Applied Energy 88 (2011) 3704–3711

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Applied Energy

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Combustion and emissions performance of a DME-enriched spark-ignitedmethanol engine at idle condition

Chen Liang, Changwei Ji ⇑, Xiaolong LiuCollege of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China

a r t i c l e i n f o

Article history:Received 3 September 2010Received in revised form 2 April 2011Accepted 30 April 2011Available online 11 June 2011

Keywords:MethanolDimethyl ether (DME)SI enginesIdle conditionEmissions

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.04.056

⇑ Corresponding author. Tel./fax: +86 1067392126.E-mail address: chwji@bjut.edu.cn (C. Ji).

a b s t r a c t

Dimethyl ether (DME) and methanol are thought to be one of the most promising alternative fuels for ICengines. Meanwhile, previous investigations also have pointed out the good prospects for adopting DMEand methanol in IC engines. The experiments in this paper were carried out at idle condition to investi-gate the effect of applying the methanol/DME blended fuel in a SI engine. The engine was modified to befueled with the mixture of methanol and DME which were injected into the engine intake ports simul-taneously. Various DME fractions were selected to investigate the effect of DME addition on engine per-formance. The experimental results showed that indicated thermal efficiency was increased by 25% andcoefficient of cyclic variation in engine speed was decreased by 29.2% at the DME energy fraction of 85.2%in the total fuel. In addition, both flame development and propagation durations were shortened with theincrease of DME enrichment level at idle condition. Meanwhile, the largest drop of HC emissions wasnearly 50% compared with the original methanol engine at stoichiometric condition. However, CO andNOx emissions increase with the addition of DME.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

With the tremendous increase of transportation energy con-sumption, the environmental pollution and energy shortage haveemerged as a main societal problem and a focus for more and morepeople. In order to protect the environment and realize the sustain-able development, finding better substitutes for fossil fuels has be-come a major work for most of the researchers studying internalcombustion (IC) engines. Due to the low emissions, being environ-mentally friendly, renewable, sustainable, and large reserves, alter-native fuels have been considered as one of the best choices forfuture IC engines [1–4].

Compared with other alternatives, dimethyl ether (DME) andmethanol are suitable alternative fuels for CI and SI engines,respectively. DME can be produced from a variety of feedstockssuch as natural gas, coal, crude oil, residual oil, waste productsand biomass. To verify the suitability of DME in a diesel engine,many investigations have been carried out [5–8]. The high cetanenumber and low boiling point of DME symbolize the short ignitiondelay, low auto-ignition temperature and almost instantaneousvaporization when DME is injected into the cylinder. Moreover,as DME is non-toxic and environmentally benign, whatever atlow or high mole fractions (percent by volume) in air, it hardlyhas any odor and causes negative health effects. The DME has a

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low carbon-to-hydrogen ratio (C:H), a high oxygen content(around 35% by mass) and no C–C bonds in its molecular structure.DME can realize the smoke-free combustion and was also used asan ignition promoter in order to ignite a low cetane number fuel insome researches [9–15].

In addition, methanol also can be produced from synthesis gaswhich is formed by steam reforming of natural gas or by gasifica-tion of coal, or from biomass, all of which are available in abun-dance or renewable [16,17]. When used in SI engines, as methanol has a high octane number, high latent heat of vaporizationand high oxygen content (around 50% by mass), it will result in agood antiknock performance, dense fuel–air charge and excellentlean combustion [18]. Although methanol is a good alternative fuelfor IC engines, however, the low vapor pressure and high latent heatof vaporization of methanol also cause cold start difficulties at lowambient and in-cylinder temperatures.

Because vehicle engines always run at complex and variableconditions, it is difficult for a mono-fuel to satisfy the requirementsof different operating conditions, especially for the alternativefuels. There are plenty of researches focusing on investigating thedual-fuel combustion or the effect of DME or methanol additionon the traditional IC engine performance. Gong et al. [17] made acomparative study of a SI engine fueled with methanol, LPG andmethanol–LPG mixture, respectively. They found that the ambienttemperature was the most significant factor for a reliable cold startof the methanol-fueled engine, and when the ambient temperaturewas below 16 �C, the methanol-fueled engine could not be started

Fig. 1. Methanol and DME fuel rails and injectors installed on the intake manifolds.

