is designated as M15. In regard to the
methanol-gasoline blend, the previously reported work was
mainly focused on the engine performance, including thermal
efficiency, power output, and emissions related to moderate or
heavy-duty conditions. Few works were found in this fundamental aspect of the combustion characteristics of methanol/
gasoline blended fuels at a relatively low temperature. From
these considerations above, the main objective of this study is
to explore the possibility of rapid and reliable cold start for the
use of methanol/gasoline blends. Because the temperature is
the leading parameter affecting the formation of the fuel/air
mixture, it is hence discussed in detailed herein. Mixtures with
various equivalence ratios are ignited centrally in a combustion
bomb to investigate the combustion process and flame propagation characteristics for methanol, gasoline, and blended fuels
of M15 and M30.
Experimental Procedures
Figure 1. Schematic diagram of the experimental system.
significant effect on SI engine performance when a blended fuel
of 15 volume percent methanol and 85% gasoline is used. Kelkar
et al. made a comparative study of methanol, ethanol as motor
fuels in both the pure form and blended with gasoline. They
found that 10:90% alcohol/gasoline blends can be used to fuel
the engine without modifications. From recent investigations,
it is found that moderate methanol addition into gasoline
improved energy utilization, increased engine output, and
improved knock resistance. In conclusion, methanol is also being
regarded as one of the promising alternative fuels or an
oxygenate additive in the engine.
In view of environmental protection, a major difficulty in
meeting the rigorous emissions standard is the initial cold-start
transient, where the HC emitted remain at a high level because
of the richer fuel/air mixture supplied as well as the lower
efficiency of the catalytic converter. It has been suggested that
a significant reduction in cold-start HC emissions could be
obtained by operating the engine at a stoichiometric ratio or
(10) Kirwan, J. E.; Quader, A. A.; Grieve, M. J. Advanced engine
management using on-board gasoline partial oxidation reforming for meeting
super-low emissions standards. SAE paper number 1999-01-2927, 1999.
(11) Heinrich, W.; Marquardt, K. J.; Schaefer, A. J. Methanol as a fuel
for commercial vehicle. SAE paper number 861581, 998-1010, 1986.
Idealized engine combustion studies, conducted in a constant
volume combustion bomb, can be used to simulate the engine
combustion process near the top dead center at the end of the
compression stroke. Shown in Figure 1 are the schematic diagrams
of the combustion bomb and the optical system used for recording
the flame growth. The combustion bomb has an inside size of 108
× 108 × 135 mm, as shown in Figure 1A. Two sides of this bomb
are transparent to make the inside observable; these sides are to
provide the optical access, and the other four sides are enclosed
with resistance coils to heat the bomb to the desired preheat
temperature. The inlet/outlet valve is used to let fresh air or
combustion product in or out. The liquid fuel needed is precalculated
corresponding to the given equivalence ratio, and it is then injected
into the combustion chamber using a small capability injector. The
function of the perforated plate is to simulate the airflow in the
engine intake port, to enhance the fuel evaporation. Two extended
stainless steel electrodes are used to form the spark gap at the center
of this bomb. A conventional battery-coil ignition system is used
for producing the spark. The history of the shape and size of the
developing flame kernel is recorded by a REDLAKE HG-100K
high-speed CCD camera, operating at 5000 pictures per second with
a schlieren optical system, and the detailed experimental setup about
the schlieren system is given in Figure 1B. The dynamic pressure
is measured with a spark ignition with a piezoelectric absolute
pressure transducer, model Kistler 4075A, with a calibrating element
Kistler 4618A.
Combustion Analysis and Flame Speed. To extract information
on the combustion process, a cycle-resolved mass burning rate
calculation is performed using a quasi-dimensional two-zone
combustion model. Assuming the flame front is a thin reactive sheet,
the model divides the combustion chamber into two zones, i.e.,
the burned and unburned zones, on the basis of the following
assumptions: (1) The unburned mixture consists of fresh air
(oxygen gas and nitrogen gas) and fuel. This mixture will be burned
immediately after entering the flame region; (2) The components
(12) Henein, N. A.; Tagomori, M. K. Cold-start hydrocarbon emissions
in port-injected gasoline engines. Prog. Energy Combust. Sci. 1999, 25,
563-593.
86 Energy & Fuels, Vol. 20, No. 1, 2006
Liao et al.
of burned gas within the flame kernel are estimated by chemical
equilibrium computation, where NASA polynomial curve fits13 are
used to calculate the thermodynamic properties, namely, specific
heat, enthalpy, and entropy, while the properties of gasoline are
determined from empirical correlation;14 (3) The pressure is uniform
throughout the burned and unburned zones.
