Performance and emission characteristics of a turbocharged CNG engine fueled by hydrogen-enriched compressed natural gas with high hydrogen ratio

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 6 4 3 8 e6 4 4 7

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Performance and emission characteristics of a turbocharged CNG engine fueled by hydrogen-enriched compressed natural gas with high hydrogen ratio Fanhua Ma*, Mingyue Wang, Long Jiang, Renzhe Chen, Jiao Deng, Nashay Naeve, Shuli Zhao State Key Laboratory of Automotive Safety and Energy Tsinghua University, Beijing 100084, PR China

article info

abstract

Article history:

This paper investigates the effect of high hydrogen volumetric ratio of 55% on performance

Received 1 February 2010

and emission characteristics in a turbocharged lean burn natural gas engine. The experi-

Received in revised form

mental data was conducted under various operating conditions including different spark

24 March 2010

timing, excess air ratio (lambda), and manifold pressure. It is found that the addition of

Accepted 25 March 2010

hydrogen at a high volumetric ratio could significantly extend the lean burn limit, improve the engine lean burn ability, decrease burn duration, and yield higher thermal efficiency.

Keywords:

The CO, CH4 emissions were reduced and NOx emission could be kept an acceptable low

High hydrogen addition

level with high hydrogen content under lean burn conditions when ignition timing were

Combustion

optimized.

Emission

1.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Introduction

With increasing concerns about the environmental protection and energy shortage, more and more attention in the auto industry has been shifted to the use of alternative fuels. Natural gas, which is primarily composed of methane, is considered as the most promising alternative fuel for engine application due to its availability and potential economic and environmental benefits. However, due to the slow burning velocity of natural gas and the poor lean burn capability, the natural gas spark ignited engine has the disadvantage of large cycle-by-cycle variations and poor lean burn capability, and these will decrease the engine power output and increase fuel consumption. Hydrogen is regarded as the best gaseous candidate for addition into natural gas because it has some unique and

highly desirable properties for application in SI engines. Some thermal and chemical properties of hydrogen and methane, which is the main component of natural gas, are compared in Table 1. As can be seen, compared to natural gas, hydrogen has a wider flammable mixture range, lower ignition energy and faster flame propagation rates, all of which are helpful to improve engine’s lean burn capability. Over the past years, there has been much research related to hydrogen-enriched fuels, some focus on the influence of the engine’s overall performance after the hydrogen addition. These research include the engine’s power, efficiency, combustion, emission performance under various engine speed, excess air ratio l, ignition timings, MAP (Manifold Absolute Pressure) at middle load, low load and even idle. Such research were performed by Bauer and Forest [1], Swain and Yusuf [2], Hoekstra and Lynch [3], Sierens and Rossel [4],

* Corresponding author. Tel./fax: þ86 10 62785946. E-mail address: [email protected] (F. Ma). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.03.111

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Table 1 e Comparison of hydrogen and methane [1]. Fuel characteristics Equivalence ratio ignition lower limit in NTP air Mass lower heating value (kJ/kg) Density of gas NTP (kg/m3) Volumetric lower heating value at NTP (kJ/m3) Stoichiometric air-to-fuel ratio Volumetric fraction of fuel in air, l ¼ 1 Volumetric lower heating value in air l ¼ 1 Burning speed in NTP air (cm/s) Flame temperature in air (K)

Hydrogen (H2)

Methane (CH4)

0.1

0.53

119,930

50,000

0.083764 10,046

0.65119 32,573

34.20 0.290

17.19 0.095

2913

3088

265e325 2318

37e45 2148

NTP denotes normal temperature (293.15 K) and pressure (1 atm).

Huang et al. [5] and Ma et al. [6] These studies generally showed that HC and CO concentrations could be decreased by hydrogen addition.

