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Manufacturing and testing of an α-type Stirling engine
Applied Thermal Engineering 130 (2018) 1373–1379
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Manufacturing and testing of an a-type Stirling engine Can Çınar a, Fatih Aksoy b, Hamit Solmaz a, Emre Yılmaz a,⇑, Ahmet Uyumaz c a
Automotive Engineering Department, Faculty of Technology, Gazi University, 06500, Teknikokullar, Ankara, Turkey Automotive Engineering Department, Faculty of Technology, Afyon Kocatepe University, 03200 Afyon, Turkey c Department of Automotive Technology, Vocational High School of Technical Sciences, Mehmet Akif Ersoy University, 15100 Burdur, Turkey b
h i g h l i g h t s An
a-type Stirling engine was manufactured and tested.
Engine was tested at different charge pressures and hot source temperatures. The maximum engine power was obtained as 30.7 W at 437 rpm engine speed. Using helium as working fluid caused an increase in engine power.
a r t i c l e
i n f o
Article history: Received 15 June 2017 Revised 15 November 2017 Accepted 25 November 2017 Available online 2 December 2017 Keywords: Stirling engine V-type Alpha engine Manufacturing
a b s t r a c t This paper presents construction and performance tests of an a-type Stirling engine. Experiments were performed within the range of 1–4 bars charge pressure with air and helium as the working fluid. The outer surface of the expansion cylinder was heated with an electrical heater within the range of heater temperature 800–1000 °C with 50 °C increments. The outer surface of the compression cylinder was cooled by circulating water. Heater temperature and charge pressure were assumed as operating parameters and the engine torque was measured for different engine speeds with air and helium. The engine produced a maximum power of 30.7 W at 437 rpm engine speed, at the heater temperature of 1000 °C and 3.5 bars charge pressure with helium. Engine torque increased with the increase of charge pressure up to 3 bar then started to decrease. By looking the point of engine speed range of the engine, 1000 °C hot end temperature and 3 bar charge pressure was the optimal values. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Concerns about global warming and climate change are growing all around the world. Countries are increasingly taking steps to combat climate change and global warming. Reduction of carbon dioxide (CO2) emission, which is a major greenhouse gas, is the most important measure in combating global warming [1–3]. CO2 is mostly resulted from the combustion of fossil-based fuels. In order to achieve low CO2 release level, diversity of the energy sources must be increased. On the other hand, the world is faced with the risk of depletion of fossil fuels [4,5]. Because of these concerns, supports to researches on renewable energy and high efficiency energy conversion technologies has been increased [6,7]. It was reported that the production of the renewable energy in Europe increased by 84.4% between 2003 and 2013 [8]. Renewable ⇑ Corresponding author. E-mail addresses: [email protected] (C. Çınar), [email protected] (F. Aksoy), [email protected] (H. Solmaz), [email protected] (E. Yılmaz), auyumaz@ mehmetakif.edu.tr (A. Uyumaz). https://doi.org/10.1016/j.applthermaleng.2017.11.132 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
energy can be derived from several energy sources such as wind, internal heat of the earth, solar radiation, wind, flowing water, and biomasses such as energy crops, and agricultural wastes [9,10]. However, in order to meet energy demand, high efficient, low-cost and long-lasting devices that can produce energy from these sources are required. Stirling engine, which was invented by Robert Stirling in 1816 [11], is capable to run with any kind of energy including waste heat [12,13]. Stirling engines draw attention because of their high thermal efficiency. The efficiency of a Stirling engine can reach Carnot efficiency theoretically [14–16]. Moreover they need lower maintenance cost and they can run with different kinds of energy source [17–19]. The Stirling engine is an externally heated reciprocating device, which operates on a closed thermodynamic cycle consisting of two isothermal and two constant volume processes. Although the theoretical thermal efficiency of Stirling cycle is high enough compared to Carnot cycle, heat transfer resistance, energy losses such as gas leakages and heat losses can be seen for real Stirling cycle resulting in lower thermal efficiency [20]. These leakages
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Nomenclature hc hh hp hu
distance between the cold cylinder head and piston top (m) distance between the hot cylinder head and piston top (m) the length of the piston (m) the length of the heating passage of piston (m)
are seen between compression and expansion during displacercylinder gap. Furthermore, flow configuration of the working gas affects the mentioned gas leakages and thermal efficiency [21,22]. It can be also pointed that charge pressure of working fluid and the heater temperature are the other important parameters effecting engine efficiency. Because heat losses could be reduced through expansion cylinder with higher heating temperature and more charge mixture caused to obtain higher thermal efficiency [23–25]. Stirling engines are generally classified in two groups as free piston and kinematic engines. In free piston engines piston and displacer synchronization is provided by springs and gas pressure. Kinematic Stirling engines includes a drive mechanism such as crank [17], rhombic [18], lever [19], ross-yoke etc. [26]. According to piston configurations, Stirling engines are also classified in three groups as alpha, (a) beta (b) and gamma (c) [27]. Beta