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Experimental study on factors affecting lean combustion limit of S.I engine fueled with compressed natural gas and hydrogen blends
Energy 38 (2012) 58e65
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Experimental study on factors affecting lean combustion limit of S.I engine fueled with compressed natural gas and hydrogen blends Xin Wang a, b, Hongguang Zhang a, *, Baofeng Yao a, c, Yan Lei a, Xiaona Sun a, Daojing Wang b, Yunshan Ge b a
College of Environmental and Energy Engineering, Beijing University of Technology, Pingleyuan No. 100, 100124 Beijing, China College of Mechanical and Vehicle Engineering, Beijing Institute of Technology, Zhongguancun South Street No. 5, Beijing, China c School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Shangyuancun No. 3, 100044 Beijing, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 March 2011 Received in revised form 25 November 2011 Accepted 28 December 2011 Available online 21 January 2012
In order to study factors affecting lean combustion limit, an experimental study was carried out on a sparkignited engine fueled with compressed natural gas and hydrogen blends. Effects of ignition timing, hydrogen fraction, engine speed, throttle opening, coolant and oil temperature were investigated. Experiments were conducted at a low and moderate level of engine load and speed with a wide range of hydrogen fraction, varying from 0 to 40 percent by volume. The results indicated that lean combustion limit could be obviously extended by adding hydrogen into compressed natural gas. In addition, leaner but more stable combustion can be acquired under higher throttle opening or lower engine speed conditions. Ignition timing was studied separately, a series of timings were adjusted around lean combustion limit under various conditions. Experiments also demonstrated that both over-advanced and over-retarded ignition could lead to a reduction in lean operating area of the engine. Further experiments on coolant and lubricant oil pointed out that lean combustion limit was positively correlated with coolant temperature; while an oil temperature increase corresponded to an initial decrease followed by an increase in lean combustion limit. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Lean combustion limit Compressed natural gas Hydrogen
1. Introduction Nowadays, energy supply and air pollution have posed serious challenges for us all. Searching for alternatives to traditional fossil fuels should be prioritized. Hydrogen, with its fantastic thermal efficiency and non-carbon emission [1], is an ideal, potential alternative fuel to internal combustion engines. However, it is difficult to acquire and store in a sufficient amount at an affordable price. In view of this, majority of current use of hydrogen has still been limited to being additives in other fuels [2]. Natural gas, a common and easy-to-get source, is another option. But its low flame speed and high combustion temperature makes it suffer from reduced thermal efficiency and increased NOx emissions [3]. According to previous studies [4e6], hydrogen enrichment is beneficial to promoting thermal efficiency. But incylinder temperature is difficult to lower due to the even higher combustion temperature of the mixture with hydrogen enrichment. So thermal efficiency is increased at the cost of higher NOx Abbreviations: MBT, maximum brake torque; HCNG, compressed natural gas and hydrogen blends; dP/d4, rate of pressure rise; BTDC, before top dead center; CA, crank angle. * Corresponding author. Tel.: þ86 10 6739 2469; fax: þ86 10 6739 2774. E-mail address: [email protected] (H. Zhang). 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.12.042
emissions; making it necessary to combine the hydrogen enrichment with lean combustion. Lean combustion, by which lower combustion temperature can be achieved, has been proved as an important and effective way to control NOx emissions [4e7], in particular spark-ignited ones [8]. Presently, to reduce emission and meet legislations, methane (including natural gas and landfills) powered engines always associates with retarded ignition, enlarged EGR rate and after treatments, hence relative clean emission is at the cost of engine dynamic characteristics and fuel economy. This situation could be got rid of by achieving lean combustion with hydrogen enrichment. With optimization of excess air ratio and ignition timing, HCNG could reduce NOx as much as 80 percent [9]. And the result of reference [7] indicated that emission legislation could be fulfilled by fueling with landfill gas and hydrogen blend. As demonstrated in Eq. (1), for a certain compression ratio, specific heat ratio is negatively correlated with mixture intensity. Hence leaner mixture owns higher thermal efficiency [10].
h ¼ 1
k1 1 r
where, r represents for compression ratio of the engine; k represents for specific heat ratio of the mixture.
(1)
X. Wang et al. / Energy 38 (2012) 58e65
Lean combustion, useful as it is, has its limit (Lean combustion limit, also called lean operation limit. The author of Ref. [11] recommended to use stable operating limit). Over-lean mixture may lead to over-frequent misfire and engine instability [8]. Therefore, lean combustion limit becomes a significant parameter indicating the capability of engine lean operation. The desire for better fuel economy and cleaner emission has urged the researchers to conduct series of experimental and theoretical studies on bench and combustion chamber [6,7,12e15]. All these studies have yielded the positive impact of hydrogen enrichment on lean combustion limit extension. In addition, lean combustion limit can be further extended by appropriate settings of engine operating parameters. Ma et al. performed an experimental study on a turbocharged S.I engine fueled with HCNG [16]. The study indicated that both advancing ignition timing in the appropriate range and increasing load level could enlarge lean combustion limit. Besides, increasing engine speed could extend operating area at a low load level while it was reversed at a high load level. Ref. [17] declaimed that lean combustion limit could be extended by increasing compression ratio. Besides, higher intake temperature promoted lean combustion performance, while higher humidity of intake did not. Majority of engine operating parameters have been thoroughly investigated, studies on coolant and lubricant oil temperature have attracted little attention. Coolant temperature can influence the heat transfer between cylinder wall and flame front as well as quenching distance. And oil temperature has correlations with different regimes of lubricated friction. Therefore, it is necessary to conduct experiments to examine their influences. Moreover, lots of researchers have adopted constant ignition timing condition in their research. This method provides convenience to experiment designing and conducting. However, it to some extent ignores the effect of inappropriate ignition timing on combustion. Refs. [18,19] has investigated the influences of ignition timing on engine performance. Hence, to better understand the impacts of each parameter, lean combustion limit timing (L.L.T) was introduced, which involves the influence of ignition timing. Several methodologies to describe lean combustion limit have been founded ever since it was first put forward in the 1970s, as it is always observed accompanied with by phenomena such as excess torque fluctuation, cyclic variation and hydrocarbon emissions [11]. From the torque fluctuation point of view, Kuroda et al. stated that lean combustion limit was where the COVimep (coefficient of variation of imep) was 10 percent approximately [20]. Eq. (2) gives out the calculation of COVimep.
COVimep ¼
simep imep
100
Excess air ratio is equal to a proportion of real A/F to that of stoichiometric. Stoichiometric A/F was derived from chemical Eq. (3).
CH4 þ aH2 þ ð2 þ 0:5aÞðO2 þ 3:76N2 Þ/CO2 þ ð2 þ aÞH2 O þ ð7:52 þ 1:88aÞN2 (3) where, a represents for hydrogen fraction in the blend by volume. Some researchers also introduced optimal ignition timing in their studies, the most common regime of which is MBT timing. Although by using optimal ignition experiment workload increases sharply, it is still recognized and adopted by some researchers [16,26]. The purpose of using optimal ignition condition (including MBT) was to accomplish objective and comparable combustion under each condition. Generally, engine operating parameters differ from one condition to another. For this reason, adopting a constant ignition timing condition optimizes only a few operating conditions. That’s to say, relatively perfect combustion is guaranteed under conditions matching the given timing. However, combustion under the other conditions has not exerted its best performance. As shown in Fig. 1a, engine speed was kept at 2000 r/min and throttle opening was a constant of 15 percent, dP/d4 under three excess air ratio (lambda, or l in the figures) conditions were illustrated. Ignition timing was set to 53 CA BTDC constantly, which was favorable for the mixture with lambda 1.4. Due to the fact that this timing was over-advanced for the mixture with lambda 1.2, excessively early
(2)
where, imep represents for indicated mean effective pressure; simep represents for root-mean-square error of imep. Besides, lean combustion limit was defined from other perspectives as well. Kubesh et al. has demonstrated that lean combustion limit could be effectively detected by measuring hydrocarbon emissions [21]. Tokuta et al. defined lean combustion limit by using cylinder-to-cylinder variation [22]. Wang et al. [23] and Yao et al. [24] have conducted experimental studies to validate the methods of COVimep and hydrocarbon emissions, respectively. The results proved that both methods were reliable. This paper adopted the Kuroda’s definition. However, Ref. [20] did not declare the minimum sufficient cycle number when calculating COVimep. Ref. [25] performed experiments to validate the minimum sufficient number and stated that it was 50. Hence, in this present paper, lean combustion limit was defined as the excess air ratio when COVimep reaches 10 percent for continuous 200 cycles.
