Experimental Study on the Operating Range Restrictions of a Lean

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Experimental Study on the Operating Range Restrictions of a Lean-burn Turbocharged SI Natural Gas Engine Du Wang,* Xiaojian Mao, Junxi Wang, and Bin Zhuo School of Mechanical Engineering, Shanghai Jiao Tong UniVersity, Shanghai, People’s Republic of China ReceiVed February 15, 2009. ReVised Manuscript ReceiVed May 4, 2009

An experimental investigation aiming at extending the operating range of lean-burn turbocharged sparkignition (SI) CNG engine was conducted on a six-cylinder throttle body injection CNG engine. Cylinder pressure was recorded by using a high sampling frequency transducer, thus allowing the analysis of pressure characteristics introduced by knock. Coefficient of variations (COV) in indicated mean effective pressure (IMEP) was used to define lean misfire. The effects of various engine parameters on an engine’s lean burn capability, knock limit, and preturbo temperature limit were examined at various operating conditions. The results indicate that a lean-burn SI CNG engine’s operating range is sensitively affected by the mechanical structure of the engine, excess air ratio, spark timing, load, engine speed, intake temperature, ambient humidity, etc. For the turbocharged engine, preturbo temperature is also a key consideration in extending the operating range of the engine.

1. Introduction In a vehicle, an engine needs to operate over a wide range of operating conditions, from idle to high loads and high speeds. Many other factors, such as fuel economy, emissions, and performance also require an engine having a wide range of operating conditions. However, the restrictions of misfire, knocking, and compressor efficiency limit the operating range of a lean-burn engine, especially for a turbocharged SI engine. To extend the operating range, mechanical structure of the engine, control strategy, and calibration method should be optimized. Many researches related to extending the operating range of engines have been conducted during past decade or even earlier. Fiveland et al.1 numerically explored the effect of reformed fuels on both fundamental flame stability and the performance/ emissions tradeoffs of the engine. Evans2 improved combustion chamber design to enhance turbulence generation and introduced a small quantity of fuel through the spark plug to provide a relatively rich mixture near the spark electrodes during the ignition process. Both techniques were found to be able to extend the lean-limit of operation, increase the burning rate of very lean air-fuel mixtures, thereby providing greater specific power output with a corresponding reduction in brake specific fuel consumption. Syrimis et al.3 analyzed gasoline engine knock via using cylinder pressure data. There are several efforts for CNG engines carried out by some researchers. For example, Raju et al.4 studied the effects of compression ratio (CR) on full throttle performance of a lean* To whom correspondence should be addressed. Phone: +86-2134206138; e-mail: [email protected]. (1) Fiveland, S. B.; Bailey, B. M.; Willi, M. L.; Hiltner, J. D.; Parsinejad, F.; Metghalchi, H. Proc. Fall Tech, Conf. ASME Intern. Combust. Eng. DiV. 2004, (10), 659–667. (2) Evans, R. L. Int. J. EnViron. Stud. 2006, 63 (4), 441–452. (3) Brunt, M. F. J.; Pond, C. R.; Biundo, J. Gasoline Engine Knock Analysis Using Cylinder Pressure Data. SAE Paper 980896; 1998. (4) Raju, A. V. S. R.; Ramesh, A.; Nagalingan, B. J. Inst. Eng. India Mech. Eng. DiV. 2000, 80 (4), 144–147.