Then, mass and energy conservation yield
dmb dmu
+
)0
dt
dt
(1)
d(mu)k
dVk dmk
dQk
+p
hk )
k ) u and b
dt
dt
dt
dt
(2)
From the combined equations of the state and volume conservation
pVk ) mkRkTk
k ) u and b
(3)
V ) Vu + Vb
(4)
The following equations can be obtained:15
[
(
)
dTu
1
dp dQu
Vu +
)
dt
mucpu
dt
dt
(
(5)
)
]
dTb
dmb Ru dp dQu
1
dV
dp
)
+
p - (RbTb - RuTu)
V
+V
dt
mbRb dt
dt
cpu u dt
dt
dt
(6)
Ru dmb
- (ub - uu) - cvb Tb - Tu
Rb dt
cvu cvb Ru dQu
dQu dQb
+
+
cpu cpu Rb dt
dt
dt
dp
)
(7)
dt
cvu cvb Ru
cvb
Vu
+V
cpu cpu Rb
Rb
[
(
(
) (
(
)
)]
)
Namely, the unstretched flame speed, Sl, can be obtained as the
intercept value at R ) 0, in the plot of Sn against R.
Results and Discussion
Figure 2 plots the peak pressure rise because of combustion
for various fuels. It is known that the equivalence ratio is
generally defined as the ratio of gaseous fuel to air in a
normalized form by the stoichiometry air amount. Whereas,
when gasoline is partially evaporated, it is difficult to estimate
the actual fuel amount vaporized in the combustion chamber,
owing to the complexity of gasoline components; thus, it is
clarified that the equivalence ratio used in this work is
considered as the stoichiometric ratio of the fuel injected into
the chamber to air, obviously different from its conventional
definition. It can be seen that the equivalence ratios corresponding to the peak combustion pressures are different. It is obvious
that this equivalence ratio of blended fuels (M15 and M30) is
about 1.3, less than that of gasoline (about 1.6). As we know,
the boiling point of methanol is about 350 K, while only when
the preheated temperature is above about 500 K, gasoline
contents could be evaporated completely. Furthermore, it is
known that the maximum combustion pressure occurs almost
for the stoichiometric mixture, and the evaporation pressures
for liquid fuels are mainly dependent upon the preheated
temperature. Thus, it is understandable that equivalence ratios
corresponding to the peak combustion pressures for mixtures
of M15, M30, and gasoline in air are bigger than a unit, because
these fuels are partially evaporated at these tested temperatures.
where the heat losses dQu/dt, dQb/dt are estimated from Annand’s
formulas.14 The mass burning rate dmb/dt (m̆b) can then be solved
numerically with a fourth order Runge-Kutta scheme for eqs 5-7.
To give a quantitative analysis of flame initialization, the ignition
delay time is explored, defined as the time interval from the ignition
beginning to the ending timing of 10% mass burned, also named
as 10% mass fraction burned time (10% MFBT) herein.
The schlieren system is used to visualize the expanding flames.
The stretched flame speed can be well-defined as the increased
rate of flame size, i.e.,
Sn ) dru/dt
(8)
The definition of the flame stretch, R, of a flame front in a quiescent
mixture is given by
R)
1 dA
A dt
(9)
where A is the area of the flame. Thereby, in regard to a spherically
outwardly expanding flame front, the flame stretch is well-defined
as
R)
1 dA 2 dru 2
)
) Sn
A dt ru dt
ru
(10)
As described in refs 16 and 17, the linear relationship between flame
speeds and the total stretch rates is given in eq 11
Sl - Sn ) LbR
(11)
(13) Brokaw R. Alignment charts for transport properties, viscosity, and
diffusion coefficients for nonpolar gases and gas mixtures at low density.
Technical Report TR R-81; NASA, 1961.
Figure 2. Maximum combustion pressure for various fuels.
Effect of Methanol Addition into Gasoline
Energy & Fuels, Vol. 20, No. 1, 2006 87
Figure 4. 10% MFBT for various fuels.
Figure 3. Comparisons of normalized apparent mass burning rates
for various fuels at different equivalence ratios (T0 ) 358 K).
Because the normalized apparent mass burning rate has a very
significant importance on the pressure rise rate related with the
NOx emissions, the burning rate analysis is also often used to
analyze the combustion process in the chamber. Figure 3 shows
the normalized apparent mass burning rates for various fuels at
358 K. As indicated in this figure, the normalized apparent mass
burning rate is a mainly relevant parameter of the fuel/air ratio.
Over the presented results, the biggest difference can be found
for fuel/air mixtures with an equivalence ratio of 1.6, and the most
likely behaviors also occur near the equivalence ratio of 1.3.