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Fig. 1. Comparison between constant and optimal ignition timings.
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and shrunken peak dP/d4 value was obtained. In a similar way, for the mixture with lambda 1.5, ignition setting was over-delayed; thereby peak dP/d4 was shrunken as well, but postponed. Hence experiment results have confirmed that making comparison under constant ignition timing was not objective enough. Comparing to constant ignition timing, optimal ignition timing condition is more plausible; due to the fact that ignition timing is set appropriately, regardless of MBT or L.L.T adopted in this paper. In this way, in-cylinder combustion was ensured fabulous under each operating condition. Fig. 1b illustrated the dP/d4 curves of five excess air ratios under optimal ignition timing condition. Engine speed, throttle opening and fuel component settings were exactly the same as that in Fig. 1a. But ignition timing was optimized according to their lambda. It is clear that the peak dP/d4 timings are almost consentaneous, which differs from that in Fig. 1a greatly. Furthermore, when the engine was fueled with mixture of same lambda, using optimized ignition made the peak dP/d4 values in Fig. 1b obviously higher than that in Fig. 1a. This phenomenon proved the negative impacts of inappropriate ignition timing on in-cylinder combustion. 2. Experiment setup Fig. 2 gives out a sketch map of experiment system, which consists of an internal combustion engine, a dynamometer, a piezoelectric crystal sensor, an optical encoder apparatus, a lambda analyzer and a computer. The internal combustion engine type JL465Q5 used in this study was manufactured by Chongqing Changan Co. Ltd. Table 1 lists a brief specification of the prototype engine. The fuel classification of this experimental engine was modified from gasoline to natural gas. To adapt to the new fuel, a couple of gaseous fuel injectors were fixed behind the throttle body. By redeveloping and standardizing of ECU, real-time communication between ECU and host computer was realized. At the same time, adjustment of ignition timing, fuel injection pulse width could be achieved. Thus, excess air ratio became controllable. In this paper, the engine was operated under open-looped conditions. Eddy current dynamometer subsystem produced by Xiangyi Co. Ltd, which mainly contains dynamometer type GW160, throttle excitation unit FC2020, multifunctional control panel FC2030, etc.
Fig. 2. Sketch map of experiment system.
Table 1 Specifications of engine type JL465Q5. Item
Value
Engine type
In-line four cylinders, spark ignition, electronic single-point injection 1012 8.8 65.5 74 39 78
Displacement volume (mL) Compression ratio Bore (mm) Stroke (mm) Maximum power (kW) Maximum torque (Nm)
Controlling precision of dynamometer is 1 r/min and the average error of throttle excitation does not exceed 0.1 percent at full openness. Kistler 6117BFD piezoelectric crystal sensor was used to collect in-cylinder pressure data and then calculated imep and COVimep. Function of spark plug is assembled on this sensor, making it convenient to fix in the present plug hole on the engine. Before calculated by the combustion analyzing software, output of the sensor needs getting through an amplifier type Kistler 5011B, as it is in the form of micro electric charge. A crank angle datum is indispensable in calculation of imep. This signal is provided by the optical encoder ALF-A fixed at the front of crankshaft with flexible couplings. The precision of this device is 0.2 CA. The function of lambda analyzer is to observe the real-time excess air ratio. The device introduced in this study was produced by HORIBA, type MEXA-700l. A wide range oxygen sensor was fixed at about 10 cm away from the exhaust manifold exit. The sensor can detect emission components and then calculate and show the lambda value. As to software, the communication between ECU and host computer was realized by using self-developed software based on Microsoft Visual Basic via RS-232, whose function was transferring control signals from computer to ECU and providing indirect support for real-time combustion analysis. Real-time combustion analysis software was based on National Instruments LabView. Apparatus of fuel supply subsystem was shown as Fig. 3. The function of this subsystem was to deliver homogenous compressed natural gas and hydrogen blend to the engine. However, there is large density difference between natural gas and hydrogen, which may cause stratification during blending progress. So during the experiments, the blends were premixed and then consumed directly. In this way, negative influence of heterogeneity can be avoided as much as possible. Compressed natural gas and hydrogen were stored in the special steel tanks respectively. A pressure regulator was set at the exit of each tank; this installation could realize controllable exit pressure. In order to guarantee high air tightness, the tanks were connected by stainless pipes. Hydrogen fraction in the blends was concocted by using Dalton’s law of partial pressure. The law states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressure of each individual component in a gas mixture. By this manner, a high accuracy manometer was adopted to indicate the partial pressure. The average error of this manometer is less than 0.4 percent. During blending process, main switch valve and exhaust switch valve were switched off. The blending process was conducted very slowly, in order to guarantee high accuracy of blend components. When natural gas switch valve was open while hydrogen switch valve was shut up, the manometer screened the partial pressure of natural gas in the blend, and vice versa. In this way, pressure control of each component could be done. During the experiments, valves of both fuels were closed and main switch
X. Wang et al. / Energy 38 (2012) 58e65
Fig. 3. Sketch map of fuel supply subsystem.
valve was open. Opening the valve of blend tank could provide fuel for the engine. At that time, manometer was measuring the exit pressure of blend tank. Usually, the initial pressure of blend tank is approximately 10 MPa, and the minimum pressure is limited by 0.5 MPa to ensure that another secondary pressure regulator set before the injector works normally. In order to make sure the safety of this subsystem, a flame arrestor was introduced to prevent fire flowing back to the flammable fuel tanks, which was integrated with secondary pressure regulator. Leak check before experiments were dispensable and good ventilation must be guaranteed during the experiments. Commercial compressed natural gas in Beijing (main component is methane, occupied approximately 91 percent by volume) and industrial hydrogen (purity is 99 percent) were used in this present paper.
especially under ultra lean or with enormous ignition advance conditions, there was quite huge difference in the results by using these two ignition settings. Therefore, in order to better study lean combustion limit, L.L.T was introduced in this paper instead of MBT. In the experiments, the dynamometer was adjusted to constant engine speed and throttle opening mode. A great number of previous experiments were designed to figure out the L.L.T under each operating condition, which has made later experiments much more convenient. Flow chart of searching L.L.T illustrated in Fig. 4 Fig. 4 drafted a searching process of L.L.T. The initial value of ignition timing was set in accordance of enormous former experiment results and retarded to some extent. This configuration made sure that COVimep exceeded 10 percent at the very beginning of the searching process. When adjusting ignition timing, a judgment of COVimep varying trend was indispensable. Trend judgment was to observe whether an ignition timing adjustment caused a consequent increase or decrease in COVimep (for at least 3 times empirically). Experiments could be continued only after a right trend was ensured. A right trend represented for that the adjustment of ignition timing has made COVimep approach 10 percent, gradually. If the trend violated this principle, ignition timing was retarded by 15 CA (this value was selected empirically), and then the experiments could go on. At last, the ignition timing that made the lean combustion limit maximum was selected and named L.L.T under this operating condition.
3. Experiment procedure In this paper, ignition timing, hydrogen fraction, engine speed, throttle opening, coolant and lubricant oil temperature were investigated. For each factor, a series of operating conditions were chosen, the parameter settings of experiments were given in Table 2. In Table 2, L.L.T demonstrates the ignition timing when maximum lean combustion limit is acquired under a certain fuel component, engine speed and throttle opening condition, which is a type of optimal ignition timing condition. The difference between MBT timing and L.L.T is that the former focused on torque while the latter paid more attention to lean combustion limit. According to previous experiment results on this specific engine, L.L.T is in accordance with MBT for most times. However, under some special conditions,
Table 2 Operating parameters in this work. Factors
Value
Ignition timing
Overall from 32 to 72 CA BTDC, for each operating condition 20 CA around L.L.T approximately 0, 10, 20, 30, 40
Hydrogen fraction (percent by volume) Engine speed (r/min) Throttle opening (%) Oil temperature ( C) Coolant temperature ( C)
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1500, 2000, 2500, 3000 10, 20, 30, 40 75, 80, 85, 90, 95 65, 70, 75, 80, 85 Fig. 4. Flow chart of searching L.L.T under various operating conditions.