C. Liang et al. / Applied Energy 88 (2011) 3704–3711 3705

reliably without auxiliary startaids. Meanwhile, the methanol–LPGmixture-fueled engine could realize the better combustion at coldstart after the injection timings of methanol and LPG were con-trolled reasonably. Abu-Zaid et al. [19] studied the effect of meth-anol addition on the performance of a SI gasoline engine. Theyobserved that methanol had a significant effect on the performanceof the gasoline engine and the best engine performance appearedat the methanol volume fraction in the blended fuel of 15%. Weiet al. [20] used the detailed kinetic mechanism of primary refer-ence fuels (iso-C8H18/n-C7H16) to simulate the production mecha-nisms of HCHO and CH3CHO emissions from the methanol–gasoline mixture-fueled engine. The results showed that HCHOmainly originated from C1 radicals and HCHO from the M85 enginewas about five times more than that from the gasoline engine.CH3OH emission was the part of unconsumed methanol and de-creased with the increase of the engine speed. Yeom and Bae[21] tested a HCCI engine fueled with gasoline–DME fuel mixtures.Their test results showed that the start of combustion was inde-pendent of the gasoline injection quantity, but the burning dura-tion (CA20-90) and combustion pressure rise rate weredependent on the fuel quantity and timing of intake valve opening(IVO). The indicated mean effective pressure (imep) was deter-mined by the fuel quantity. The knocking probability and intensityincreased with the increase of the peak combustion pressure. Inaddition, HC emissions increased as the IVO timing was retardedand the total excess air ratio was decreased. Zhen et al. [22,23]experimentally investigated a DME-fueled HCCI engine and foundthat the pure DME combustion in a HCCI engine would lead to avery early self-ignition timing when the compression ratio was16.5, and the negative compression work increased. Anotherexperimental result indicated that the application of CO2 could re-duce the double-peak heat release, delay the positions of theircrank angles, and drop the peak pressure of DME combustion ina HCCI engine. Yao et al. [24,25] applied DME to the compressednatural gas (CNG) and methanol HCCI combustion processes topromote ignition. The results showed that DME addition could im-prove ignition and broaden the operating range of CNG and meth-anol HCCI engines.

Up to now, most work on methanol or DME has concentrated onHCCI or traditional gasoline engines, while little work is reportedon the DME-enriched SI methanol engine. Although methanol en-gines have lower emissions and higher thermal efficiency, theoperational range is narrow and running condition is unstable atlow speed and load conditions, because of the high octane numberand high vaporization heat. In order to widen the operational rangeand improve the performance of SI methanol engines, DME isintroduced into the SI methanol engine. According to the previousstudies introduced above, as DME is easy to vaporize and autoig-nite at low temperatures, it can accomplish almost the instanta-neous vaporization in air when injected into the intake ports dueto its low boiling point and realize the low temperature combus-tion. In SI engines, DME addition can also enhance the fuel com-bustion and produce lower HC and NOx emissions than theoriginal engine because of its high oxygen content [26]. Moreover,methanol blended with DME can be easily ignited and burnt at idlecondition.

Nowadays, SI engines have been using a fixed octane rating fuelsuch as #93 gasoline to produce a wide range of torque and speed.However, for the optimal operation of SI engines to produce thepossibly highest thermal efficiency and lowest emissions, SI en-gines should be fueled with the variable octane rating fuels for var-ious engine operating conditions, e.g. at cold start, the low octanerating fuel is used to facilitate the engine start, and at high loads,the high octane rating fuel is utilized to avoid the engine knock.As methanol is a high octane fuel and DME is a high cetane fuel,DME-rich methanol has a lower octane rating than the pure meth-

anol, which avails the high efficiency and low emissions operationof the methanol engine at low load conditions. The higher DMEaddition level is, the lower octane rating of the DME–methanol fuelmixture is. But the experimental study on the DME-enriched SImethanol engines has not been reported. In order to investigatethe effect of DME addition on the performance of a SI methanol en-gine at idle condition, this paper introduced an experiment inwhich DME and methanol were synchronously injected and mixedin the intake ports at the specified DME-to-methanol ratios. Thecombustion and emissions performance of a DME-enriched meth-anol engine was experimentally investigated at various DME-to-methanol energy ratios and idle condition.

