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 Fuel 159 (2015) 796–802

Contents lists available at ScienceDirect

Fuel
journal homepage: www.elsevier.com/locate/fuel

Effect of intake pre-heating and injection timing on combustion

and emission characteristics of a methanol fumigated diesel engine
at part load
Quangang Wang, Chunde Yao ⇑, Zhancheng Dou, Bin Wang, Taoyang Wu
State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China

h i g h l i g h t s

 BTE at light load is improved by 7.3% by raising intake temperature.

 Combustion process is highly affected by intake temperature and injection timing.
 DMDF combustion becomes single-stage combustion as injection timing retarded.
 Soot-NOx trade-off is completely broken at low intake temperature and high MSP.

a r t i c l e i n f o a b s t r a c t

Article history: Diesel–methanol dual fuel (DMDF) engines at light loads suffer from low thermal efficiency and high
Received 27 January 2015 unburned percentages of fuel. Pilot fuel injection timing and intake temperature are two important
Received in revised form 7 July 2015 parameters which affect the combustion process in DMDF engines. In present experimental work, the
Accepted 9 July 2015
combined effects of intake temperature and injection timing on the performance of a DMDF engine have
Available online 18 July 2015
been studied. The experiments were conducted on a methanol-fumigated diesel engine at 25% of full load
and the results concerning performance, combustion characteristics and emissions were analyzed.
Keywords:
Results show that the low efficiency at light loads can be improved significantly by raising the intake
Dual fuel
Methanol fumigation
temperature and advancing the injection timing of direct-injected diesel. Increasing the intake tempera-
Combustion characteristics ture also significantly decreases the heat release rate of premixed combustion and increases the combus-
Emissions tion rate of methanol burned by flame propagation. Flame propagation of the methanol–air mixture
Light loads disappears gradually and DMDF combustion transforms into single stage combustion as the injection
timing is retarded. When injection timing is retarded after 4.6° crank angle, misfire occurs at higher
methanol substitute percent (MSP) and lower intake temperature, while the auto-ignition of methanol
occurs at lower MSP and higher intake temperature. Under DMDF operation, soot and nitrogen oxides
trade-off dilemma is completely broken at lower intake temperature and higher MSP.
Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction economy with high compression ratio and no throttling loss.

However, the conventional CI engine sustains with high nitrogen
Compared with spark-ignition engine, compression-ignition oxides (NOx) and particulate matter (PM) emissions. Hence, the
(CI) engine has attracted more attentions due to its better fuel heavy-duty CI diesel engine has been a hot topic over the last
two decades. Moreover, the sources of fossil fuel are dwindling,
Abbreviations: DMDF, diesel methanol dual fuel; CI, compression-ignition; PM, which results in raising price of petroleum oil, posing challenges
particulate matter; DMCC, diesel methanol compound combustion; NOx, nitrogen to the availability of fossil fuel. Under these circumstances, the
oxides; BTE, brake thermal efficiency; EGR, exhaust gas recirculation; LPG, liquefied substitute for conventional fuels is significant to address energy
petroleum gas; CNG, compressed natural gas; ECU, electronic control unit; CA, security issues. Among the alternative fuels, methanol has received
crank angle; AHRR, apparent heat release rate; ATDC, after top dead center; MSP,
considerable attention as suitable diesel replacement. In particular,
methanol substitution percent; BTDC, before top dead center; SOC, start of
combustion; FSN, filter smoke number. methanol is readily available from the conversion of biomass, coal
⇑ Corresponding author at: Tianjin University, No. 92 Weijin Road, Nankai and natural gas [1]. Moreover, the storage, transportation, distribu-
District, Tianjin 300072, China. Tel.: +86 22 2740 6649; fax: +86 22 2738 3362. tion, and application of methanol are similar to those of traditional
E-mail address: arcdyao@tju.edu.cn (C. Yao).