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4. Experiment result and analysis
4.2. Effect of hydrogen fraction on lean combustion limit
4.1. Effect of ignition timing on lean combustion limit
Fig. 6 shows the effect of hydrogen enrichment of different fraction by volume, from 0 to 40 percent. The diagram demonstrated that lean combustion limit of the engine could be obviously extended by hydrogen enrichment. As expected, an extension of lean combustion limit was corresponded to higher hydrogen fraction in the blend, which could be explained by analysis of properties of hydrogen. Some fundamental physical and chemical properties of hydrogen and methane are compared in Table 3 [16]. First, ignition energy of hydrogen is only one fifteenth of that of methane in NTP air. With hydrogen enrichment, initiation of flame kernel became easier, shortening flame development duration and stabling the flame kernel itself. Second, blending hydrogen cut down combustion duration as the laminar flame speed of hydrogen is approximately eight times higher than that of methane, as to the proportion of turbulent flame speed is even higher. Higher flame speed has been found to enhance the rate of flame kernel growth under a certain engine speed condition [27]. The shorter quenching distance of hydrogen is another perspective that benefits the extension of lean combustion limit. It increases in-cylinder spreading volume of the flame, thus making the combustion more complete. Furthermore, hydrogen, which can be ignited in a much wider range, facilitates lean combustion limit. Similar with natural gas, hydrogen has very high self-ignition temperature, which makes the blend more capably restrain knock trend, especially under large ignition advance conditions. From the view of chemical kinetics, introducing hydrogen has two advantages. On one side, the quantity of hydroxyl radical was increased [16]. On the other side, higher adiabatic temperature of hydrogen assisted in higher combustion temperature. Both of these impacts have boosted in-cylinder combustion chemical reactions, thereby promoting stability of the engine. In sum, ascendant combustion stability of hydrogen contributes to achieving leaner but more stable combustion of the alternative, which coincides with reference [28].
Fig. 5 illustrated ignition timing’s effect on lean combustion limit under three different conditions. In this diagram, lean combustion limit first increased with the advance of ignition timing then decreased. This phenomenon can be explained from a couple of views. First of all, over-retarded ignition timing, which has strengthened post-combustion phasing, resulted in an increased amount of fuel combusted in expansion stroke instead of late stage of compression stroke. Thus, combustion-cylinder volume phasing got worsened, leading to an increase of engine instability and a reduction of dynamic characteristics. This behavior had reflected on severer torque fluctuation, larger COVimep and then shrunken lean combustion limit. In addition, loss of heat transfer increased by using over-retarded ignition timing, due to over-strengthened post-combustion. More energy was taken away by exhaust, which weakened combustion stability, thereby hazarded the extension of lean combustion limit [16]. On the other hand, over-advanced ignition would encounter problem of inadequate compression before ignition. Then, the initial temperature and pressure were relatively low when spark plug gap was broken through. Such an adverse initial in-cylinder circumstance would result in low imep directly, a condition not beneficial to enhancing the operating stability and dynamic characteristics of the engine. Low temperature would also harass the initiation of flame kernel, thereby lengthening the flame development duration. Hardness on initiation of flame kernel, which could easily lead to the appearance of over-frequent misfire, had a very negative impact on engine operating stability. This behavior could be even more serious under ultra lean conditions. Excessively early appearance of peak in-cylinder pressure was another abnormal phenomenon caused by over-advanced ignition, making the compression work and possibility of knock rise concurrently. Knock would produce fatal influence on stable operation of the engine, evidently. Above all, the effect of ignition timing on lean combustion limit can be summarized as that neither over-retarded nor overadvanced ignition timing is favorable for lean combustion limit extension.
Fig. 5. Effect of ignition timing on lean combustion limit.
4.3. Effect of engine speed on lean combustion limit Fig. 7 gives out the curves that demonstrated the correlation between engine speed and lean combustion limit. As illustrated,
Fig. 6. Effect of hydrogen fraction on lean combustion limit.
X. Wang et al. / Energy 38 (2012) 58e65 Table 3 Properties of hydrogen and methane in NTP air. Property
Hydrogen
Methane
Density (kg/m3) Specific heat ratio Minimum ignition energy (mJ) Laminar burning speed (cm/s) Quenching distance (cm) Flammable limit by volume (%) Adiabatic flame temperature (K)
0.084 1.412 0.02 265e325 0.064 4e72 2318
0.651 1.315 0.29 37e45 0.203 5e15 2148
a decrease of engine speed was followed by a positive response of lean combustion limit. During the experiments, engine speed was consequently adjusted while throttle opening remained the initial value. Thus, load level of the engine was raised when engine speed was reduced. Higher load was beneficial to stable operation of the engine [29]. Besides, the higher the engine speed, the stronger the overall turbulence intensity. Under lean combustion limit condition, flame kernel was quite weak and easy to be blown out. Over-strong turbulence may increase the possibility of this behavior, thereby hazarding combustion stability. As a result, lean combustion limit had a negative correlation with engine speed [27]. It is worth noting that, combustion duration toward 1 CA is almost independent of engine speed under lean combustion limit conditions [8]. In fact, when the engine speed decreased, the absolute time elapsed toward 1 CA was lengthened. More sufficient time means that combustion could be more complete under lower engine speed conditions. As a result, flame speed was not depended on as much. In another word, although flame speed was slowed down by fueling with leaner mixture, there was enough time for the flame to propagate throughout the cylinders. Therefore, leaner mixture can be still applied to the engine. The increase of engine speed also results in an acceleration of intake air flow velocity in the manifold. Given that head loss is correlated with intake velocity’s square; the higher the engine speed, the severer the head loss in intake system. For this reason, insufficient charge under high speed circumstances brought about instable possibility. Besides, low pressure at intake port caused by head loss has led to low exhaust pressure P4. This behavior
Fig. 7. Effect of engine speed on lean combustion limit.
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endangered blow-down process and increased coefficient of residual gas fres. Eq. (4) lists the calculation of coefficient of residual gas.
fres
sffiffiffiffiffiffiffiffiffiffiffiffi 1 k Pe ¼ $ P4 r
(4)
Where, Pe represents for the exhaust back pressure; P4 represents for the in-cylinder pressure when exhaust valves open. Both low exhaust pressure and inadequate scavenge by fresh charge caused severe accumulated residual gas in-cylinder, especially under high speed conditions. Dilution by residual gas has made the mixture leaner and combust less stable. This impact hazarded further extension of operating area. 4.4. Effect of throttle opening on lean combustion limit Fig. 8 shows the correlation between throttle opening and lean combustion limit. As expected, larger throttle opening was beneficial to achieving leaner combustion, due to the fact that enlarging throttle opening improves intake motion characteristics. The influences of throttle opening are discussed in several aspects: load level of the engine, pump losing, volumetric efficiency and residual gas. Contrary to the study of engine speed, the experiments of this section were conducted with continuous adjustment of throttle opening under constant engine speed condition. Load level of the engine increased with enlargement of throttle opening, extending operating area. The reasons have been stated in the former section. As to the influence of pump losing, generally speaking, increasing openness of throttle can reduce head loss in intake system. This reduction comes from two perspectives. One is that at throttle body position, adding openness can decrease head loss for valve, evidently. The other is head loss. Throttle plays a role of a variable pressure ratio nozzle. By adding openness, the area after throttle valve was enlarged, making the proportion of throttle entrance area to exit area decreased. This change slowed down the velocity of air flow in manifold, and helped rising intake absolute pressure. Besides, higher volumetric efficiency of the engine could be achieved by introducing larger throttle opening, rendering the precombustion duration and promoting lean combustion capability. Moreover, with more sufficient fresh charge and stronger
Fig. 8. Effect of throttle opening on lean combustion limit.
