- Email: [email protected]
The effects of different intake charge diluents on the combustion and emission characteristics of a spark ignition natural gas engine
Accepted Manuscript The effects of different intake charge diluents on the combustion and emission characteristics of a spark ignition natural gas engine Zhuoyao He, Qijian Jing, Lei Zhu, Wugao Zhang, Zhen Huang PII:
S1359-4311(15)00631-6
DOI:
10.1016/j.applthermaleng.2015.06.072
Reference:
ATE 6765
To appear in:
Applied Thermal Engineering
Received Date: 1 November 2014 Accepted Date: 24 June 2015
Please cite this article as: Z. He, Q. Jing, L. Zhu, W. Zhang, Z. Huang, The effects of different intake charge diluents on the combustion and emission characteristics of a spark ignition natural gas engine, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.06.072. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
The effects of different intake charge diluents on the combustion and emission
2
characteristics of a spark ignition natural gas engine
3 4 5
Zhuoyao He, Qijian Jing, Lei Zhu*, Wugao Zhang, Zhen Huang*
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
ABDC: after bottom dead center ATDC: after top dead center BBDC: before bottom dead center BTDC: before top dead center CAD: crank angle degree : specific heat capacity at constant pressure for intake mixture : specific heat capacity at constant pressure for component i
26 27 28 29 30 31 32
: specific heat capacity at constant volume for intake mixture : specific heat capacity at constant volume for component i ECU: electronic control unit EGR: exhaust gas recirculation EOC: end of combustion HC: hydrocarbon , : lower heating value of natural gas
33 34 35 36 37 38 39 40 41 42 43
IVO: intake valve opening k: specific heat ratio of intake mixture : mass flow rate of natural gas
: engine speed p: cylinder pressure R: gas constant ST: spark timing TDC: top dead center THC: total hydrocarbon T: mean in-cylinder temperature : engine torque
Corresponding authors： E-mail address: [email protected] (Lei Zhu*) E-mail address: [email protected] (Zhen Huang*)
RI PT
Key Lab. for Power machinery and Engineering of M. O. E., Shanghai Jiao Tong University, 200240 Shanghai, P R China
M AN U
SC
* Corresponding author address: Key laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China. Tel: +86-21-34205949 Fax: +86-21-34205949
AC C
EP
TE D
Nomenclature： ：
1 / 23
58 59 60 61 62
[CO]: CO emission concentration [23 ]4 : intake oxygen concentration [52 ]: NOx emission concentration [THC]: total hydrocarbon emission concentration
SC
: peak mean in-cylinder temperature : exhaust manifold temperature V: cylinder volume : mass percentage of component i : mass fraction burnt θ: crank angle θ : crank angle for spark timing θ : crank angle for end of combustion θ ! : crank angle for 10 percent mass fraction burnt θ"!: crank angle for 90 percent mass fraction burnt ∆$% : ignition delay ∆$&% : combustion duration η : effective thermal efficiency ()*+, : net heat release rate (-
AC C
EP
TE D
M AN U
44 45 46 47 48 49 50 51 52 53 54 55 56 57
RI PT
ACCEPTED MANUSCRIPT
2 / 23
ACCEPTED MANUSCRIPT Abstract
M AN U
SC
RI PT
Exhaust gas recirculation (EGR) is the most common method to control NOx emission of internal combustion engine. The major components of EGR are CO2 and N2, which have different influences on engine combustion and pollutants formation through thermal, dilution and chemical effects. The main objective of this work is to investigate the different influences of CO2 and N2 on engine combustion and emission on a four-cylinder, turbo charged, spark ignition natural gas engine with electronically control unit, simultaneously to separate the thermal effect with the comparison with Ar. It was found that the peak in-cylinder pressure and heat release rate both decreased along with the increase of intake dilution extent regardless of the diluent’s type. For each diluent gas, NOx emission decreases while HC emission increases with the increased dilution ratio. However, CO emission firstly decreased and then increased. Results also revealed that NOx and CO emission could be simultaneously reduced by intake charge dilution at a little sacrifice of HC emission. The effects of three diluents are different compared with each other. Among these three diluents, it can be found that CO2 is the most effective on reducing NOx and CO emission followed by N2. However, both CO2 and N2 dilution deteriorates the thermal efficiency while Ar dilution improved it. Besides, when NOx emission was reduced to the same level, the thermal efficiency is the highest and CO emission is the lowest for Ar dilution.
EP
TE D
Key words: Intake charge dilution, Natural gas engine, Combustion, Emission
AC C
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82
3 / 23
ACCEPTED MANUSCRIPT 83
1. Introduction With the increasing price of crude oil, demand for energy provision and the tightening of emission
85
regulations, nowadays, seeking for an alternative fuel for oil is getting more and more urgent. Among many
86
alternative fuels, natural gas is found to be the most promising and attractive for its strong sustainability and
87
cleanliness. The application of natural gas also contributes to the reduction of CO2 emission for its high
88
hydrogen-carbon ratio. However, natural gas engine is also encountered with substantial conventional
89
emissions like NOx, hydrocarbon (HC) and CO.
SC
RI PT
84
In order to solve the aforementioned problems in spark ignition natural gas engine, numerous studies have
91
been conducted by a series of researchers. Jiao Deng et al. [1] suggested that HC and CO emission could be
92
lowered by blending a portion of hydrogen with natural gas at little sacrifice of increasing NOx emission.
93
Antonio Mariani et al. [2] found that hydrogen addition into natural gas drastically accelerated the in-cylinder
94
combustion process and improved engine operating thermal efficiency, moreover, it also contributed a lot to
95
lowering the cyclic variation of the engine.
TE D
M AN U
90
Some other researchers like Arvind Thiruvengadam et al.[3] adopted after-treatment to handle the
97
conventional emissions of natural gas engine and gained encouraging achievements. Be that as it may, the
98
adoption of catalyst entailed a complex controlling system and was expensive. The most promising way for
99
controlling conventional emissions, especially NOx emission, under middle and low load for engines was EGR
100
for its simplicity and high controllability, which has been investigated by several researchers. Amr Ibrahim et
101
al.[4] found that the application of EGR on a spark ignition natural gas engine substantially reduced NOx
102
emission through experimental and simulative study in 2009. Moreover, Amr Ibrahim et al.[5] also declared
103
that not only NOx emission could be decreased, but also the thermal efficiency of the engine could be
104
improved through EGR strategy at EGR rate of approximately 5% in 2010. Actually, EGR contains high
AC C
EP
96
4 / 23
ACCEPTED MANUSCRIPT 105
specific heat capacity gases like N2, CO2 and H2O(g), these gases intrinsically act as diluents which affect the
106
combustion process of engine thermally and chemically[6]. To obtain deeper understanding about EGR, some researchers focused on the independent effect of the
108
main components, CO2 and N2 and H2O, of EGR. N. Ladommatos et al.[7-10] conducted a series of
109
experiments on diesel engine to investigate the effects of intake charge diluents like EGR, CO2 and N2 and
110
H2O on the emissions of the engine. Results showed that intake charge dilution lowered the NOx emission
111
significantly, especially for diluents CO2 and H2O. Lei Zhu[11] and coworkers investigated the effect of CO2
112
on the gaseous and particulate emissions of a diesel engine fueled with biodiesel and biodiesel blended with
113
methanol and ethanol. He found that CO, HC and particulate emission all increased while NOx emission
114
decreased evidently by CO2 intake dilution. In diesel engine, diffusion combustion dominates the combustion
115
process. While, in spark ignition engine, stoichiometric air-fuel mixture is inducted into the cylinder
116
homogeneously thus premixed combustion dominates the combustion process. Thus, the effect of intake
117
dilution on the combustion and emission in spark ignition engine may be different from that in diesel engine.
118
So far, such intake charge dilution experiments on spark ignition engine have been conducted by a few
119
researchers. V. Subramanian et al.[12] conducted intake charge dilution experiments on a hydrogen fueled
120
spark ignition engine with CO2, N2 and EGR being diluents, and pointed out that intake charge dilution
121
lowered NOx emission. Nevertheless, the impact of intake charge dilution on the formation of HC and CO
122
emission was not evaluated as hydrogen was not a carbonaceous fuel. Zhongshu Wang et al.[13] conducted
123
intake charge dilution with N2 being diluents on a spark ignition natural gas engine, results indicated that N2
124
dilution could substantially decrease NOx emission in spite of increasing CO and HC emission. In addition to
125
N2 and CO2, Ar, which is monotonic gas exists in air with a small amount (0.93% vol.), was also used for such
126
fundamental studies in engines. Hany A. Moneib[14] and coworkers investigated the effect of Ar intake
AC C
EP
TE D
M AN U
SC
RI PT
107
5 / 23
ACCEPTED MANUSCRIPT dilution on the engine performances and found that Ar dilution could effectively reduce NOx emission and
128
improve thermal efficiency at the same time. WeiFeng Li and coworkers[15] assessed the dilution and thermal
129
effect of CO2 and N2 by comparing with Ar dilution and and the following conclusions were drown: 1) CO2
130
had greater effects on NOx formation than N2 at the same dilution ratio, 2) Ar dilution improved thermal
131
efficiency of the engine. Still, the effect of the diluents CO2, N2, and Ar on combustion and emission were not
132
comprehensively compared in the above mentioned works.
RI PT
127
The main components of EGR are CO2 and N2, which have different effects on engine combustion and
134
emission formation [6]. In one hand, both N2 and CO2 in combustion chamber would introduce higher heat
135
capacity into the engine cylinder, and then affect the engine combustion and emission (thermal effect). In
136
another hand, CO2 was believed to decompose and involve in a complex chemical reaction during the
137
combustion process (chemical effect). The main objective of this work is to investigate the different influences
138
of CO2 and N2 on engine combustion and emission, simultaneously to separate the thermal effect with
139
comparison of Ar .
