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Full load performance and emission characteristics of hydrogen-compressed natural gas engines with valve overlap changes
Fuel 123 (2014) 101–106
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full load performance and emission characteristics of hydrogen-compressed natural gas engines with valve overlap changes Cheolwoong Park a,⇑, Sungwon Lee a, Gihun Lim b, Young Choi a, Changgi Kim a a Department of Engine Research, Environmental and Energy System Research Division, Korea Institute of Machinery and Materials, 156 Gajeongbuk-Ro, Yuseong-Gu, Daejeon 305-343, Republic of Korea b Environment and Energy Mechanical Engineering, University of Science and Technology, 217 Gajeong-Ro, Yuseong-gu, Daejeon 305-700, Republic of Korea
h i g h l i g h t s Altering the cam with a decreased valve overlap decreases torque at the lean limit. THC and the CH4 reductions are approximately 41% for the HCNG fuel with reduced valve overlap. The NOx from the camshaft with decreased valve overlap is higher than that of original camshaft.
a r t i c l e
i n f o
Article history: Received 12 August 2013 Received in revised form 2 January 2014 Accepted 11 January 2014 Available online 31 January 2014 Keywords: Hydrogen-compressed natural gas Valve overlap Full load performance Lean-burn limit Methane emission
a b s t r a c t Natural gas vehicle engines are preferred over diesel engines because they release significantly lower amounts of nitrogen oxides (NOx) and carbon dioxide emissions. Hydrogen-compressed natural gas (HCNG) technology is a promising alternative to that used in conventional compressed natural gas engines because hydrogen possesses stable lean combustion characteristics. Although the NOx emissions requirements are satisfied, methane emissions must be filtered using an oxidation catalyst. An alternative is required for methane reduction because improving the conversion efficiency of methane is expensive. In this study, the strategy of varying the valve overlap was employed to reduce methane emissions. Although a torque valve cannot meet engine emission specifications, the hydrocarbon and methane emissions were reduced by approximately 41% by decreasing the valve overlap duration while using HCNG fuel. The level of NOx emissions was approximately equivalent to or slightly higher than that of a conventional camshaft. Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction The world energy market is attempting to diversify energy sources because of the rapid increase in the use of fossil fuels and the problem of global warming that is partially caused by automotive vehicles. Natural gas vehicles (NGVs) have the advantages of low carbon dioxide (CO2) emissions and a wide range of application; the can be used in passenger vehicles, trucks, and buses [1,2]. NGVs were initially promoted for the consumption of overproduced natural gas. However, now, they also save energy or can be used as an alternative fuel. NGVs are preferable to diesel engine-based vehicles because of their low harmful emission characteristics. However, although the development of NGVs is a prior⇑ Corresponding author. Tel.: +82 42 868 7928; fax: +82 42 868 7305. E-mail addresses: [email protected] (C. Park), [email protected] (S. Lee), [email protected] (G. Lim), [email protected] (Y. Choi), [email protected] (C. Kim).
ity, it faces challenges such as the development of technology for clean diesel engines and insufficient infrastructure for recharging. Recently, the development of NGV technology has focused, especially in the case heavy duty vehicles, on reducing NOx and achieving high thermal efficiency through the improvement in fuel consumption [3,4]. Compressed natural gas (CNG)-hybrid engines significantly increase the cost of production, although they achieve the improvement required by the aforementioned goals. Furthermore, hydrogen-compressed natural gas (HCNG) is a promising fuel technology that can improve the performance of CNG engines. HCNG is considered to be a type of mono-fuel, and it is a mixture of 20–30 vol% of hydrogen and natural gas. The advantages of HCNG use in vehicles were proved through tests in the United States and Canada. Many research results for HCNG engines have been reported which indicate that HCNG engines can satisfy future emission regulations (e.g., as outlined in EURO-VI) and are economically feasible [5–8].
http://dx.doi.org/10.1016/j.fuel.2014.01.041 0016-2361/Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
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An HCNG engine exhibits a leaner combustion than a CNG engine, which is possible because of the stable lean combustion characteristics of hydrogen. Consequently, nitrogen oxides (NOx) and CO2 emissions can be significantly reduced. HCNG technology can be applied in traditional CNG engines and their fuel supply systems without any modifications. Additionally, the employment of NOx after-treatment systems is not required. NOx emissions caused by lean combustion can meet the EURO-VI standard (0.46 g/kW h). However, carbon monoxide (CO) and total hydrocarbon (THC) emissions, including methane (CH4), must be filtered through an oxidation catalyst (OC) to meet their corresponding standard (0.5 g/kW h) [9–11]. CH4 has a very stable molecular structure, and improvement in the conversion efficiency of the OC requires an increase in the amount of precious metal used in the OC. In order to ensure that the HCNG engine is cost-effective, a less expensive alternative is required for CH4 reduction. In the present study, a strategy of decreasing the valve overlap (VO) was employed to reduce CH4 emissions. Closing the exhaust valve earlier can lessen the level of hydrocarbon and CH4 emissions. This is because the unburned mixture retained in the crevices emerges around the valve seat as the exhaust valve opens and the piston approaches the top dead center (TDC) [12–14]. The effects of decreasing the VO on the full load performance and the emission characteristics are evaluated. The applicability of a camshaft with a reduced VO duration was assessed by taking into account its potential usability in the field.