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scavenging, residual gas was reduced, a positive effort benefiting lean combustion as well. 4.5. Effect of coolant temperature on lean combustion limit As demonstrated in Fig. 9, lean combustion limit was positively related to coolant temperature under the given engine operating condition. Coolant temperature has a strong correlation with cylinder wall temperature. The higher the former, the warmer the latter. High cylinder wall temperature not only facilitated blending of the mixture, but also shortened quenching distance. Besides, warmer cylinder wall provided more appropriate circumstance for the initiation of flame kernel, which was regarded as an essential way to reduce over-frequent misfire. Homogenous mixture reduced cyclic variation effectively, thereby promoting operating stability of the engine [27]. Shortened quenching distance benefited flame propagation and constant volume combustion, which enhanced combustion stability of the engine from another aspect. Also, due to the fact that temperature difference between flame front and cylinder wall was lessened by warming up coolant, quantity of heat transfer was reduced to some extent. This is helpful to extend lean combustion limit, but the effort is relatively slight. 4.6. Effect of oil temperature on lean combustion limit Fig. 10 illustrates the correlation between lean combustion limit and lubricant oil temperature. The diagram indicates that an oil temperature increase corresponded to an initial decrease followed by an increase of lean combustion limit. The influences of heating lubricant oil have two opposite aspects, one is on lubrication, and the other is on thermal load. Warming up has reduced viscosity and improved liquidity of the oil, which enhanced lubrication and decreased friction within moving parts, namely, crankshaft and camshaft bearings, piston rings and cylinder wall, etc. Thus, the engine could be driven by consuming less energy. This behavior allowed the engine in fueling with leaner mixture, thereby extending the operating area. However, with the rise of oil temperature, thermal load within the moving parts has increased concurrently, which led to expansion of these parts and higher friction. The combined effort of these two aspects has probably resulted in the curve in this figure. For the specific engine adopted in this present paper, the influence of thermal load was stronger than
Fig. 9. Effect of coolant temperature on lean combustion limit.
Fig. 10. Effect of oil temperature on lean combustion limit.
that of lubrication, when the oil temperature was lower than 80 C, so lean combustion limit first decreased. When at 80 C, these two influences were balanced and made this temperature a turning point. Then when oil temperature was over 80 C, the influence of reduced viscosity and better liquidity exceeded that of thermal load, lean combustion limit was extended by higher oil temperature. Another possible explanation to this phenomenon is to consider the variation of coefficient of friction f. As stated in reference [11], f has a correlation with expression mN/s. In the expression, m represents for dynamic viscosity of lubricant oil; N represents for rotatory speed of engine crankshaft; s represents for normal force within moving parts per area. Given that the experiments were conducted with unchanged engine speed, N was a constant, actually. m becomes smaller when oil temperature rises. As to s, empirically, a tiny increase is corresponded to an increase of temperature. In sum, expression mN/s shrinks under higher oil temperature conditions. When oil temperature was lower than 80 C, the friction status was in mixed lubrication as described in Fig. 11. In this field, a higher oil temperature correlates a larger f, making lean combustion limit shrink with warming up of oil. On the contrary, when oil
Fig. 11. Coefficient of fraction f versus expression mN/s. [9]
X. Wang et al. / Energy 38 (2012) 58e65
temperature was more than 80 C, friction status entered hydrodynamic lubrication field, where f has a positive correlation with oil temperature. 5. Conclusion In this present paper, six operating parameters and their effects on lean combustion limit of an engine fueled with compressed natural gas and hydrogen blend were examined and discussed. Lean combustion limit timing, a type of optimal ignition timing was adopted to search lean combustion limit during the experiments. Conclusion was drawn as follows. 1. Both over-retarded and over-advanced ignition timing are not beneficial to accomplishing stable combustion at lean combustion limit under a certain operating condition. 2. It is obvious that lean combustion limit of the engine was extended with hydrogen enrichment; lean combustion limit of the engine increased with hydrogen fraction in the blend. 3. With a reduction of engine speed, lean combustion limit of the engine could be extended with fixed throttle opening. 4. Lean combustion limit increased with throttle opening when engine speed remained a constant. 5. Lean combustion limit had a positive correlation with coolant temperature under a certain operating condition. 6. A lubricant oil temperature increase corresponded to an initial decrease followed by an increase in lean combustion limit under a certain operating condition. Among these factors, hydrogen fraction is the most significant one. This result has indicated that optimizing fuel components and properties making it more appropriate for lean combustion is still and will be always the most effective way in extension of lean combustion limit. The conclusions on engine speed and throttle opening demonstrates that more attention needs paying to further extension of lean combustion limit under high engine speed or low throttle opening conditions. Temperature factors, both coolant and lubricant oil, do have impact on lean combustion limit, comparatively tiny but indispensable. Further study on detailed mechanism of lubricant oil temperature’s effects is needed in the future. Acknowledgments This work was sponsored by “Xing Huo Fund” for undergraduate students in Beijing University of Technology (Grant No. XH-201005-01) and “Innovation Foundation” for excellent doctor students in Beijing Jiaotong University (Grant No. 141050522). References [1] Rakopoulos CD, Scott MA, Kyritsis DC, Giakoumis EG. Availability analysis of hydrogen/natural gas blends combustion in internal combustion engines. Energy 2008;33:248e55. [2] Bauer CG, Forest TW. Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: effect on S.I. engine performance. International Journal of Hydrogen Energy 2001;26:55e70. [3] Jahirui MI, Masjuki HH, Saidur R, Kalam MA, Jayed MH, Wazed MA. Comparative engine performance and emission analysis of CNG and gasoline in a retrofitted car engine. Applied Thermal Engineering 2010;30:2219e26. [4] Huang Zuohua, Liu Bin, Zeng Ke, Huang Yinyu, Jiang Deming, Wang Xibin, et al. Experimental study on engine performance and emissions for an engine fuelled with natural gas-hydrogen mixtures. Energy & Fuels 2006;20:2131e6.
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[5] Ma Fanhua, Wang Yu, Liu Haiquan, Yong Li, Wang Junjun, Zhao Shuli. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. International Journal of Hydrogen Energy 2007;32:5067e75. [6] Bysveen Marie. Engine characteristics of emissions and performance using mixture of natural gas and hydrogen. Energy 2007;32:482e9. [7] Kornbluth Kurt, Greenwood Jason, McCaffary Zach, Vernon David, Erickson Paul. Extension of the lean limit through hydrogen enrichment of a LFG-fuelled spark-ignition engine and emissions reduction. International Journal of Hydrogen Energy 2010;35:1412e9. [8] Quader Ather A. Lean combustion and misfire limit in spark ignition engines. SAE Papers 1974;741055. [9] Park Cheolwoong, Kim Changgi, Choi Young, Won Sangyeon, Moriyoshi Yasuo. The influence of hydrogen on the performance and emission characteristics of a heavy duty natural gas engine. International Journal of Hydrogen Energy 2011;36:3739e45. [10] Zhan Zhangsong. A theoretical analysis and experimental research on lean and fast burn of spark ignited engines. Transactions of the Chinese Society for Agricultural Machinery 2000;31(4):10e3 (In Chinese). [11] Heywood John B. Internal combustion engine fundamentals. New York: McGraw-Hill; 1988. [12] Jiang Lei, Li Xinghu, Wang Yu, Zhang Jichun, Liu Haiquan, Li Yong. An experimental study on lean burn limits and emission characteristics of an engine fuelled with CNG/hydrogen blends. Automotive Engineering 2008;30: 197e201 (In Chinese). [13] Wang Yu, Ma Fanhua, Liu Haiquan, Li Yong. Influence factors of the lean operation limit for a spark ignition hydrogen enriched natural gas engine. Transactions of the Chinese Society for Argricaltural Machinery 2008;39:9e13 (In Chinese). [14] Tunestal Per, Christensen Magnus, Einewall Patrik, Andersson Tobias, Johansson Bengt. Hydrogen addition for improved lean burn capability of slow and fast burning natural gas combustion chambers. SAE papers, 200201-2686; 2002. [15] Guo Hongsheng, Smallwood Gregory J, Liu Fengshan, Ju Yiguang, Gülder Ömer L. The effect of hydrogen addition on flammability limit and NOx emission in ultra-lean counterflow CH4/air premixed flames. Proceedings of the Combustion Institute 2005;30:303e11. [16] Ma Fanhua, Wang Yu. Study on the extension of lean operation limit through hydrogen enrichment in a natural gas spark-ignition engine. International Journal of Hydrogen Energy 2008;33:1416e24. [17] Wang Du, Mao Xiaojian, Sheng Li, Zhuo Bin. Extension of operating range for lean-burn turbocharged SI natural gas engine. Transaction of CSICE 2009;27: 511e7 (In Chinese). [18] Ma Fanhua, Liu Haiquan, Wang Yu, Li Yong, Wang Junjun, Zhao