Table 2 e Main composition of the tested NG fuel. Component CH4 C3H8 n-C4H10 O2 N2

Since hydrogen can enhance the lean burn capability thereby solving the problem of NOx emissions through allowing ultra-lean operation. There is still some literature focusing on the influence of combustion characteristics including those performed by Karim [7], Tully and Heywood [8], Goldwitz and Heywood [9] and Huang et al. [10], Ma et al. [11]. The results of these researches showed with consistency that both the flame development and flame propagation duration can be reduced through introducing hydrogen. In addition, hydrogen addition improves the combustion, so that the average effective pressure increases, at this time reducing the ignition delay increase the torque output so that it is comparable to using pure CNG fuel. However, most of these studies mentioned above investigate the combustion and emissions characteristics based on a low hydrogen volume ratio (0e30%) in the HCNG fuel. While for the higher hydrogen content (above 50%) fuels, there has never been research completed for the airefuel ratio or ignition advance angle characteristics at a given operating condition aimed at even more superior power, economy, emissions performance.

Component

Volumetric ratio (%)

96.51 0.18 0.02 0.01 0.22

C2H6 i-C4H10 i-C5H12 CO2

1.2 0.02 0.01 1.81

This paper specifically deals with the performance and emission characteristics of a turbocharged CNG engine fueled by hydrogen-enriched compressed natural gas with a high hydrogen ratio of 55%. Main composition of the tested NG fuel show in Table 2.In this study, it is found that high hydrogen ratio addition can significantly extend the lean burn limit while increasing thermal efficiency, and the cycle-by-cycle variations are also greatly reduced under lean burn conditions. In addition, the CO and HC emissions are also reduced and the NOx emission can be kept an acceptable level with lean burn condition.

2. NG natural gas CNG compressed natural gas HCNG hydrogen-enriched compressed natural gas SI spark ignition MAP manifold absolute pressure MBT maximum brake torque CFR cooperative fuel research CCV cycle-by-cycle variations MFB mass fraction burned CA crank angle ATDC after top dead center lambda (l) excess air ratio IMEP indicated mean effective pressure COVimep coefficient of variation of IMEP

Volumetric ratio (%)

Experimental apparatus and test method

The experiments were carried out on a six-cylinder, single point injection, SI NG engine (see Table 3 for specifications). The engine was coupled to an eddy-current dynamometer for the measurement and control of speed and load. The exhaust concentration of HC, NOx, CO, H2 and the air/fuel ratio were monitored by HORIBA-MEXA-7100DEGR emission monitoring system and a HORIBA wide-range lambda analyzer, respectively. A high speed YOKOGAWA ScopeCorder was used to record the cylinder pressure from a Kistler 6117B piezoelectric high pressure transducer. Corresponding crankshaft positions were measured by a Kistler 2613B crank angle encoder with a resolution of 1 CA. The record length of pressure data in this study is 250 K (about 347 cycles) which is enough for CCV analysis (Table 4 for measurement instruments specifications). An online mixing system was developed in order to blend desired amount of hydrogen with natural gas in a pressure stabilizing tank just before entering the engine. The tank was divided into two chambers with a damping line used to improve the mixture uniformity [12]. Fig. 1 is a schematic of

Table 3 e Engine Specifications. Item Engine make Engine type Aspiration method Compression ratio Bore (mm) Stroke (mm) Displacement volume (L) Rated power/speed Maximum torque/speed Full-load minimum fuel consumption rate

Value DONGFENG MOTOR CO., LTD, China In-line 6 cylinders, spark ignition Turbocharged intercooled 10.5 105 120 6.234 154 kw/2800 rpm 620 N$m/1600 rpm 198 g/kW h

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Table 4 e Measurement instruments specifications. Instruments