2. Experimental setup and procedure

2.1. Experimental setup

A 1.6 L, port fuel injection, four-cylinder, SI engine manufac-tured by Beijing Hyundai Motors is used in this experiment. Theengine has a bore of 85 mm, a compression ratio of 10:1, a ratedpower of 82.32 kW at 6000 rpm and a rated torque of 143.28 Nmat 4500 rpm. The engine intake manifolds used in this study ismodified to the one (see Fig. 1) on which each of four DME injec-tors is mounted near the intake port of each cylinder, and metha-nol is injected by the original engine gasoline injectors, so thatDME and methanol can be mixed in the intake ports online.

The DME used in the study is produced from the methanolreforming with the impurity content below 0.1 vol.% and storedin a stainless steel bottle in liquid state at the pressure of0.7 MPa. The high pressure DME can be introduced into the gas-eous DME rail at 0.2 MPa after one step of pressure reduction.The methanol used in the study is produced from synthesis gaswhich is formed from coal gasification, whose impurity contentis below 0.4 vol.%. The key properties of DME and methanol areshown in Table 1 [27,28]. A hybrid electronic control unit (HECU)is developed and adopted to govern the spark timings, injectiontimings and durations of DME and methanol. The HECU acquiresthe engine sensor data from the original ECU (OECU) and commu-nicates with a calibration computer. The injection durations ofDME and methanol can be adjusted freely through the commandsfrom the calibration computer to accomplish the specified DME en-ergy fraction in the total methanol–DME fuel mixture.

The experimental system is shown in Fig. 2. A GW160 eddy cur-rent dynamometer is coupled to the engine to control and measure

Table 1Properties of methanol and DME.

Methanol DME

Chemical structure CH3OH CH3OCH3

Density (kg/m3) 0.79 (liquid) 2.06 (gaseity)Boiling point (�C) 65 �25.1Auto-ignition point (�C) 450 235Low heat value (MJ/kg) 19.9 27.6Vaporization heat (kJ/kg) 1110 460Octane number 111 –Cetane number <15 55–60A/Fst 6.5 9.0Flame speed (cm/s) 45–52.3 45–52Oxygen content (wt.%) 50 34.8Carbon content (wt.%) 37.5 52.2

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the engine speed and torque output (measurement uncertainties:±1 rpm in speed and ±0.4 Nm in torque, respectively). The metha-nol mass flow rate is measured by a FC2210 fuel mass flow meter(measurement uncertainty: ±0.4% F.S.). The cylinder pressure dataare detected by a Kistler 6117BFD17 pressure transducer with aspark plug (measurement accuracy: ±0.6% F.S.), which is screwedinto the fourth cylinder of the test engine to acquire the combus-tion cylinder pressure and enforce the ignition of the fourth cylin-der. A Kistler 2613B optical encoder is connected to the front of theengine crankshaft producing 1800 pulses per rotation to trigger thepressure acquisition system (crank angle resolution: 0.2�CA, mea-surement deviation <0.01�CA). A cylinder pressure transducerand an optical encoder are connected to a DEWETRON combustionanalyzer made in Austria via the screened cables. Cylinder pressureand crank angle signals for over 200 consecutive cycles are sam-pled and treated via a DEWE-CA combustion analysis softwareembedded in the DEWETRON combustion analyzer to obtain pro-files of cylinder pressure and heat release rate against crank angle,and curves of indicated thermal efficiency, CA0-10, CA10-90,COVimep against DME energy fraction, etc.

Fig. 2. Schematics of the experimental system. (1) DME cylinder assembly, (2) DME presarrestor, (6) DME injector, (7) throttle, (8) air mass flow meter, (9) idle motor, (10) origitank, (14) methanol mass flow meter, (15) methanol pump, (16) methanol injector, (17encoder, (20) charge amplifier, (21) A/D converter, (22) combustion analyzer, (23) O2 semissions analyzer. (a) Signals from the OECU to the HECU. (b1) Calibration and control sithe calibration computer.