Shuli. Combustion and emission characteristics of a port-injection HCNG engine under various ignition timings. International Journal of Hydrogen Energy 2008;33:816e22. [19] Huang Zuohua, Wang Jinhua, Liu Bing, Zeng Ke, Yu Yinrong, Jiang Deming. Combustion characteristics of a direct-injection engine fuelled with natural gas-hydrogen blends under different ignition timings. Fuel 2007;86:381e7. [20] Kuroda H, Nakajima Y, Sugihara K, Takagi Y, Muranaka S. The fast burn with heavy EGR, new approach for low NOx and improved fuel economy. SAE Papers 1978;780006. [21] Kubesh John T, Podnar Daniel J, Colucci Christopher P. Lean limit and performance improvements for a heavy-duty natural gas engine. SAE Papers 1996;961939. [22] Inoue Tokuta, Matsushita Souichi, Nakanishi Kiyoshi, Okano Hiroshi. Toyota lean combustion system e the third generation system. SAE Papers 1993; 930873. [23] Wang Zhen-suo, Li Li-guang, Xiao Zong-cheng, Wang Huiping, Deng Baoqing, Su Yan. Study on the criterions of lean burn limit for an LPG EFI engine. Transactions of CSICE 2005;23:524e31 (In Chiense). [24] Yao Bao-feng, Liu Jian-hua, Yu Yu-song, Guo Wen-fang. Experimental study on lean combustion limit of natural gas engine. Beijing Automotive Engineering 2007;(6):30e3 (In Chinese). _ Determination of lu Bülent, Kaya Ferhat, Vokan Öner I. [25] Akif Ceviz M, Çavus¸og cycle number for real in-cylinder pressure cycle analysis in internal combustion engines. Energy 2011;36:2465e72. [26] Jiang Deming, Huang Zuohua. Combustion of alternative in internal combustion engine. Xi’an Jiangtong University Press; 2007 (In Chinese). [27] Ozdor Nir, Dulger Mark, Sher Eran. Cyclic variability in spark ignition engines a literature survey. SAE Papers 1994;940987. [28] El-Ghafour SAA, El-dein AHE, Aref AAR. Combustion characteristics of natural gas-hydrogen hybrid fuel turbulent diffusion flame. International Journal of Hydrogen Energy 2010;35:2556e65. [29] Ma Fanhua, Ding Shangfen, Wang Yu, Wang Yefu, Wang Junjun, Zhao Shuli. Study on combustion behaviors and cycle-by-cycle variations in a turbocharged lean burn natural gas S.I engine with hydrogen enrichment. International Journal of Hydrogen Energy 2008;33:7245e55.
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Experimental study on factors affecting lean combustion limit of S.I engine fueled with compressed natural gas and hydrogen blends Xin Wang a, b, Hongguang Zhang a, *, Baofeng Yao a, c, Yan Lei a, Xiaona Sun a, Daojing Wang b, Yunshan Ge b a
College of Environmental and Energy Engineering, Beijing University of Technology, Pingleyuan No. 100, 100124 Beijing, China College of Mechanical and Vehicle Engineering, Beijing Institute of Technology, Zhongguancun South Street No. 5, Beijing, China c School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Shangyuancun No. 3, 100044 Beijing, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 March 2011 Received in revised form 25 November 2011 Accepted 28 December 2011 Available online 21 January 2012
In order to study factors affecting lean combustion limit, an experimental study was carried out on a sparkignited engine fueled with compressed natural gas and hydrogen blends. Effects of ignition timing, hydrogen fraction, engine speed, throttle opening, coolant and oil temperature were investigated. Experiments were conducted at a low and moderate level of engine load and speed with a wide range of hydrogen fraction, varying from 0 to 40 percent by volume. The results indicated that lean combustion limit could be obviously extended by adding hydrogen into compressed natural gas. In addition, leaner but more stable combustion can be acquired under higher throttle opening or lower engine speed conditions. Ignition timing was studied separately, a series of timings were adjusted around lean combustion limit under various conditions. Experiments also demonstrated that both over-advanced and over-retarded ignition could lead to a reduction in lean operating area of the engine. Further experiments on coolant and lubricant oil pointed out that lean combustion limit was positively correlated with coolant temperature; while an oil temperature increase corresponded to an initial decrease followed by an increase in lean combustion limit. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Lean combustion limit Compressed natural gas Hydrogen
1. Introduction Nowadays, energy supply and air pollution have posed serious challenges for us all. Searching for alternatives to traditional fossil fuels should be prioritized. Hydrogen, with its fantastic thermal efficiency and non-carbon emission [1], is an ideal, potential alternative fuel to internal combustion engines. However, it is difficult to acquire and store in a sufficient amount at an affordable price. In view of this, majority of current use of hydrogen has still been limited to being additives in other fuels [2]. Natural gas, a common and easy-to-get source, is another option. But its low flame speed and high combustion temperature makes it suffer from reduced thermal efficiency and increased NOx emissions [3]. According to previous studies [4e6], hydrogen enrichment is beneficial to promoting thermal efficiency. But incylinder temperature is difficult to lower due to the even higher combustion temperature of the mixture with hydrogen enrichment. So thermal efficiency is increased at the cost of higher NOx Abbreviations: MBT, maximum brake torque; HCNG, compressed natural gas and hydrogen blends; dP/d4, rate of pressure rise; BTDC, before top dead center; CA, crank angle. * Corresponding author. Tel.: þ86 10 6739 2469; fax: þ86 10 6739 2774. E-mail address: [email protected] (H. Zhang). 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.12.042
emissions; making it necessary to combine the hydrogen enrichment with lean combustion. Lean combustion, by which lower combustion temperature can be achieved, has been proved as an important and effective way to control NOx emissions [4e7], in particular spark-ignited ones [8]. Presently, to reduce emission and meet legislations, methane (including natural gas and landfills) powered engines always associates with retarded ignition, enlarged EGR rate and after treatments, hence relative clean emission is at the cost of engine dynamic characteristics and fuel economy. This situation could be got rid of by achieving lean combustion with hydrogen enrichment. With optimization of excess air ratio and ignition timing, HCNG could reduce NOx as much as 80 percent [9]. And the result of reference [7] indicated that emission legislation could be fulfilled by fueling with landfill gas and hydrogen blend. As demonstrated in Eq. (1), for a certain compression ratio, specific heat ratio is negatively correlated with mixture intensity. Hence leaner mixture owns higher thermal efficiency [10].
h ¼ 1
k1 1 r
where, r represents for compression ratio of the engine; k represents for specific heat ratio of the mixture.
(1)
X. Wang et al. / Energy 38 (2012) 58e65
Lean combustion, useful as it is, has its limit (Lean combustion limit, also called lean operation limit. The author of Ref. [11] recommended to use stable operating limit). Over-lean mixture may lead to over-frequent misfire and engine instability [8]. Therefore, lean combustion limit becomes a significant parameter indicating the capability of engine lean operation. The desire for better fuel economy and cleaner emission has urged the researchers to conduct series of experimental and theoretical studies on bench and combustion chamber [6,7,12e15]. All these studies have yielded the positive impact of hydrogen enrichment on lean combustion limit extension. In addition, lean combustion limit can be further extended by appropriate settings of engine operating parameters. Ma et al. performed an experimental study on a turbocharged S.I engine fueled with HCNG [16]. The study indicated that both advancing ignition timing in the appropriate range and increasing load level could enlarge lean combustion limit. Besides, increasing engine speed could extend operating area at a low load level while it was reversed at a high load level. Ref. [17] declaimed that lean combustion limit could be extended by increasing compression ratio. Besides, higher intake temperature promoted lean combustion performance, while higher humidity of intake did not. Majority of engine operating parameters have been thoroughly investigated, studies on coolant and lubricant oil temperature have attracted little attention. Coolant temperature can influence the heat transfer between cylinder wall and flame front as well as quenching distance. And oil temperature has correlations with different regimes of lubricated friction. Therefore, it is necessary to conduct experiments to examine their influences. Moreover, lots of researchers have adopted constant ignition timing condition in their research. This method provides convenience to experiment designing and conducting. However, it to some extent ignores the effect of inappropriate ignition timing on combustion. Refs. [18,19] has investigated the influences of ignition timing on engine performance. Hence, to better understand the impacts of each parameter, lean combustion limit timing (L.L.T) was introduced, which involves the influence of ignition timing. Several methodologies to describe lean combustion limit have been founded ever since it was first put forward in the 1970s, as it is always observed accompanied with by phenomena such as excess torque fluctuation, cyclic variation and hydrocarbon emissions [11]. From the torque fluctuation point of view, Kuroda et al. stated that lean combustion limit was where the COVimep (coefficient of variation of imep) was 10 percent approximately [20]. Eq. (2) gives out the calculation of COVimep.