140
2. Experimental setup and methodology
TE D
M AN U
SC
133
In this research, experiments have been carried out on a four-cylinder, water cooled, turbo-charged,
142
four-stroke, spark ignition natural gas engine. The displacement of this engine was 5.3L and the compression
143
ratio was 11. The specification of the test engine is shown in Table 1. An eddy current dynamometer was used
144
to control the engine speed and torque. The schematic diagram of this test system is shown in Fig1. Natural gas
145
was supplied to the natural gas injection assembly, on which six natural gas injectors were mounted, via a
146
pressure reducer from CNG bottles. The amount of natural gas fuel was controlled by ECU and recorded. The
147
main component of the natural gas used in this study was 96% methane with some other impurities. The
148
diluents, CO2, N2 and Ar, with purity of 99.99%, were supplied to the intake pipe of the engine from gas
AC C
EP
141
6 / 23
ACCEPTED MANUSCRIPT bottles via a pressure regulator as shown in Fig. 1. A mixer was installed on the inlet pipe before the
150
compressor to assist the mixing between inlet air and diluent gas. The oxygen concentration in the inlet pipe
151
was used as an indicator to evaluate the extent of dilution in this test. As shown by Fig. 1, the inlet mixture was
152
sampled to NIDR analyzer after being filtered and dehumidified for oxygen concentration measurement.
RI PT
149
In this study, in-cylinder pressure of the second cylinder was measured with Kistler piezoelectric
154
transducer (type 6118BFD35 Q03A41) for combustion analysis. The resolution of the transducer was 10 Pa.
155
After being amplified by Kistler charge amplifier (type 5015), cylinder pressure was supplied to combustion
156
analyzer (D2T Evolution 3) along with crank angle signal and recorded by computer. The crank angle signal
157
was generated by a 60-teeth gear with a single missing tooth and sensed by a magneto-electric tachometric
158
transducer. The cylinder pressure was recorded every 0.5 crank angle degree. CO emission was measured by
159
non-dispersive infra-red analyzer (NIDR CAI 602P). Total hydrocarbon (THC) and NOx emission were
160
measured by heated flame ionization detector (HFID CAI 600) and heat chemiluminescent analyzer (HCLD
161
CAI 600) respectively. A self-developed procedure with Labview Software was used for emission data
162
acquisition.
TE D
M AN U
SC
153
Based on the first law of thermal dynamics and ideal gas assumption, a zero dimensional model, which
164
neglected the leakage through the piston ring and assumed specific heat of each intake component to be
165
constant, was used to make combustion analysis[16]. Net heat release rate
166
temperature T(θ) were determined through the followings equations:
168 169 170
AC C
167
EP
163
789:; (=) 7?
A
= AB p(θ)
7C(?) 7?
T(θ) =
+ AB V(θ)
7E(?) 7?
(?)F(?)
k=
&I
R = −
7?
and mean in-cylinder
(a)
(b)
G &H
789:; (=)
(c) (d) 7 / 23
ACCEPTED MANUSCRIPT 171
= ∑
(e)
172
= ∑
(f)
where p(θ) was measured cylinder pressure (Pa) and θ was crank angle (CAD), R, , and k were gas
174
constant (kJ/(kg·K)), specific heat capacity at constant pressure (kJ/(kg·K)), specific heat capacity at constant
175
volume (kJ/(kg·K)) and the specific heat ratio of intake mixture, respectively. was the mass percentage of
176
component i in the intake mixture, and were the specific heat at constant pressure (kJ/(kg·K)) and at
177
constant volume (kJ/(kg·K)) of component i in the intake charge. The end of combustion(EOC) was detected
178
through the method developed by Brunt et al.[17], which considered the crank angle at which pV1.15 peaked as
179
the end of combustion. Burn rate (mass fraction burnt) (θ) is caculated through equation (g). Crank angle
180
for 10 percent and 90 percent mass fraction burnt, θ
181
equation (i), respectively.
M AN U
SC
RI PT
173
!
and θ"!, are obtained by solving equation (h) and
-
($) =
185 186
EP
184
(g)
($) = 0.1
(h)
($) = 0.9
(i)
Effective thermal efficiency η is calculated through the following equation:
AC C
183
()*+, %= MN (-OPQ ()*+, %= L(MN
L-
TE D
182
∗4∗3W/Y!
η = Z [\, ]^__
∗`a,[\ ∗ !!!
(j)
187
where , n, and , were engine torque (Nm), engine speed (rpm), mass flow rate (kg/h) and lower
188
heating value (kJ/kg) of natural gas, respectively.
189
The maximum torque of the engine was 645Nm and its corresponding rotating speed, 1500rpm, was
190
chosen to be the tested speed in this study. The tested conditions in this study were 45%, 50%, 65% of the
191
maximum load. Under a specific load condition, diluent gases were added gradually through the inlet pipe until
192
the engine running was unstable. For all the tests, the spark ignition advance was fixed at 20 crank angle 8 / 23
ACCEPTED MANUSCRIPT degree before top dead center (BTDC) and equivalence ratio was fixed at 0.73. Engine coolant and lubrication
194
oil temperature were controlled to be 78~80℃ and 80~85℃, respectively. All data were recorded when the
195
engine running reached its steady state. Cylinder pressure of 200 consecutive cycles were saved and averaged
196
to make combustion analysis. In this study, firstly, CO2 was inducted as intake charge diluent under different
197
loads to study its effect on the combustion and emission characteristics. Secondly, experiments were carried
198
out with N2 and Ar being intake charge diluents under a single load and the comparison between these different
199
diluents was made.
SC
RI PT
193
M AN U
200 201
3. Results and Discussion
202
3.1 Combustion characteristics
203
3.1.1 Effects of CO2 as an intake charge diluent on combustion characteristics In this study, dilution test was firstly carried out for diluent CO2 under three loads, namely 35%, 50%, 65%
TE D
204
of the maximum load, to investigate the effects of intake charge dilution on the combustion and emission
206
characteristics. Generally, the induction of CO2 restrains the combustion temperature and slows down the
207
flame propagation speed of the air-fuel mixture. This is supported by the investigation conducted by Halter, F.
208
et al.[18]. He suggested that the existence of CO2 slowed down the combustion process in two ways: firstly,
209
the addition of CO2 increased the heat capacity of the intake charge mixture and lowered the adiabatic
210
combustion temperature. Secondly, the dissociation process of CO2 also contributed to worsening the
211
combustion process. These two effects were concluded as thermal and chemical effects by N. Ladommatos et
212
al.[6] and they suggested that the dissociation of CO2 may somehow be involved in combustion reactions. So,
213
as shown by Fig. 2, the peak pressure decreases with the increase of intake dilution extent ignoring its load
214
condition and the peak pressure occurs further away from TDC with increased intake dilution. In addition, the
AC C
EP
205
9 / 23
ACCEPTED MANUSCRIPT heat release process is elongated and the peak value of heat release rate is decreased and retarded. All these
216
implies that the existence of CO2 reduces the flame speed and slows down the heat release process, which is in
217
accordance with results of Halter, F. et al.[18]. The possible result of prolonged and retarded heat release
218
process is the decrease of degree of constant volume[19], and it could be concluded that the brake thermal
219
efficiency of the engine also decreases with the increase of dilution extent. In order to maintain a constant load,
220
more fuel-air mixture should be inducted under higher dilution extent. This is one of the reasons that the
221
in-cylinder pressure shows a slight increasing trend with the increased dilution in Fig.2. Another reason for this
222
phenomenon is that the increasing amount of diluents, along with the increasing of dilution extent, itself would
223
contribute to increase the bulk volume intake charge. This phenomenon is in accordance with the experiments
224
done by Zhongshu Wang and coworkers[13]. In this paper, ignition delay (∆$% ) and combustion duration
225
(∆$&% ) are calculated through equation (k) and equation (l), respectively.
M AN U
SC
RI PT
215
∆$% = $
!
TE D
226
− $
∆$&% = $"! − $
227
!
(k) (l)
Due to the elongated ignition delay, lengthened combustion process and lowered adiabatic flame
229
temperature by CO2 dilution, the peak mean in-cylinder temperature drops monotonously along with the
230
increase of CO2 dilution extent as shown in Fig. 3. Moreover, the elongated ignition delay and combustion
231
process also results in more energy released further away from TDC during the expansion stroke. Therefore,
232
the exhaust manifold temperature ( ) increases monotonously along with the increase of intake dilution
233
extent.
AC C
EP
228
234 235
3.1.2 Comparison between the effects of CO2, N2, Ar as intake charge diluents on combustion
236
characteristics 10 / 23
ACCEPTED MANUSCRIPT The effects of different intake charge diluents, CO2, N2 and Ar, on the combustion characteristics have
238
been displayed in Fig. 4. Compared with the case of no dilution, the peak pressures for all these three dilution
239
cases are comparatively lower. Under the same dilution extent, i.e. intake charge O2 concentration being 19.7%,
240
CO2 behaves the strongest impact on combustion process followed by N2 and then Ar. The ignition delay and
241
combustion duration are remarkably affected by intake dilution and increase with the increased dilution, as
242
shown by Fig. 5. According to Williams’[20] theory, it is easy to conclude that the initiation of flame would be
243
more difficult for deeper diluted mixtures under a fixed ignition energy as dilution restrains the adiabatic
244
temperature of the flame. So, as shown by Fig. 5(a), ignition delay increases with dilution extent whatever the
245
diluent is. As shown in table 2, the specific heat capacity is the highest for CO2 and lowest for Ar. Thus, under
246
the same intake dilution extent, the ignition delay for Ar dilution is the shortest, for CO2 dilution is the longest
247
and lies between for N2 dilution. Moreover, these differences in ignition delay become more pronounced with
248
the increased dilution.
TE D
M AN U
SC
RI PT
237
The flame speed is reduced by decreased flame temperature when dilution gases are added. As shown in
250
Fig. 5 (b), deeper dilution results in lower flame speed and longer combustion duration. This is in good
251
agreement with C. Prathap’s[21] research that dilution gas lowers the flame temperature and flame velocity
252
evidently, and the flame speed decreases along with the increase of dilution extent. Among these three dilution
253
cases, argon dilution yields the highest flame temperature owing to its lowest specific heat capacity thus its
254
combustion duration is always the shortest as shown in Fig. 5 (b). This phenomenon becomes more remarkable
255
under higher dilution extent.
AC C
EP
249
256
The peak mean in-cylinder temperature variation along with intake oxygen concentration is shown in Fig.