Range

Sensitivity

Linearity

Cylinder pressure 0e20 Mpa 16.8 pC/bar 0.6%FSO sensor (Kistler6117B) Charge Amplifier 10e 9.99 0.01e9.99 0.05%FS (Kistler5011B)  105 (10 V FS)  104 pC/M.U.; 0.01e9.99  104 mV/M.U. 4 0.1e6 e Crank angle generator 1e2.0  10 r/min (Kistler2613B) 1% 0.24% Mass air flow meter 0e1000 (ToCeil20N100114LI) Nm3/h

the fuel supply system. The flow rate of CNG and H2 were measured by a Micro Motion flow meter that uses the principle of Coriolis force for a direct measure of mass flow. An ALICAT flow control valve was used to adjust the flow rate of the hydrogen according to the flow rate of CNG and obtain the target hydrogen fraction. It was validated that the fuel mixing system worked well under all conditions with the absolute error in hydrogen fraction being less than 1.5% in all cases [13]. The experiment was conducted under various operating conditions including different spark timings, equivalent air/ fuel ratios, manifold pressure and engine speed in order to evaluate performance and emission characteristics with the addition of hydrogen at a 55% volumetric ratio to the CNG fuel. In order to avoid knocking and to avoid exceeding the limit of maximum cylinder pressure, the experimental strategy was to optimize the spark timing at lean burn conditions. The experiment was divided into three parts, investigating the effects of ignition timing, air/fuel ratio, manifold absolute pressure on combustion characteristics and CCV, Table 5 shows the engine operating conditions used in this study. The manifold absolute pressure (MAP) was chosen as an indicator of load rather than the conventional method of using the throttle position because the aim is to keep the amount of intake air constant. In a turbocharged engine, a fixed throttle position cannot be directly translated into the fixed amount of intake air, due to the effects of the turbocharger. Also note that due to the lower sensitivity (compared

to a CFR engine or engines with smaller displacement) of the test engine’s torque output to spark timing, in this study the MBT spark timing was determined in a special way: it is assumed that spark timings that make 50% MFB occur at 9 ATDC and these were selected as the MBT spark timings in this experiment. Based on Cooper’s experimental results [14], when MBT spark timing is achieved, the 50% MFB point generally occurs between 8 and 10 ATDC.

3.

Results and discussion

3.1. The effect of high hydrogen addition on lean operation limit and combustion duration Lean operation limit is an important parameter to represent the fuel’s lean burn ability. It is generally accepted that a COVimep above 10% will be perceived by a driver as a poor running condition [15]. Therefore, the lean operation limit was defined as the excess air ratio (reciprocal of equivalence ratio) at which COVimep reaches 10%. COVimep was defined as the standard deviation in IMEP divided by the mean IMEP [16]. Fig. 2 shows the variation of COVimep versus excess air ratio for 55% hydrogen volumetric ratio (mass ratio 13.253%) compared with NG and 30% hydrogen fractions (mass fractions 5.085%)under the engine operation conditions of an engine speed of 1200 rpm, a manifold pressure of 105 kPa and MBT spark timing. It is found that a high hydrogen ratio can significantly extend the lean burn limit. As can be seen in Fig. 2, the lean burn limit under the engine operation conditions of 1200 rpm engine speed and 105MAP manifold pressure occurs at a lambda of 2.5, while for fuel blends containing NG and 30% hydrogen were 1.71, and 2.09, respectively. Quader’s researches have shown that the combustion duration is nearly the same when the engine reaches its lean limit no matter what type of fuel used [17]. This is to say although combustion duration will be prolonged as the engine is gradually leaned out, it has an upper limit which is independent on fuel type and once the combustion duration exceeds this upper limit, the engine will become unstable due to

Fig. 1 e Schematic of the fuel supply system.

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Section

Item

1

Value

Engine Speed Hydrogen fraction by volume Intake manifold absolute pressure Spark timing

2

1200 rpm 0%, 30%, 55% 105 kPa Ranges that contain MBT with a step of 2 CA 1200 rpm 55%

Engine Speed Hydrogen fraction by volume Intake manifold absolute pressure Excess air ratio

50 kPa, 105 kPa, Wide open throttle 1.6

Spark to 10%MFBBurn Duration/CA deg

Table 5 e Test engine specifications.