2. Experimental procedures
tremely low temperature could cause malfunctioning of the fuel supply system because of the expansion of the compressed fuel in the regulator. The flow rate of CNG was measured using a mass flow meter and that of hydrogen was directly controlled using a mass flow controller. The individual in-cylinder pressure was measured using piezoelectric pressure transducers (Kistler Co., Ltd., 6117BFD17). For the analysis of the in-cylinder data, a high-precision rotary encoder with a pulse per revolution of 1800 and a combustion analyzer (Dewetron Co., Dewe 800) were used to observe the combustion stability. The excess air ratio of the air–fuel charge was monitored using a lambda meter (ETAS Co., LA4) located at the exhaust runner of an individual cylinder and downstream of the turbine. To evaluate the effect of the VO, the torque, in-cylinder pressure, and emissions were measured for each VO condition. The VO with the original camshaft had a crank angle degree (CAD) of 32. In comparison, the duration of the VO with the modified camshaft decreased by 50% (to 16 CAD). The intake and exhaust valve timings for each camshaft are summarized in Table 2. For each cylinder, an exhaust gas-sampling probe was installed between the exhaust valve and the exhaust manifold. Then, the common emission of the engine was measured after the turbocharger. The composition of the exhaust gas, which included CO2, CO, THC, and oxygen (O2), was analyzed using an exhaust gas analyzer (AVL Co., AMA i60). The temperatures and pressures in the major parts of the engine, fuel flow, and exhaust gas concentrations were measured, and the resulting data was stored using a data acquisition system (GRAPHTEC Co., GL820).
2.1. Experimental setup
2.2. Experimental methods
An 11-L 6-cylinder CNG engine (Doosan Infracore Inc., GL11K) that satisfied the EURO-V emission regulations was used in the test. The detailed engine specifications are listed in Table 1. The speed and torque of the engine were controlled and monitored using an eddy current dynamometer (Schenck). The spark ignition timing and the fuel flow rate were controlled using a computerbased universal engine control unit. An electronic throttle control and a waste gate control for the turbocharger system were employed to control the excess air ratio under each operating condition. Fig. 1 shows the experimental setup for the engine tests. The CNG from the compressed fuel vessel, which has a pressure of approximately 15 MPa, was decompressed to 0.67 MPa using a conventional regulator for vehicles. Next, it was supplied by an eight-gas fuel-metering valve connected to a gas mixer. A series of compressed tanks for hydrogen, which had a pressure of approximately 12 MPa, were employed and connected to the pressure controller. A 3:7 volumetric ratio of hydrogen to natural gas was selected for the HCNG. This is because a 30% hydrogen concentration is considered to be the optimal ratio based on previous research [15]. The CNG and hydrogen, after passing through the regulator, were heated to 40 °C using a heat exchanger. This is because an ex-
The engine was operated at 1260 rpm with 1150 Nm, which are the operating conditions of a heavy-duty natural gas engine at maximum torque with a wide-open throttle. The spark ignition timing was selected to maximize the thermal efficiency, and the maximum brake torque (MBT) was determined for each operating condition. The excess air ratio was increased, in increments of 0.1, from k = 1.3 to the lean limit. The combustion performance and exhaust emissions were analyzed with respect to the excess air ratio for each fuel and VO duration. The temperatures of the CNG and intake air downstream of the intercooler were maintained at 42.5 ± 2.5 °C, and the engine-inlet coolant temperature was maintained at 82.5 ± 2.5 °C. Water-cooled heat exchangers were used for maintaining the temperatures. An important criterion of the full load performance is the maximum torque value as compared to that in the engine specifications. This value is easily affected by a change in the VO that is accompanied by a change in the volumetric efficiency. The test conditions were evaluated by measuring the boost pressure with respect to the excess air ratio while maintaining the throttle angle opening. The performance and exhaust emissions for HCNG fuel were compared to those of CNG. The mass fraction burned is a factor of combustion performance and is usually expressed as a percentage. The mass fractions burned are derived from pressure data; the timing of the mass fraction burned was compared to the spark advance timing. Mass fractions burned of 10% and 90% are typically observed at the start and end of combustion, respectively [16–18].
Table 1 Engine specifications. Type
Description
Number of cylinders Bore (mm) Stroke (mm) Displacement volume (cc) Compression ratio Max. power Max. torque
6 123 155 11,050 11.5 222 kW @ 2100 rpm 1150 Nm @ 1260 rpm
3. Results and discussion 3.1. Full load performance characteristics The level of NOx required by the enforced emissions standards can be satisfied via combustion in a lean-burning natural gas en-
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Fig. 1. Schematic of experimental setup.