257
6 (a). It can be seen that the peak mean in-cylinder temperature drops along with the increase of dilution extent
258
for all these three diluents and that it is the highest for Ar dilution and lowest for CO2 dilution. The variations 11 / 23
ACCEPTED MANUSCRIPT of the exhaust manifold temperature along with intake oxygen concentration for these three diluents show
260
different trends as shown in Fig. 6 (b). Exhaust manifold temperature increases for CO2 dilution and decreases
261
for Ar dilution with the increasing dilution extent. However, it exhibits a down-up trend for N2 dilution. In
262
comparison, exhaust manifold temperature is always the highest for CO2 dilution case followed by N2 dilution
263
and then Ar dilution, which is contrary to the situation for peak mean in-cylinder temperature. Lower exhaust
264
manifold temperature indicates less heat loss through exhaust and and thus thermal efficiency may tend to be
265
higher. This is verified by Fig. 7. The thermal efficiency increases slightly when the intake oxygen
266
concentration is above 19.5 and then decreases slowly with the increase of Ar dilution in spite of a decrease in
267
peak mean in-cylinder temperature. While, for both CO2 and N2 dilution, the thermal efficiency drops
268
remarkably along with the increase of dilution extent.
M AN U
SC
RI PT
259
269 3.2 Emission Characteristics
271
3.2.1 Effects of CO2 as an intake charge diluent on emission characteristics
TE D
270
The variation of CO emission along with the intake charge oxygen content under different loads is shown
273
in Fig. 8. It can be clearly seen that CO emission firstly decreases to some extent and increases along with the
274
increase of dilution extent irrespective of the load condition.
AC C
EP
272
275
In spark ignition engine, it is believed that CO concentration is close to equilibrium in the immediate post
276
flame zone at the condition peak cycle temperature (about 2800K) and pressure (about 15 to 40bar)[22], which
277
means that higher temperature leads to higher concentration of CO because of the existence of chemical
278
equilibrium between CO and CO2 through an endothermic equation (m)[23]. However, CO deviates from its
279
equilibrium when the burnt gases cools down in the expansion stroke and undergoes kinetically controlled
280
oxidization process[22]. 12 / 23
ACCEPTED MANUSCRIPT CO3 ⇌ CO + O3
281
(m)
At the first, the immediate post flame zone temperature is suppressed due to the increased heat capacity of
283
the intake charge when slight amount of CO2 is inducted. Thus, CO concentration in the burnt gas is reduced in
284
the post flame zone. Besides, the increased temperature during the expansion stroke, which could be
285
demonstrated by the exhaust manifold temperature variation in Fig. 3 (b), favors the oxidation process of CO.
286
The above two factors bring about a decreasing trend for CO emission when slight dilution is taken as shown
287
in Fig. 8 (a). However, when dilution extent further increases, the immediate post flame zone temperature
288
drops remarkably so that CO concentration no longer reaches its equilibrium and stays very high level.
289
Therefore, CO emission increases along with the increased CO2 dilution.
M AN U
SC
RI PT
282
The main HC formation mechanisms are suggested as the followings: (1) flame quenching at the
291
combustion chamber wall, (2) crevice volume originated HC emission, (3) oil layer absorption of fuel, (4)
292
incomplete combustion[22], among which quenching and crevice effect are considered to be the key factors in
293
this study. With load increases, the mean in-cylinder temperature during the combustion process is enhanced
294
and the cylinder wall temperature is improved thus lead to a shorter quenching distance[24]. In addition, as
295
indicated by Fig. 3, the boosted exhaust temperature for higher loads favors the post oxidization process of
296
unburnt HC component. So, HC emission is lower for higher load condition as revealed by Fig. 8 (b). However,
297
HC emission increases monotonously along with the increase of CO2 dilution extent. This should be ascribed
298
to the following two reasons: 1) the inducted CO2 suppresses the combustion temperature, causing lower
299
laminar flame speed as well as larger quenching distance[24], 2) the elongated combustion process renders
300
fuel-air mixture staying in the crevice for longer time and at last escapes the primary combustion process.
301
These two key factors jointly induce the increasing trend of HC emission with the increased dilution.
302
AC C
EP
TE D
290
The NOx emission of the engine along with varying dilution extent under high, middle and low load is 13 / 23
ACCEPTED MANUSCRIPT shown in Fig. 8 (c). The NOx emission shows a decreasing trend with the increased dilution extent
304
monotonously regardless of the load conditions. Ordinarily, the formation of NOx in the combustion of air-fuel
305
mixture entails the following requests: high temperature, high O2 concentration and long time duration. In this
306
study, the equivalence ratio and the engine speed is fixed at 0.73 and 1500rpm, respectively. The dominant
307
factor that affects the formation of NOx is the maximum combustion temperature. This is verified by the
308
approximate linear relationship between ln(NOx) value and the peak mean in-cylinder temperature (Tmax)
309
indicated by Fig. 8 (c). As previously discussed, diluent CO2 suppresses combustion temperature thus restrains
310
the formation of NOx.
M AN U
SC
RI PT
303
Throughout Fig. 8, the effect of CO2 dilution on HC emission is not distinct when intake oxygen content
312
is above 20.3%, while NOx and CO emission drop evidently along with the increased of dilution. At middle
313
load, when intake oxygen content changed from 20.93% (no dilution) to 20.3%, both CO and NOx emissions
314
are lowered by 28.2% and 81.9%, while HC emission increases slightly. In conclusion, with slight intake
315
dilution (intake O2 concentration changed from 20.93 to 20.3), NOx emission could be decreased significantly
316
at a little sacrifice of HC emission regardless of the loads. At the same time, CO emission decreased evidently.
TE D
311
EP
317
3.2.2 Comparison of the effects between CO2, N2, Ar as intake charge diluents on emission
319
characteristics
AC C
318
320
The effects of CO2, N2 and Ar on emission characteristics are shown in Fig. 9. The effects of all three
321
diluents on CO emission characteristic are very similar as indicated by Fig. 9 (a). CO emission exhibits a
322
downward-upward trend with varying dilution. However, the point that minimum CO emission is achieved for
323
CO2 dilution case occurs under the lightest dilution extent followed by N2 and then Ar dilution case. Before
324
these points, CO2 behaves the strongest effect on reducing CO emission. When intake oxygen concentration is 14 / 23
ACCEPTED MANUSCRIPT 325
diluted to be 20.3%, CO concentration was lowered by 25.8%, 13.5%, 7.8% for CO2, N2 and Ar dilution,
326
respectively. However, the minimum CO emission value could be reached by Ar dilution is the lowest followed
327
by N2 dilution and then CO2 dilution. There seems to be no very remarkable differences between the effects of these three diluents on HC
329
emission before the points that minimum CO emission values are achieved and HC emission increases slowly
330
along with the increased dilution. When too more diluent gas is inducted, the combustion process would be
331
influenced (flame speed being slowed, flame propagation being seriously interfered), which is proved by the
332
peak mean in-cylinder temperature from Fig. 9 (c), thus brought about a rapid increase in HC emission as
333
indicated in Fig. 9 (b).
M AN U
SC
RI PT
328
The NOx emission decreases along with the increase of dilution extent regardless of the diluents. Among
335
these three diluents, CO2 is the most effective for reducing NOx emission while Ar is the least effective one.
336
When oxygen concentration was diluted to be 20.3%, NOx emission was reduced by 80.5%, 51.6% and 33.2%
337
for CO2, N2 and Ar dilution, respectively as shown in Fig. 9 (c). It was considered that both thermal and
338
chemical interaction should be responsible for the NOx reduction, among which thermal effect was considered
339
to be the primary factor[25]. Both N2 and Ar were inert gases thus wouldn’t involve chemical effect[6]. The
340
heat capacity is higher for N2 as shown in Table 2. Therefore, the results of thermal impact on NOx emission
341
could be described by comparing Ar dilution and N2 dilution as shown in Fig. 9 (c). It is obvious that thermal
342
effect is the dominant factor on lowering NOx emission. As has been discussed in section 3.2.1, NOx emission
343
is highly dependent on combustion temperature. So, owing to the comparatively higher specific heat capacity,
344
CO2 shows more effective impact on lowing the combustion temperature and this is demonstrated by the peak
345
mean in-cylinder temperature along with dilution extent shown in Fig. 9. Moreover, it was also suggested that
346
hydrocarbon radical reacts with NOx and lead to the formation of hydrogen cyanide, which ultimately lead to
AC C
EP
TE D
334
15 / 23
ACCEPTED MANUSCRIPT 347
the formation of N2, thus help to reduce NOx emission[8]. Under high dilution extent, HC emission is found to
348
be high, thus the possible reaction between HC and NOx in the post-flame and exhaust would help to restrain
349
NOx in the exhaust. So, ultimately, extreme low NOx emission is obtained. Throughout the comparison between these three diluents in terms of the combustion and emission
351
characteristics, it could be found that both CO2 and N2 could lower NOx emission effectively, while, thermal
352
efficiency decreases noticeably. However, Ar dilution almost does not worsen thermal efficiency while lowers
353
NOx emission. Actually, for Ar dilution, thermal efficiency increases at first and then decreases along with the
354
increased dilution as shown in Fig. 7. When NOx is lowered to the same level (100ppm, for example, shown in
355
Fig. 9, points A, B and C), it can be clearly found that the thermal efficiency is the highest for Ar dilution
356
followed by CO2 and then N2 as shown in Fig. 7. Besides, CO emission is the highest for CO2 dilution and
357
lowest for Ar dilution as indicated by Fig. 9 (a).
M AN U
SC
RI PT
350
359
4. Conclusions
TE D
358
In this paper, the effects of intake charge diluents, CO2, N2 and Ar, on the combustion and emission
361
characteristics of a spark ignition natural gas engine have been investigated and the comparison between these
362
three diluents has been carried out. It was found that intake charge dilution had an evident effect on the
363
combustion process and the emission of the spark ignition natural gas engine. Under a fixed load condition,
364
with the increase of dilution extent, the peak in-cylinder pressure and the maximum value of heat release rate
365
decreases and their corresponding crank angle shifts further away from the crank angle for spark timing.