The exhaust emissions of NOx, HC and CO from the test engineare measured by a Horiba MEXA-7100DEGR emissions analyzer.NOx emissions are measured by the chemiluminescent method;HC emissions are measured by the hydrogen flame ionizationdetection method, and CO is measured by the non-dispersive infra-red method. The analyzer is calibrated before the experiment withzero and span gases to ensure its accuracy and repeatability. Themeasurement uncertainties of NOx, HC and CO are less than ±1%of their measured values, respectively. The measurement sensitiv-ities of NOx, HC and CO are all 1 ppm. The air and methanol massflow rates are monitored by a EPI-800 and a D07-9EM thermalmass flow meters (measurement uncertainty for air or methanol:±1% F.S.), respectively.

For a specified DME mass flow rate, methanol injection durationcan be adjusted to gain the stoichiometric or lean condition. Theexcess air ratio is monitored by a Horiba MEXA-110 A/F analyzerwith an oxygen sensor inserted into the exhaust pipe (uncertainty:±0.1 A/F at A/F = 6.5). The H/C atom molar ratio setting of theinstrument can be adjusted in terms of the measured methanoland DME mass flow rates, so that it can well measure the A/F ratioof a DME-enriched SI methanol engine. The calculated and mea-sured excess air ratios have a good consensus with the error below±5%. We use the regression way to analysis the collected data anddraw the pictures, the regression confidence lever (R2) we used is95%.

2.2. Experimental procedure

The test was carried out at idle condition which is a typicaloperating condition of IC engines, under which engines alwaysbear an unstable combustion process and serious cyclic variation,because of the low engine speed, unhomogeneity of the in-cylindercharge, high residual gas fraction and the variation in fuel to air ra-tio. The control of engine idle fuel consumption and emissions hasbeen of great importance, and many researchers have been tryingto improve the engine idle performance [29–33].

sure adjusting valve, (3) DME pressure meter, (4) DME mass flow meter, (5) backfirenal ECU (OECU), (11) hybrid ECU (HECU), (12) calibration computer, (13) methanol) ignition module, (18) cylinder pressure transducer with a spark plug, (19) opticalensor, (24) A/F analyzer, (25) emissions sampling pipe, (26) Horiba MEXA-7100D

gnals from the calibration computer to the HECU. (b2) Data signals from the HECU to

Fig. 3. Effect of DME enrichment on COVn at idle condition.

Fig. 4. Effect of DME enrichment on fuel energy flow rate at idle condition.

C. Liang et al. / Applied Energy 88 (2011) 3704–3711 3707

It is general accepted that the too high or too low coolant andlubricant temperatures would lead to unstable operation conditionand bad combustion condition [34]. In order to reduce their nega-tive effects on the tests and facilitate comparisons, the coolant andlubricant oil temperatures were kept at 90 and 95 ± 1 �C during theexperiment. For a given DME mass flow rate, the methanol massflow rate was gradually reduced to make the engine operate atstoichiometric or lean condition to accomplish DME energy frac-tion in the total fuel (DME + methanol) varying from 0% to 90%.In this study, DME energy fraction in the total fuel (bDME), the glo-bal excess air ratio of the DME–methanol–air mixture (k) are de-fined by Eqs. (1) and (2), respectively as below.

bDME ¼ _VDMEqDMEqDME=ð _VDMEqDMEqDME þ _mmethanolqmethanolÞ� 100% ð1Þ

k ¼_Vairqair

_VDMEqDMEAFst;DME þ _mmethanolAFst;methanol

ð2Þ

In Eqs. (1) and (2), _mmethanol represents the measured methanol massflow rate (g/min). _Vair and _VDME represent air and DME volume flowrates at normal conditions (L/min); qair and qDME symbolize air andDME densities at normal condition (g/L); AFst,methanol and AFst,DME arethe stoichiometric air-to-fuel ratios of methanol and DME (6.5 and9.0); qmethanol and qDME are the low heating values of methanol andDME (19.9 and 28.8 kJ/g), respectively.