Table 2 Intake and exhaust valve timings for each camshaft. Item
Original camshaft Camshaft with reduced valve overlap duration
Intake valve (CAD)
Exhaust valve (CAD)
Valve overlap (CAD)
Open at
Close at
Open at
Close at
BTDC 18 BTDC 10
ABDC 34 ABDC 42
BBDC 46 BBDC 54
ATDC 14 ATDC 6
gine, which improves the thermal efficiency. However, hydrocarbon emissions, including CH4, should be filtered by an OC to satisfy the required level reported in a previous study [10]. The EURO-VI standard decreases the CH4 level by more than one-half compared to the EURO-V standard (1.1 g/kW h). To improve the conversion efficiency, the precious metal content of the OC must be increased. This leads to both a decrease in the cost-effectiveness of the HCNG engine and an increase in the potential for thermal aging. Therefore, an alternative method is critically required for CH4 reduction, without an after-treatment system such as an OC, during the combustion process. It is established that the majority of THC emissions are produced as a result of crevices in the piston ring or the head gasket. Wentworth reported that THC emissions were dependent on the width and depth of the top ring land and the unburned mixture, retained in the crevices, that is emitted as the exhaust valve opens [19]. In the present study, a modified camshaft with a reduced VO duration was utilized. This was because altering the combustion chamber geometry required a complicated redesign of the main components, such as the piston and engine head. Some of the THC emitted from the cylinder could be reduced by closing the exhaust valve early when the piston approached the TDC. Fig. 2 shows the torque values at the MBT spark timing with respect to the excess air ratios for each VO condition. The torque values satisfy the requirements based on the engine specifications,
32 16
Fig. 2. Full load performance for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
except at k = 1.7 (the lean-burn limit). The torque value using CNG and the original camshaft is sufficiently high at the lean-burn limit. However, when using HCNG, the value decreases slightly because of the decreased exhaust gas energy. The introduction of HCNG fuel results in an improvement in the thermal efficiency. However, at a specific level of input energy, the increase in thermal
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efficiency corresponds to a decrease in the exhaust gas energy, as well as the temperature and the flow rate. Regardless of the fuel type, changing the camshaft with a decreased VO duration decreases the torque values at the lean-burn limit. The decrease in volumetric efficiency due to the decreased VO results in a reduced torque for the modified camshaft. The boost pressure of the modified camshaft is higher than that of the original camshaft. This is because the amount of the fuel/air mixture must be maintained to accomplish a full load performance, even with a decreased volumetric efficiency, as shown in Fig. 3. The required boost pressure increases with increase in the excess air ratio. However, the amount of mixture at the lean-burn limit is insufficient for the required torque. Consequently, the decreased exhaust gas energy strongly affects the decrease in the boost pressure and the torque values [20]. Under a certain excess air ratio condition, the increase in boost pressure can be realized by increasing the input energy (the fuel flow rate). As a result, the thermal efficiency decreases, and an increase in the exhaust gas temperature is associated with the modified camshaft, as shown in Fig. 4. Similar levels of exhaust gas temperatures at the lean-burn limit indicate that the decrease in the exhaust gas energy is caused by the flow rate and not by the temperature. Thus, the extent of the thermal efficiency decrease is less than that of the torque reduction. Fig. 5 shows the spark advance timings and the timings for each percentage of mass fraction burned with respect to the excess air ratios. The spark advance timing did not change with the VO because an increase in the mixture does not affect the overall combustion at a given excess air ratio. However, the timings for a mass fraction burned of 10%, which is considered the start of combustion, increase with decrease in the VO duration. The increase in the residual gas, which is caused by a decreased VO duration, adds thermal energy to the uncompressed mixture, and consequently, the ignition delay decreases. These results show a remarkably different trend compared to certain previous research [21]. However, exhaust gas recirculation (EGR) occurs owing to the slowing of residual gas oxidation kinetics, which is closely related to the combustion speed. As a result, the timing for a mass fraction burned of 90%, which is considered the end of combustion, is comparable to that for the CAD.
Fig. 4. Exhaust gas temperature for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
Fig. 5. Ignition delay and combustion speed for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
The torque values due to a decreased VO duration at the leanburn limit do not meet the required values for the engine specifications. However, the effects of the modified camshaft on the THC and CH4 emissions are another indication that the torque values can be satisfied by operating at a richer mixture condition or by
improving the boosting system. Figs. 6 and 7 show the THC and CH4 emissions for each fuel and camshaft condition, respectively. By employing a camshaft with a decreased VO duration, reduced THC and CH4 emissions were achieved for each fuel. The level of the THC increases steeply as the piston moves to the TDC because the unburned mixture, which remains in the cylinder via various storage mechanisms, is emitted. However, by closing the exhaust valve early, a certain amount of the unburned mixture and burned gas is trapped within the cylinder. This strategy aids in the reduction of THC and CH4 when the amount of residual gas increases.
Fig. 3. Boost pressure for each fuel vs. excess air ratio using original camshaft/ modified camshaft with reduced valve overlap duration.
Fig. 6. Total hydrocarbon emissions for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
3.2. Emission characteristics
C. Park et al. / Fuel 123 (2014) 101–106
Fig. 7. Methane emissions for each fuel vs. excess air ratio using original camshaft/ modified camshaft with reduced valve overlap duration.