burn SI engine operating on natural gas. Ramesh et al.5 dealt with the nature of cycle-by-cycle variations in important parameters such as indicated mean effective pressure (IMEP), peak pressure, rate of pressure rise, and heat release characteristics in a single cylinder, lean-burn natural-gas-fuelled SI engine operated at the constant speed of 1500 rpm under variable equivalence ratio, fixed throttle conditions. The changes in humidity and temperature of the intake air also have an affect on engine performance for a lean-burn prechamber natural gas engine.6 Hydrogen enrichment in a CNG SI engine aiming at extending operating range was studied in some literatures,7-10 and the resultant benefit was obvious. However, the additional supply of hydrogen equipment will increase the cost of the engine and decrease the security of automobile application. For the purpose of augmenting the knock resistance characteristics and extending the engine operational limits, some studies added diluent carbon dioxide or nitrogen in the fuel mixtures.11 Some researcher established the knocking model for avoiding knock.12 Considering the interference between shrinking lean limit region and shrinking knock limit region, Badr et al.13 carried out a parametric study on the lean misfiring and knocking limits of a natural-aspirated CNG engine. However, these studies above were only conducted for an individual factor such as lean limit, knock limit, and boost, not fully taking into consideration the influence on operating region brought up by all factors. This study focuses on the analysis of (5) Ramesh, A.; Tazerout, M.; Le, C. O. ASME Intern. Combust Engine DiV. Publ. ICE 2003, 40 (9), 321–329. (6) Wimmer, A.; Schnessl, E. ASME Intern. Combust. Engine DiV. Publ. ICE, 2006, 9. (7) Ma, F. H.; Wang, Y. J. Hydrogen Energy 2008, 33 (4), 1416–1424. (8) Huang, Z. H.; Liu, B.; Zeng, K.; Huang, Y. Y.; Jiang, D.; Wang, X.; Miao, H. Energy Fuels 2006, 20 (5), 2131–2136. (9) Sierens, R.; Rosseel, E. J. Eng. Gas Turbines Power 2000, 122 (1), 135–140. (10) Raju, A.V. S. R.; Ramesh, A.; Nagalingam, B. J. Inst. Energy 2000, 73 (496), 143–148. (11) Bade, S. S. O.; Rodrigues, R. Proc. Inst. Mech. Eng. Part A J. Power Eng. 2008, 222 (6), 587–597. (12) Soylu, S. Energy ConVers. Manage. 2005, 46 (1), 121–138. (13) Badr, O.; Alsayed, N.; Manaf, M. Appl. Thermal Eng. 1998, 18 (7)), 579–594.

10.1021/ef900131v CCC: $40.75  2009 American Chemical Society Published on Web 05/19/2009

A Lean-burn Turbocharged SI Natural Gas Engine

Energy & Fuels, Vol. 23, 2009 3055

Table 1. Specifications of the Experimental Engine parameter

definition

engine type aspiration intercooler fuel type bore × stroke compression ratio displacement rated power/speed maximum torque/speed combustion chamber

in-line six cylinders, spark ignition turbocharger air-to-water compressed natural gas 112 mm × 132 mm 11 7.8 L 191 kW/2300r/min 980 N m/1400r/min ω type

all key restrictions on the range of operating conditions of a four-stroke lean-burn turbocharged SI CNG engine and evaluates the approaches to extend this range based on experimental data in a six-cylinder CNG engine. The authors hope that this study could give some practical guidance to the development and calibration of lean-burn turbocharged SI CNG engines. 2. Experimental Setup The experiments were carried out on a six-cylinder, single point injection, SI CNG engine (see Table 1 for specifications). The experimental engine was coupled to an eddy-current dynamometer for engine speed and load measurement and control. An electronic control system acquired sensor signal, controlled the actuators, and provided access to all calibration parameters, allowing the user to set the desired equivalence ratio (by adjusting fuel injection duration), throttle position, ignition timing, and boost pressure. The wastegate on the turbocharger was designed to dump exhaust pressure around the turbine to reduce boost. So ECU could set the boost pressure by controlling the opening of the wastegate. In-cylinder pressure is the most important classical diagnostic in engine studies, providing information on the burn rate and overall engine performance. In knock studies in particular, cylinder pressure also provides measures of knock intensity. Such knock indicators include the amplitude of the pressure fluctuation, the rate of pressure rise, the third derivative of pressure, the burn duration, and the rate of change of net heat release rate.14 In-cylinder pressure data were taken with a watercooled piezoelectric pressure transducer (Kistler6125A) and indicator system (AVL Indimeter619).

Figure 1. The experimental setup.

Figure 2. Restrictions on the range of operating conditions.