COVimep ¼
simep imep
100
Excess air ratio is equal to a proportion of real A/F to that of stoichiometric. Stoichiometric A/F was derived from chemical Eq. (3).
CH4 þ aH2 þ ð2 þ 0:5aÞðO2 þ 3:76N2 Þ/CO2 þ ð2 þ aÞH2 O þ ð7:52 þ 1:88aÞN2 (3) where, a represents for hydrogen fraction in the blend by volume. Some researchers also introduced optimal ignition timing in their studies, the most common regime of which is MBT timing. Although by using optimal ignition experiment workload increases sharply, it is still recognized and adopted by some researchers [16,26]. The purpose of using optimal ignition condition (including MBT) was to accomplish objective and comparable combustion under each condition. Generally, engine operating parameters differ from one condition to another. For this reason, adopting a constant ignition timing condition optimizes only a few operating conditions. That’s to say, relatively perfect combustion is guaranteed under conditions matching the given timing. However, combustion under the other conditions has not exerted its best performance. As shown in Fig. 1a, engine speed was kept at 2000 r/min and throttle opening was a constant of 15 percent, dP/d4 under three excess air ratio (lambda, or l in the figures) conditions were illustrated. Ignition timing was set to 53 CA BTDC constantly, which was favorable for the mixture with lambda 1.4. Due to the fact that this timing was over-advanced for the mixture with lambda 1.2, excessively early
(2)
where, imep represents for indicated mean effective pressure; simep represents for root-mean-square error of imep. Besides, lean combustion limit was defined from other perspectives as well. Kubesh et al. has demonstrated that lean combustion limit could be effectively detected by measuring hydrocarbon emissions [21]. Tokuta et al. defined lean combustion limit by using cylinder-to-cylinder variation [22]. Wang et al. [23] and Yao et al. [24] have conducted experimental studies to validate the methods of COVimep and hydrocarbon emissions, respectively. The results proved that both methods were reliable. This paper adopted the Kuroda’s definition. However, Ref. [20] did not declare the minimum sufficient cycle number when calculating COVimep. Ref. [25] performed experiments to validate the minimum sufficient number and stated that it was 50. Hence, in this present paper, lean combustion limit was defined as the excess air ratio when COVimep reaches 10 percent for continuous 200 cycles.
59
Fig. 1. Comparison between constant and optimal ignition timings.
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X. Wang et al. / Energy 38 (2012) 58e65
and shrunken peak dP/d4 value was obtained. In a similar way, for the mixture with lambda 1.5, ignition setting was over-delayed; thereby peak dP/d4 was shrunken as well, but postponed. Hence experiment results have confirmed that making comparison under constant ignition timing was not objective enough. Comparing to constant ignition timing, optimal ignition timing condition is more plausible; due to the fact that ignition timing is set appropriately, regardless of MBT or L.L.T adopted in this paper. In this way, in-cylinder combustion was ensured fabulous under each operating condition. Fig. 1b illustrated the dP/d4 curves of five excess air ratios under optimal ignition timing condition. Engine speed, throttle opening and fuel component settings were exactly the same as that in Fig. 1a. But ignition timing was optimized according to their lambda. It is clear that the peak dP/d4 timings are almost consentaneous, which differs from that in Fig. 1a greatly. Furthermore, when the engine was fueled with mixture of same lambda, using optimized ignition made the peak dP/d4 values in Fig. 1b obviously higher than that in Fig. 1a. This phenomenon proved the negative impacts of inappropriate ignition timing on in-cylinder combustion. 2. Experiment setup Fig. 2 gives out a sketch map of experiment system, which consists of an internal combustion engine, a dynamometer, a piezoelectric crystal sensor, an optical encoder apparatus, a lambda analyzer and a computer. The internal combustion engine type JL465Q5 used in this study was manufactured by Chongqing Changan Co. Ltd. Table 1 lists a brief specification of the prototype engine. The fuel classification of this experimental engine was modified from gasoline to natural gas. To adapt to the new fuel, a couple of gaseous fuel injectors were fixed behind the throttle body. By redeveloping and standardizing of ECU, real-time communication between ECU and host computer was realized. At the same time, adjustment of ignition timing, fuel injection pulse width could be achieved. Thus, excess air ratio became controllable. In this paper, the engine was operated under open-looped conditions. Eddy current dynamometer subsystem produced by Xiangyi Co. Ltd, which mainly contains dynamometer type GW160, throttle excitation unit FC2020, multifunctional control panel FC2030, etc.
Fig. 2. Sketch map of experiment system.
Table 1 Specifications of engine type JL465Q5. Item
Value
Engine type
In-line four cylinders, spark ignition, electronic single-point injection 1012 8.8 65.5 74 39 78
Displacement volume (mL) Compression ratio Bore (mm) Stroke (mm) Maximum power (kW) Maximum torque (Nm)
Controlling precision of dynamometer is 1 r/min and the average error of throttle excitation does not exceed 0.1 percent at full openness. Kistler 6117BFD piezoelectric crystal sensor was used to collect in-cylinder pressure data and then calculated imep and COVimep. Function of spark plug is assembled on this sensor, making it convenient to fix in the present plug hole on the engine. Before calculated by the combustion analyzing software, output of the sensor needs getting through an amplifier type Kistler 5011B, as it is in the form of micro electric charge. A crank angle datum is indispensable in calculation of imep. This signal is provided by the optical encoder ALF-A fixed at the front of crankshaft with flexible couplings. The precision of this device is 0.2 CA. The function of lambda analyzer is to observe the real-time excess air ratio. The device introduced in this study was produced by HORIBA, type MEXA-700l. A wide range oxygen sensor was fixed at about 10 cm away from the exhaust manifold exit. The sensor can detect emission components and then calculate and show the lambda value. As to software, the communication between ECU and host computer was realized by using self-developed software based on Microsoft Visual Basic via RS-232, whose function was transferring control signals from computer to ECU and providing indirect support for real-time combustion analysis. Real-time combustion analysis software was based on National Instruments LabView. Apparatus of fuel supply subsystem was shown as Fig. 3. The function of this subsystem was to deliver homogenous compressed natural gas and hydrogen blend to the engine. However, there is large density difference between natural gas and hydrogen, which may cause stratification during blending progress. So during the experiments, the blends were premixed and then consumed directly. In this way, negative influence of heterogeneity can be avoided as much as possible. Compressed natural gas and hydrogen were stored in the special steel tanks respectively. A pressure regulator was set at the exit of each tank; this installation could realize controllable exit pressure. In order to guarantee high air tightness, the tanks were connected by stainless pipes. Hydrogen fraction in the blends was concocted by using Dalton’s law of partial pressure. The law states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressure of each individual component in a gas mixture. By this manner, a high accuracy manometer was adopted to indicate the partial pressure. The average error of this manometer is less than 0.4 percent. During blending process, main switch valve and exhaust switch valve were switched off. The blending process was conducted very slowly, in order to guarantee high accuracy of blend components. When natural gas switch valve was open while hydrogen switch valve was shut up, the manometer screened the partial pressure of natural gas in the blend, and vice versa. In this way, pressure control of each component could be done. During the experiments, valves of both fuels were closed and main switch
X. Wang et al. / Energy 38 (2012) 58e65
Fig. 3. Sketch map of fuel supply subsystem.