The ignition delay time (10% MFBT) is proposed to
quantitatively describe the ignitability of a fuel/air mixture in
(14) Heywood, J. B. Internal Combustion Engine Fundamentals; McGrawHill: New York, 1988.
(15) Liao, S. Y.; Jiang, D. M.; Gao, J.; Zeng, K. Effects of different
frequency component in turbulence on accelerating turbulent premixed
combustion. Proc. Inst. Mech. Engr., Part D 2003, 217, 1023-1030.
this work. Obviously, the addition of the methanol content has
an important influence on the early flame initialization, because
it is indicated that the blended fuel and methanol show a similar
performance in the ignition delay time, as shown in Figure 4.
Whereas, this figure also presents that the minimum values of
10% MFBT for blended fuels keep the comparable values
quantitatively, and the equivalence ratio corresponding to this
minimum 10% MFBT is relatively less than that of gasoline.
Generally, we think that a short ignition delay time means a
rapid engine start. Thus, for the convenience in the discussion,
this equivalence ratio can then be defined as the optimization
equivalence ratio related to the cold start in this work.
Figure 5 summarizes the effect of the methanol volume
fraction on the shortest 10% MFBT of fuel at different preheated
temperatures. The optimization equivalence ratios for different
blends are plotted as well. It reports that M15 has a relatively
shorter 10% MFBT than gasoline, which is owing to the
enhancement evaporation of methanol addition into gasoline.
However, for M30, this phenomenon is not obvious, maybe
mainly because of the negative effect of the relatively slow flame
speed of methanol than that of gasoline. The reductions of the
combustion duration with the increase of the preheated temperature are also illustrated in these figures. Because the
chemical kinetics and the evaporation of fuel are temperaturedependent parameters, it is expectable that the ignition delay
time and the optimization equivalence ratio decrease with the
increase of the preheated temperature. It is also illustrated that,
in this figure, with the increasing methanol, the optimization
equivalence ratio of injected fuel to air would decrease, and it
(16) Gu, X. J.; Haq, M. Z.; Lawes, M.; Woolley, R. Laminar burning
velocity and Markstein lengths of methane-air mixtures. Combust. Flame
2000, 121, 41-58.
88 Energy & Fuels, Vol. 20, No. 1, 2006
Liao et al.
Figure 5. Variations of the shortest 10% MFBT and optimization equivalence ratio with the increasing methanol volume fraction.
Figure 6. HC emissions for various fuel against the equivalence ratio
at 358 K. The reference point is the gasoline-air mixture (φ ) 1.0),
whose HC emission is 100%.
is predictable that this value would be up to about a unit of the
pure methanol with the increasing methanol volume fraction.
The effects of methanol addition on the pollution emissions
are investigated as well. The experimental measurement is
conducted in terms of HC, CO, and NOx emissions at 358 K.
Shown in Figure 6 are the HC emissions for various fuels.
Because methanol is in full gasification at 358 K, we can see
that the addition of methanol has led to a significant increase
of HC emissions through fuel evaporation for rich fuel mixtures.
The above discussions have indicated that a relatively lean
mixture can be supplied to realize the rapid combustion for
blended fuel, compared to gasoline. This means that we should
compare the emission characteristics at their separated optimization equivalence ratio, to further explore the effect of methanol
addition on the emissions. Figure 7 plots the pollution emissions
at the separated optimization equivalence ratio for the cold start.
Obviously, because of the enhancement of evaporation, for
blended fuels, the fuel injected into the chamber decreases. As
a consequence, the HC and CO emissions can be reduced
slightly at the separated optimization equivalence ratio; that is
to say, from the view of a rapid cold start, the methanol addition
into gasoline also results in the reduction of HC and CO
emissions, compared to pure gasoline. However, Figures 6 and
7 also demonstrate that, with the increasing methanol level in
blended fuel, the HC emissions would increase gradually, as
shown in the comparison of M30 with M15, which means that
Figure 7. Variations in HC, CO, and NOx emissions for various fuel at the optimization equivalence ratio for cold start. The reference fuel is
gasoline, and gasoline emissions are 100%.
Effect of Methanol Addition into Gasoline
Energy & Fuels, Vol. 20, No. 1, 2006 89
Figure 8. Typical growing schlieren flame kernels for stoichiometric mixtures of M30 and gasoline in air. The time interval is 4 ms, and T0 ) 358
K.
Figure 9. Typical stretched flame speeds for various fuels of various
fuels in air, where initial conditions are 0.1 MPa and 358 K.
the reduction of HC emissions during the cold start is only
justified for moderate methanol addition into gasoline.