Exhaust gases were measured online by an AVL exhaust analyzer, in which hydrocarbon (HC) was analyzed with a flame ionization detector (FID), CO was analyzed with a nondispersive infrared analyzer (NDIR), and NOx was measured with a chemiluminescent detector (CLD). CO, HC, and NOx emissions were the average values of acquired data at each steady state of operating condition. All the tests were repeated. The air/fuel ratio was monitored by an ETAS wide range λ Meter. Figure 1 shows the experimental setup consisting of several main subsystems, namely the CNG engine system, an electronic control system and the calibration tools, the λ meter and the exhaust analyzers, and a PC system equipped with in-cylinder pressure monitor system. The dynamometer is not presented at Figure 1.

3. Operating Range Restrictions The operating range of the lean-burn turbocharged SI CNG engine is usually confined to a relatively small region. The size and shape of the operating region are largely decided by knock occurrence probability, lean misfire limit, turbocharger matching, etc. The major restrictions are shown schematically in Figure 2. Knocking in an engine has some unfavorable effects such as increase in engine pollution, decrease in engine efficiency, considerable rise in engine specific fuel consumption (SFC), and potential of structural harm to engines in the long-run. Lean

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Figure 5. Effect of maximum squish velocity on burn duration.

Figure 3. In-cylinder pressure curve of light-knocking and nonknocking.

Figure 6. Squish velocity calculation.

Figure 4. Burned and unburned zones of combustion chamber.

misfire and nonmatched turbocharger also result in bad emissions, poor power performance, and even unsafe driving security. 4. Knock Limit Many factors can affect knocking region, including CR, burn rate, excess air ratio, intake pressure and temperature, spark advance angle, engine speed, and load. Major factors are analyzed on the basis of experimental data in this study, so as to find the methods of shrinking knock region and to improve emissions, performance, and fuel economy. There are three monitoring methods for judging knocking occurrence, which are in-cylinder pressure (using pressure transducer), burn noise (monitoring metal resonance), and engine vibration (using a knock sensor). In this study, in-cylinder pressure is monitored real time and used to judge knocking occurrence. Figure 3 illustrates the in-cylinder pressure of both light-knock (engine speed of 1500 rpm, MAP of 207 kPa, spark advance of 22 dBTDC and λ of 1.16) and nonknocking (the same engine speed, MAP and spark advance, with different λ of 1.28). 4.1. Effect of In-cylinder Turbulence on Knock. Figure 4 demonstrates the process of knocking occurrence. The unburned gas zone ahead of flame front, or end gas, is compressed by extending burned gases and piston. Chemical reaction in end gas is accelerated by the increase in temperature, and the autoignition of end gas prior to flame arrival leads to knock. In-cylinder turbulence can promote flame propagation, increase burn rate and combustion stability, and reduce heat transfer to components and likelihood of knock. One of the (14) Puzinauskas. Examination of Methods Used to Characterize Engine Knock. SAE Paper 920808; 1992.

Figure 7. Effect of combustion chamber design on squish.

efficient means of increasing turbulence is to optimize piston geometry, which is proportional to squish velocity. The variation of burn duration versus maximum squish velocity with constant λ is plotted in Figure 5. As expected, increasing maximum squish velocity will result in shorter flame propagation duration (characterized as the duration between 10 and 90% MFB). Squish velocity can be calculated by means of eq 1. Figure 7 shows the change of squish velocity resulting from the difference of combustion chamber parameters. V)

Sp × D B 4z D

2

[( )

-1

] Ac × Vbz + Vb

(1)

4.2. Effect of Compression Ratio on Knock. Compression ratio is a key factor in shrinking knock region, and the components of CNG exert significant influence on the critical CR of engine knocking. Figure 8 shows knock resistance of fuels and the less number of carbon atoms in molecule, the greater critical compression ratio. CNG mainly consists of methane, which has a high critical CR, about 14. However, CNG composition varies drastically with location, time of year, and time of day. Table 2 shows the components of CNG in this study. Critical CR is about 13.



Figure 10. Effect of excess air ratio on in-cylinder temperature.