type Stirling engine has one cylinder and both power piston and displacer are placed coaxially in this cylinder. a and gamma type engines have two cylinder. In a-type engine both of the cylinders contain a piston. However, in gamma type Stirling engines, one of the cylinder contains a power piston and the other cylinder contains a displacer [27,28]. In the past century, numerous studies were performed about Stirling engines. The main researches were conducted on manufacturing, testing and development [29–32]. There are also numerical studies to predict the engine performance and make an optimization [24,33–35]. Trayser and Eibling [36] performed a design study to develop 50 W portable solar-generator for rural applications. It was depicted that a portable, lightweight and reliable solar-powered Stirling engine can be built with a reasonable cost. The overall efficiency was 7.5% and the total cost of the device was 470 dollars. Markman et al. [37] conducted a study on a beta-type Stirling engine. The performance parameters were investigated to reach 200 W power density by measuring the thermal flux and mechanical power loss. Hirata et al. [38] performed a study on a 100 W displacer type Stirling engine to improve the engine performance efficiently. An analysis model was developed to simulate the engine performance of Ecobody-SCM81 engine, which was developed by Japan Society of Mechanical Engineers. Pressure loss in the regenerator, buffer space loss and mechanical loss were taken into consideration in the analysis. Most of the studies were focused on beta-type Stirling engine because of its higher power density [39]. Therefore there are only a few studies conducted with atype Stirling engines. Podesser [40] designed and constructed an a type Stirling engine that was heated with flue gas of a biomass furnace. The aim of the study was to meet the electrical demand in rural villages. a type engine was preferred because of its low cost and maintenance expenditures. Shaft power of the engine reached 3.2 kW with a working gas pressure of 33 bar at 600 rpm. Engine efficiency was 25% for same operation conditions. Batmaz and Ustun [41] designed and manufactured a v-type a Stirling engine for solar applications. Their engine was having two heater cylinder. The engine was designed to obtain 500 W output power, however, the maximum power was obtained as 118 W at 1 bar charge pressure with a hot source temperature of 950 °C. Thermal
r lb h bc bh
radius of the crankshaft (m) the length of the connecting rod (m) crank angle (rad) the angle between connecting rod and vertical axis (rad) the angle between connecting rod and horizontal axis (rad)
efficiency of the engine was 11%. It was reported that the low power and thermal efficiency was caused by excessive leakage in the system and high dead volume. In an experimental study, carried out by Demir and Gungor [42] an air-charged v-type a Stirling engine was manufactured and tested. The performance characteristics of the engine were obtained under different heater temperature and charge pressure values. The maximum power output was obtained as 31 W at 1050 °C heater temperature and 1.5 bar charge pressure. Çınar et al. [43] manufactured and tested a beta-type Stirling engine with rhombic mechanism. They tested the engine with different charge pressures with air and helium working fluid. They obtained maximum power and torque as 95.77 W at 575 rpm and 1.98 Nm at 410 rpm respectively. Karabulut et al. [19] showed test results of the Stirling engine working with a lever controlled displacer driving mechanism. Helium was used in the experiments. They determined that maximum torque and power were measured 3.99 Nm and 183 W at 4 bars charge pressure and 260 °C hot end temperature. Kazimierski and Wojewoda [44] simulated the two stroke externally heated air valve engine (EHVE) which is different from typical Stirling engine. They compared the EHVE and Stirling engine under the same maximum pressures. They pointed that EHVE showed reasonable performance compared to typical Stirling engine. Almajri et al. [45] investigated the performance of V- a type Stirling engine at different operating conditions via combining thermodynamic model with 3D CFD analysis. They developed 3D CFD model and compared the results with thermodynamic model. Maximum power could be obtained with the increase of porosity up to 80%. They also pointed out that using CFD modelling was useful in order to enhance the V- a type Stirling engine performance. Shendage et al. [46] investigated the design methodology of Beta type Stirling engine with rhombic drive mechanism. They researched the optimization of the phase angle and the effects of overlapping between the compression and expansion processes. Abuelyamen et al. [47] performed a parametric study on a b-type Stirling engine with ANSYS fluent 14.5 software. They investigated the effects of initial charge pressure, thermal boundary condition; and three different types of working fluids (Air, He and H2). It was determined that the best performance was obtained with H2 gas. It was also found that small pressure difference across the engine chambers. Most of the studies related to Stirling engines were performed on beta type engines. There are very few studies related about Vtype a engines. In the present study, design, construction and performance tests of an a-type Stirling engine are presented. Helium and air are used as working fluid. An electrical furnace is used as the heat source. 2. Engine specifications 2.1. Test engine The schematic view and the photograph of the test engine are illustrated in Figs. 1 and 2, respectively. The engine consists of two pistons, called the hot and cold pistons, a crankcase, a crankshaft, two connecting rods, a flywheel and a connection pipe.