valve was open. Opening the valve of blend tank could provide fuel for the engine. At that time, manometer was measuring the exit pressure of blend tank. Usually, the initial pressure of blend tank is approximately 10 MPa, and the minimum pressure is limited by 0.5 MPa to ensure that another secondary pressure regulator set before the injector works normally. In order to make sure the safety of this subsystem, a flame arrestor was introduced to prevent fire flowing back to the flammable fuel tanks, which was integrated with secondary pressure regulator. Leak check before experiments were dispensable and good ventilation must be guaranteed during the experiments. Commercial compressed natural gas in Beijing (main component is methane, occupied approximately 91 percent by volume) and industrial hydrogen (purity is 99 percent) were used in this present paper.
especially under ultra lean or with enormous ignition advance conditions, there was quite huge difference in the results by using these two ignition settings. Therefore, in order to better study lean combustion limit, L.L.T was introduced in this paper instead of MBT. In the experiments, the dynamometer was adjusted to constant engine speed and throttle opening mode. A great number of previous experiments were designed to figure out the L.L.T under each operating condition, which has made later experiments much more convenient. Flow chart of searching L.L.T illustrated in Fig. 4 Fig. 4 drafted a searching process of L.L.T. The initial value of ignition timing was set in accordance of enormous former experiment results and retarded to some extent. This configuration made sure that COVimep exceeded 10 percent at the very beginning of the searching process. When adjusting ignition timing, a judgment of COVimep varying trend was indispensable. Trend judgment was to observe whether an ignition timing adjustment caused a consequent increase or decrease in COVimep (for at least 3 times empirically). Experiments could be continued only after a right trend was ensured. A right trend represented for that the adjustment of ignition timing has made COVimep approach 10 percent, gradually. If the trend violated this principle, ignition timing was retarded by 15 CA (this value was selected empirically), and then the experiments could go on. At last, the ignition timing that made the lean combustion limit maximum was selected and named L.L.T under this operating condition.
3. Experiment procedure In this paper, ignition timing, hydrogen fraction, engine speed, throttle opening, coolant and lubricant oil temperature were investigated. For each factor, a series of operating conditions were chosen, the parameter settings of experiments were given in Table 2. In Table 2, L.L.T demonstrates the ignition timing when maximum lean combustion limit is acquired under a certain fuel component, engine speed and throttle opening condition, which is a type of optimal ignition timing condition. The difference between MBT timing and L.L.T is that the former focused on torque while the latter paid more attention to lean combustion limit. According to previous experiment results on this specific engine, L.L.T is in accordance with MBT for most times. However, under some special conditions,
Table 2 Operating parameters in this work. Factors
Value
Ignition timing
Overall from 32 to 72 CA BTDC, for each operating condition 20 CA around L.L.T approximately 0, 10, 20, 30, 40
Hydrogen fraction (percent by volume) Engine speed (r/min) Throttle opening (%) Oil temperature ( C) Coolant temperature ( C)
61
1500, 2000, 2500, 3000 10, 20, 30, 40 75, 80, 85, 90, 95 65, 70, 75, 80, 85 Fig. 4. Flow chart of searching L.L.T under various operating conditions.
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4. Experiment result and analysis
4.2. Effect of hydrogen fraction on lean combustion limit
4.1. Effect of ignition timing on lean combustion limit
Fig. 6 shows the effect of hydrogen enrichment of different fraction by volume, from 0 to 40 percent. The diagram demonstrated that lean combustion limit of the engine could be obviously extended by hydrogen enrichment. As expected, an extension of lean combustion limit was corresponded to higher hydrogen fraction in the blend, which could be explained by analysis of properties of hydrogen. Some fundamental physical and chemical properties of hydrogen and methane are compared in Table 3 [16]. First, ignition energy of hydrogen is only one fifteenth of that of methane in NTP air. With hydrogen enrichment, initiation of flame kernel became easier, shortening flame development duration and stabling the flame kernel itself. Second, blending hydrogen cut down combustion duration as the laminar flame speed of hydrogen is approximately eight times higher than that of methane, as to the proportion of turbulent flame speed is even higher. Higher flame speed has been found to enhance the rate of flame kernel growth under a certain engine speed condition [27]. The shorter quenching distance of hydrogen is another perspective that benefits the extension of lean combustion limit. It increases in-cylinder spreading volume of the flame, thus making the combustion more complete. Furthermore, hydrogen, which can be ignited in a much wider range, facilitates lean combustion limit. Similar with natural gas, hydrogen has very high self-ignition temperature, which makes the blend more capably restrain knock trend, especially under large ignition advance conditions. From the view of chemical kinetics, introducing hydrogen has two advantages. On one side, the quantity of hydroxyl radical was increased [16]. On the other side, higher adiabatic temperature of hydrogen assisted in higher combustion temperature. Both of these impacts have boosted in-cylinder combustion chemical reactions, thereby promoting stability of the engine. In sum, ascendant combustion stability of hydrogen contributes to achieving leaner but more stable combustion of the alternative, which coincides with reference [28].
Fig. 5 illustrated ignition timing’s effect on lean combustion limit under three different conditions. In this diagram, lean combustion limit first increased with the advance of ignition timing then decreased. This phenomenon can be explained from a couple of views. First of all, over-retarded ignition timing, which has strengthened post-combustion phasing, resulted in an increased amount of fuel combusted in expansion stroke instead of late stage of compression stroke. Thus, combustion-cylinder volume phasing got worsened, leading to an increase of engine instability and a reduction of dynamic characteristics. This behavior had reflected on severer torque fluctuation, larger COVimep and then shrunken lean combustion limit. In addition, loss of heat transfer increased by using over-retarded ignition timing, due to over-strengthened post-combustion. More energy was taken away by exhaust, which weakened combustion stability, thereby hazarded the extension of lean combustion limit [16]. On the other hand, over-advanced ignition would encounter problem of inadequate compression before ignition. Then, the initial temperature and pressure were relatively low when spark plug gap was broken through. Such an adverse initial in-cylinder circumstance would result in low imep directly, a condition not beneficial to enhancing the operating stability and dynamic characteristics of the engine. Low temperature would also harass the initiation of flame kernel, thereby lengthening the flame development duration. Hardness on initiation of flame kernel, which could easily lead to the appearance of over-frequent misfire, had a very negative impact on engine operating stability. This behavior could be even more serious under ultra lean conditions. Excessively early appearance of peak in-cylinder pressure was another abnormal phenomenon caused by over-advanced ignition, making the compression work and possibility of knock rise concurrently. Knock would produce fatal influence on stable operation of the engine, evidently. Above all, the effect of ignition timing on lean combustion limit can be summarized as that neither over-retarded nor overadvanced ignition timing is favorable for lean combustion limit extension.
Fig. 5. Effect of ignition timing on lean combustion limit.
4.3. Effect of engine speed on lean combustion limit Fig. 7 gives out the curves that demonstrated the correlation between engine speed and lean combustion limit. As illustrated,
Fig. 6. Effect of hydrogen fraction on lean combustion limit.
X. Wang et al. / Energy 38 (2012) 58e65 Table 3 Properties of hydrogen and methane in NTP air. Property
Hydrogen
Methane
Density (kg/m3) Specific heat ratio Minimum ignition energy (mJ) Laminar burning speed (cm/s) Quenching distance (cm) Flammable limit by volume (%) Adiabatic flame temperature (K)
0.084 1.412 0.02 265e325 0.064 4e72 2318
0.651 1.315 0.29 37e45 0.203 5e15 2148
a decrease of engine speed was followed by a positive response of lean combustion limit. During the experiments, engine speed was consequently adjusted while throttle opening remained the initial value. Thus, load level of the engine was raised when engine speed was reduced. Higher load was beneficial to stable operation of the engine [29]. Besides, the higher the engine speed, the stronger the overall turbulence intensity. Under lean combustion limit condition, flame kernel was quite weak and easy to be blown out. Over-strong turbulence may increase the possibility of this behavior, thereby hazarding combustion stability. As a result, lean combustion limit had a negative correlation with engine speed [27]. It is worth noting that, combustion duration toward 1 CA is almost independent of engine speed under lean combustion limit conditions [8]. In fact, when the engine speed decreased, the absolute time elapsed toward 1 CA was lengthened. More sufficient time means that combustion could be more complete under lower engine speed conditions. As a result, flame speed was not depended on as much. In another word, although flame speed was slowed down by fueling with leaner mixture, there was enough time for the flame to propagate throughout the cylinders. Therefore, leaner mixture can be still applied to the engine. The increase of engine speed also results in an acceleration of intake air flow velocity in the manifold. Given that head loss is correlated with intake velocity’s square; the higher the engine speed, the severer the head loss in intake system. For this reason, insufficient charge under high speed circumstances brought about instable possibility. Besides, low pressure at intake port caused by head loss has led to low exhaust pressure P4. This behavior
Fig. 7. Effect of engine speed on lean combustion limit.