Figure 8. Critical compression ratio of some fuels. Table 2. Components of CNG component

chemical formula

proportion

methane ethane propane 2-methylpropane n-butane 2-methylbutane n-pentane 2,2-dimethylbutaan nitrogen

CH4 C2H6 C3H8 i-C4H10 n-C4H10 i-C5H12 n-C5H12 C6H14 N2

91.68 4.64 1.09 0.19 0.23 0.08 0.07 0.28 1.69

Figure 9 shows the impact of CR on knock limit λ (KLL) at a speed of 2000 rpm, MAP of 200 kPa, and spark advance of 24 dBTDC. The increase of CR lifts up frame propagation rates, shortens ignition delay, and extends knocking region. 4.3. Effect of Excess Air Ratio on Knock. When operating lean of air-fuel ratio, knock sensitivity is slightly improved due to slowing of fuel burn and more energy and time required to ignite. Figures 10 and 11 show the influence of excess air ratio on in-cylinder temperature and pressure. When λ equals 1.28, the max in-cylinder temperature is about 2300 °C, and the max in-cylinder pressure is about 8.5 MPa. However, at the same operating condition of engine and when λ equals 1.54, the max in-cylinder temperature is down to about 1800 °C, and the max in-cylinder pressure is down to about 6 MPa. The richer mixture accelerates the burn, as well as increases in-cylinder pressure and temperature, which increases the possibility of the autoignition of end gas and will results in higher chance of knocking and higher NOx emissions. 4.4. Effect of Spark Timing on Knock. In the process of engine calibration, spark timing could be adjusted frequently for the purpose of optimizing performance, emissions, and fuel

Figure 9. The effect of CR on KLL.

Figure 11. Effect of excess air ratio on in-cylinder pressure.

Figure 12. Effect of spark timing on knock.

economy. Spark timing has a close relationship with knock possibility, and usually, the spark timing causing the best engine torque is just close to the knock limit. Therefore it is important to find the effect of spark timing on knock for avoiding it. Figure 12 shows the effect of spark timing on knock at different speeds and a MAP of 124 kPa. The knocking region tends to widen and the operating region to narrow with increasing spark advance, since greater angles of spark advance give the end gases longer time to react before being consumed by the propagating flame front and thus enhance knocking. Due to the knocking sensitivity to spark timing, ECU realtime analyzes the vibration frequency of engine acquired from the knock sensor installed in the engine. In case the where vibration frequency exceeds a calibrated threshold value, ECU will reduce the spark advance to avoiding knocking. In Figure 12, it can be found that the knocking limit λ shows a decrease with the speed increased. The cause is similar to the increase of spark advance discussed above.

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Figure 15. The effect of CR on lean burn limit. Figure 13. Effect of intake temperature on KLL.

Figure 14. Effect of load on knock.

4.5. Effects of Intake Temperature on Knock. Turbocharging significantly increases air inlet temperatures and has much greater effect on KLL, which is proofed in Figure 13. The KLL increases with the increase of intake temperature at different engine speed. Decreasing temperature after intercooler will extend knock-free operating range. 4.6. Effect of Load on Knock. On SI engines, MAP is an explicit indicator of load. In this study, MAP can be changed by electronic throttle and wastegate, which controls the boost pressure. The change of MAP caused by ETC or wastegate will differently affect KLL. The increase of MAP achieved by increasing throttle opening will lead to less pumping loss and less residual gas. The resulting reduction in combustion duration not only brings about improved level of constant volume combustion but also leads to higher pressure rise, higher cylinder temperature, and hence increases the possibility of knock. The increase of MAP achieved by increasing boost pressure will result in higher intake temperature and density. As discussed above, the increase of intake temperature will increase KLL. As a result, the increase of load level has a negative effect on extending operating range. As shown in Figure 14, the knocking region extends slightly with the load increased. 5. Lean Limit literature,13