http://dx.doi.org/10.1016/j.fuel.2015.07.032
0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
 Q. Wang et al. / Fuel 159 (2015) 796–802 797

fossil oil such as gasoline and diesel as a liquid [2–5]. Therefore, the the fuel delivery advance angle, which is more effective at low
substitution of diesel with methanol is of great significances in engine load [31,32]. Paykani et al. found that the use of exhaust
countries such as China which has rich coal reserve, especially gas recirculation (EGR) at high levels seems to be unable to
the huge amount of coke-oven gas resources [6]. improve the engine performance at part loads [23]. Experiments
However, the foremost drawback for the utilization of methanol conducted by Poonia et al. showed that the intake temperature
in diesel engines is probably the low cetane number of methanol, does not seem to have a significant effect on the heat release at
which, depending on the measurement method, typically ranges these conditions [33]. However, the above researches were all
from only 2 to 12 [7]. The very high latent heat of vaporization also about LPG-diesel (liquefied petroleum gas (LPG)) dual fuel or
weakens its auto-ignition property [8–10]. In this regard, the most CNG-diesel (compressed natural gas (CNG)) dual fuel combustion,
favored method to introduce methanol into diesel engines is fumi- and there is hardly any researchers conducted the experiment con-
gation, which requires just a minor modification to the original cerning DMDF combustion. In this paper, tests were conducted to
engines as methanol injectors are fixed at the intake manifold investigate the effect of intake pre-heating and injection timing
[11–13]. However, methanol fumigation is unfavorable for cold of pilot diesel on the performance, combustion characteristics
start and low load operation. Based on the method of fumigation, and emissions on a direct-injected diesel engine fueled with
Yao et al. [14,15] developed a diesel/methanol compound combus- fumigated methanol.
tion (DMCC) system. Under DMCC mode, the engine operates on
pure diesel at cold start and low speed conditions to ensure cold
starting capability and to avoid aldehyde production. At medium 2. Experimental apparatus and method
to high loads, the engine operates on diesel methanol dual fuel
(DMDF) mode, during which methanol is fumigated into intake 2.1. Test engine and fuels
manifold and the homogeneous air–methanol mixture is ignited
by the diesel directly injected. The advantages of DMCC system The original engine was an in-line four-cylinder, direct injec-
include the following: (1) there is no cold start difficulty when tion, turbocharged diesel engine with an electronically controlled
the engine operates at dual fuel mode, (2) in case of lacking metha- unit injection pump. Technical specifications of the engine are
nol supply, this engine could still run as the diesel cycle by switch- listed in Table 1. Fig. 1 shows the schematic of the engine layout.
ing from dual fuel mode to neat diesel mode [16] and (3) The engine was modified to run on DMDF mode with introducing
distinguished from natural gas-fumigated fuel engine, there is no methanol by 3 electronically controlled methanol injectors fixed
simultaneous reduction of air supply [17], thus the compression at the intake manifold. The methanol was injected at a pressure
pressure and the mean effective pressure of the engine would of 0.4 MPa and the mass of methanol injected was controlled by
not be decreased but even boosted with methanol fumigation. an electronic control unit (ECU) developed by ourselves. Intake
Many previous investigations have been performed with a temperature was varied in the range of 35–115 °C by the coordina-
DMCC system. Recently, using a 4-cylinder direct-injection diesel tion of an intercooler and an electric heater, with a precision of
engine with fumigated methanol, Cheng et al. [18] showed that 2 °C. Injection timing and quantity of diesel were controlled by
the concentration of nitrogen oxides (NOx) is significantly reduced the ECU of the original diesel engine. The engine was coupled to
except under full load conditions. There is also a reduction in the an electronically controlled hydraulic dynamometer. Engine speed
smoke opacity and the particulate matter mass concentration. and torque could be controlled by the EMC2020 engine test sys-
With the same engine setup and operating conditions, Zhang tem, which allowed changing engine speed and load as required.
et al. [19] found that under low engine loads, the brake thermal The pressure trace in cylinder was measured with a Kistler
efficiency (BTE) decreases with the increase of fumigation metha- 6125CU20 piezoelectric pressure transducer in series with an
nol; but under high loads, it is slightly boosted with the increase AVL 612 IndiSmart combustion analyzer, which had a signal ampli-
of fumigation methanol. On a direct injection, turbocharged diesel fier for piezo inputs. A shaft encoder with 720 pulses per revolution
engine with an electronically controlled unit injection pump, Geng was used to send engine speed, which supplied a resolution of 0.5°
et al. [20] observed that the mass and number concentrations of crank angle (CA). For each engine operating point, 100 consecutive
particulate matter significantly decrease at low and medium loads, cycles of cylinder pressure data were recorded. The collected cycles
while they increase when the tested engine is operated at high were ensemble averaged to yield a representative cylinder pres-
loads. Li et al [21] developed a multi-dimensional model to inves- sure trace, which was used to calculate the apparent heat release
tigate the combustion and emission characteristics of a fumigated rate (AHRR) by the AVL 612 IndiSmart combustion analyzer.
methanol and diesel reactivity controlled compression ignition Diesel injection timing and injection quality were controlled by
engine. They found that methanol addition is an effective way to the ECU of the original engine. The methanol injection system
achieve the efficient and clean combustion and all the emissions was wholly independent of the diesel ECU. Diesel and methanol
are reduced with moderate methanol addition. fuel consumption was independently measured gravimetrically
However, the operation of dual fuel engines at lower loads still using two coriolis meters with a precision of 0.1 g. Gaseous emis-
suffers from lower thermal efficiency and higher unburned per- sions in the exhaust pipe were sampled by a Horiba MEXA
centages of fuel [22–29]. Results from our previous study showed 7100DEGR analyzer. Engine coolant temperature and inlet air
that the worsened DMDF combustion progress resulted in the
reduction of BTE from 25% to 22% at light loads, while it was
Table 1
boosted at medium and high load [30]. However, the trend to Parameters of the engine.
knock is considerable at high load when engine operates at dual
Parameters Value
fuel mode. Therefore, numerous researches have also been carried
out to improve BTE at light load when diesel engines operate with Number of cylinders Four in-line
dual fuel mode. Abd Alla et al. found that the low efficiency and Displacement 4.214 L
Bore  stoke 108  115 mm
poor emissions at light loads can be improved significantly by Compression ratio 17:1
advancing the injection timing of the pilot fuel [22]. Huang et al. Maximum power 103 kW@1600 r/min
conducted the experiments in a CI engine fueled with die- Inlet valve opening 130.3°CA ATDC
sel/methanol blend and found that the rapid burn duration and Exhaust valve opening 112.2°CA ATDC
Injection pressure 28 MPa
the total combustion duration increased with the advancing of
798 Q. Wang et al. / Fuel 159 (2015) 796–802