In this experiment, the spark timing was governed by the OECUto be near the TDC. The DEWE-CA combustion analyzer and its rel-evant analysis software were used to record cylinder pressure andcrank angle data, calculate heat release fraction (HRF) and the per-centage coefficient of variation in indicated mean effective pres-sure (COVimep) for over 200 consecutive engine cycles. Theengine combustion and emissions parameters for various DMEaddition levels and excess air ratios were compared and analyzed.

3. Results and discussion

3.1. Coefficient of variation in engine speed (COVn)

COVn is an important parameter to evaluate the engine operat-ing stability and engines always show high cyclic variation at idlecondition [35]. According to Lyon’s research, engine thermal effi-ciency could be increased by 6% if there was no cyclic variation[36]. It is general accepted that if all the fuels in cylinder can be ig-nited at the same time, both of COVn and COVimep could be elimi-nated [34,37–40].

In the experiment, the engine speed is held at 791 rpm by theidle step motor which is controlled by the OECU. Fig. 3 showsthe COVn with DME energy fraction in the total fuel at idle condi-tion. The coefficient of variation in engine speed is defined as:

COVn ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXk

i¼1

ðni � �nÞ2=ðk� 1Þ

vuut0@

1A=�n

24

35� 100% ð3Þ

where �n is the average engine speed for k engine rotations, ni is theaverage speed of the ith rotation calculated by crank angle pulsegenerated from a Kistler 2613B optical encoder (rpm), and k is thenumber of engine rotations.

It can be seen from Fig. 3 that COVn is 0.24% at stoichiometriccondition and 0.48% at lean condition without DME addition,respectively, and then decreases with the addition of DME. Espe-cially at lean condition, COVn is decreased by 29.2% at DME energyfraction of 87%.

The reduction of COVn can be attributed to the following rea-sons: Firstly, as is shown in Fig. 7a, the peak in-cylinder tempera-

ture was continuously increased with the increase of DME energyfraction in the total fuel. The enhanced in-cylinder temperaturemeans that the combustion quality was improved after DME addi-tion. Secondly, the gaseous state of DME [41] results in the well-mixed charge of air–fuel mixture and therefore benefits the com-plete combustion of the blended fuel. Thirdly, the low-temperatureexothermic oxidation of DME is also helpful for improving thecombustion condition and shortening the combustion duration[9,42,43]. Lastly, the relatively fast flame speed of DME and lowtemperature oxidation also results in the shortened combustionduration, which also leads to the low cycle-to-cycle variations afterDME addition.

Thus, based on the above discussions, it is reasonable to statethat the addition of DME could improve the combustion conditionand therefore reduces the cyclic variation of the original methanolengine at idle condition.

3.2. Fuel energy flow rate

Fig. 4 shows the plot of fuel energy flow rate versus DME energyfraction in the total fuel at idle condition. The energy of theblended fuel not only influences the peak cylinder pressure andimep, but also concerns the engine torque output as well as indi-

Fig. 5. Effect of DME enrichment on indicated thermal efficiency at idle condition.

3708 C. Liang et al. / Applied Energy 88 (2011) 3704–3711

cated thermal efficiency. The fuel energy flow rate ( _ETotalFuel) is de-fined as:

_ETotalFuel ¼ 2ð _VDMEqDMEqDME þ _mmethanolqmethanolÞ=n ðkJ=cycÞ ð4Þ

where n is the engine speed (rpm) and the others are the same asthose defined in Eqs. (1) and (2).

In Fig. 4, it is apparent that the fuel energy flow rate decreaseswith the increase of DME energy fraction at idle condition. At stoi-chiometric condition, the fuel energy flow rate decreases from0.86 kJ/cyc of the original methanol engine to 0.65 kJ/cyc of theDME-enriched methanol engine at DME energy fraction of 86.3%,which is reduced by 24.5%. In addition, at lean condition ofk = 1.3, the fuel energy flow rate drops from 0.90 kJ/cyc of the ori-ginal methanol engine to 0.68 kJ/cyc of the DME-enriched metha-nol engine at bDME = 85.2%.