Moreover, the flame propagation characteristics are also
discussed in the present work. Shown in Figure 8 is a typical
flame kernel growth. As mentioned above, from the measurement for these expanding flame kernels, the flame speed can
be well-obtained. Figure 9 plots partial measurement results of
the flame speed against the flame size. It is known that the flame
speed is greatly dependent upon the type of fuel and fuel/air
ratio. Over the testing conditions, it is indicated that the flame
speeds commonly have a lower initialization and gradually
increase to a stabilized growth period, because of the relatively
higher flame stretch and heat loss during the early stage of the
flame propagation.16 The flame stretch is well-defined for the
outward expanding spherical flame; thereby, we can plot the
stretched flame speed against the flame stretch rate, to induce
the unstretched flame speed, as indicated in eq 11. Figure 10
plots the results. The mixture of methanol-air has the fastest
flame speed, and in turn, mixtures of M15, M30, and gasoline
in air at the stoichiometric condition, while it shows a reverse
tendency for richer cases (φ ) 1.6). The blends of M15 and
M30 have also shown the fastest flame propagation when the
fuel/air ratio is 1.3, compared to 1.6 for gasoline and 1.0 for
methanol. Generally, at high rates of the stretch (small flame
radius), the flame speed is small. With the expanding flame,
the flame speed slowly increases because of the reduced flame
stretch. As reported previously,17 the ignition energy, initial
temperature, and pressure have a strong influence on the flame
propagation. Therefore, the flames, restricted within the range
in which the spark, temperature, and pressure effects can be
Figure 10. Variations of flame speeds with different stretch rates. Lines
are first-order fits through experimental data within the stretch rate
(flame radius) range, in which spark, temperature, and pressure effects
are discounted.
90 Energy & Fuels, Vol. 20, No. 1, 2006
Liao et al.
Figure 11. General variation of unstretched flame speeds on the fuel/air ratio.
discounted, are used to induce the unstretched flame speed, as
described in ref 18. The unstretched flame speeds are plotted
in Figure 11. From this figure, it also can be found that the
ideal fuel/air ratio of M15 and M30 at 358 K combustion is
about at an equivalence ratio of 1.3, and their flame speeds are
bigger than those of pure methanol and gasoline with the same
equivalence ratio.
temperature, HC and CO emissions can be reduced as moderate
addition used. The flame speeds are investigated as well. It is
also revealed that, for methanol-gasoline blends with a
methanol content below 30%, the suitable fuel/air ratio to realize
fast flame propagation is about 1.3.
Acknowledgment. This work is supported by the state key
project of fundamental research plan, number 2001CB209206, and
the NSFC Awarding Fund for Excellent State Key Laboratory
(number 50323001).
Conclusions
This experimental study is conducted in a closed combustion
chamber to investigate the equivalence ratio on the combustion
characteristics of methanol-gasoline at a low temperature,
which is related to the cold-start operation of the engine. It
presents the effects of the fuel amount injected into combustion
vessel on the combustion process, and some key characteristic
parameters, such as ignition delay time, mass burning rate, and
flame speeds, have been explored, respectively. The results show
that the optimization equivalence ratios for M15 and M30
combustion are approximately the equivalence ratio of 1.3, at a
temperature range from 358 to 400 K. The exhaust emissions
are measured in terms of unburned HC, CO, and NOx emissions.
It is presented that the emissions of HC during the rich
combustion at a relatively low temperature are increased with
the addition of methanol into gasoline. However, in the view
of the optimization equivalence ratio of combustion at a low
(17) Liao, S. Y.; Jiang, D. M.; Gao, J.; Huang, Z. H. Measurement of
Markstein numbers and laminar burning velocities for natural gas-air
mixtures. Energy Fuels 2004, 18, 316-326.
(18) Liao, S. Y.; Jiang, D. M.; Gao, J.; Huang, Z. H. Measurements of
Markstein numbers and laminar burning velocities for liquefied petroleum
gas-air mixtures. Fuel 2004, 83, 1281-1288.
Nomenclature
A ) flame area
cp ) specific heat at constant pressure
cv ) specific heat at constant volume
h ) enthalpy
Hu ) low heat value
m ) mass
m̆b ) mass burning rate
p ) pressure
Q ) heat loss
R ) gas constant
ru ) flame radius
Sn ) stretched flame speed
Sl ) unstretched flame speed
t ) elapsed time from ignition
T ) temperature
u ) internal energy
V ) volume
F ) density
R ) flame stretch
10% MFBT ) 10% mass fraction burned time
Subscript
u ) unburned gas
b ) burned gas
o ) initial condition
EF0502352