Fig. 1. Schematic diagram of experiment setup.

temperature were recorded by a resistance thermometer sensor which could be reached by a short circuit in the air flow circuit
and exhaust temperature was recorded using K type thermocou- (which means the compressed air from the turbocharger goes
ples with an accuracy of 0.1 °C. The diesel used in the test was directly into the engine, without passing through the intercooler).
commercial diesel fuel, while the methanol used was industrial 115 °C was selected as it was the maximum temperature produced
grade with a purity of 99.99%. by the electric heater. First, tests were conducted at diesel mode to
obtain the performance, combustion and emission characteristics
2.2. Engine operating method and test conditions of baseline engine at 25% of full load. Then the diesel was reduced
and the rest of input energy was supplied by methanol. During
The engine was fixed at 1660 r/min (as 1660 r/min represents engine tests at 25% of full load, the intake temperature is firstly
the speed A in Europe steady-state cycle test) and 25% of full load fixed at 35 °C, and the injection timings were adjusted according
(420 N m) throughout the test. Intake temperature and injection to the parameters in Table 2. And then intake temperature at
timing variations are shown in Table 2. Intake temperature of 75 °C and 115 °C was tested successively. At each intake tempera-
35 °C was selected as it was the right temperature when the orig- ture and injection timing, 30% and 60% methanol substitution
inal engine operated at this load. 75 °C was selected because it percents (MSP) were conducted, defined as MSP30 and MSP60.
referred to the intake temperature right after the turbocharger, At each test point, the period of operation was maintained for
about 3 min, and experimental data were the weighted average
of the data stream. Based on the engine load and the mass
Table 2
consumption rates of diesel and methanol, the BTE and MSP can
Engine test conditions.
be calculated by using Eqs. (1) and (2), respectively [18].
Injection timing (°CA Intake temperature Methanol substitution
ATDC) (°C) ratio (%)
35 30/60
Pb
17.4 BTE ¼  100% ð1Þ
75 30/60 ðqm0 d  Q LHV 0 d Þ þ ðqm0 m  Q LHV 0 m Þ
105 30/60
11.4 35 30/60
75 30/60
where Pb = brake power, kW; qm’d = mass consumption rate of diesel
105 30/60 fuel, kg/s; qm’m = mass consumption rate of methanol, kg/s;
7.4 35 30/60
QLHV’d = lower heating value of diesel fuel, kJ/kg; QLHV’m = lower
75 30/60 heating value of methanol, kJ/kg.
105 30/60
1.4 35 30/60 qm0 dd  qm0 dm
75 30/60 MSP ¼ ð2Þ
105 30/60
qm0 dd
4.6 35 30
75 30/60 where qm’dd = diesel fuel consumption rate in neat diesel mode;
105 30/60
qm’dm = diesel fuel consumption rate in DMDF mode.
 Q. Wang et al. / Fuel 159 (2015) 796–802 799