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combustion. This is similar to the analysis results of the abovementioned full load performance. The change in NOx emissions must be evaluated because the slight increase in the residual gas affects the ignition delay and the combustion speed. Fig. 8 shows the results of the NOx emission variation with respect to the excess air ratio conditions for different camshafts. The level of NOx emissions for the camshaft with a reduced VO duration is slightly higher than or comparable to that for the original camshaft. The increase in the EGR rate, caused by the trapped residual gas, generally reduces the combustion speed and temperature. Thus, the level of NOx emissions was expected to reduce via the dilution effect. This countertrend is assumed to emerge because of the additional thermal energy of the residual gas. As a result, the apparent heat release rate of the camshaft, with a reduced VO duration, is higher than that of the original camshaft, as shown in Fig. 9. The combustion duration is longer owing to the decreased combustion speed. These results imply the deterioration of the thermal efficiency because the heat release is increased and the combustion duration is longer. Thus, the power output is lowered for a given operation point, even with a higher heat release peak. 4. Conclusions The effects of the VO on the full load performance and emission characteristics of an HCNG engine were investigated. The torque values and emissions resulting from modifying the camshaft, by decreasing the VO duration, were examined for a heavy-duty natural gas. The performance, implementing an operational strategy change, was also evaluated to determine whether the engine’s specifications were satisfied. The results from the experiments are summarized as follows:
Fig. 8. Nitrogen oxides emissions for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
(1) The camshaft with a reduced VO duration has lower torque values at the lean-burn limit because of the decrease in volumetric efficiency. The decrease in exhaust gas energy also significantly decreases the boost pressure and torque values. (2) The increase in residual gas, caused by the reduced VO duration, shortens the ignition delay through the addition of thermal energy to the intake mixture. On the other hand, it decreases the speed of the oxidation kinetics, which is closely related to the combustion speed. (3) The trapping of the unburned mixture and burned gas, caused by closing the exhaust valve early, is the main mechanism of the THC and CH4 emission reduction. The THC and the CH4 emission reduction is approximately 41% for the HCNG fuel. (4) The level of the NOx emissions from the camshaft with a reduced VO duration is slightly higher than or comparable to the emissions from the original camshaft. This is because of the additional thermal energy of the residual gas.
References Fig. 9. Heat release rate for HCNG fuel at an excess air ratio of 1.6 vs. crank angle using original camshaft/modified camshaft with reduced valve overlap duration.
The THC and CH4 reductions average approximately 28% and 41% for CNG and HCNG, respectively. The reduction ratio is different for each fuel because of the lower levels of THC and CH4 in HCNG. However, the extents of the reduction are consistent for each excess air ratio and average approximately 0.82 and 0.63 g/kW h for THC and CH4, respectively. The trapping of the unburned mixture and burned gas through closing the exhaust valve early is believed to be the main reason for emission reduction. The reduced VO duration negligibly influences the oxidation kinetics during
[1] Jahirul 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. Appl Therm Eng 2010;30:2219–26. [2] Lin W, Zhang N, Gu A. LNG (liquefied natural gas): a necessary part in China’s future energy infrastructure. Energy 2010;35:4383–91. [3] Abdullah S, Kurniawan WH, Khamas M, Ali Y. Emission analysis of a compressed natural gas directinjection engine with a homogenous mixture. Int J Automot Technol 2011;12:29–38. [4] Cho HM, He B-Q. Combustion and emission characteristics of a lean burn natural gas engine. Int J Automot Technol 2008;9:415–22. [5] Collier K, Mulligan N, Shin D, Brandon S. Emission results from the new development of a dedicated hydrogen – enriched natural gas heavy duty engine. Warrendale, PA: SAE International; 2005. [6] Karim GA, Wierzba I, Al-Alousi Y. Methane-hydrogen mixtures as fuels. Int J Hydrog Energy 1996;21:625–31.