366
Moreover, the ignition delay and combustion duration both are prolonged by intake charge dilution. However,
367
among the three diluents, CO2 shows its strongest effect on the combustion processes followed by N2 and then
368
Ar.
AC C
EP
360
16 / 23
ACCEPTED MANUSCRIPT With the increasing dilution extent, NOx emission decreases while HC emission increases regardless of
370
the diluents. However, CO emission exhibits a down-up trend. Reducing NOx and CO emission simultaneously
371
could be achieved when appropriate dilution extent is chosen at a little sacrifice of HC emission for all these
372
three diluents. In comparison, CO2 is the most effective for reducing NOx and CO emission followed by N2
373
and then Ar. However, both CO2 and N2 dilution lower thermal efficiency while Ar dilution improves thermal
374
efficiency. When NOx is lowered to the same level, the thermal efficiency is the highest and CO emission is the
375
lowest for Ar dilution.
SC
RI PT
369
With the comparison between N2 and Ar, the thermal effect on combustion and NOx emission is separated
377
in Fig. 9 (c). It could be found that thermal effect helps to lowering NOx emission remarkably. Similar analysis
378
on combustion is given in Fig. 4 and results show that thermal effect lowers peak in-cylinder pressure as well
379
as peak heat release rate.
M AN U
376
381
Acknowledgements
TE D
380
The author would like to thank National Science Foundation of China (Project No. 51306117; 51436005),
383
Ministry of Science and Technology (MOST) of China (Project No.2012BAD47B04) and Foundation of
384
Shanghai Jiao Tong University to support this research.
AC C
385
EP
382
17 / 23
ACCEPTED MANUSCRIPT 386
References
387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428
[1] J. Deng, F. Ma, S. Li, Y. He, M. Wang, L. Jiang, S. Zhao, Experimental study on combustion and emission characteristics of a hydrogen-enriched compressed natural gas engine under idling condition, International Journal of Hydrogen Energy, 36 (2011) 13150-13157. [2] A. Mariani, M.V. Prati, A. Unich, B. Morrone, Combustion analysis of a spark ignition i. c. engine fuelled 1616-1623.
RI PT
alternatively with natural gas and hydrogen-natural gas blends, International Journal of Hydrogen Energy, 38 (2013) [3] A. Thiruvengadam, D.K. Carder, M. Krishnamurthy, A. Oshinuga, M. Gautam, Effect of an economical oxidation catalyst formulation on regulated and unregulated pollutants from natural gas fueled heavy duty transit buses, Transportation Research Part D: Transport and Environment, 16 (2011) 469-473.
[4] A. Ibrahim, S. Bari, A comparison between EGR and lean-burn strategies employed in a natural gas SI engine using a two-zone combustion model, Energy Conversion and Management, 50 (2009) 3129-3139.
SC
[5] A. Ibrahim, S. Bari, An experimental investigation on the use of EGR in a supercharged natural gas SI engine, Fuel, 89 (2010) 1721-1730.
[6] N. Ladommatos, S. Abdelhalim, H. Zhao, Control of oxides of nitrogen from diesel engines using diluents while
M AN U
minimising the impact on particulate pollutants, Applied Thermal Engineering, 18 (1998) 963-980. [7] N. Ladommatos, S. Abdelhalim, H. Zhao, Z. Hu, The dilution, chemical, and thermal effects of exhaust gas recirculation on diesel engine emissions-part 1: Effect of reducing inlet charge oxygen, in, SAE Technical Paper, 1996.
[8] N. Ladommatos, S.M. Abdelhalim, H. Zhao, Z. Hu, 97/02332 The dilution, chemical, and thermal effects of exhaust gas recirculation on diesel engine emissions. Part 2: Effects of carbon dioxide, Fuel and Energy Abstracts, 38 (1997) 186.
TE D
[9] N. Ladommatos, S.M. Abdelhalim, H. Zhao, Z. Hu, The Dilution, Chemical, and Thermal Effects of Exhaust Gas Recirculation on Disesel Engine Emissions-Part 4: Effects of Carbon Dioxide and Water Vapour, in, SAE Technical Paper, 1997.
[10] N. Ladommatos, S.M. Abdelhalim, H. Zhao, Z. Hu, The dilution, chemical, and thermal effects of exhaust gas recirculation on diesel engine emissions-Part 3: effects of water vapour, in, SAE Technical Paper, 1997.
EP
[11] L. Zhu, C. Cheung, W. Zhang, Z. Huang, Effect of charge dilution on gaseous and particulate emissions from a diesel engine fueled with biodiesel and biodiesel blended with methanol and ethanol, Applied Thermal Engineering, 31 (2011) 2271-2278.
AC C
[12] V. Subramanian, J.M. Mallikarjuna, A. Ramesh, Intake charge dilution effects on control of nitric oxide emission in a hydrogen fueled SI engine, International Journal of Hydrogen Energy, 32 (2007) 2043-2056. [13] Z. Wang, H. Zuo, Z. Liu, W. Li, H. Dou, Impact of N2 dilution on combustion and emissions in a spark ignition CNG engine, Energy Conversion and Management, 85 (2014) 354-360. [14] H.A. Moneib, M. Abdelaal, M.Y.E. Selim, O.A. Abdallah, NOx emission control in SI engine by adding argon inert gas to intake mixture, Energy Conversion and Management, 50 (2009) 2699-2708. [15] W. Li, Z. Liu, Z. Wang, Y. Xu, Experimental investigation of the thermal and diluent effects of EGR components on combustion and NOx emissions of a turbocharged natural gas SI engine, Energy Conversion and Management, 88 (2014) 1041-1050. [16] R. Krieger, G.L. Borman, Computation of apparent heat release for internal combustion engines, in: MECHANICAL ENGINEERING, Vol. 89, ASME-AMER SOC MECHANICAL ENG 345 E 47TH ST, NEW YORK, NY 10017, 1967, pp. 59-&. [17] M.F. Brunt, A.L. Emtage, Evaluation of burn rate routines and analysis errors, in, SAE Technical Paper, 1997. 18 / 23
ACCEPTED MANUSCRIPT [18] F. Halter, F. Foucher, L. Landry, C. Mounaïm-Rousselle, Effect of Dilution by Nitrogen and/or Carbon Dioxide on Methane and Iso-Octane Air Flames, Combustion Science and Technology, 181 (2009) 813-827. [19] T. Shudo, Improving thermal efficiency by reducing cooling losses in hydrogen combustion engines, International Journal of Hydrogen Energy, 32 (2007) 4285-4293. [20] G. Williams, Combustion theory, (1985). [21] C. Prathap, A. Ray, M.R. Ravi, Investigation of nitrogen dilution effects on the laminar burning velocity and flame stability of syngas fuel at atmospheric condition, Combustion and Flame, 155 (2008) 145-160. [23] S.R. Turns, An introduction to combustion, McGraw-hill New York, 1996. [24] S.R. Turns, An introduction to combustion, McGraw-hill New York, 1996.
RI PT
[22] J.B. Heywood, Internal combustion engine fundamentals, Mcgraw-hill New York, 1988.
[25] A.A. Quader, Why intake charge dilution decreases nitric oxide emission from spark ignition engines, in, SAE Technical Paper, 1971.
SC
429 430 431 432 433 434 435 436 437 438 439 440 441 442
AC C
EP
TE D
M AN U
443
19 / 23
ACCEPTED MANUSCRIPT 444
Table Captions
445 Table 1. Engine specifications
447
Table 2. The specific heat capacity at constant pressure of the diluents at standard condition
RI PT
446
448
AC C
EP
TE D
M AN U
SC
449
20 / 23
ACCEPTED MANUSCRIPT 450 Table 1. Engine specifications Number of cylinders
4
Number of strokes
4
Displacement/L
5.3
Intake valve opening/CAD BTDC
22.5
Bore/mm
114
Intake valve closing/ CAD ABDC
34.5
Connecting rod length/mm
130
Exhaust valve opening/ CAD BBDC
67.5
Compression ratio
11
Exhaust valve closing/ CAD ATDC
25.5
RI PT
451
AC C
EP
TE D
M AN U
SC
452 453
21 / 23
ACCEPTED MANUSCRIPT 454 455
Table 2. The specific heat capacity at constant pressure of the diluents at standard condition Diluents
CO2
N2
Ar
Cp/kJ/(kmol·K)
37.198
29.071
20.785
AC C
EP
TE D
M AN U
SC
RI PT
456 457
22 / 23
ACCEPTED MANUSCRIPT 458
Figure Captions
459 Fig. 1. The schematic layout of the experimental setup
461
Fig. 2. Cylinder pressure and net heat release rate along with crank angle under 35, 50, 65% load for CO2
462
dilution
RI PT
460
Fig. 3. The peak mean in-cylinder temperature and exhaust manifold temperature variation along with intake
464
O2 concentration under 35%, 50%, 65% load for CO2 dilution
465
Fig. 4. Cylinder pressure and neat heat release rate along with crank angle under 50% load for CO2, N2 and Ar
466
dilution when intake oxygen concentration is 19.7%
467
Fig. 5. The ignition delay and combustion duration variation along with intake O2 concentration under 50%
468
load for CO2, N2 and Ar dilution
469
Fig. 6. The peak mean in-cylinder temperature and exhaust manifold temperature variation along with intake
470
O2 concentration for CO2, N2 and Ar dilution under 50% load
471
Fig. 7. Effective thermal efficiency variation along with intake oxygen concentration for CO2, N2 and Ar
472
dilution under 50% load (points A, B and C correspond to [52 ] = 100ppm)
473
Fig. 8. CO, HC and NOx emission variation along with intake O2 concentration for CO2 dilution under
474
different loads
475
Fig. 9. CO, HC and NOx emission variation along with intake O2 concentration for CO2, N2 and Ar dilution
476
under 50% load (points A, B and C correspond to [52 ] = 100ppm, points A, D and E correspond to
477
[23 ]4 = 20.3%)
AC C
EP
TE D
M AN U
SC
463
478
23 / 23
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
1, CO2 is the most effective on reducing NOx and CO emission followed by N2 and then Ar. 2, NOx and CO emission could be simultaneously reduced by intake charge dilution regardless of the diluents when appropriate dilution extent is chosen. 3, Both CO2 and N2 dilution worsen while Ar dilution improves thermal efficiency. 4, Thermal effect is a dominant factor for reducing NOx emission.