In it is discussed that there are several ways of defining engine lean limit, and a unique definition is not

available. In practice, the λ at which HC emission start to soar is usually defined as lean misfire limit. However, it is difficult to judge precisely when HC emission does start to soar. In this study, lean limit is defined as the excess air ratio at which COV in IMEP reaches 10%.15 This is because it is generally accepted that a COV above 10% will be perceived by a driver as a poor running condition. COV here was defined as the standard deviation divided by the mean and was calculated from a set of 150 consecutive cycles at each test point. Mass fraction burned profile, which is a commonly used indicator of the combustion process, is determined from the experimental cylinder pressure data using the improved R-W method.15,16 5.1. Effect of Compression Ratio on Lean Limit. The increase in CR will bring about the decrease in clearance volume, which causes an increase in heat transfer and a decrease in the amount of residual gases. The corresponding increase in gas temperature and pressure at the moment of spark discharge helps the reaction rate and flame propagation. Moreover, the increase on CR can also result in an intensification of the swirl motion near the TDC where ignition occurs. The resultant increase in turbulence level helps the ignition process. As Figure 15 shows, at a speed of 1500 rpm, MAP of 150 kPa, spark advance of 15 dBTDC, and CR of 9, COVIMEP reaches 10% before λ up to 1.45. However, when CR is 12, COVIMEP reaches 10% after λ up to 1.6. Namely, the increase of CR can shrink the lean misfire region. Of course, engine structure restrictions and knocking possibility also must be considered while determining CR. 5.2. Effect of Engine Speed on Lean Limit. The engine speed, which is an important operating parameter, indirectly affects the lean limit through its effects on turbulence, flame initiation and propagation, heat transfer, and cyclic time. However, the changes of engine speed usually bring the change of spark timing, intake pressure, and boost pressure, which is disadvantageous to analyze the individual effect of engine speed on lean limit. To uncouple such combined effects, in this study, the spark timing is fixed, and the MAP controlled by electronic throttle and wastegate are restricted in several operating points. The increase in the engine speed affects positively the shrinking lean limit region, with one example enhancing the in-cylinder turbulence. However, at the same time, some negative effects can be caused, such as the increase of residual (15) Gupta, M.; Bell, S. R.; Tillman, S. T. Trans. ASME J. Energy Resour. Technol., 1996, 118, 145–51. (16) Rasswieler, G. M.; Withrow, L. Motion pictures of engine flames correlated with pressure cards. SAE paper 800131; 1980. (17) Shayler, P. J.; Wiseman, M. W.; Ma, T. ImproVing the determination of mass fraction burned. SAE Paper 900351; 1990.



Figure 17. The effect of spark timing on COV in IMEP. Figure 16. The effect of engine speed on lean limit for different MAP.

gas because of decreasing time per cycle for intake and exhaust processes and increasing of throttling loss, the incomplete combustion since the time for flame propagation is shortened, and the increase of COV in IMEP. In addition, heat losses, having the opposing effect on the shrinking lean limit region, become higher with the speed increased in the low speed range, and vice versa in the high one. Figure 16 gives the lean limit λ (LLL) for different speed and MAP with fixed spark timing. The influence of engine speed on lean limit is less obvious, because, as discussed above, there are many positive and negative effects. The LLL slightly increases with speed increased at low load. However, at high load, the increase of speed leads to the decrease of LLL, because the effect of speed on reducing residual gas is small at high load; however, other negative effects on extending lean limit are still enhanced with speed increased. 5.3. Effect of Spark Timing on Lean Limit. Because of the density of the charge and the conductivity of natural gas, lean burn turbocharged CNG engines have more chances of lean misfire. To avoid lean misfire, on one hand, the ignition system requires a smaller spark plug gap, higher energy, and longer ignition duration to ensure the spark. On the other hand, spark timing have a significant influence on the engine lean limit, since lean limits are sensitive to flame initiation and flame propagation, so the spark timing should be adjusted on the basis of operating condition of engine. Overadvanced spark timing results in inadequacy of precombustion compression of the cylinder charge, therefore its temperature and pressure are relatively low. There are negative effects on the flame initiation and following propagation process, and the COVIMEP will be increased. Over-retarded spark timing is also not good for burn process and brings about the increase of the COVIMEP due to the shorter period of thermal transfer. It can be proven from Figure 17, at a constant engine speed of 1300 rpm and excess air ratio of 1.1. Figure 18 shows the variation in lean limit as a function of spark timing for different engine speed and MAP. The increase in spark timing is beneficial to shrinking lean limit region when the spark advances slightly. However, when spark advance becomes more, the increase in spark timing has an opposite effect on shrinking lean limit region, because, as discussed above, overadvanced or over-retarded spark timing will increase COVIMEP. 5.4. Effect of Load on Lean Limit. As discussed earlier, the change of MAP caused by ETC or wastegate will differently affect the operating range. The increase of MAP achieved by increasing throttle opening will result in higher in-cylinder pressure and temperature, hence reducing the possibility of misfire.