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Fig. 1. Schematic view of the test engine.
The expansion and compression spaces are formed in separate cylinders, situated at 90° angles to each other. Two pistons were connected to the same crankshaft rod journal by two lightweight connecting rots made of aluminum. Pistons were made of sphero cast iron considering its low friction. Pistons were extended with hubs made of stainless steel. A 1 mm gap was left between piston hubs and cylinders. The gap between the cylinder wall and the hot piston hub was used as heating passage and the gap between the cylinder wall and the cold piston hub was used as cooling passage (Fig. 1). The contact surfaces of pistons were machined to superfinish quality by grinding and polishing. Inner surfaces of the cylinders were machined by boring and grinding. A 0.02 mm working clearance was left between pistons and cylinders. The external surface of the compression cylinder was cooled by circulating water trough the water jacket around it. The crankshaft was dynamically balanced to reduce engine vibration. Construction details of the engine are given by Cinar [48]. Technical specifications of the engine are shown in Table 1. 2.2. Principle of engine operation and kinematic relations The theoretical cycle of the Stirling engine consists of four processes namely isothermal compression, constant volume heating, isothermal expansion and constant volume cooling processes (Fig. 3). As shown in Fig. 1, when the crank pin moves from 1 to 2, both pistons move simultaneously upward. During this process, the working fluid gives heat to the cooling water and the isothermal compression is performed. As the crank pin moves from 2 to
3, the hot piston moves downward and the cold piston moves upward. During this process, the working fluid is expelled from the cold space to the hot space and constant volume heating process is performed. While the crank pin moves from 3 to 4, both pistons move downward and the isothermal expansion process is realized. While the crank pin moves from 4 to 1, the cold piston moves downward and the hot piston moves upward and constant volume heat rejection is performed. The variation of cold volume, hot volume and total volume versus the crank angle is illustrated in Fig. 4. Implementation procedure and details of the analysis were presented by Cinar [48]. To calculate the values of cold volume and hot volume the kinematic relations,
hh ¼ r cos h þ lb cos bh þ hp þ hu hc ¼ r cos
p
h þ lb cos bc þ hp þ hu
2
ð1Þ ð2Þ
are used. In these equations, bc and bh are angles made by cold piston rod and hot piston rod with cylinder axis, respectively and are shown in Fig. 1. Relations between h and bc , bh are,
bc ¼ Arcsin
r sinh ‘b
bh ¼ Arcsin
p r sin h lb 2
ð3Þ
ð4Þ
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Fig. 2. Photograph of the test engine.
Table 1 Technical specifications of the engine. Parameters
Specification
Mechanical configuration Expansion cylinder
a
Compression cylinder
Bore (mm) Stroke (mm) Swept volume (cc) Bore (mm) Stroke (mm) Swept volume (cc)
Phase angle Heater temperature (°C) Cooler temperature (°C) Charge pressure (bar) Working fluid Cooling system Maximum engine power
50 50 98 50 50 98 90° 800 – 1000 °C 30 °C 1–4 Air and Helium Water cooled 30.7 W at 437 rpm Fig. 4. Variation of cold volume, hot volume and total volume with crank angle.
3. Experimental apparatus
Fig. 3. Theoretical Stirling cycle.
The schematic view of the test equipment is shown in Fig. 5. An electrical heater was used as the heat source. The temperature of the heater was adjustable between 0 °C and 1200 °C with an accuracy of 1 °C. The hot end of the expansion cylinder was inserted into the heater and it was heated within the heater temperature of 800–1000 °C. A prony brake dynamometer was used to measure the engine torque. The speed of the engine was measured by a digital tachometer with an accuracy of 1 rpm. Temperatures were measured by means of multi point temperature indicator (type
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Fig. 5. Schematic view of the test equipment.