S1359-4311(15)00631-6
DOI:
10.1016/j.applthermaleng.2015.06.072
Reference:
ATE 6765
To appear in:
Applied Thermal Engineering
Received Date: 1 November 2014 Accepted Date: 24 June 2015
Please cite this article as: Z. He, Q. Jing, L. Zhu, W. Zhang, Z. Huang, The effects of different intake charge diluents on the combustion and emission characteristics of a spark ignition natural gas engine, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.06.072. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
The effects of different intake charge diluents on the combustion and emission
2
characteristics of a spark ignition natural gas engine
3 4 5
Zhuoyao He, Qijian Jing, Lei Zhu*, Wugao Zhang, Zhen Huang*
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
ABDC: after bottom dead center ATDC: after top dead center BBDC: before bottom dead center BTDC: before top dead center CAD: crank angle degree : specific heat capacity at constant pressure for intake mixture : specific heat capacity at constant pressure for component i
26 27 28 29 30 31 32
: specific heat capacity at constant volume for intake mixture : specific heat capacity at constant volume for component i ECU: electronic control unit EGR: exhaust gas recirculation EOC: end of combustion HC: hydrocarbon , : lower heating value of natural gas
33 34 35 36 37 38 39 40 41 42 43
IVO: intake valve opening k: specific heat ratio of intake mixture : mass flow rate of natural gas
: engine speed p: cylinder pressure R: gas constant ST: spark timing TDC: top dead center THC: total hydrocarbon T: mean in-cylinder temperature : engine torque
Corresponding authors： E-mail address: [email protected] (Lei Zhu*) E-mail address: [email protected] (Zhen Huang*)
RI PT
Key Lab. for Power machinery and Engineering of M. O. E., Shanghai Jiao Tong University, 200240 Shanghai, P R China
M AN U
SC
* Corresponding author address: Key laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China. Tel: +86-21-34205949 Fax: +86-21-34205949
AC C
EP
TE D
Nomenclature： ：
1 / 23
58 59 60 61 62
[CO]: CO emission concentration [23 ]4 : intake oxygen concentration [52 ]: NOx emission concentration [THC]: total hydrocarbon emission concentration
SC
: peak mean in-cylinder temperature : exhaust manifold temperature V: cylinder volume : mass percentage of component i : mass fraction burnt θ: crank angle θ : crank angle for spark timing θ : crank angle for end of combustion θ ! : crank angle for 10 percent mass fraction burnt θ"!: crank angle for 90 percent mass fraction burnt ∆$% : ignition delay ∆$&% : combustion duration η : effective thermal efficiency ()*+, : net heat release rate (-
AC C
EP
TE D
M AN U
44 45 46 47 48 49 50 51 52 53 54 55 56 57
RI PT
ACCEPTED MANUSCRIPT
2 / 23
ACCEPTED MANUSCRIPT Abstract
M AN U
SC
RI PT
Exhaust gas recirculation (EGR) is the most common method to control NOx emission of internal combustion engine. The major components of EGR are CO2 and N2, which have different influences on engine combustion and pollutants formation through thermal, dilution and chemical effects. The main objective of this work is to investigate the different influences of CO2 and N2 on engine combustion and emission on a four-cylinder, turbo charged, spark ignition natural gas engine with electronically control unit, simultaneously to separate the thermal effect with the comparison with Ar. It was found that the peak in-cylinder pressure and heat release rate both decreased along with the increase of intake dilution extent regardless of the diluent’s type. For each diluent gas, NOx emission decreases while HC emission increases with the increased dilution ratio. However, CO emission firstly decreased and then increased. Results also revealed that NOx and CO emission could be simultaneously reduced by intake charge dilution at a little sacrifice of HC emission. The effects of three diluents are different compared with each other. Among these three diluents, it can be found that CO2 is the most effective on reducing NOx and CO emission followed by N2. However, both CO2 and N2 dilution deteriorates the thermal efficiency while Ar dilution improved it. Besides, when NOx emission was reduced to the same level, the thermal efficiency is the highest and CO emission is the lowest for Ar dilution.
EP
TE D
Key words: Intake charge dilution, Natural gas engine, Combustion, Emission
AC C
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82
3 / 23
ACCEPTED MANUSCRIPT 83
1. Introduction With the increasing price of crude oil, demand for energy provision and the tightening of emission
85
regulations, nowadays, seeking for an alternative fuel for oil is getting more and more urgent. Among many
86
alternative fuels, natural gas is found to be the most promising and attractive for its strong sustainability and
87
cleanliness. The application of natural gas also contributes to the reduction of CO2 emission for its high
88
hydrogen-carbon ratio. However, natural gas engine is also encountered with substantial conventional
89
emissions like NOx, hydrocarbon (HC) and CO.
SC
RI PT
84
In order to solve the aforementioned problems in spark ignition natural gas engine, numerous studies have
91
been conducted by a series of researchers. Jiao Deng et al. [1] suggested that HC and CO emission could be
92
lowered by blending a portion of hydrogen with natural gas at little sacrifice of increasing NOx emission.
93
Antonio Mariani et al. [2] found that hydrogen addition into natural gas drastically accelerated the in-cylinder
94
combustion process and improved engine operating thermal efficiency, moreover, it also contributed a lot to
95
lowering the cyclic variation of the engine.
TE D
M AN U
90
Some other researchers like Arvind Thiruvengadam et al.[3] adopted after-treatment to handle the
97
conventional emissions of natural gas engine and gained encouraging achievements. Be that as it may, the
98
adoption of catalyst entailed a complex controlling system and was expensive. The most promising way for
99
controlling conventional emissions, especially NOx emission, under middle and low load for engines was EGR
100
for its simplicity and high controllability, which has been investigated by several researchers. Amr Ibrahim et
101
al.[4] found that the application of EGR on a spark ignition natural gas engine substantially reduced NOx
102
emission through experimental and simulative study in 2009. Moreover, Amr Ibrahim et al.[5] also declared
103
that not only NOx emission could be decreased, but also the thermal efficiency of the engine could be
104
improved through EGR strategy at EGR rate of approximately 5% in 2010. Actually, EGR contains high
AC C
EP
96
4 / 23
ACCEPTED MANUSCRIPT 105
specific heat capacity gases like N2, CO2 and H2O(g), these gases intrinsically act as diluents which affect the
106
combustion process of engine thermally and chemically[6]. To obtain deeper understanding about EGR, some researchers focused on the independent effect of the
108
main components, CO2 and N2 and H2O, of EGR. N. Ladommatos et al.[7-10] conducted a series of
109
experiments on diesel engine to investigate the effects of intake charge diluents like EGR, CO2 and N2 and
110
H2O on the emissions of the engine. Results showed that intake charge dilution lowered the NOx emission
111
significantly, especially for diluents CO2 and H2O. Lei Zhu[11] and coworkers investigated the effect of CO2
112
on the gaseous and particulate emissions of a diesel engine fueled with biodiesel and biodiesel blended with
113
methanol and ethanol. He found that CO, HC and particulate emission all increased while NOx emission
114
decreased evidently by CO2 intake dilution. In diesel engine, diffusion combustion dominates the combustion
115
process. While, in spark ignition engine, stoichiometric air-fuel mixture is inducted into the cylinder
116
homogeneously thus premixed combustion dominates the combustion process. Thus, the effect of intake
117
dilution on the combustion and emission in spark ignition engine may be different from that in diesel engine.
118
So far, such intake charge dilution experiments on spark ignition engine have been conducted by a few
119
researchers. V. Subramanian et al.[12] conducted intake charge dilution experiments on a hydrogen fueled
120
spark ignition engine with CO2, N2 and EGR being diluents, and pointed out that intake charge dilution
121
lowered NOx emission. Nevertheless, the impact of intake charge dilution on the formation of HC and CO
122
emission was not evaluated as hydrogen was not a carbonaceous fuel. Zhongshu Wang et al.[13] conducted
123
intake charge dilution with N2 being diluents on a spark ignition natural gas engine, results indicated that N2
124
dilution could substantially decrease NOx emission in spite of increasing CO and HC emission. In addition to
125
N2 and CO2, Ar, which is monotonic gas exists in air with a small amount (0.93% vol.), was also used for such
126
fundamental studies in engines. Hany A. Moneib[14] and coworkers investigated the effect of Ar intake
AC C
EP
TE D
M AN U
SC
RI PT
107
5 / 23
ACCEPTED MANUSCRIPT dilution on the engine performances and found that Ar dilution could effectively reduce NOx emission and
128
improve thermal efficiency at the same time. WeiFeng Li and coworkers[15] assessed the dilution and thermal
129
effect of CO2 and N2 by comparing with Ar dilution and and the following conclusions were drown: 1) CO2
130
had greater effects on NOx formation than N2 at the same dilution ratio, 2) Ar dilution improved thermal
131
efficiency of the engine. Still, the effect of the diluents CO2, N2, and Ar on combustion and emission were not
132
comprehensively compared in the above mentioned works.
RI PT
127
The main components of EGR are CO2 and N2, which have different effects on engine combustion and
134
emission formation [6]. In one hand, both N2 and CO2 in combustion chamber would introduce higher heat
135
capacity into the engine cylinder, and then affect the engine combustion and emission (thermal effect). In
136
another hand, CO2 was believed to decompose and involve in a complex chemical reaction during the
137
combustion process (chemical effect). The main objective of this work is to investigate the different influences
138
of CO2 and N2 on engine combustion and emission, simultaneously to separate the thermal effect with
139
comparison of Ar .