3. Results and discussion evaporation, accelerates the reaction rate of the mixture, widens
its flammability limits and sustains flame propagation within rela-
In this section, we give experimental results concerning the tively leaner mixtures, which result in better combustion efficiency
combined effects of air inlet pre-heating and diesel injection and higher BTE. On the other hand, the BTE remains unchanged
timing on performance, combustion characteristics and pollutant when the injection timing is before 7.4°CA BTDC and decreases
emissions of a DMDF engine. sharply as the injection timing is further retarded. With advanced
injection timing, the ignition delay is longer and more energy is
3.1. Engine performance given by the diesel injection to onset multiple propagation flames.
In contrast, retarding injection timing means later combustion, and
Fig. 2 provides the variation of BTE under DMDF combustion as therefore an incomplete combustion of methanol and the result is
a function of injection timing at different intake temperatures. At a reduction in BTE. However, when the intake temperature was at
the intake temperature of 35 °C, compared with the baseline, the 115 °C and MSP is 30%, the BTE is not decreased apparently with
BTE of DMDF combustion is lower, and even lower at MSP60. But retarded injection timing. This is due to the auto-ignition of metha-
at 75 °C and 115 °C, remarkable improvement in BTE is obtained nol before diesel injection so that the start of combustion (SOC)
under DMDF combustion when the injection timing is before and combustion duration are appropriate for higher BTE.
7.4°CA BTDC (before top dead center (BTDC)), especially at Combustion characteristics of each condition are discussed in
115 °C. The best result in terms of BTE was obtained at 115 °C detail in Section 3.2.
and MSP60, increasing by 7.3%. The reduction of BTE at 35 °C is
mainly due to the cooling effect of methanol that results in the 3.2. Combustion characteristics
reduction of average combustion temperature in the combustion
chamber. Fig. 3 shows the temperature drop after methanol injec- Figs. 4–6 show the cylinder pressure and AHRR variations of
tors. The intake temperature drops by 50% after methanol injectors, DMDF combustion at various injection timings and different intake
as the heat is absorbed by the atomization of methanol. Since the temperatures. The rate of heat release rete reveals three stages of
methanol is not fully vaporized and the intake quantity is more combustion, and for the purpose of analysis the heat release curve
at low temperature, the temperature drops became smaller. Low is divided artificially into three stages as indicated (with respect to
intake temperature worsens the combustion process at low the heat release curve obtained at an intake temperature of 75 °C
temperature conditions. As the intake temperature is enhanced, and an injection timing of 17.4°CA BTDC, which is shown in
the increase of intake temperature improves the methanol Fig. 5) [33]. In the first stage, heat is mainly released due to pre-
mixed burning of part or whole of the pilot diesel in addition to
combustion of a small part of the methanol entrained in the spray.
The end of the first stage of combustion is indicated by a dip in the
rate of heat release and lasts till 5–6°CA. During the second stage of
combustion, the remaining liquid fuel burns in a diffusion con-
trolled mode while the methanol–air mixture in the close vicinity
of pilot spray is ignited and burned. The third stage corresponds to
the burning of the methanol–air mixture by flame propagation ini-
tiated from spray zone, which is a continuation of the second stage.
The presence of stage 2 and stage 3 and their duration depend on
the different DMDF combustion conditions.
Fig. 4 shows the cylinder pressure and heat release rate at the
intake temperature of 35 °C. At 35 °C, the combustion is a combi-
nation of stage 1 and stage 3. In other words, stage 2 is not obvi-
ously distinguishable, or it takes place together with stage 1. At
the intake temperature of 35 °C, the delay period is longer and
more diesel combusts in premixed mode. Thus the ignition energy
Fig. 2. BTE versus injection timing at different intake temperature and MSPs. given by premixed diesel is enough to onset multipoint ignition of