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[7] Ma F, Ding S, Wang Y, Wang M, Jiang L, Naeve N, et al. Performance and emission characteristics of a spark-ignition (SI) hydrogen-enriched compressed natural gas (HCNG) engine under various operating conditions including idle conditions. Energy Fuels 2009;23:3113–8. [8] Thipse SS, Rairikar SDK, Chitnis PP. Development of a six cylinder HCNG engine using an optimized lean burn concept. Warrendale, PA: SAE, International; 2009. [9] Park C, Lee S, Lim G, Choi Y, Kim C. Effect of mixer type on cylinder-to-cylinder variation and performance in hydrogen-natural gas blend fuel engine. Int J Hydrog Energy 2013;38:4809–15. [10] Park C, Kim C, Choi Y, Lee J. Operating strategy for exhaust gas reduction and performance improvement in a heavy-duty hydrogen-natural gas blend engine. Energy 2013;50:262–9. [11] Ma F, Wang M, Jiang L, Deng J, Chen R, Naeve N, et al. Performance and emission characteristics of a turbocharged spark-ignition hydrogen-enriched compressed natural gas engine under wide open throttle operating conditions. Bio-Ethanol Renew Sour Reforming Process Sustain Hydrog Prod 2010;35:12502–9. [12] Boam DJ, Finlay IC, Biddulph TW, Ma TA, Lee R, Richardson SH, et al. The sources of unburnt hydrocarbon emissions from spark ignition engines during cold starts and warm-up. Arch Proc Inst Mech Eng Part J Automob Eng 1989– 1996 1994;203–210(208):1–11. [13] Choi K, Lee H, Hwang IG, Myung C-L, Park S. Effects of various intake valve timings and spark timings on combustion, cyclic THC and NOx emissions
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during cold start phase with idle operation in CVVT engine. J Mech Sci Technol 2009;22:2254–62. Myung CL, Choi KH, Hwang IG, Lee KH, Park S. Effects of valve timing and intake flow motion control on combustion and time-resolved HC and NOx formation characteristics. Int J Automot Technol 2009;10:161–6. Park C, Kim C, Choi Y, Won S, Moriyoshi Y. The influences of hydrogen on the performance and emission characteristics of a heavy duty natural gas engine. Int J Hydrog Energy 2011;36:3739–45. Chun KM, Heywood JB. Estimating heat-release and mass-of-mixture burned from spark-ignition engine pressure data. Combust Sci Technol 1987;54:133–43. Ma F, Wang Y, Liu H, Li Y, Wang J, Zhao S. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. Int J Hydrog Energy 2007;32:5067–75. Metghalchi M, Keck JC. Laminar burning velocity of propane–air mixtures at high temperature and pressure. Combust Flame 1980;38:143–54. Wentworth JT. Piston and ring variables affect exhaust hydrocarbon emissions. Warrendale, PA: SAE International; 1968. Park C, Kim C, Choi Y. Power output characteristics of hydrogen-natural gas blend fuel engine at different compression ratios. Int J Hydrog Energy 2012;37:8681–7. Jang J, Yeom K, Bae C. Effects of exhaust throttling on engine performance and residual gas in an SI engine. Warrendale, PA: SAE International; 2004.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full load performance and emission characteristics of hydrogen-compressed natural gas engines with valve overlap changes Cheolwoong Park a,⇑, Sungwon Lee a, Gihun Lim b, Young Choi a, Changgi Kim a a Department of Engine Research, Environmental and Energy System Research Division, Korea Institute of Machinery and Materials, 156 Gajeongbuk-Ro, Yuseong-Gu, Daejeon 305-343, Republic of Korea b Environment and Energy Mechanical Engineering, University of Science and Technology, 217 Gajeong-Ro, Yuseong-gu, Daejeon 305-700, Republic of Korea
h i g h l i g h t s Altering the cam with a decreased valve overlap decreases torque at the lean limit. THC and the CH4 reductions are approximately 41% for the HCNG fuel with reduced valve overlap. The NOx from the camshaft with decreased valve overlap is higher than that of original camshaft.
a r t i c l e
i n f o
Article history: Received 12 August 2013 Received in revised form 2 January 2014 Accepted 11 January 2014 Available online 31 January 2014 Keywords: Hydrogen-compressed natural gas Valve overlap Full load performance Lean-burn limit Methane emission
a b s t r a c t Natural gas vehicle engines are preferred over diesel engines because they release significantly lower amounts of nitrogen oxides (NOx) and carbon dioxide emissions. Hydrogen-compressed natural gas (HCNG) technology is a promising alternative to that used in conventional compressed natural gas engines because hydrogen possesses stable lean combustion characteristics. Although the NOx emissions requirements are satisfied, methane emissions must be filtered using an oxidation catalyst. An alternative is required for methane reduction because improving the conversion efficiency of methane is expensive. In this study, the strategy of varying the valve overlap was employed to reduce methane emissions. Although a torque valve cannot meet engine emission specifications, the hydrocarbon and methane emissions were reduced by approximately 41% by decreasing the valve overlap duration while using HCNG fuel. The level of NOx emissions was approximately equivalent to or slightly higher than that of a conventional camshaft. Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction The world energy market is attempting to diversify energy sources because of the rapid increase in the use of fossil fuels and the problem of global warming that is partially caused by automotive vehicles. Natural gas vehicles (NGVs) have the advantages of low carbon dioxide (CO2) emissions and a wide range of application; the can be used in passenger vehicles, trucks, and buses [1,2]. NGVs were initially promoted for the consumption of overproduced natural gas. However, now, they also save energy or can be used as an alternative fuel. NGVs are preferable to diesel engine-based vehicles because of their low harmful emission characteristics. However, although the development of NGVs is a prior⇑ Corresponding author. Tel.: +82 42 868 7928; fax: +82 42 868 7305. E-mail addresses: [email protected] (C. Park), [email protected] (S. Lee), [email protected] (G. Lim), [email protected] (Y. Choi), [email protected] (C. Kim).