140
2. Experimental setup and methodology
TE D
M AN U
SC
133
In this research, experiments have been carried out on a four-cylinder, water cooled, turbo-charged,
142
four-stroke, spark ignition natural gas engine. The displacement of this engine was 5.3L and the compression
143
ratio was 11. The specification of the test engine is shown in Table 1. An eddy current dynamometer was used
144
to control the engine speed and torque. The schematic diagram of this test system is shown in Fig1. Natural gas
145
was supplied to the natural gas injection assembly, on which six natural gas injectors were mounted, via a
146
pressure reducer from CNG bottles. The amount of natural gas fuel was controlled by ECU and recorded. The
147
main component of the natural gas used in this study was 96% methane with some other impurities. The
148
diluents, CO2, N2 and Ar, with purity of 99.99%, were supplied to the intake pipe of the engine from gas
AC C
EP
141
6 / 23
ACCEPTED MANUSCRIPT bottles via a pressure regulator as shown in Fig. 1. A mixer was installed on the inlet pipe before the
150
compressor to assist the mixing between inlet air and diluent gas. The oxygen concentration in the inlet pipe
151
was used as an indicator to evaluate the extent of dilution in this test. As shown by Fig. 1, the inlet mixture was
152
sampled to NIDR analyzer after being filtered and dehumidified for oxygen concentration measurement.
RI PT
149
In this study, in-cylinder pressure of the second cylinder was measured with Kistler piezoelectric
154
transducer (type 6118BFD35 Q03A41) for combustion analysis. The resolution of the transducer was 10 Pa.
155
After being amplified by Kistler charge amplifier (type 5015), cylinder pressure was supplied to combustion
156
analyzer (D2T Evolution 3) along with crank angle signal and recorded by computer. The crank angle signal
157
was generated by a 60-teeth gear with a single missing tooth and sensed by a magneto-electric tachometric
158
transducer. The cylinder pressure was recorded every 0.5 crank angle degree. CO emission was measured by
159
non-dispersive infra-red analyzer (NIDR CAI 602P). Total hydrocarbon (THC) and NOx emission were
160
measured by heated flame ionization detector (HFID CAI 600) and heat chemiluminescent analyzer (HCLD
161
CAI 600) respectively. A self-developed procedure with Labview Software was used for emission data
162
acquisition.
TE D
M AN U
SC
153
Based on the first law of thermal dynamics and ideal gas assumption, a zero dimensional model, which
164
neglected the leakage through the piston ring and assumed specific heat of each intake component to be
165
constant, was used to make combustion analysis[16]. Net heat release rate
166
temperature T(θ) were determined through the followings equations:
168 169 170
AC C
167
EP
163
789:; (=) 7?
A
= AB p(θ)
7C(?) 7?
T(θ) =
+ AB V(θ)
7E(?) 7?
(?)F(?)
k=
&I
R = −
7?
and mean in-cylinder
(a)
(b)
G &H
789:; (=)
(c) (d) 7 / 23
ACCEPTED MANUSCRIPT 171
= ∑
(e)
172
= ∑
(f)
where p(θ) was measured cylinder pressure (Pa) and θ was crank angle (CAD), R, , and k were gas
174
constant (kJ/(kg·K)), specific heat capacity at constant pressure (kJ/(kg·K)), specific heat capacity at constant
175
volume (kJ/(kg·K)) and the specific heat ratio of intake mixture, respectively. was the mass percentage of
176
component i in the intake mixture, and were the specific heat at constant pressure (kJ/(kg·K)) and at
177
constant volume (kJ/(kg·K)) of component i in the intake charge. The end of combustion(EOC) was detected
178
through the method developed by Brunt et al.[17], which considered the crank angle at which pV1.15 peaked as
179
the end of combustion. Burn rate (mass fraction burnt) (θ) is caculated through equation (g). Crank angle
180
for 10 percent and 90 percent mass fraction burnt, θ
181
equation (i), respectively.
M AN U
SC
RI PT
173
!
and θ"!, are obtained by solving equation (h) and
-
($) =
185 186
EP
184
(g)
($) = 0.1
(h)
($) = 0.9
(i)
Effective thermal efficiency η is calculated through the following equation:
AC C
183
()*+, %= MN (-OPQ ()*+, %= L(MN
L-
TE D
182
∗4∗3W/Y!
η = Z [\, ]^__
∗`a,[\ ∗ !!!
(j)
187
where , n, and , were engine torque (Nm), engine speed (rpm), mass flow rate (kg/h) and lower
188
heating value (kJ/kg) of natural gas, respectively.
189
The maximum torque of the engine was 645Nm and its corresponding rotating speed, 1500rpm, was
190
chosen to be the tested speed in this study. The tested conditions in this study were 45%, 50%, 65% of the
191
maximum load. Under a specific load condition, diluent gases were added gradually through the inlet pipe until
192
the engine running was unstable. For all the tests, the spark ignition advance was fixed at 20 crank angle 8 / 23
ACCEPTED MANUSCRIPT degree before top dead center (BTDC) and equivalence ratio was fixed at 0.73. Engine coolant and lubrication
194
oil temperature were controlled to be 78~80℃ and 80~85℃, respectively. All data were recorded when the
195
engine running reached its steady state. Cylinder pressure of 200 consecutive cycles were saved and averaged
196
to make combustion analysis. In this study, firstly, CO2 was inducted as intake charge diluent under different
197
loads to study its effect on the combustion and emission characteristics. Secondly, experiments were carried
198
out with N2 and Ar being intake charge diluents under a single load and the comparison between these different
199
diluents was made.
SC
RI PT
193
M AN U
200 201
3. Results and Discussion
202
3.1 Combustion characteristics
203
3.1.1 Effects of CO2 as an intake charge diluent on combustion characteristics In this study, dilution test was firstly carried out for diluent CO2 under three loads, namely 35%, 50%, 65%
TE D
204
of the maximum load, to investigate the effects of intake charge dilution on the combustion and emission
206
characteristics. Generally, the induction of CO2 restrains the combustion temperature and slows down the
207
flame propagation speed of the air-fuel mixture. This is supported by the investigation conducted by Halter, F.
208
et al.[18]. He suggested that the existence of CO2 slowed down the combustion process in two ways: firstly,
209
the addition of CO2 increased the heat capacity of the intake charge mixture and lowered the adiabatic
210
combustion temperature. Secondly, the dissociation process of CO2 also contributed to worsening the
211
combustion process. These two effects were concluded as thermal and chemical effects by N. Ladommatos et
212
al.[6] and they suggested that the dissociation of CO2 may somehow be involved in combustion reactions. So,
213
as shown by Fig. 2, the peak pressure decreases with the increase of intake dilution extent ignoring its load
214
condition and the peak pressure occurs further away from TDC with increased intake dilution. In addition, the
AC C
EP
205
9 / 23
ACCEPTED MANUSCRIPT heat release process is elongated and the peak value of heat release rate is decreased and retarded. All these
216
implies that the existence of CO2 reduces the flame speed and slows down the heat release process, which is in
217
accordance with results of Halter, F. et al.[18]. The possible result of prolonged and retarded heat release
218
process is the decrease of degree of constant volume[19], and it could be concluded that the brake thermal
219
efficiency of the engine also decreases with the increase of dilution extent. In order to maintain a constant load,
220
more fuel-air mixture should be inducted under higher dilution extent. This is one of the reasons that the
221
in-cylinder pressure shows a slight increasing trend with the increased dilution in Fig.2. Another reason for this
222
phenomenon is that the increasing amount of diluents, along with the increasing of dilution extent, itself would
223
contribute to increase the bulk volume intake charge. This phenomenon is in accordance with the experiments
224
done by Zhongshu Wang and coworkers[13]. In this paper, ignition delay (∆$% ) and combustion duration
225
(∆$&% ) are calculated through equation (k) and equation (l), respectively.
M AN U
SC
RI PT
215
∆$% = $
!
TE D
226
− $
∆$&% = $"! − $
227
!
(k) (l)
Due to the elongated ignition delay, lengthened combustion process and lowered adiabatic flame
229
temperature by CO2 dilution, the peak mean in-cylinder temperature drops monotonously along with the
230
increase of CO2 dilution extent as shown in Fig. 3. Moreover, the elongated ignition delay and combustion
231
process also results in more energy released further away from TDC during the expansion stroke. Therefore,
232
the exhaust manifold temperature ( ) increases monotonously along with the increase of intake dilution
233
extent.
AC C
EP
228
234 235
3.1.2 Comparison between the effects of CO2, N2, Ar as intake charge diluents on combustion
236
characteristics 10 / 23
ACCEPTED MANUSCRIPT The effects of different intake charge diluents, CO2, N2 and Ar, on the combustion characteristics have
238
been displayed in Fig. 4. Compared with the case of no dilution, the peak pressures for all these three dilution
239
cases are comparatively lower. Under the same dilution extent, i.e. intake charge O2 concentration being 19.7%,
240
CO2 behaves the strongest impact on combustion process followed by N2 and then Ar. The ignition delay and
241
combustion duration are remarkably affected by intake dilution and increase with the increased dilution, as
242
shown by Fig. 5. According to Williams’[20] theory, it is easy to conclude that the initiation of flame would be
243
more difficult for deeper diluted mixtures under a fixed ignition energy as dilution restrains the adiabatic
244
temperature of the flame. So, as shown by Fig. 5(a), ignition delay increases with dilution extent whatever the
245
diluent is. As shown in table 2, the specific heat capacity is the highest for CO2 and lowest for Ar. Thus, under
246
the same intake dilution extent, the ignition delay for Ar dilution is the shortest, for CO2 dilution is the longest
247
and lies between for N2 dilution. Moreover, these differences in ignition delay become more pronounced with
248
the increased dilution.
TE D
M AN U
SC
RI PT
237
The flame speed is reduced by decreased flame temperature when dilution gases are added. As shown in
250
Fig. 5 (b), deeper dilution results in lower flame speed and longer combustion duration. This is in good
251
agreement with C. Prathap’s[21] research that dilution gas lowers the flame temperature and flame velocity
252
evidently, and the flame speed decreases along with the increase of dilution extent. Among these three dilution
253
cases, argon dilution yields the highest flame temperature owing to its lowest specific heat capacity thus its
254
combustion duration is always the shortest as shown in Fig. 5 (b). This phenomenon becomes more remarkable
255
under higher dilution extent.
AC C
EP
249
256
The peak mean in-cylinder temperature variation along with intake oxygen concentration is shown in Fig.