ELIMKO-6000) having 1 °C accuracy. As the working fluid, air and helium were used. For the regulation of pressure of working fluid, a pressure-reducing valve was used. Cooling water outlet temperature was kept at 30 °C supplying sufficient water. 4. Experimental results and discussion In the experimental investigation a temperature adjustable heater was used as the heat source. The engine starts to run at 520 °C heater temperature. Systematic testing was performed within the range of 800–1000 °C heater temperature with 50 °C increments and 1–4 bars charge pressure with 0.5 bar increments. Heater temperature and charge pressure were assumed as operating parameters and the engine torque was measured with an accuracy of 0.003 Nm at various engine speeds by loading with the dynamometer. The output power was calculated from;
P¼2pT n
ð5Þ
where n is engine speed and T is engine torque.Fig. 6 shows the variation of torque with engine speed at 1000 °C heater temperature and different rates of air charge pressures from 1 to 3.5 bars. The maximum engine torque was obtained at 215 rpm as 0.7 Nm. Increasing charge pressure resulted with a higher torque level
Fig. 6. Variation of engine torque versus engine speed.
because of increased working fluid mass. However, after charge pressure of 3 bar, the engine torque started to decrease again. This indicated that the working fluid cannot be effectively used because of insufficient heat transfer surface area in the cylinder. It is seen that higher torque is obtained at low engine speeds because of the longer heating-cooling period. It is also seen that the operation speed range of the engine enlarges with the increased charge pressure. Increasing charge pressure implies the increase of the working fluid mass. Higher cyclic work generation can be obtained with higher mass of working fluid and as a result of this engine speed can reach a higher level. However, over increasing the mass of the working fluid by comparing the heat transfer surface area, speed range of the engine may reduce because of the insufficient heat transfer to the working gas. Fig. 7 shows the variation of power output as a function of engine speed for various heater temperatures between 800 °C and 1000 °C and 3 bars air charge pressure. An increase in the power output was obtained depend on the heater temperature. As the heater temperature increases, the difference between the hot and cold space temperatures increases. At the heater temperature of 1000 °C, the maximum power output is obtained as 18.13 W. With a lower heater temperature of 800 °C, the maximum power output is obtained as 6.5 W. As seen in Fig. 7, engine power
Fig. 7. Variation of power output versus engine speed.
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35
Power Output (W)
30 25 20 1.5 bar air
15
2.0 bar air 2.5 bar air
10
3.0 bar air 2.0 bar He
5
3.0 bar He 3.5 bar He
0 150
200
250
300
350
400
450
500
550
600
650
700
Engine Speed (rpm) Fig. 8. Variation of power output versus heater temperature.
Fig. 10. Variation of power output versus engine speed.
increases with increase of the engine speed up to a certain value of speed then starts to decrease. This behavior is an expected behavior for most of the engines, because, cyclic work generation starts to decrease at higher engine speeds. This reduction in cyclic work results from insufficient time for heat transfer and insufficient heat transfer surface area. The variation of power output with heater temperature using air and helium is shown in Fig. 8. It is observed that, an increase in the heater temperature results in an increase in power output. Because of higher heat transfer capabilities of helium, it has a higher power output than air for the same heater temperature and charge pressure. At the heater temperature of 1000 °C and 3 bars charge pressure, the maximum power output was measured as 18.13 W at 286 rpm engine speed for air while it was measured as 27.33 W at 418 rpm engine speed for helium. The working fluids having higher specific heat capacities such as hydrogen, might be contributed to increase the engine performance in this manner. The maximum power of the engine was 30.7 W at 437 rpm engine speed, at the heater temperature of 1000 °C and 3.5 bars charge pressure with helium. The engine manufactured by Batmaz and Ustun [41] had a maximum power output of 118 W. The difference in power outputs of the engines may be resulted because of the heating performances of the engines. Their engine was using double acting heater cylinder. Fig. 9 shows the variation of power output with charge pressure at 900 and 1000 °C heater temperatures using air and helium. Power output increases with the increase of charge pressure until a certain power value. More power output was obtained with helium compared to air working fluid with higher heating
temperature. In addition, maximum power was obtained with helium at 1000 °C and 3.5 bar charge pressure. Fig. 10 shows the variation of power output with engine speed at a constant heater temperature of 1000 °C and different rates of air and helium charge pressures. Comparison of curves obtained for 3 bars charge pressure air and helium shows that the maximum power output is obtained at 286 rpm for air and at 417 rpm for helium. As seen in Fig. 10, depending on the engine speed the power output is increased at a certain level. A decrease in the power is seen after that speed, due to insufficient heat transfer rate.