1200rpm MAP=105kPa MBT spark timing λ =1.71

34 32 30 28

λ =2.09

λ =2.5

0% H2 30% H2 55% H2

26 24 22 20 18 16 14 12 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6

Excess air ratio λ

combustion instability. Therefore, a certain type of fuel will have greater lean operation ability if it provides shorter combustion duration at a given lambda, because it may require leaner fueleair mixtures for the combustion duration to reach the upper limit. The above analysis makes it clear that examining the effect of high hydrogen addition on combustion duration is essential to the analysis of hydrogen’s ability to extend lean limit. Figs. 3 and 4 confirm the improvements in flame development speed (characterized as the duration between the spark and 10% mass fraction burned) and propagation speed (characterized as the duration between 10% and 90% mass fraction burned). Clearly, both durations were significantly reduced with a high hydrogen volumetric ratio of 55%. Fundamentally, the addition of hydrogen provides a large pool of H and OH radicals whose increase makes the combustion reaction much easier and faster, thus leading to shorter burn duration [18]. It can also be seen from Figs. 3 and 4 that at low excess air ratios, high hydrogen addition contributes more impact in speeding up the flame development phase of combustion than to speeding up the flame propagation phase. This is because the additional H and OH radicals brought by the hydrogen addition increases the mixture’s laminar flame speed, which has a greater effect on flame growth rate before the flame becomes

Fig. 3 e Spark to 10% MFB burn duration versus excess air ratio for 55% hydrogen volumetric ratio compared with NG and 30% hydrogen fractions, engine speed of 1200 rpm, MAP [ 105 kPa, MBT spark timing.

fully turbulent [19]. However, as the dilution level further increases, the curves for the 10e90% burn durations begin to separate considerably as the hydrogen addition has an increasingly positive effect on the turbulent flame propagation. This increase in turbulent flame speed is still thought to be caused by the increase in laminar flame speed, the reason for the difference at different excess air ratios is because the effects of the laminar flame speed on the following turbulent flame propagation is more obvious at leaner fuel/air mixtures. This is because under the unfavorable condition of leaner mixtures, faster flame development can provide a more stable and rapid flame propagation process. Fig. 5 shows that high hydrogen addition contributes more impact in slow down the decline in power. It can be seen that with l increases, power output declines, because in this paper 1200rpm MAP=105kPa MBT spark timing λ =2.09 λ =2.5 λ =1.71

0.5

λ=1.71

λ=2.09

0% H 30% H

λ=2.5

55% H

COV

/%

0.4

0.3

0.2

0.1

0.0 1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Excess air ratio λ

Fig. 2 e COVimep versus excess air ratio for 55% hydrogen volumetric ratio compared with NG and 30% hydrogen fractions, engine speed of 1200 rpm, MAP [ 105 kPa, MBT spark timing.

10%-90% MFB Burn Duration/CA deg

45

1200rpm MAP=105kPa MBT spark timing

40 35

0% H2 30% H2 55% H2

30 25 20 15 10 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6

Excess air ratio λ

Fig. 4 e 10e90% MFB burn duration versus excess air ratio for 55% hydrogen volumetric ratio compared with NG and 30% hydrogen fractions, engine speed of 1200 rpm, MAP [ 105 kPa, MBT spark timing.

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n=1200rpm MAP=105kPa MBT spark timing

n=1200rpm MAP=105kPa MBT spark timing

60

7000

NOXconcentration /ppm

power output /kw

40

30

0% H2

20

30% H2 55% H2

10

0% H2

6000

50

1.0

1.2

30% H2 5000

55% H2

4000 3000 2000 1000 0

1.4

1.6

1.8

2.0

2.2

2.4

1.0

2.6

1.2

1.4

Fig. 5 e Engine’s power performance versus excess air ratio for 55% hydrogen volumetric ratio compared with NG and 30% hydrogen fractions, engine speed of 1200 rpm, MAP [ 105 kPa, MBT spark timing.