257
6 (a). It can be seen that the peak mean in-cylinder temperature drops along with the increase of dilution extent
258
for all these three diluents and that it is the highest for Ar dilution and lowest for CO2 dilution. The variations 11 / 23
ACCEPTED MANUSCRIPT of the exhaust manifold temperature along with intake oxygen concentration for these three diluents show
260
different trends as shown in Fig. 6 (b). Exhaust manifold temperature increases for CO2 dilution and decreases
261
for Ar dilution with the increasing dilution extent. However, it exhibits a down-up trend for N2 dilution. In
262
comparison, exhaust manifold temperature is always the highest for CO2 dilution case followed by N2 dilution
263
and then Ar dilution, which is contrary to the situation for peak mean in-cylinder temperature. Lower exhaust
264
manifold temperature indicates less heat loss through exhaust and and thus thermal efficiency may tend to be
265
higher. This is verified by Fig. 7. The thermal efficiency increases slightly when the intake oxygen
266
concentration is above 19.5 and then decreases slowly with the increase of Ar dilution in spite of a decrease in
267
peak mean in-cylinder temperature. While, for both CO2 and N2 dilution, the thermal efficiency drops
268
remarkably along with the increase of dilution extent.
M AN U
SC
RI PT
259
269 3.2 Emission Characteristics
271
3.2.1 Effects of CO2 as an intake charge diluent on emission characteristics
TE D
270
The variation of CO emission along with the intake charge oxygen content under different loads is shown
273
in Fig. 8. It can be clearly seen that CO emission firstly decreases to some extent and increases along with the
274
increase of dilution extent irrespective of the load condition.
AC C
EP
272
275
In spark ignition engine, it is believed that CO concentration is close to equilibrium in the immediate post
276
flame zone at the condition peak cycle temperature (about 2800K) and pressure (about 15 to 40bar)[22], which
277
means that higher temperature leads to higher concentration of CO because of the existence of chemical
278
equilibrium between CO and CO2 through an endothermic equation (m)[23]. However, CO deviates from its
279
equilibrium when the burnt gases cools down in the expansion stroke and undergoes kinetically controlled
280
oxidization process[22]. 12 / 23
ACCEPTED MANUSCRIPT CO3 ⇌ CO + O3
281
(m)
At the first, the immediate post flame zone temperature is suppressed due to the increased heat capacity of
283
the intake charge when slight amount of CO2 is inducted. Thus, CO concentration in the burnt gas is reduced in
284
the post flame zone. Besides, the increased temperature during the expansion stroke, which could be
285
demonstrated by the exhaust manifold temperature variation in Fig. 3 (b), favors the oxidation process of CO.
286
The above two factors bring about a decreasing trend for CO emission when slight dilution is taken as shown
287
in Fig. 8 (a). However, when dilution extent further increases, the immediate post flame zone temperature
288
drops remarkably so that CO concentration no longer reaches its equilibrium and stays very high level.
289
Therefore, CO emission increases along with the increased CO2 dilution.
M AN U
SC
RI PT
282
The main HC formation mechanisms are suggested as the followings: (1) flame quenching at the
291
combustion chamber wall, (2) crevice volume originated HC emission, (3) oil layer absorption of fuel, (4)
292
incomplete combustion[22], among which quenching and crevice effect are considered to be the key factors in
293
this study. With load increases, the mean in-cylinder temperature during the combustion process is enhanced
294
and the cylinder wall temperature is improved thus lead to a shorter quenching distance[24]. In addition, as
295
indicated by Fig. 3, the boosted exhaust temperature for higher loads favors the post oxidization process of
296
unburnt HC component. So, HC emission is lower for higher load condition as revealed by Fig. 8 (b). However,
297
HC emission increases monotonously along with the increase of CO2 dilution extent. This should be ascribed
298
to the following two reasons: 1) the inducted CO2 suppresses the combustion temperature, causing lower
299
laminar flame speed as well as larger quenching distance[24], 2) the elongated combustion process renders
300
fuel-air mixture staying in the crevice for longer time and at last escapes the primary combustion process.
301
These two key factors jointly induce the increasing trend of HC emission with the increased dilution.
302
AC C
EP
TE D
290
The NOx emission of the engine along with varying dilution extent under high, middle and low load is 13 / 23
ACCEPTED MANUSCRIPT shown in Fig. 8 (c). The NOx emission shows a decreasing trend with the increased dilution extent
304
monotonously regardless of the load conditions. Ordinarily, the formation of NOx in the combustion of air-fuel
305
mixture entails the following requests: high temperature, high O2 concentration and long time duration. In this
306
study, the equivalence ratio and the engine speed is fixed at 0.73 and 1500rpm, respectively. The dominant
307
factor that affects the formation of NOx is the maximum combustion temperature. This is verified by the
308
approximate linear relationship between ln(NOx) value and the peak mean in-cylinder temperature (Tmax)
309
indicated by Fig. 8 (c). As previously discussed, diluent CO2 suppresses combustion temperature thus restrains
310
the formation of NOx.
M AN U
SC
RI PT
303
Throughout Fig. 8, the effect of CO2 dilution on HC emission is not distinct when intake oxygen content
312
is above 20.3%, while NOx and CO emission drop evidently along with the increased of dilution. At middle
313
load, when intake oxygen content changed from 20.93% (no dilution) to 20.3%, both CO and NOx emissions
314
are lowered by 28.2% and 81.9%, while HC emission increases slightly. In conclusion, with slight intake
315
dilution (intake O2 concentration changed from 20.93 to 20.3), NOx emission could be decreased significantly
316
at a little sacrifice of HC emission regardless of the loads. At the same time, CO emission decreased evidently.
TE D
311
EP
317
3.2.2 Comparison of the effects between CO2, N2, Ar as intake charge diluents on emission
319
characteristics
AC C
318
320
The effects of CO2, N2 and Ar on emission characteristics are shown in Fig. 9. The effects of all three
321
diluents on CO emission characteristic are very similar as indicated by Fig. 9 (a). CO emission exhibits a
322
downward-upward trend with varying dilution. However, the point that minimum CO emission is achieved for
323
CO2 dilution case occurs under the lightest dilution extent followed by N2 and then Ar dilution case. Before
324
these points, CO2 behaves the strongest effect on reducing CO emission. When intake oxygen concentration is 14 / 23
ACCEPTED MANUSCRIPT 325
diluted to be 20.3%, CO concentration was lowered by 25.8%, 13.5%, 7.8% for CO2, N2 and Ar dilution,
326
respectively. However, the minimum CO emission value could be reached by Ar dilution is the lowest followed
327
by N2 dilution and then CO2 dilution. There seems to be no very remarkable differences between the effects of these three diluents on HC
329
emission before the points that minimum CO emission values are achieved and HC emission increases slowly
330
along with the increased dilution. When too more diluent gas is inducted, the combustion process would be
331
influenced (flame speed being slowed, flame propagation being seriously interfered), which is proved by the
332
peak mean in-cylinder temperature from Fig. 9 (c), thus brought about a rapid increase in HC emission as
333
indicated in Fig. 9 (b).
M AN U
SC
RI PT
328
The NOx emission decreases along with the increase of dilution extent regardless of the diluents. Among
335
these three diluents, CO2 is the most effective for reducing NOx emission while Ar is the least effective one.
336
When oxygen concentration was diluted to be 20.3%, NOx emission was reduced by 80.5%, 51.6% and 33.2%
337
for CO2, N2 and Ar dilution, respectively as shown in Fig. 9 (c). It was considered that both thermal and
338
chemical interaction should be responsible for the NOx reduction, among which thermal effect was considered
339
to be the primary factor[25]. Both N2 and Ar were inert gases thus wouldn’t involve chemical effect[6]. The
340
heat capacity is higher for N2 as shown in Table 2. Therefore, the results of thermal impact on NOx emission
341
could be described by comparing Ar dilution and N2 dilution as shown in Fig. 9 (c). It is obvious that thermal
342
effect is the dominant factor on lowering NOx emission. As has been discussed in section 3.2.1, NOx emission
343
is highly dependent on combustion temperature. So, owing to the comparatively higher specific heat capacity,
344
CO2 shows more effective impact on lowing the combustion temperature and this is demonstrated by the peak
345
mean in-cylinder temperature along with dilution extent shown in Fig. 9. Moreover, it was also suggested that
346
hydrocarbon radical reacts with NOx and lead to the formation of hydrogen cyanide, which ultimately lead to
AC C
EP
TE D
334
15 / 23
ACCEPTED MANUSCRIPT 347
the formation of N2, thus help to reduce NOx emission[8]. Under high dilution extent, HC emission is found to
348
be high, thus the possible reaction between HC and NOx in the post-flame and exhaust would help to restrain
349
NOx in the exhaust. So, ultimately, extreme low NOx emission is obtained. Throughout the comparison between these three diluents in terms of the combustion and emission
351
characteristics, it could be found that both CO2 and N2 could lower NOx emission effectively, while, thermal
352
efficiency decreases noticeably. However, Ar dilution almost does not worsen thermal efficiency while lowers
353
NOx emission. Actually, for Ar dilution, thermal efficiency increases at first and then decreases along with the
354
increased dilution as shown in Fig. 7. When NOx is lowered to the same level (100ppm, for example, shown in
355
Fig. 9, points A, B and C), it can be clearly found that the thermal efficiency is the highest for Ar dilution
356
followed by CO2 and then N2 as shown in Fig. 7. Besides, CO emission is the highest for CO2 dilution and
357
lowest for Ar dilution as indicated by Fig. 9 (a).
M AN U
SC
RI PT
350
359
4. Conclusions
TE D
358
In this paper, the effects of intake charge diluents, CO2, N2 and Ar, on the combustion and emission
361
characteristics of a spark ignition natural gas engine have been investigated and the comparison between these
362
three diluents has been carried out. It was found that intake charge dilution had an evident effect on the
363
combustion process and the emission of the spark ignition natural gas engine. Under a fixed load condition,
364
with the increase of dilution extent, the peak in-cylinder pressure and the maximum value of heat release rate
365
decreases and their corresponding crank angle shifts further away from the crank angle for spark timing.
366
Moreover, the ignition delay and combustion duration both are prolonged by intake charge dilution. However,
367
among the three diluents, CO2 shows its strongest effect on the combustion processes followed by N2 and then
368
Ar.