Figure 18. The effect of spark timing on lean limit at different speed and MAP.

Figure 19. The effect of MAP on lean limit for different engine speed.

The increase of MAP achieved by increasing boost pressure will result in higher intake temperature and density. Higher intake temperature increases the λ of lean limit. However, on the other hand, increasing the intake density decreases the available ignition energy per unit mass of the gas in the vicinity of the spark plug, which has little negative effect on shrinking lean limit region. However this negative effect can be easily solved by enhancing ignition energy. The analyses above indicate that the increase in load level is beneficial to increasing LLL and extending operating range. Figure 19 can prove it. 5.5. Effect of Intake Temperature on Lean Limit. Intake temperature increases the reaction rates and also increases heat losses particularly at high speeds. The former increases LLL and the latter has an opposite effect.

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Figure 20. Effect of intake temperature on lean limit.

Wang et al.

Figure 22. Comparison of power and torque between TC and NA engines.

Figure 21. The effect of humidity on lean limit.

As showed in figure 20, the increase of intake temperature increases LLL at low speeds because of the dominant effect of the increased reaction rates. However, at high speeds and relatively low intake temperature, the dominant effect of the heat losses results in the decrease of LLL. As reaching higher intake temperature (>65° C), the LLL increases again due to the dominant effect of the increased reaction rates. 5.6. Effect of Humidity on Lean Limit. The lean operation boundary is also sensitive to the variation in ambient humidity, which affects both ignition and the heat transfer. When the ambient specific humidity increases, the operating region shrinks, as indicated by Figure 21. The humidity sensor is installed on the engine for the humidity correction strategies, including spark timing correction, desired excess air ratio correction, and acquisition correction for air-fuel ratio. 6. Boost and Preturbo Temperature Limit As compared with gasoline, CNG has a decrease of power density. Turbocharging technique is one of the most effective methods of power recovery. Figure 22 shows the power and torque of TC engines have a significant rise compared with NA engines. However, the selection of turbocharger and boost calibration are restricted to some factors, such as client’s requirement for power and operating characteristic of engine, surge limit of compressor, and preturbo temperature. From Figure 23 it can be seen that preturbo temperature changes with different speeds, spark timing, and λ at a MAP of 124 kPa. For one thing, it is obvious that preturbo temperature decreases with the increase of λ because lean mixture leads to low in-cylinder temperature and pressure. For another thing, the increase of engine speed

Figure 23. Preturbo temperature versus n, ST, and λ.

increases preturbo temperature while the spark timing has an opposite effect. 7. Final Development of Operating Range Through analyzing the effects of operating range, the mechanical structure of engine, control strategy and operating Maps of engine are optimized. On the basis of meeting the Stage-IV emission standard in China, the main objective of optimization is fuel economy. The optimized set of parameters can be summarized mainly as follows. (1) Considering knocking restriction and the octane number varying with location and time, the spark advance angle is 4∼5 dBTDC less than the knocking limit. (2) Considering lean restriction and misfire limit varying drastically with humidity, the excess air ratio is 10% less than the misfire limit. (3) When calibrating the parameters of external characteristic, the preturbo temperature limit must be considered. (4) At speeds that ranged from 600 to 1300 rpm and at MAPs that ranged from 70 to 140 kPa, the final excess air ratio is 5% less than the excess air ratio of the best fuel economy, for the purpose of improving the power response at low speeds. At the same time, the spark timing can be adjusted for keeping more than 10% knocking margin. (5) At speeds that ranged from 1300 to 2100 rpm, the fuel economy is the main aim for deciding spark timing and excess air ratio. The spark timing and excess air ratio optimized for the purpose of best fuel economy are shown in Figures 24 and 25.


Figure 24. Spark timing for best fuel economy.