we change l by means of changing the injection pulse width: l increases the injected fuel quality reduced, resulting in an engine input energy reduction In addition, it can be seen that when l < 1.6, the enriched hydrogen makes a slight decline in power, while l > 1.6, the situation is just the opposite. This is because when l < 1.6, various fuels can attain normal burning, due to the volume calorific value of hydrogen is less than natural gas, enriched hydrogen reduces the gas mixture energy, resulting in power declined, while l > 1.6, the pure natural gas is too thin, which slow down combustion velocity, makes incomplete combustion more quite serious, while the hydrogen-enriched help improve the burning velocity and improve the incomplete combustion, so even at this time volume calorific value of the mixture is slightly lower, but after hydrogen-enriched the combustion efficiency and

1.8

2.0

2.2

2.4

2.6

Fig. 7 e NOx emission versus excess air ratio for 55% hydrogen volumetric ratio compared with NG and 30% hydrogen fractions, engine speed of 1200 rpm, MAP [ 105 kPa, MBT spark timing.

thermal power conversion efficiency has been enhanced, resulting in a higher power performance.

3.2. Engine thermal efficiency and emission characteristics versus excess air ratio at MBT for 55% high hydrogen ratio The thermal efficiency and NOx emission characteristics obtained in this part of the test are shown in Figs. 6 and 7, respectively. Fig. 6 shows that the indicated thermal efficiency for the addition high hydrogen content at a volumetric ratio of 55% is 33.7% under the operating conditions of a lambda of 2.4, which is only 5% lower than that under the conditions of a lambda of 1.6. It is found that the addition of hydrogen at a high ratio can significantly extend the lean burn limit and yield a higher thermal efficiency.

n=1200rpm MAP=105kPa MBT spark timing

0.40

1.6

Excess air ratio λ

Excess air ratio λ

n=1200rpm MAP=105kPa MBT spark timing

0H2 CH4 concentration /ppm

Indicated thermal efficiency

10000

0.35

0.30

0.25

0% H2

0.20

30% H2

30H2

8000

55H2

6000

4000

2000

55% H2

0.15

0

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Excess air ratio λ

Fig. 6 e Indicated thermal efficiency versus excess air ratio for 55% hydrogen volumetric ratio compared with NG and 30% hydrogen fractions, engine speed of 1200 rpm, MAP [ 105 kPa, MBT spark timing.

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

Excess air ratio λ

Fig. 8 e CH4 emission versus excess air ratio for 55% hydrogen volumetric ratio compared with NG and 30% hydrogen fractions, engine speed of 1200 rpm, MAP [ 105 kPa, MBT spark timing.

2.6

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CO concentration /ppm

4000

0% H2

3500

30% H2

3000

55% H2

2500 2000 1500 1000 500 0

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Excess air ratio λ

Fig. 9 e CO emission versus excess air ratio for a 55% hydrogen volumetric ratio compared with NG and 30% hydrogen fractions, engine speed of 1200 rpm, MAP [ 105 kPa, MBT spark timing.

Fig. 11 e In-cylinder pressure for 55% hydrogen volumetric ratio at engine speed of 1200 rpm, MAP [ 105 kPa, l [ 1.6, MBT spark timing.

Fig. 7 shows that when the spark timing is adjusted to MBT, the NOx emission at a 55% hydrogen volumetric ratio is similar to the NOx emissions at lower hydrogen volumetric ratios. It can be seen that in the range of excess air ratios above 1.8, NOx can be reduced to very low levels and high hydrogen addition does not visible increase NOx emission higher than that of pure natural gas operation. NOx emission can be kept an acceptable level at lean burn conditions and with high hydrogen addition. Furthermore, it can be seen from Fig. 6 that high hydrogen addition can improve thermal efficiency after spark timing optimization and that as hydrogen addition is increased, the more efficiency rise is gained. But, compared with the experimental results in Sierens’ study, the thermal efficiency gain in this study due to the hydrogen addition is quite small. Siererns et al. achieved a maximum of 6% efficiency rise when adding 10% hydrogen in volume into NG [6].