5. Conclusions In this study, a small Stirling engine with 98 cc total swept volume was manufactured and performance tests were performed. The test engine has reached a maximum power output of 30.7 W at 437 rpm engine speed, at the heater temperature of 1000 °C and 3.5 bars charge pressure using helium as working fluid. An increase trend was observed in engine torque with the increase of charge pressure up to 3 bar then it started to decrease. With an increase in the heater temperature, the power output and torque of the engine was increased. For the same heater temperature, higher power output was obtained with helium due to higher thermal conductivity compared to air. However, the use of air as the working fluid in Stirling engines has great advantages, such as abundant, readily available and free of charge except for compression cost. References
Fig. 9. Variation of power output versus charge pressure.
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Manufacturing and testing of an a-type Stirling engine Can Çınar a, Fatih Aksoy b, Hamit Solmaz a, Emre Yılmaz a,⇑, Ahmet Uyumaz c a
Automotive Engineering Department, Faculty of Technology, Gazi University, 06500, Teknikokullar, Ankara, Turkey Automotive Engineering Department, Faculty of Technology, Afyon Kocatepe University, 03200 Afyon, Turkey c Department of Automotive Technology, Vocational High School of Technical Sciences, Mehmet Akif Ersoy University, 15100 Burdur, Turkey b
h i g h l i g h t s An
a-type Stirling engine was manufactured and tested.
Engine was tested at different charge pressures and hot source temperatures. The maximum engine power was obtained as 30.7 W at 437 rpm engine speed. Using helium as working fluid caused an increase in engine power.
a r t i c l e
i n f o
Article history: Received 15 June 2017 Revised 15 November 2017 Accepted 25 November 2017 Available online 2 December 2017 Keywords: Stirling engine V-type Alpha engine Manufacturing
a b s t r a c t This paper presents construction and performance tests of an a-type Stirling engine. Experiments were performed within the range of 1–4 bars charge pressure with air and helium as the working fluid. The outer surface of the expansion cylinder was heated with an electrical heater within the range of heater temperature 800–1000 °C with 50 °C increments. The outer surface of the compression cylinder was cooled by circulating water. Heater temperature and charge pressure were assumed as operating parameters and the engine torque was measured for different engine speeds with air and helium. The engine produced a maximum power of 30.7 W at 437 rpm engine speed, at the heater temperature of 1000 °C and 3.5 bars charge pressure with helium. Engine torque increased with the increase of charge pressure up to 3 bar then started to decrease. By looking the point of engine speed range of the engine, 1000 °C hot end temperature and 3 bar charge pressure was the optimal values. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Concerns about global warming and climate change are growing all around the world. Countries are increasingly taking steps to combat climate change and global warming. Reduction of carbon dioxide (CO2) emission, which is a major greenhouse gas, is the most important measure in combating global warming [1–3]. CO2 is mostly resulted from the combustion of fossil-based fuels. In order to achieve low CO2 release level, diversity of the energy sources must be increased. On the other hand, the world is faced with the risk of depletion of fossil fuels [4,5]. Because of these concerns, supports to researches on renewable energy and high efficiency energy conversion technologies has been increased [6,7]. It was reported that the production of the renewable energy in Europe increased by 84.4% between 2003 and 2013 [8]. Renewable ⇑ Corresponding author. E-mail addresses: [email protected] (C. Çınar), [email protected] (F. Aksoy), [email protected] (H. Solmaz), [email protected] (E. Yılmaz), auyumaz@ mehmetakif.edu.tr (A. Uyumaz). https://doi.org/10.1016/j.applthermaleng.2017.11.132 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
energy can be derived from several energy sources such as wind, internal heat of the earth, solar radiation, wind, flowing water, and biomasses such as energy crops, and agricultural wastes [9,10]. However, in order to meet energy demand, high efficient, low-cost and long-lasting devices that can produce energy from these sources are required. Stirling engine, which was invented by Robert Stirling in 1816 [11], is capable to run with any kind of energy including waste heat [12,13]. Stirling engines draw attention because of their high thermal efficiency. The efficiency of a Stirling engine can reach Carnot efficiency theoretically [14–16]. Moreover they need lower maintenance cost and they can run with different kinds of energy source [17–19]. The Stirling engine is an externally heated reciprocating device, which operates on a closed thermodynamic cycle consisting of two isothermal and two constant volume processes. Although the theoretical thermal efficiency of Stirling cycle is high enough compared to Carnot cycle, heat transfer resistance, energy losses such as gas leakages and heat losses can be seen for real Stirling cycle resulting in lower thermal efficiency [20]. These leakages
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Nomenclature hc hh hp hu
distance between the cold cylinder head and piston top (m) distance between the hot cylinder head and piston top (m) the length of the piston (m) the length of the heating passage of piston (m)