Fig. 4. Cylinder pressure and AHRR for various injection timings at the intake
Fig. 3. Temperature drop after methanol injectors with DMDF combustion. temperature of 35 °C.
800 Q. Wang et al. / Fuel 159 (2015) 796–802

the diesel is injected into the cylinder when the piston is moving
down, there is less liquid fuel impingement on the piston bowl,
and the in-cylinder temperature is higher than conditions at earlier
fuel injection timing; thus, the liquid fuel is totally vaporized and
well mixed at the start of combustion. Completely vaporized diesel
fuel provides more ignition energy and almost all the methanol–air
mixture is ignited and burned rapidly. Therefore, there is only one
stage of combustion. On the other hand, with higher MSP, SOC is
delayed for about 2°CA, and the peak of premixed combustion
slightly decreases and peak of flame propagation slightly increases
with a long tail of late combustion caused by flame propagation.
When the MSP is at 30%, the mass and duration of injected diesel
are longer and more energy is given by the diesel injection to onset
multiple propagation flames, which result in higher peak of pre-
mixed combustion and shorter duration of flame propagation.
The intake temperature after methanol injection is 7.55 °C at
Fig. 5. Cylinder pressure and AHRR for various injection timings at the intake
temperature of 75 °C.
MSP60, decreasing by 27.45 °C. Combustion efficiency is highly
reduced and the BTE is highly worsened at such low temperature.
The variation in cylinder pressure and AHRR with injection tim-
methanol–air mixture, which results in the combination of stage 1 ing at the intake temperature of 75 °C and 115 °C are indicated in
and stage 2. The excess methanol–air mixture away from pilot Figs. 5 and 6. Three stages of combustion are more obvious at
spray combusts with a lower flame propagation speed as the tem- higher intake temperature. When the intake temperature
perature is quite low and the mixture is very dilute, causing a long increases, the delay period becomes shorter and less diesel fuel is
tail of late combustion in stage 3. When the methanol concentra- mixed within combustible limits during the delay, so the peak pre-
tion increases from MSP30 to MSP60, the rate of heat release in mixed heat release rate decreases in stage 1. However, as the
the first stage of combustion slightly decreased and rate of the intake temperature increases, the methanol–air mixture in the
third stage exhibits a slight increase. The main reasons are the close vicinity of pilot spray is ignited and burned faster, which
reduction in premixed diesel and the increase in the methanol– results in an increase of AHRR in stage 2. Meanwhile, flame prop-
air ratio. The reduction in pilot diesel lowers the quality of pre- agation is accelerated at higher intake temperature and thus the
mixed combustion of diesel, resulting in lower rate of heat release stage 3 of combustion ends at an earlier crank angle. Therefore,
in stage 1. The increase in the methanol–air ratio accelerates the combustion starts up at an earlier crank angle and also finished
flame propagation speed, resulting in higher rate of heat release at an earlier crank angle, resulting in good combustion efficiency
in stage 3. and high brake thermal efficiency. With any intake temperature,
At the intake temperature of 35 °C, as the injection timing is delay period becomes longer as the pilot quantity is reduced or
delayed, the combustion event begins at a later crank angle and as the inducted methanol–gas mixture becomes richer. As MSP
the peak cylinder pressure decreases significantly. Finally misfire increases, peak release rate in stage 1 decreases while it increases
occurs when the injection timing is retarded after 4.6°CA ATDC in stage 2, and peak release rate in stage 2 is even much higher
at MSP60, as the intake temperature is much lower so that the than that in stage 1 at MSP60 and 115°CA. It is because the metha-
directly injected diesel cannot be compressed to be ignited. As nol–air mixture becomes richer at MSP60, and the combustion
the injection timing is retarded, the rate of heat release in the first speed is faster at higher intake temperature. There is also an inter-
stage of combustion slightly reduces. This is probably due to the esting phenomenon that occurs at the intake temperature of 115 °C
reduction of premixed diesel at the SOC, since the delay period and injection timing of 4.6°CA ATDC that premixed methanol–air
decreases with the postponement of injection timing. When the auto-ignites before the direct injected diesel is injected into the
diesel injection timing is further delayed after top dead center, cylinder. In other words, the diesel is ignited by methanol. At any
two stages of combustion gradually become single stage combus- intake temperature, the BTE of DMDF combustion is quite lowered
tion and the heat release rate of combustion increases. Because at such injection timings. Such delayed injection timing causes late
combustion and long combustion duration in stage 3, which
decrease the efficiency of work extraction since it is in the expan-
sion stroke. But the BTE at this particular condition is not worsened
due to the auto-ignition of methanol which advances the SOC.
Combustion takes place at an earlier crank angle, which results
in higher efficiency of work extraction and better brake thermal
efficiency.