ity, it faces challenges such as the development of technology for clean diesel engines and insufficient infrastructure for recharging. Recently, the development of NGV technology has focused, especially in the case heavy duty vehicles, on reducing NOx and achieving high thermal efficiency through the improvement in fuel consumption [3,4]. Compressed natural gas (CNG)-hybrid engines significantly increase the cost of production, although they achieve the improvement required by the aforementioned goals. Furthermore, hydrogen-compressed natural gas (HCNG) is a promising fuel technology that can improve the performance of CNG engines. HCNG is considered to be a type of mono-fuel, and it is a mixture of 20–30 vol% of hydrogen and natural gas. The advantages of HCNG use in vehicles were proved through tests in the United States and Canada. Many research results for HCNG engines have been reported which indicate that HCNG engines can satisfy future emission regulations (e.g., as outlined in EURO-VI) and are economically feasible [5–8].
http://dx.doi.org/10.1016/j.fuel.2014.01.041 0016-2361/Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
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An HCNG engine exhibits a leaner combustion than a CNG engine, which is possible because of the stable lean combustion characteristics of hydrogen. Consequently, nitrogen oxides (NOx) and CO2 emissions can be significantly reduced. HCNG technology can be applied in traditional CNG engines and their fuel supply systems without any modifications. Additionally, the employment of NOx after-treatment systems is not required. NOx emissions caused by lean combustion can meet the EURO-VI standard (0.46 g/kW h). However, carbon monoxide (CO) and total hydrocarbon (THC) emissions, including methane (CH4), must be filtered through an oxidation catalyst (OC) to meet their corresponding standard (0.5 g/kW h) [9–11]. CH4 has a very stable molecular structure, and improvement in the conversion efficiency of the OC requires an increase in the amount of precious metal used in the OC. In order to ensure that the HCNG engine is cost-effective, a less expensive alternative is required for CH4 reduction. In the present study, a strategy of decreasing the valve overlap (VO) was employed to reduce CH4 emissions. Closing the exhaust valve earlier can lessen the level of hydrocarbon and CH4 emissions. This is because the unburned mixture retained in the crevices emerges around the valve seat as the exhaust valve opens and the piston approaches the top dead center (TDC) [12–14]. The effects of decreasing the VO on the full load performance and the emission characteristics are evaluated. The applicability of a camshaft with a reduced VO duration was assessed by taking into account its potential usability in the field.
2. Experimental procedures
tremely low temperature could cause malfunctioning of the fuel supply system because of the expansion of the compressed fuel in the regulator. The flow rate of CNG was measured using a mass flow meter and that of hydrogen was directly controlled using a mass flow controller. The individual in-cylinder pressure was measured using piezoelectric pressure transducers (Kistler Co., Ltd., 6117BFD17). For the analysis of the in-cylinder data, a high-precision rotary encoder with a pulse per revolution of 1800 and a combustion analyzer (Dewetron Co., Dewe 800) were used to observe the combustion stability. The excess air ratio of the air–fuel charge was monitored using a lambda meter (ETAS Co., LA4) located at the exhaust runner of an individual cylinder and downstream of the turbine. To evaluate the effect of the VO, the torque, in-cylinder pressure, and emissions were measured for each VO condition. The VO with the original camshaft had a crank angle degree (CAD) of 32. In comparison, the duration of the VO with the modified camshaft decreased by 50% (to 16 CAD). The intake and exhaust valve timings for each camshaft are summarized in Table 2. For each cylinder, an exhaust gas-sampling probe was installed between the exhaust valve and the exhaust manifold. Then, the common emission of the engine was measured after the turbocharger. The composition of the exhaust gas, which included CO2, CO, THC, and oxygen (O2), was analyzed using an exhaust gas analyzer (AVL Co., AMA i60). The temperatures and pressures in the major parts of the engine, fuel flow, and exhaust gas concentrations were measured, and the resulting data was stored using a data acquisition system (GRAPHTEC Co., GL820).