are seen between compression and expansion during displacercylinder gap. Furthermore, flow configuration of the working gas affects the mentioned gas leakages and thermal efficiency [21,22]. It can be also pointed that charge pressure of working fluid and the heater temperature are the other important parameters effecting engine efficiency. Because heat losses could be reduced through expansion cylinder with higher heating temperature and more charge mixture caused to obtain higher thermal efficiency [23–25]. Stirling engines are generally classified in two groups as free piston and kinematic engines. In free piston engines piston and displacer synchronization is provided by springs and gas pressure. Kinematic Stirling engines includes a drive mechanism such as crank [17], rhombic [18], lever [19], ross-yoke etc. [26]. According to piston configurations, Stirling engines are also classified in three groups as alpha, (a) beta (b) and gamma (c) [27]. Beta type Stirling engine has one cylinder and both power piston and displacer are placed coaxially in this cylinder. a and gamma type engines have two cylinder. In a-type engine both of the cylinders contain a piston. However, in gamma type Stirling engines, one of the cylinder contains a power piston and the other cylinder contains a displacer [27,28]. In the past century, numerous studies were performed about Stirling engines. The main researches were conducted on manufacturing, testing and development [29–32]. There are also numerical studies to predict the engine performance and make an optimization [24,33–35]. Trayser and Eibling [36] performed a design study to develop 50 W portable solar-generator for rural applications. It was depicted that a portable, lightweight and reliable solar-powered Stirling engine can be built with a reasonable cost. The overall efficiency was 7.5% and the total cost of the device was 470 dollars. Markman et al. [37] conducted a study on a beta-type Stirling engine. The performance parameters were investigated to reach 200 W power density by measuring the thermal flux and mechanical power loss. Hirata et al. [38] performed a study on a 100 W displacer type Stirling engine to improve the engine performance efficiently. An analysis model was developed to simulate the engine performance of Ecobody-SCM81 engine, which was developed by Japan Society of Mechanical Engineers. Pressure loss in the regenerator, buffer space loss and mechanical loss were taken into consideration in the analysis. Most of the studies were focused on beta-type Stirling engine because of its higher power density [39]. Therefore there are only a few studies conducted with atype Stirling engines. Podesser [40] designed and constructed an a type Stirling engine that was heated with flue gas of a biomass furnace. The aim of the study was to meet the electrical demand in rural villages. a type engine was preferred because of its low cost and maintenance expenditures. Shaft power of the engine reached 3.2 kW with a working gas pressure of 33 bar at 600 rpm. Engine efficiency was 25% for same operation conditions. Batmaz and Ustun [41] designed and manufactured a v-type a Stirling engine for solar applications. Their engine was having two heater cylinder. The engine was designed to obtain 500 W output power, however, the maximum power was obtained as 118 W at 1 bar charge pressure with a hot source temperature of 950 °C. Thermal
r lb h bc bh
radius of the crankshaft (m) the length of the connecting rod (m) crank angle (rad) the angle between connecting rod and vertical axis (rad) the angle between connecting rod and horizontal axis (rad)
efficiency of the engine was 11%. It was reported that the low power and thermal efficiency was caused by excessive leakage in the system and high dead volume. In an experimental study, carried out by Demir and Gungor [42] an air-charged v-type a Stirling engine was manufactured and tested. The performance characteristics of the engine were obtained under different heater temperature and charge pressure values. The maximum power output was obtained as 31 W at 1050 °C heater temperature and 1.5 bar charge pressure. Çınar et al. [43] manufactured and tested a beta-type Stirling engine with rhombic mechanism. They tested the engine with different charge pressures with air and helium working fluid. They obtained maximum power and torque as 95.77 W at 575 rpm and 1.98 Nm at 410 rpm respectively. Karabulut et al. [19] showed test results of the Stirling engine working with a lever controlled displacer driving mechanism. Helium was used in the experiments. They determined that maximum torque and power were measured 3.99 Nm and 183 W at 4 bars charge pressure and 260 °C hot end temperature. Kazimierski and Wojewoda [44] simulated the two stroke externally heated air valve engine (EHVE) which is different from typical Stirling engine. They compared the EHVE and Stirling engine under the same maximum pressures. They pointed that EHVE showed reasonable performance compared to typical Stirling engine. Almajri et al. [45] investigated the performance of V- a type Stirling engine at different operating conditions via combining thermodynamic model with 3D CFD analysis. They developed 3D CFD model and compared the results with thermodynamic model. Maximum power could be obtained with the increase of porosity up to 80%. They also pointed out that using CFD modelling was useful in order to enhance the V- a type Stirling engine performance. Shendage et al. [46] investigated the design methodology of Beta type Stirling engine with rhombic drive mechanism. They researched the optimization of the phase angle and the effects of overlapping between the compression and expansion processes. Abuelyamen et al. [47] performed a parametric study on a b-type Stirling engine with ANSYS fluent 14.5 software. They investigated the effects of initial charge pressure, thermal boundary condition; and three different types of working fluids (Air, He and H2). It was determined that the best performance was obtained with H2 gas. It was also found that small pressure difference across the engine chambers. Most of the studies related to Stirling engines were performed on beta type engines. There are very few studies related about Vtype a engines. In the present study, design, construction and performance tests of an a-type Stirling engine are presented. Helium and air are used as working fluid. An electrical furnace is used as the heat source. 2. Engine specifications 2.1. Test engine The schematic view and the photograph of the test engine are illustrated in Figs. 1 and 2, respectively. The engine consists of two pistons, called the hot and cold pistons, a crankcase, a crankshaft, two connecting rods, a flywheel and a connection pipe.