Figure 25. Excess air ratio for best fuel economy.

Figure 26. Final development of operating range.

Figure 26 shows initial nonoptimized widow, first optimized window, and final development window, at 1500 rpm of speed and full load. After the optimization, the operating range of engine is extended. The larger excess air ratio at which engine can operate improves the fuel economy and preturbo temperature, and engine’s emissions meet Stage-IV emission standard in China. The larger range for the spark timing can improve power performance and transient acceleration performance. 8. Conclusions The results of our investigations on extending operating range of the CNG engine carried out in this paper can be summarized mainly as follows: The design of mechanical structure of engine greatly affects the likelihood of misfire and knocking. Increasing CR results in the decrease of the likelihood of misfire and the increase of the likelihood of knocking. The final CR is decided to be 11.


Improving piston geometry can increase maximum squish velocity, shorten flame propagation duration, and reduce the possibility of knocking. The shape of combustion chamber is designed like a straight barrel. Advancing spark timing is generally beneficial to improving engine’s lean burn capability. However, overadvanced spark timing would adversely affect the engine’s lean burn capability due to inadequate charge precombustion compression. Moreover, overadvanced spark timing is also the key cause of knocking. Increase in excess air ratio helps to decrease in-cylinder pressure and temperature and slow fuel burn, and results in less likelihood to knock. Spark timing and excess air ratio are optimized for the purposes of meeting the Stage-IV emission standard in China and the best fuel economy. The increase of load may lead to less pumping loss, less residual gas, and higher intake temperature and density, which are helpful in increasing LLL and shrinking the lead limit region. However, the resultant rise in in-cylinder pressure and temperature enhances the knocking. Therefore, the excess air ratio increases with the increase of load at most of speeds. The increase of engine speed gives the end gases longer time to react before being consumed by the propagating flame front and thus enhance knocking. The effects of speed on lean limit depends on the load, because of the different effects of in-cylinder turbulence and residual gas with different load level. At low load level the increase in engine speed is beneficial to shrinking the lean limit region, but this is not applicable at high load level. Increasing temperature after intercooler will shrink the knockfree operating range and extend lean limit range at low speeds, or at high speeds and high temperature. The only exception is the behavior at high speeds and relatively low intake temperature, which the lean limit region shrinks with the increase of intake temperature. Ambient humidity affects both ignition and the heat transfer. The operating region will shrink in high humidity. So the humidity offset strategy is used for the acquisition of excess air ratio. Boost can recover the power of engine fuelling CNG and bring some restrictions including preturbo temperature limit. The increase of λ and spark timing causes the decrease of preturbo temperature, and engine speed has an opposite effect. Compared with initial nonoptimized operating range of engine, the final development is considerably extended. Therefore, vehicle’s fuel economy and emissions benefit brought about by using lean-burn turbocharged CNG engine are ensured, and the performance of the engine does not drop. Acknowledgment. This study is supported by National 863 Project of China (No. 2006AA11A1A9). The authors acknowledge the colleagues at Shanghai Jiaotong University and at Guangxi Yuchai Machinery Co. Ltd. for their help with the experiment and preparation of the manuscript.

Nomenclature A/F ) air/fuel ratio Ac ) cylinder area B ) bore diameter c ) piston to head when TDC CNG ) compressed natural gas CR ) compression ratio D ) bowl diameter dATDC ) degree after top dead center dBTDC ) degree before top dead center ECU ) electronic control unit

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ETC ) electronic throttle control FT ) fuel temperature KLL ) knock limit λ λ ) excess air ratio LLL ) lean limit λ MAP ) manifold absolute pressure MAT ) manifold absolute temperature MFB ) mass fraction burned n ) engine speed NA ) natural-aspirated PC ) personal computer rpm ) revolutions per minute

Wang et al.

SI ) spark ignition Sp ) piston speed ST ) spark timing TDC ) top dead center TC ) turbocharged TIP ) throttle inlet pressure UEGO ) universal exhaust gas oxygen V ) squish velocity Vb ) bowl volume z ) piston to head EF900131V

Experimental Study on the Operating Range Restrictions of a Lean

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