2.1. Experimental setup
2.2. Experimental methods
An 11-L 6-cylinder CNG engine (Doosan Infracore Inc., GL11K) that satisfied the EURO-V emission regulations was used in the test. The detailed engine specifications are listed in Table 1. The speed and torque of the engine were controlled and monitored using an eddy current dynamometer (Schenck). The spark ignition timing and the fuel flow rate were controlled using a computerbased universal engine control unit. An electronic throttle control and a waste gate control for the turbocharger system were employed to control the excess air ratio under each operating condition. Fig. 1 shows the experimental setup for the engine tests. The CNG from the compressed fuel vessel, which has a pressure of approximately 15 MPa, was decompressed to 0.67 MPa using a conventional regulator for vehicles. Next, it was supplied by an eight-gas fuel-metering valve connected to a gas mixer. A series of compressed tanks for hydrogen, which had a pressure of approximately 12 MPa, were employed and connected to the pressure controller. A 3:7 volumetric ratio of hydrogen to natural gas was selected for the HCNG. This is because a 30% hydrogen concentration is considered to be the optimal ratio based on previous research [15]. The CNG and hydrogen, after passing through the regulator, were heated to 40 °C using a heat exchanger. This is because an ex-
The engine was operated at 1260 rpm with 1150 Nm, which are the operating conditions of a heavy-duty natural gas engine at maximum torque with a wide-open throttle. The spark ignition timing was selected to maximize the thermal efficiency, and the maximum brake torque (MBT) was determined for each operating condition. The excess air ratio was increased, in increments of 0.1, from k = 1.3 to the lean limit. The combustion performance and exhaust emissions were analyzed with respect to the excess air ratio for each fuel and VO duration. The temperatures of the CNG and intake air downstream of the intercooler were maintained at 42.5 ± 2.5 °C, and the engine-inlet coolant temperature was maintained at 82.5 ± 2.5 °C. Water-cooled heat exchangers were used for maintaining the temperatures. An important criterion of the full load performance is the maximum torque value as compared to that in the engine specifications. This value is easily affected by a change in the VO that is accompanied by a change in the volumetric efficiency. The test conditions were evaluated by measuring the boost pressure with respect to the excess air ratio while maintaining the throttle angle opening. The performance and exhaust emissions for HCNG fuel were compared to those of CNG. The mass fraction burned is a factor of combustion performance and is usually expressed as a percentage. The mass fractions burned are derived from pressure data; the timing of the mass fraction burned was compared to the spark advance timing. Mass fractions burned of 10% and 90% are typically observed at the start and end of combustion, respectively [16–18].
Table 1 Engine specifications. Type
Description
Number of cylinders Bore (mm) Stroke (mm) Displacement volume (cc) Compression ratio Max. power Max. torque
6 123 155 11,050 11.5 222 kW @ 2100 rpm 1150 Nm @ 1260 rpm
3. Results and discussion 3.1. Full load performance characteristics The level of NOx required by the enforced emissions standards can be satisfied via combustion in a lean-burning natural gas en-
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Fig. 1. Schematic of experimental setup.
Table 2 Intake and exhaust valve timings for each camshaft. Item
Original camshaft Camshaft with reduced valve overlap duration
Intake valve (CAD)
Exhaust valve (CAD)
Valve overlap (CAD)
Open at
Close at
Open at
Close at
BTDC 18 BTDC 10
ABDC 34 ABDC 42
BBDC 46 BBDC 54
ATDC 14 ATDC 6
gine, which improves the thermal efficiency. However, hydrocarbon emissions, including CH4, should be filtered by an OC to satisfy the required level reported in a previous study [10]. The EURO-VI standard decreases the CH4 level by more than one-half compared to the EURO-V standard (1.1 g/kW h). To improve the conversion efficiency, the precious metal content of the OC must be increased. This leads to both a decrease in the cost-effectiveness of the HCNG engine and an increase in the potential for thermal aging. Therefore, an alternative method is critically required for CH4 reduction, without an after-treatment system such as an OC, during the combustion process. It is established that the majority of THC emissions are produced as a result of crevices in the piston ring or the head gasket. Wentworth reported that THC emissions were dependent on the width and depth of the top ring land and the unburned mixture, retained in the crevices, that is emitted as the exhaust valve opens [19]. In the present study, a modified camshaft with a reduced VO duration was utilized. This was because altering the combustion chamber geometry required a complicated redesign of the main components, such as the piston and engine head. Some of the THC emitted from the cylinder could be reduced by closing the exhaust valve early when the piston approached the TDC. Fig. 2 shows the torque values at the MBT spark timing with respect to the excess air ratios for each VO condition. The torque values satisfy the requirements based on the engine specifications,
32 16
Fig. 2. Full load performance for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
except at k = 1.7 (the lean-burn limit). The torque value using CNG and the original camshaft is sufficiently high at the lean-burn limit. However, when using HCNG, the value decreases slightly because of the decreased exhaust gas energy. The introduction of HCNG fuel results in an improvement in the thermal efficiency. However, at a specific level of input energy, the increase in thermal
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efficiency corresponds to a decrease in the exhaust gas energy, as well as the temperature and the flow rate. Regardless of the fuel type, changing the camshaft with a decreased VO duration decreases the torque values at the lean-burn limit. The decrease in volumetric efficiency due to the decreased VO results in a reduced torque for the modified camshaft. The boost pressure of the modified camshaft is higher than that of the original camshaft. This is because the amount of the fuel/air mixture must be maintained to accomplish a full load performance, even with a decreased volumetric efficiency, as shown in Fig. 3. The required boost pressure increases with increase in the excess air ratio. However, the amount of mixture at the lean-burn limit is insufficient for the required torque. Consequently, the decreased exhaust gas energy strongly affects the decrease in the boost pressure and the torque values [20]. Under a certain excess air ratio condition, the increase in boost pressure can be realized by increasing the input energy (the fuel flow rate). As a result, the thermal efficiency decreases, and an increase in the exhaust gas temperature is associated with the modified camshaft, as shown in Fig. 4. Similar levels of exhaust gas temperatures at the lean-burn limit indicate that the decrease in the exhaust gas energy is caused by the flow rate and not by the temperature. Thus, the extent of the thermal efficiency decrease is less than that of the torque reduction. Fig. 5 shows the spark advance timings and the timings for each percentage of mass fraction burned with respect to the excess air ratios. The spark advance timing did not change with the VO because an increase in the mixture does not affect the overall combustion at a given excess air ratio. However, the timings for a mass fraction burned of 10%, which is considered the start of combustion, increase with decrease in the VO duration. The increase in the residual gas, which is caused by a decreased VO duration, adds thermal energy to the uncompressed mixture, and consequently, the ignition delay decreases. These results show a remarkably different trend compared to certain previous research [21]. However, exhaust gas recirculation (EGR) occurs owing to the slowing of residual gas oxidation kinetics, which is closely related to the combustion speed. As a result, the timing for a mass fraction burned of 90%, which is considered the end of combustion, is comparable to that for the CAD.