Based on the these results it can be conclude that high hydrogen addition, at a 55% volume ratio not only keeps an acceptable level of the exhaust NOx but also greatly improves the engines thermal efficiency. As Fig. 8 shows significantly lower CH4 emission can be reached by adding a 55% hydrogen volumetric ratio compared with NG and 30% hydrogen fractions. This can be explained by the fact that hydrogen can speed up flame propagation and reduce quenching distance, thus decreasing the possibilities of incomplete combustion [20]. Of course, the fact that the carbon concentration of the fuel blends is decreased due to hydrogen addition should also be accounted for. The formation of carbon monoxide is mainly caused by incomplete combustion. As can be seen in Fig. 9, the CO concentration first dropped gradually, reached a minimum value at an excess air ratio of 2.0, and then started climbing rapidly due to poor combustion conditions as the engine was

Fig. 10 e In-cylinder pressure for 55% hydrogen volumetric ratio at engine speed of 1200 rpm, MAP [ 50 kPa, l [ 1.6, MBT spark timing.

Fig. 12 e In-cylinder pressure for 55% hydrogen volumetric ratio at engine speed of 1200 rpm, WOT, l [ 1.6, MBT spark timing.

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n=1200rpm λ =1.6

MAP=50kPa(Load=20%) MAP=105kPa(Load=50%) Wide Open Throttle

5

MAP=50kPa(Load=20%) MAP=105kPa(Load=50%) Wide Open Throttle

n=1200rpm λ =1.6 50

40

3

power output /kw

COVimep/%

4

2

1 0

5

10

15

20

25

30

35

40

45

30

20

10

Ignition timing/°CA BTDC

Fig. 13 e COVimep versus ignition timing for 55% hydrogen volumetric ratio at three different load levels, engine speed of 1200 rpm, MBT spark timing.

further leaned. It is found that high hydrogen addition of 55% by volume resulted in much less carbon monoxide exhaust. This was also attributed to hydrogen’s ability to strengthen combustion, especially for lean fueleair mixtures.

3.3. Engine characteristics versus ignition timing at MBT for a high hydrogen ratio of 55% Figs. 10e12 shows in-cylinder pressure curve under various MAP with 50 vol% HCNG. From those there figures, as the load is bigger, the max in-cylinder pressure is bigger. We can see this from Fig. 10, when l is 1.3, ignition timing is 40 BTDC the max in-cylinder pressure is 2672.711 KPa at the point of

MAP=50kPa(Load=20%) MAP=105kPa(Load=50%) Wide Open Throttle n=1200rpm λ=1.6

0.39 0.38

Indicated thermal efficiency

0.37 0.36 0.35 0.34 0.33 0.32 0.31 0.30 0.29 0.28 0.27 0.26 0.25 0

5

10

15

20

25

30

35

40

45

Ignition timing/°CA BTDC

Fig. 14 e Indicated thermal efficiency versus ignition timing for a 55% hydrogen volumetric ratio at three different load levels, engine speed of 1200 rpm, MBT spark timing.

0

0

5

10

15

20

25

30

35

40

45

Ignition timing/°CA BTDC

Fig. 15 e Engine’s power performance versus ignition timing for a 55% hydrogen volumetric ratio at three different load levels, engine speed of 1200 rpm, MBT spark timing.