3.3. Exhaust emissions

Fig. 7 clearly shows the soot-NOx trade-off dilemma on DMDF

operation with various injection timings under different intake
temperatures and MSPs. It can be observed that the NOx emission
decreases and soot emission increases as the injection timing is
retarded at the intake temperature of 115°CA and MSP30. When
the intake temperature is decreased and methanol substitute per-
cent increases, both NOx and soot emissions decrease drastically.
The trade-off dilemma is apparent except when the injection tim-
Fig. 6. Cylinder pressure and AHRR for various injection timings at the intake ing is retarded to 4.6°CA ATDC. At the condition of 35 °C and
temperature of 115 °C. MSP60, the soot NOx trade-off dilemma completely disappears
 Q. Wang et al. / Fuel 159 (2015) 796–802 801

temperature increased. Long tail combustion of methanol

caused by flame propagation disappeared and hence the
BTE increased when the intake temperature is above 75 °C.
(3) The methanol–air mixture flame propagation disappears
gradually as the injection timing is retarded, and two stages
of combustion gradually become a single stage combustion
with increased heat release rate of combustion when the
diesel injection timing is further delayed after TDC.
(4) The rate of premixed combustion decreases while the rate of
flame propagation increases as the methanol substitution
percent increases. When injection timing is retarded after
4.6° crank angle, misfire occurs at higher MSP and lower
intake temperature, while the auto-ignition of methanol
occurs at lower MSP and higher intake temperature.
(5) Soot-NOx trade-off dilemma on DMDF operation is com-
pletely broken at lower intake temperature and higher
methanol substitute percent. A new relationship of optimal
trade-off points of DMDF operation was observed at the con-
Fig. 7. Effect of injection timings on NOx and soot emissions at different intake
temperatures and MSPs. dition of 35 °C and MSP60.

and a new optimal trade-off point of operation can be observed.

Advancing the injection timing so that combustion occurs earlier Acknowledgments
in the cycle increases the peak cylinder pressure and higher peak
cylinder pressures result in higher peak burned gas temperatures, The authors wish to thank the National Natural Science
and hence higher NOx formation rates. The availability of more Foundation of China (No. 51336005) and the Project of Doctorate
time for the oxidation process between carbon and oxygen mole- funding from the Ministry of Education of China (No.
cules results in better combustion process by the early start of fuel 20120032130009).
injection, and hence less soot emission is generated. At lower
intake temperature and higher MSPs, the combustion temperature
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