Fig. 4. Exhaust gas temperature for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
Fig. 5. Ignition delay and combustion speed for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
The torque values due to a decreased VO duration at the leanburn limit do not meet the required values for the engine specifications. However, the effects of the modified camshaft on the THC and CH4 emissions are another indication that the torque values can be satisfied by operating at a richer mixture condition or by
improving the boosting system. Figs. 6 and 7 show the THC and CH4 emissions for each fuel and camshaft condition, respectively. By employing a camshaft with a decreased VO duration, reduced THC and CH4 emissions were achieved for each fuel. The level of the THC increases steeply as the piston moves to the TDC because the unburned mixture, which remains in the cylinder via various storage mechanisms, is emitted. However, by closing the exhaust valve early, a certain amount of the unburned mixture and burned gas is trapped within the cylinder. This strategy aids in the reduction of THC and CH4 when the amount of residual gas increases.
Fig. 3. Boost pressure for each fuel vs. excess air ratio using original camshaft/ modified camshaft with reduced valve overlap duration.
Fig. 6. Total hydrocarbon emissions for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
3.2. Emission characteristics
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Fig. 7. Methane emissions for each fuel vs. excess air ratio using original camshaft/ modified camshaft with reduced valve overlap duration.
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combustion. This is similar to the analysis results of the abovementioned full load performance. The change in NOx emissions must be evaluated because the slight increase in the residual gas affects the ignition delay and the combustion speed. Fig. 8 shows the results of the NOx emission variation with respect to the excess air ratio conditions for different camshafts. The level of NOx emissions for the camshaft with a reduced VO duration is slightly higher than or comparable to that for the original camshaft. The increase in the EGR rate, caused by the trapped residual gas, generally reduces the combustion speed and temperature. Thus, the level of NOx emissions was expected to reduce via the dilution effect. This countertrend is assumed to emerge because of the additional thermal energy of the residual gas. As a result, the apparent heat release rate of the camshaft, with a reduced VO duration, is higher than that of the original camshaft, as shown in Fig. 9. The combustion duration is longer owing to the decreased combustion speed. These results imply the deterioration of the thermal efficiency because the heat release is increased and the combustion duration is longer. Thus, the power output is lowered for a given operation point, even with a higher heat release peak. 4. Conclusions The effects of the VO on the full load performance and emission characteristics of an HCNG engine were investigated. The torque values and emissions resulting from modifying the camshaft, by decreasing the VO duration, were examined for a heavy-duty natural gas. The performance, implementing an operational strategy change, was also evaluated to determine whether the engine’s specifications were satisfied. The results from the experiments are summarized as follows:
Fig. 8. Nitrogen oxides emissions for each fuel vs. excess air ratio using original camshaft/modified camshaft with reduced valve overlap duration.
(1) The camshaft with a reduced VO duration has lower torque values at the lean-burn limit because of the decrease in volumetric efficiency. The decrease in exhaust gas energy also significantly decreases the boost pressure and torque values. (2) The increase in residual gas, caused by the reduced VO duration, shortens the ignition delay through the addition of thermal energy to the intake mixture. On the other hand, it decreases the speed of the oxidation kinetics, which is closely related to the combustion speed. (3) The trapping of the unburned mixture and burned gas, caused by closing the exhaust valve early, is the main mechanism of the THC and CH4 emission reduction. The THC and the CH4 emission reduction is approximately 41% for the HCNG fuel. (4) The level of the NOx emissions from the camshaft with a reduced VO duration is slightly higher than or comparable to the emissions from the original camshaft. This is because of the additional thermal energy of the residual gas.
References Fig. 9. Heat release rate for HCNG fuel at an excess air ratio of 1.6 vs. crank angle using original camshaft/modified camshaft with reduced valve overlap duration.
The THC and CH4 reductions average approximately 28% and 41% for CNG and HCNG, respectively. The reduction ratio is different for each fuel because of the lower levels of THC and CH4 in HCNG. However, the extents of the reduction are consistent for each excess air ratio and average approximately 0.82 and 0.63 g/kW h for THC and CH4, respectively. The trapping of the unburned mixture and burned gas through closing the exhaust valve early is believed to be the main reason for emission reduction. The reduced VO duration negligibly influences the oxidation kinetics during
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