363 CA, which is much smaller than the max in-cylinder pressure 7477.349 KPa shows in Fig. 12. On the other hand, when ignition timing earlier, the max in-cylinder pressure is more near the TDC. Fig. 13 shows the COVimep versus ignition timing for the addition of hydrogen at a high volumetric ratio of 55% and at three different load levels. The COVimep is decreasing as the spark advance is increased until reaching a minimum value which is located at an angle that typical spark timing would occur, and then has a slight increase. Moreover with the load increases, COVimep present decreasing trend. From the indicated thermal efficiencies shown in Fig. 14, a conclusion can be drawn that the typical spark timing crank angles are equal to the MBT spark timings for the three different load levels in this experiment. When using very advanced ignition timing, the cylinder temperature is comparatively low. Also, the low and uneven mixture concentration in the vicinity of spark plug will cause a negative influence on the initiation and development of flame kernel. When igniting at very retarded ignition timing, the low combustion efficiency does harm to the combustion stability, therefore the ignition timing has to be adjusted to achieve a comparatively low CCV. Fig. 14 shows the indicated thermal efficiencies for 55% hydrogen volumetric ratio under various ignition timings. From the figure it can be seen that the indicated thermal efficiency is 38% at manifold pressure of 105 kPa when spark timing was adjusted to MBT, while the value are 40% at wide open throttle As spark timing deviates from MBT, the engine thermal efficiency decreases. In addition, when the spark timing is retarded past MBT the efficiency reduction is greater than when it is advanced prior to MBT. The reason why the efficiency is affected by spark advance is because of the phasing of the combustion process.

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MAP=50kPa(Load=20%) MAP=105kPa(Load=50%) Wide Open Throttle n=1200rpm λ =1.6

NOXconcentration /ppm

18

16

14

4000 3000 2000 1000 0

12

0

5

10

15

20

25

30

35

40

45

Ignition timing/°CA BTDC 10 0

5

10

15

20

25

30

35

40

45

Ignition timing/°CA BTDC

Fig. 16 e Spark to 10% burn duration versus ignition timing for a volumetric ratio of 55% hydrogen at three different load levels, engine speed of 1200 rpm, MBT spark timing.

Fig. 15 shows that the power performance of three different load. It can easily be seen that with load increases, power output ascend, because load increases the injected fuel quality also rise up, resulting in engine input energy raise Figs. 16 and 17 show the burn durations of 0e10% and 10e90% respectively, at various ignition timings. The flame development duration is defined as the crank angle interval from the ignition timing to the angle for which 10% of the MFB occurs, and the rapid combustion duration is defined as the crank angle interval between 10% of the MFB and the angle for which 90% of the MFB occurs. As can be seen, the flame development duration increases by advancing the ignition timings, and high hydrogen addition can promote flame kernel formation and flame propagation at the early stages of combustion.

Fig. 18 e NOx emission versus ignition timings for 55% hydrogen volumetric ratio at three different load levels, engine speed of 1200 rpm, MBT spark timing.

The rapid combustion duration first decreases by advancing ignition timings and then has a slight increase after reaching a minimum value, this changing trend is similar to that of the COVimep shown above. By retarding the ignition timing from MBT spark timing, the main combustion may take place in the expansion stroke where the cylinder pressure and temperature are comparably low which leads to prolonged flame propagation duration. Figs. 18e20 show the NOx, CH4 and CO emissions, respectively. According to high-temperature reaction mechanism of NO, temperature, oxygen concentration and reaction time are the three elements to produce NO. When lean burning, the condition of sufficient oxygen concentration is satisfied, the higher temperature is, the faster reaction rate will be. At this moment, the NO balance concentration will be higher, thus greater amount of NO will generate. Besides, the time of hightemperature reaction is longer; NO generation capacity will be more.

n=1200rpm λ =1.6

34

10%-90% MFB Burn Duration/CA deg

MAP=50kPa(Load=20%) MAP=105kPa(Load=50%) Wide Open Throttle

5000

32

MAP=50kPa(Load=20%) MAP=105kPa(Load=50%) Wide Open Throttle

30 28 26 24

500 450 400 350 300

22

250

20

200

0

5

10

15

20

n=1200rpm λ =1.6

550

CH4 concentration /ppm

Spark to 10% MFB Burn Duration/CA deg

20

n=1200rpm λ =1.6

6000

25

30

35

40

45

Ignition timing/°CA BTDC

Fig. 17 e 10e90% MFB burn duration versus ignition timing for a volumetric ratio of 55% hydrogen at three different load levels, engine speed of 1200 rpm, MBT spark timing.