Since the enrichment of DME results in the improved enginethermal efficiency and shortened combustion duration, the engineoriginal electronic control unit (OECU) has to reduce the opening ofidle bypass valve to maintain the engine operating at the originaltarget idle speed of 790 rpm. In addition, because DME is injectedinto the intake ports in the gaseous state and occupies the air vol-ume. In order to keep a fixed air excess ratio, the methanol injec-tion quantity needs to be adjusted separately and the mass flowrate of methanol decreases with the increase of bDME. Thus, lessair is introduced into the cylinder after DME addition. For a speci-fied excess air ratio, the fuel flow rate should be reduced accord-ingly with the decrease of air flow rate. Thereby, the overall fuelenergy is reduced after DME blending. Moreover, for a given idlespeed, the resistance work and losses are roughly kept at constant.So, the improved thermal efficiency after DME blending means lessfuel is required for the DME–methanol engine than that for thepure methanol engine at idle. All in all, the basic reason for thedropped fuel energy flow rate after DME addition could be ascribedto the enhanced combustion and thermal efficiency. However, atlean condition, in order to maintain the stable idle operation, theincreased opening of idle bypass valve is required by the OECU,which leads to more consumption of the blended fuel than at stoi-chiometric condition.

Fig. 6a. Effect of DME enrichment on CA0-10 at idle condition.

3.3. Indicated thermal efficiency

Indicated thermal efficiency versus DME energy fraction curveis shown in Fig. 5 at idle condition. Engine thermal efficiency is acrucial parameter for evaluating engine economic and overall per-formance, which can be improved by either optimizing the com-bustion system or adjusting fuel properties. Fig. 5 shows thatengine indicated thermal efficiency is gradually improved withDME addition. At stoichiometric condition, indicated thermal effi-ciency is enhanced from 27.17% of the original engine to 33.13%of the DME-enriched methanol engine at bDME = 86.3%, which is in-creased by 21.9%. The curve of lean combustion shows the sametrend and indicated thermal efficiency is increased by 25.9% whenbDME is 85.2%, compared with the original engine. The test resultsshow that the addition of DME could increase the indicated ther-mal efficiency of the SI methanol engine.

This can be attributed to that based on the gaseous state, highcetane number and other properties of DME, the engine couldget the same power output when fueled with DME/methanol mix-ture. Contrarily, compared with the same power output, the energyflow rate of the DME–methanol fuel mixture is much less than thatof the pure methanol (see Fig. 4). Thus, indicated thermal efficiencyis enhanced with DME addition. The fast diffusion velocity and lowboiling point of the gaseous DME in air also stimulate the atomiza-tion of methanol and improve the mixing of the blended fuel withair. Moreover, because DME has the low-temperature oxidation

reaction during compression stroke due to its low autoignitiontemperature and high cetane number, DME addition could improvethe ignition and burning of the SI methanol engine.

3.4. Combustion

Flame development and propagation durations are two impor-tant factors influencing engine thermal efficiency, which are sym-bolized as CA0-10 and CA10-90, respectively. CA0-10 and CA10-90are defined as the crank angle durations from spark discharge to10% and from 10% to 90% heat release of the total methanol–DME fuel mixture, respectively.

Fig. 6a displays the variation of CA0-10 with DME energy frac-tion at idle condition. Fig. 6a indicates that CA0-10 decreases withthe increase of DME addition fraction. At stoichiometric condition,CA0-10 is reduced from 27.83 �CA of the original engine to24.12 �CA of the DME-enriched methanol engine at bDME = 86.3%.Especially at lean condition and when bDME exceeds 40%, CA0-10is obviously shortened and reduced by over 30%.

The reasons are attributed to two aspects. Firstly, as the boilingpoint of DME is lower than that of methanol and DME is injected inthe gaseous state, the atomization and mixing of the blended fuelwith air are improved. Secondly, with the increase of cylinder pres-

Fig. 6b. Effect of DME enrichment on CA10-90 at idle condition.Fig. 7b. Effect of DME enrichment on Tevo at idle condition.