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Fig. 1. Schematic view of the test engine.
The expansion and compression spaces are formed in separate cylinders, situated at 90° angles to each other. Two pistons were connected to the same crankshaft rod journal by two lightweight connecting rots made of aluminum. Pistons were made of sphero cast iron considering its low friction. Pistons were extended with hubs made of stainless steel. A 1 mm gap was left between piston hubs and cylinders. The gap between the cylinder wall and the hot piston hub was used as heating passage and the gap between the cylinder wall and the cold piston hub was used as cooling passage (Fig. 1). The contact surfaces of pistons were machined to superfinish quality by grinding and polishing. Inner surfaces of the cylinders were machined by boring and grinding. A 0.02 mm working clearance was left between pistons and cylinders. The external surface of the compression cylinder was cooled by circulating water trough the water jacket around it. The crankshaft was dynamically balanced to reduce engine vibration. Construction details of the engine are given by Cinar [48]. Technical specifications of the engine are shown in Table 1. 2.2. Principle of engine operation and kinematic relations The theoretical cycle of the Stirling engine consists of four processes namely isothermal compression, constant volume heating, isothermal expansion and constant volume cooling processes (Fig. 3). As shown in Fig. 1, when the crank pin moves from 1 to 2, both pistons move simultaneously upward. During this process, the working fluid gives heat to the cooling water and the isothermal compression is performed. As the crank pin moves from 2 to
3, the hot piston moves downward and the cold piston moves upward. During this process, the working fluid is expelled from the cold space to the hot space and constant volume heating process is performed. While the crank pin moves from 3 to 4, both pistons move downward and the isothermal expansion process is realized. While the crank pin moves from 4 to 1, the cold piston moves downward and the hot piston moves upward and constant volume heat rejection is performed. The variation of cold volume, hot volume and total volume versus the crank angle is illustrated in Fig. 4. Implementation procedure and details of the analysis were presented by Cinar [48]. To calculate the values of cold volume and hot volume the kinematic relations,
hh ¼ r cos h þ lb cos bh þ hp þ hu hc ¼ r cos
p
h þ lb cos bc þ hp þ hu
2
ð1Þ ð2Þ
are used. In these equations, bc and bh are angles made by cold piston rod and hot piston rod with cylinder axis, respectively and are shown in Fig. 1. Relations between h and bc , bh are,
bc ¼ Arcsin
r sinh ‘b
bh ¼ Arcsin
p r sin h lb 2
ð3Þ
ð4Þ
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Fig. 2. Photograph of the test engine.
Table 1 Technical specifications of the engine. Parameters
Specification
Mechanical configuration Expansion cylinder
a
Compression cylinder
Bore (mm) Stroke (mm) Swept volume (cc) Bore (mm) Stroke (mm) Swept volume (cc)
Phase angle Heater temperature (°C) Cooler temperature (°C) Charge pressure (bar) Working fluid Cooling system Maximum engine power
50 50 98 50 50 98 90° 800 – 1000 °C 30 °C 1–4 Air and Helium Water cooled 30.7 W at 437 rpm Fig. 4. Variation of cold volume, hot volume and total volume with crank angle.
3. Experimental apparatus
Fig. 3. Theoretical Stirling cycle.
The schematic view of the test equipment is shown in Fig. 5. An electrical heater was used as the heat source. The temperature of the heater was adjustable between 0 °C and 1200 °C with an accuracy of 1 °C. The hot end of the expansion cylinder was inserted into the heater and it was heated within the heater temperature of 800–1000 °C. A prony brake dynamometer was used to measure the engine torque. The speed of the engine was measured by a digital tachometer with an accuracy of 1 rpm. Temperatures were measured by means of multi point temperature indicator (type
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Fig. 5. Schematic view of the test equipment.