MAP=50kPa(Load=20%) MAP=105kPa(Load=50%) Wide Open Throttle 0

5

10

15

20

25

30

35

40

Ignition timing/°CA BTDC

Fig. 19 e CH4 emission versus ignition timings for 55% hydrogen volumetric ratio at three different load levels, engine speed of 1200 rpm, MBT spark timing.

45

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n=1200rpm λ =1.6

340 320

CO concentration /ppm

300 280 260 240 220 200 180

MAP=50kPa(Load=20%) MAP=105kPa(Load=50%) Wide Open Throttle

160 140 120

0

5

10

15

20

25

30

35

40

45

Ignition timing/°CA BTDC

Fig. 20 e CO emission versus ignition timings for a 55% hydrogen volumetric ratio at three different load levels, engine speed of 1200 rpm, MBT spark timing.

In our experiment, when manifold absolute pressure is higher; and the ignition timing is more advanced, the cylinder pressure will increase faster (show in Figs. 10e12). That makes larger heat loss, result in the higher cylinder temperature and more NO generation. The NOx emissions increase with the increase of spark advance. At the same conditions of 1200 rpm engine speed, the temperature and excess air ratio are the crucial factors influencing the NOx emission. Under stoichiometric conditions, high hydrogen addition at 55% volume will increase the NOx emissions if the operating conditions are not changed which is due to the higher combustion velocity of hydrogen. However, under lean burn conditions, because of the wider combustion limit of hydrogen, the hydrogen addition will result in low NOx emission. Also, because of the high combustion velocity of hydrogen, the effect of the advanced ignition timing on CH4 emissions becomes quite weak when the hydrogen content increases. The positive impact on reducing the CH4 emission is clearly seen in the figure, which can be explained by two main reasons. On one hand, the larger amounts of hydrogen that are into CNG, the less content of hydrocarbon in the fuel and the content of unburned hydrocarbon will be reduced. On the other hand, the combustion characteristics of hydrogen will improve the combustion.

4.

Conclusion

The study presented is an experimental work aimed at investigating the effect of high hydrogen addition at a volumetric ratio of 55% on performance and emission characteristics in a turbocharged lean burn natural gas engine. The experiment was conducted under various operating conditions including different spark timings, excess air ratios (lambda) and manifold pressures. The following main conclusions were drawn from this study: 1. High Hydrogen addition can reduce COVimep at all ignition timings by retarding the ignition timings from the MBT

spark timings. Due to hydrogen’s broader burn limit and its’ fast burn speed, the addition of 55% hydrogen extended lean limit to a lambda of 2.5, compared to a lambda of 1.71 for pure NG, 2.09 for 30% HCNG. 2. 55% high hydrogen addition could simultaneously shorten flame development duration and flame propagation duration. Furthermore, it could also decrease the CCV in the duration of flame propagation, all of which are beneficial to keep down combustion instability. CCV in combustion phasing could be improved by hydrogen addition which could help to improve the effectiveness of ignition timing optimization. 3. After optimizing spark timing to MBT, engine efficiency rose with 55% hydrogen fraction. NOx emission for fuel blends with different hydrogen fraction showed little difference at this MBT spark timing. The CO and CH4 emissions are remarkable reduced, especially at the range of excess air ratio above 1.7. 4. With increasing manifold absolute pressures, the power, indicated thermal efficiency and NOx emission increase. The flame development duration and flame propagation duration and COVimep, CO emission decrease contrarily. Besides, at the range of maximal indicated thermal efficiency, COVimep arrive at their minimum value.

Acknowledgments This study was supported by the National 863 Project for new energy vehicle (2006AA11A1B7). The authors want to acknowledge all the teachers and students in the research group for their help with the experiment and their great advice for the preparation of the manuscript.

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