63
endangered blow-down process and increased coefficient of residual gas fres. Eq. (4) lists the calculation of coefficient of residual gas.
fres
sffiffiffiffiffiffiffiffiffiffiffiffi 1 k Pe ¼ $ P4 r
(4)
Where, Pe represents for the exhaust back pressure; P4 represents for the in-cylinder pressure when exhaust valves open. Both low exhaust pressure and inadequate scavenge by fresh charge caused severe accumulated residual gas in-cylinder, especially under high speed conditions. Dilution by residual gas has made the mixture leaner and combust less stable. This impact hazarded further extension of operating area. 4.4. Effect of throttle opening on lean combustion limit Fig. 8 shows the correlation between throttle opening and lean combustion limit. As expected, larger throttle opening was beneficial to achieving leaner combustion, due to the fact that enlarging throttle opening improves intake motion characteristics. The influences of throttle opening are discussed in several aspects: load level of the engine, pump losing, volumetric efficiency and residual gas. Contrary to the study of engine speed, the experiments of this section were conducted with continuous adjustment of throttle opening under constant engine speed condition. Load level of the engine increased with enlargement of throttle opening, extending operating area. The reasons have been stated in the former section. As to the influence of pump losing, generally speaking, increasing openness of throttle can reduce head loss in intake system. This reduction comes from two perspectives. One is that at throttle body position, adding openness can decrease head loss for valve, evidently. The other is head loss. Throttle plays a role of a variable pressure ratio nozzle. By adding openness, the area after throttle valve was enlarged, making the proportion of throttle entrance area to exit area decreased. This change slowed down the velocity of air flow in manifold, and helped rising intake absolute pressure. Besides, higher volumetric efficiency of the engine could be achieved by introducing larger throttle opening, rendering the precombustion duration and promoting lean combustion capability. Moreover, with more sufficient fresh charge and stronger
Fig. 8. Effect of throttle opening on lean combustion limit.
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scavenging, residual gas was reduced, a positive effort benefiting lean combustion as well. 4.5. Effect of coolant temperature on lean combustion limit As demonstrated in Fig. 9, lean combustion limit was positively related to coolant temperature under the given engine operating condition. Coolant temperature has a strong correlation with cylinder wall temperature. The higher the former, the warmer the latter. High cylinder wall temperature not only facilitated blending of the mixture, but also shortened quenching distance. Besides, warmer cylinder wall provided more appropriate circumstance for the initiation of flame kernel, which was regarded as an essential way to reduce over-frequent misfire. Homogenous mixture reduced cyclic variation effectively, thereby promoting operating stability of the engine [27]. Shortened quenching distance benefited flame propagation and constant volume combustion, which enhanced combustion stability of the engine from another aspect. Also, due to the fact that temperature difference between flame front and cylinder wall was lessened by warming up coolant, quantity of heat transfer was reduced to some extent. This is helpful to extend lean combustion limit, but the effort is relatively slight. 4.6. Effect of oil temperature on lean combustion limit Fig. 10 illustrates the correlation between lean combustion limit and lubricant oil temperature. The diagram indicates that an oil temperature increase corresponded to an initial decrease followed by an increase of lean combustion limit. The influences of heating lubricant oil have two opposite aspects, one is on lubrication, and the other is on thermal load. Warming up has reduced viscosity and improved liquidity of the oil, which enhanced lubrication and decreased friction within moving parts, namely, crankshaft and camshaft bearings, piston rings and cylinder wall, etc. Thus, the engine could be driven by consuming less energy. This behavior allowed the engine in fueling with leaner mixture, thereby extending the operating area. However, with the rise of oil temperature, thermal load within the moving parts has increased concurrently, which led to expansion of these parts and higher friction. The combined effort of these two aspects has probably resulted in the curve in this figure. For the specific engine adopted in this present paper, the influence of thermal load was stronger than
Fig. 9. Effect of coolant temperature on lean combustion limit.
Fig. 10. Effect of oil temperature on lean combustion limit.
that of lubrication, when the oil temperature was lower than 80 C, so lean combustion limit first decreased. When at 80 C, these two influences were balanced and made this temperature a turning point. Then when oil temperature was over 80 C, the influence of reduced viscosity and better liquidity exceeded that of thermal load, lean combustion limit was extended by higher oil temperature. Another possible explanation to this phenomenon is to consider the variation of coefficient of friction f. As stated in reference [11], f has a correlation with expression mN/s. In the expression, m represents for dynamic viscosity of lubricant oil; N represents for rotatory speed of engine crankshaft; s represents for normal force within moving parts per area. Given that the experiments were conducted with unchanged engine speed, N was a constant, actually. m becomes smaller when oil temperature rises. As to s, empirically, a tiny increase is corresponded to an increase of temperature. In sum, expression mN/s shrinks under higher oil temperature conditions. When oil temperature was lower than 80 C, the friction status was in mixed lubrication as described in Fig. 11. In this field, a higher oil temperature correlates a larger f, making lean combustion limit shrink with warming up of oil. On the contrary, when oil
Fig. 11. Coefficient of fraction f versus expression mN/s. [9]
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temperature was more than 80 C, friction status entered hydrodynamic lubrication field, where f has a positive correlation with oil temperature. 5. Conclusion In this present paper, six operating parameters and their effects on lean combustion limit of an engine fueled with compressed natural gas and hydrogen blend were examined and discussed. Lean combustion limit timing, a type of optimal ignition timing was adopted to search lean combustion limit during the experiments. Conclusion was drawn as follows. 1. Both over-retarded and over-advanced ignition timing are not beneficial to accomplishing stable combustion at lean combustion limit under a certain operating condition. 2. It is obvious that lean combustion limit of the engine was extended with hydrogen enrichment; lean combustion limit of the engine increased with hydrogen fraction in the blend. 3. With a reduction of engine speed, lean combustion limit of the engine could be extended with fixed throttle opening. 4. Lean combustion limit increased with throttle opening when engine speed remained a constant. 5. Lean combustion limit had a positive correlation with coolant temperature under a certain operating condition. 6. A lubricant oil temperature increase corresponded to an initial decrease followed by an increase in lean combustion limit under a certain operating condition. Among these factors, hydrogen fraction is the most significant one. This result has indicated that optimizing fuel components and properties making it more appropriate for lean combustion is still and will be always the most effective way in extension of lean combustion limit. The conclusions on engine speed and throttle opening demonstrates that more attention needs paying to further extension of lean combustion limit under high engine speed or low throttle opening conditions. Temperature factors, both coolant and lubricant oil, do have impact on lean combustion limit, comparatively tiny but indispensable. Further study on detailed mechanism of lubricant oil temperature’s effects is needed in the future. Acknowledgments This work was sponsored by “Xing Huo Fund” for undergraduate students in Beijing University of Technology (Grant No. XH-201005-01) and “Innovation Foundation” for excellent doctor students in Beijing Jiaotong University (Grant No. 141050522). References [1] Rakopoulos CD, Scott MA, Kyritsis DC, Giakoumis EG. Availability analysis of hydrogen/natural gas blends combustion in internal combustion engines. Energy 2008;33:248e55. [2] Bauer CG, Forest TW. Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: effect on S.I. engine performance. International Journal of Hydrogen Energy 2001;26:55e70. [3] Jahirui MI, Masjuki HH, Saidur R, Kalam MA, Jayed MH, Wazed MA. Comparative engine performance and emission analysis of CNG and gasoline in a retrofitted car engine. Applied Thermal Engineering 2010;30:2219e26. [4] Huang Zuohua, Liu Bin, Zeng Ke, Huang Yinyu, Jiang Deming, Wang Xibin, et al. Experimental study on engine performance and emissions for an engine fuelled with natural gas-hydrogen mixtures. Energy & Fuels 2006;20:2131e6.
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