C. Liang et al. / Applied Energy 88 (2011) 3704–3711 3709

sure and temperature during the compression stroke, the low tem-perature oxidation reaction of DME is started and thus producesOH radicals [9,42,44,45] which, to some extent, inspire the reactionof methanol with air.

Fig. 6b shows the variation of CA10-90 with DME energy frac-tion at idle. It can be seen from Fig. 6b that CA10-90 decreases withthe increase of DME energy fraction. CA10-90 drops more largely atlean condition than at stoichiometric condition. At lean condition,CA10-90 decreases from 68.5 �CA of the original methanol engineto 59 �CA of the DME-enriched methanol engine at bDME = 85.2%,which is reduced by 14.2%. For a specified DME energy fraction,CA10-90 is much shorter at stoichiometric condition than at leancondition.

The possible interpretation can be that DME is mixed well withair after injected into the intake ports because of its gas state andlow boiling point, and moreover, the low-temperature oxidationand low auto-ignition temperature of DME are also helpful forincreasing the in-cylinder temperature and improving the burningof the methanol–DME fuel mixture. DME addition accelerates themethanol burning in air, promotes the methanol–DME flame rap-idly through the whole combustion chamber, and finally reducesflame propagation duration obviously.

Peak in-cylinder temperature (Tmax) greatly influences the for-mation of engine emissions, especially NOx emissions. The in-cylin-

Fig. 7a. Effect of DME enrichment on Tmax at idle condition.

der temperature at exhaust valve opening (Tevo) reflects theexhaust losses to some extent. Tmax and Tevo are calculated basedon the gas state equation. The profiles of variations of Tmax and Tevo

with DME energy fraction at idle condition are plotted in Figs. 7aand 7b, respectively.

It is found from Fig. 7a that Tmax for both stoichiometric andlean conditions are continuously enhanced with the increase ofDME addition level. At stoichiometric condition, Tmax increasesfrom 1643 K of the original engine to 1685 K of the DME-enrichedmethanol engine at bDME = 86.3%. The possible reason is that thevaporization heat of DME is only 30% of that of methanol. DMEhas a low boiling point and could be atomized easily, which couldimprove the burning condition and increase the peak in-cylindertemperature for the methanol engine. Meanwhile, the heat releasefrom the low-temperature oxidation of DME is enhanced alongwith the increase of DME addition level, which is also a reasonfor the increase of Tmax.

From Fig. 7b, it can be found that Tevo decreases when bDME in-creases. At lean condition, Tevo is 1325 K for the original engine andreduced by 11% at bDME = 85.2%. Tevo is mainly influenced by flamepropagation duration. The addition of DME to methanol could im-prove combustion, shorten flame propagation duration and reduceTevo. Meanwhile, the high diffusion speed of DME also benefits theimprovement of the homogeneity of the in-cylinder charge, whichcould be ascribed as another reason for the fast burning in theDME-blended methanol engine.

3.5. Untreated emissions

HC, CO and NOx emissions are main toxic pollutants exhaustedfrom SI engines. DME possesses many physicochemical propertiesthat promote the methanol to be more fully combusted at idle con-dition. In this study, the untreated HC, CO and NOx emissions be-fore a three-way catalytic converter from a DME-enriched SImethanol engine are measured and analyzed.

HC emissions mainly result from the incomplete and partialburning of the fuel, which are influenced by fuel properties, A/F,spark timing and so on. The variations of HC emissions from thetest engine with DME energy fraction at idle condition are shownin Fig. 8. It can be seen from Fig. 8 that HC emissions decrease withthe addition of DME. At stoichiometric condition, HC emissions de-crease from 1330 ppm of the original engine to 739 ppm of theDME-enriched engine at bDME = 86.3%, which are reduced by44.5% compared with the original methanol engine. Because ofthe relatively high A/F, HC emissions at lean condition are higher

Fig. 8. Effect of DME addition on HC emissions at idle condition. Fig. 10. Effect of DME addition on NOx emissions at idle condition.

3710 C. Liang et al. / Applied Energy 88 (2011) 3704–3711

than those at stoichiometric condition, but also obviously decreasewith the increase of DME enrichment level.