AC C
EP
360
16 / 23
ACCEPTED MANUSCRIPT With the increasing dilution extent, NOx emission decreases while HC emission increases regardless of
370
the diluents. However, CO emission exhibits a down-up trend. Reducing NOx and CO emission simultaneously
371
could be achieved when appropriate dilution extent is chosen at a little sacrifice of HC emission for all these
372
three diluents. In comparison, CO2 is the most effective for reducing NOx and CO emission followed by N2
373
and then Ar. However, both CO2 and N2 dilution lower thermal efficiency while Ar dilution improves thermal
374
efficiency. When NOx is lowered to the same level, the thermal efficiency is the highest and CO emission is the
375
lowest for Ar dilution.
SC
RI PT
369
With the comparison between N2 and Ar, the thermal effect on combustion and NOx emission is separated
377
in Fig. 9 (c). It could be found that thermal effect helps to lowering NOx emission remarkably. Similar analysis
378
on combustion is given in Fig. 4 and results show that thermal effect lowers peak in-cylinder pressure as well
379
as peak heat release rate.
M AN U
376
381
Acknowledgements
TE D
380
The author would like to thank National Science Foundation of China (Project No. 51306117; 51436005),
383
Ministry of Science and Technology (MOST) of China (Project No.2012BAD47B04) and Foundation of
384
Shanghai Jiao Tong University to support this research.
AC C
385
EP
382
17 / 23
ACCEPTED MANUSCRIPT 386
References
387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428
[1] J. Deng, F. Ma, S. Li, Y. He, M. Wang, L. Jiang, S. Zhao, Experimental study on combustion and emission characteristics of a hydrogen-enriched compressed natural gas engine under idling condition, International Journal of Hydrogen Energy, 36 (2011) 13150-13157. [2] A. Mariani, M.V. Prati, A. Unich, B. Morrone, Combustion analysis of a spark ignition i. c. engine fuelled 1616-1623.
RI PT
alternatively with natural gas and hydrogen-natural gas blends, International Journal of Hydrogen Energy, 38 (2013) [3] A. Thiruvengadam, D.K. Carder, M. Krishnamurthy, A. Oshinuga, M. Gautam, Effect of an economical oxidation catalyst formulation on regulated and unregulated pollutants from natural gas fueled heavy duty transit buses, Transportation Research Part D: Transport and Environment, 16 (2011) 469-473.
[4] A. Ibrahim, S. Bari, A comparison between EGR and lean-burn strategies employed in a natural gas SI engine using a two-zone combustion model, Energy Conversion and Management, 50 (2009) 3129-3139.
SC
[5] A. Ibrahim, S. Bari, An experimental investigation on the use of EGR in a supercharged natural gas SI engine, Fuel, 89 (2010) 1721-1730.
[6] N. Ladommatos, S. Abdelhalim, H. Zhao, Control of oxides of nitrogen from diesel engines using diluents while
M AN U
minimising the impact on particulate pollutants, Applied Thermal Engineering, 18 (1998) 963-980. [7] N. Ladommatos, S. Abdelhalim, H. Zhao, Z. Hu, The dilution, chemical, and thermal effects of exhaust gas recirculation on diesel engine emissions-part 1: Effect of reducing inlet charge oxygen, in, SAE Technical Paper, 1996.
[8] N. Ladommatos, S.M. Abdelhalim, H. Zhao, Z. Hu, 97/02332 The dilution, chemical, and thermal effects of exhaust gas recirculation on diesel engine emissions. Part 2: Effects of carbon dioxide, Fuel and Energy Abstracts, 38 (1997) 186.
TE D
[9] N. Ladommatos, S.M. Abdelhalim, H. Zhao, Z. Hu, The Dilution, Chemical, and Thermal Effects of Exhaust Gas Recirculation on Disesel Engine Emissions-Part 4: Effects of Carbon Dioxide and Water Vapour, in, SAE Technical Paper, 1997.
[10] N. Ladommatos, S.M. Abdelhalim, H. Zhao, Z. Hu, The dilution, chemical, and thermal effects of exhaust gas recirculation on diesel engine emissions-Part 3: effects of water vapour, in, SAE Technical Paper, 1997.
EP
[11] L. Zhu, C. Cheung, W. Zhang, Z. Huang, Effect of charge dilution on gaseous and particulate emissions from a diesel engine fueled with biodiesel and biodiesel blended with methanol and ethanol, Applied Thermal Engineering, 31 (2011) 2271-2278.
AC C
[12] V. Subramanian, J.M. Mallikarjuna, A. Ramesh, Intake charge dilution effects on control of nitric oxide emission in a hydrogen fueled SI engine, International Journal of Hydrogen Energy, 32 (2007) 2043-2056. [13] Z. Wang, H. Zuo, Z. Liu, W. Li, H. Dou, Impact of N2 dilution on combustion and emissions in a spark ignition CNG engine, Energy Conversion and Management, 85 (2014) 354-360. [14] H.A. Moneib, M. Abdelaal, M.Y.E. Selim, O.A. Abdallah, NOx emission control in SI engine by adding argon inert gas to intake mixture, Energy Conversion and Management, 50 (2009) 2699-2708. [15] W. Li, Z. Liu, Z. Wang, Y. Xu, Experimental investigation of the thermal and diluent effects of EGR components on combustion and NOx emissions of a turbocharged natural gas SI engine, Energy Conversion and Management, 88 (2014) 1041-1050. [16] R. Krieger, G.L. Borman, Computation of apparent heat release for internal combustion engines, in: MECHANICAL ENGINEERING, Vol. 89, ASME-AMER SOC MECHANICAL ENG 345 E 47TH ST, NEW YORK, NY 10017, 1967, pp. 59-&. [17] M.F. Brunt, A.L. Emtage, Evaluation of burn rate routines and analysis errors, in, SAE Technical Paper, 1997. 18 / 23
ACCEPTED MANUSCRIPT [18] F. Halter, F. Foucher, L. Landry, C. Mounaïm-Rousselle, Effect of Dilution by Nitrogen and/or Carbon Dioxide on Methane and Iso-Octane Air Flames, Combustion Science and Technology, 181 (2009) 813-827. [19] T. Shudo, Improving thermal efficiency by reducing cooling losses in hydrogen combustion engines, International Journal of Hydrogen Energy, 32 (2007) 4285-4293. [20] G. Williams, Combustion theory, (1985). [21] C. Prathap, A. Ray, M.R. Ravi, Investigation of nitrogen dilution effects on the laminar burning velocity and flame stability of syngas fuel at atmospheric condition, Combustion and Flame, 155 (2008) 145-160. [23] S.R. Turns, An introduction to combustion, McGraw-hill New York, 1996. [24] S.R. Turns, An introduction to combustion, McGraw-hill New York, 1996.
RI PT
[22] J.B. Heywood, Internal combustion engine fundamentals, Mcgraw-hill New York, 1988.
[25] A.A. Quader, Why intake charge dilution decreases nitric oxide emission from spark ignition engines, in, SAE Technical Paper, 1971.
SC
429 430 431 432 433 434 435 436 437 438 439 440 441 442
AC C
EP
TE D
M AN U
443
19 / 23
ACCEPTED MANUSCRIPT 444
Table Captions
445 Table 1. Engine specifications
447
Table 2. The specific heat capacity at constant pressure of the diluents at standard condition
RI PT
446
448
AC C
EP
TE D
M AN U
SC
449
20 / 23
ACCEPTED MANUSCRIPT 450 Table 1. Engine specifications Number of cylinders
4
Number of strokes
4
Displacement/L
5.3
Intake valve opening/CAD BTDC
22.5
Bore/mm
114
Intake valve closing/ CAD ABDC
34.5
Connecting rod length/mm
130
Exhaust valve opening/ CAD BBDC
67.5
Compression ratio
11
Exhaust valve closing/ CAD ATDC
25.5
RI PT
451
AC C
EP
TE D
M AN U
SC
452 453
21 / 23
ACCEPTED MANUSCRIPT 454 455
Table 2. The specific heat capacity at constant pressure of the diluents at standard condition Diluents
CO2
N2
Ar
Cp/kJ/(kmol·K)
37.198
29.071
20.785
AC C
EP
TE D
M AN U
SC
RI PT
456 457
22 / 23
ACCEPTED MANUSCRIPT 458
Figure Captions
459 Fig. 1. The schematic layout of the experimental setup
461
Fig. 2. Cylinder pressure and net heat release rate along with crank angle under 35, 50, 65% load for CO2
462
dilution
RI PT
460
Fig. 3. The peak mean in-cylinder temperature and exhaust manifold temperature variation along with intake
464
O2 concentration under 35%, 50%, 65% load for CO2 dilution
465
Fig. 4. Cylinder pressure and neat heat release rate along with crank angle under 50% load for CO2, N2 and Ar
466
dilution when intake oxygen concentration is 19.7%
467
Fig. 5. The ignition delay and combustion duration variation along with intake O2 concentration under 50%
468
load for CO2, N2 and Ar dilution
469
Fig. 6. The peak mean in-cylinder temperature and exhaust manifold temperature variation along with intake
470
O2 concentration for CO2, N2 and Ar dilution under 50% load
471
Fig. 7. Effective thermal efficiency variation along with intake oxygen concentration for CO2, N2 and Ar
472
dilution under 50% load (points A, B and C correspond to [52 ] = 100ppm)
473
Fig. 8. CO, HC and NOx emission variation along with intake O2 concentration for CO2 dilution under
474
different loads
475
Fig. 9. CO, HC and NOx emission variation along with intake O2 concentration for CO2, N2 and Ar dilution
476
under 50% load (points A, B and C correspond to [52 ] = 100ppm, points A, D and E correspond to
477
[23 ]4 = 20.3%)
AC C
EP
TE D
M AN U
SC
463
478
23 / 23
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
1, CO2 is the most effective on reducing NOx and CO emission followed by N2 and then Ar. 2, NOx and CO emission could be simultaneously reduced by intake charge dilution regardless of the diluents when appropriate dilution extent is chosen. 3, Both CO2 and N2 dilution worsen while Ar dilution improves thermal efficiency. 4, Thermal effect is a dominant factor for reducing NOx emission.