The proper explanation could be that the low boiling point ofDME is helpful for increasing the diffusion speed and improvingthe homogeneity of the methanol–DME–air mixture. Meanwhile,the low-temperature oxidation heat release of DME could enhancethe in-cylinder temperature before ignition, which could also be areason for the complete burning of the methanol–DME–air mixtureto realize the low HC emissions.

The effect of DME addition on CO emission is displayed in Fig. 9.As is shown in Fig. 9, CO emission increases with the addition ofDME. CO emission is about 0.4% and 0.16% for the original engineat stoichiometric and lean conditions, and is increased by 27.5%and 18.75%, at bDME = 86.3% and 85.2%, respectively. CO emissionresults from the fuel incomplete burning, which is not only relatedwith the air–fuel ratio, but also directly influenced by the carboncontent in the fuel. Because the carbon fraction of DME is higherthan that of methanol, DME would tend to produce more CO emis-sion after burning. In addition, due to the high flame speed of DME,the combustion of DME quickly consumes the adjacent air, causingthe formation of some lean-oxygen areas where the increased COemission is produced. Therefore, CO emission increases with theincrease of DME enrichment level.

Fig. 9. Effect of DME addition on CO emission at idle condition.

NOx emissions mainly depend on combustion temperatureand oxygen concentration in the cylinder. NOx emissions in-crease with the increase of DME addition fraction, which isshown in Fig. 10. The peak in-cylinder temperature is increasedwith DME addition, so NOx emissions increase with the increaseof DME addition level. It is also seen from Fig. 10 that at stoichi-ometric condition, NOx emissions increase from 21 ppm of theoriginal methanol engine to 25 ppm of the DME-enriched meth-anol engine at bDME = 86.3%. Comparatively, at lean condition,NOx emissions increase from 4 ppm of the original engine to12 ppm at bDME = 85.2%.

Because of the high vaporization heat of methanol and idle con-dition, the burning temperature is lower than the required temper-ature for NOx emissions to be produced, which causes the low NOx

emissions. However, as DME has a fast burning velocity and lowautoignition temperature, the DME–methanol fuel mixture canbe more easily ignited and combusted than the pure methanol atidle. Therefore, DME addition can result in the enhancement ofthe peak in-cylinder temperature. As a result, NOx emissions lightlyincreased with the increase of DME energy fraction.

4. Conclusions

An experimental study aiming at investigating the effect of DMEaddition on improving a SI methanol engine performance at idlecondition was introduced in this paper. The experiment was per-formed on a modified 1.6 L SI engine on which a DME injection sys-tem was added to the intake manifolds to realize DME portinjection. The injection timings and durations of DME and metha-nol can be adjusted online through a self-developed HECU. The en-gine was running at idle condition, and the spark timing wasadjusted by the OECU. The main conclusions are listed as below:

(1) Under the experimental condition, the average engine speedis controlled close to the target speed, and DME enrichmentresults in the reduction of COVn. In addition, the _ETotalFuel

drops with the increase of DME addition fraction at idle con-dition. At stoichiometric condition, indicated thermal effi-ciency obviously increases from 27.17% to 33.13% afterDME enrichment, which is increased by 21.9%.

(2) The enrichment of DME would enhance peak in-cylindertemperature and shorten both the flame development per-iod and propagation duration. Meanwhile, Tevo is reducedby 11% at bDME = 85.2% and lean condition.

C. Liang et al. / Applied Energy 88 (2011) 3704–3711 3711

(3) HC emissions are obviously reduced with the increase ofDME addition level. CO and NOx emissions increase withthe addition of DME. The biggest drop of HC for the DME-blended methanol engine is 27.5% when DME energy frac-tion is 86.3% at stoichiometric condition, compared with thatof the original one.

(4) In view of the above, DME addition is an effective way forimproving the economic and emissions performance of theSI methanol engine. Meanwhile, DME addition is also helpfulfor realizing the stable operation, especially at idle and leanconditions.

Acknowledgment

This work was supported by National Natural Science Founda-tion of China (Grant No. 50976005).

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