ELIMKO-6000) having 1 °C accuracy. As the working fluid, air and helium were used. For the regulation of pressure of working fluid, a pressure-reducing valve was used. Cooling water outlet temperature was kept at 30 °C supplying sufficient water. 4. Experimental results and discussion In the experimental investigation a temperature adjustable heater was used as the heat source. The engine starts to run at 520 °C heater temperature. Systematic testing was performed within the range of 800–1000 °C heater temperature with 50 °C increments and 1–4 bars charge pressure with 0.5 bar increments. Heater temperature and charge pressure were assumed as operating parameters and the engine torque was measured with an accuracy of 0.003 Nm at various engine speeds by loading with the dynamometer. The output power was calculated from;
P¼2pT n
ð5Þ
where n is engine speed and T is engine torque.Fig. 6 shows the variation of torque with engine speed at 1000 °C heater temperature and different rates of air charge pressures from 1 to 3.5 bars. The maximum engine torque was obtained at 215 rpm as 0.7 Nm. Increasing charge pressure resulted with a higher torque level
Fig. 6. Variation of engine torque versus engine speed.
because of increased working fluid mass. However, after charge pressure of 3 bar, the engine torque started to decrease again. This indicated that the working fluid cannot be effectively used because of insufficient heat transfer surface area in the cylinder. It is seen that higher torque is obtained at low engine speeds because of the longer heating-cooling period. It is also seen that the operation speed range of the engine enlarges with the increased charge pressure. Increasing charge pressure implies the increase of the working fluid mass. Higher cyclic work generation can be obtained with higher mass of working fluid and as a result of this engine speed can reach a higher level. However, over increasing the mass of the working fluid by comparing the heat transfer surface area, speed range of the engine may reduce because of the insufficient heat transfer to the working gas. Fig. 7 shows the variation of power output as a function of engine speed for various heater temperatures between 800 °C and 1000 °C and 3 bars air charge pressure. An increase in the power output was obtained depend on the heater temperature. As the heater temperature increases, the difference between the hot and cold space temperatures increases. At the heater temperature of 1000 °C, the maximum power output is obtained as 18.13 W. With a lower heater temperature of 800 °C, the maximum power output is obtained as 6.5 W. As seen in Fig. 7, engine power
Fig. 7. Variation of power output versus engine speed.
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35
Power Output (W)
30 25 20 1.5 bar air
15
2.0 bar air 2.5 bar air
10
3.0 bar air 2.0 bar He
5
3.0 bar He 3.5 bar He
0 150
200
250
300
350
400
450
500
550
600
650
700
Engine Speed (rpm) Fig. 8. Variation of power output versus heater temperature.
Fig. 10. Variation of power output versus engine speed.
increases with increase of the engine speed up to a certain value of speed then starts to decrease. This behavior is an expected behavior for most of the engines, because, cyclic work generation starts to decrease at higher engine speeds. This reduction in cyclic work results from insufficient time for heat transfer and insufficient heat transfer surface area. The variation of power output with heater temperature using air and helium is shown in Fig. 8. It is observed that, an increase in the heater temperature results in an increase in power output. Because of higher heat transfer capabilities of helium, it has a higher power output than air for the same heater temperature and charge pressure. At the heater temperature of 1000 °C and 3 bars charge pressure, the maximum power output was measured as 18.13 W at 286 rpm engine speed for air while it was measured as 27.33 W at 418 rpm engine speed for helium. The working fluids having higher specific heat capacities such as hydrogen, might be contributed to increase the engine performance in this manner. The maximum power of the engine was 30.7 W at 437 rpm engine speed, at the heater temperature of 1000 °C and 3.5 bars charge pressure with helium. The engine manufactured by Batmaz and Ustun [41] had a maximum power output of 118 W. The difference in power outputs of the engines may be resulted because of the heating performances of the engines. Their engine was using double acting heater cylinder. Fig. 9 shows the variation of power output with charge pressure at 900 and 1000 °C heater temperatures using air and helium. Power output increases with the increase of charge pressure until a certain power value. More power output was obtained with helium compared to air working fluid with higher heating
temperature. In addition, maximum power was obtained with helium at 1000 °C and 3.5 bar charge pressure. Fig. 10 shows the variation of power output with engine speed at a constant heater temperature of 1000 °C and different rates of air and helium charge pressures. Comparison of curves obtained for 3 bars charge pressure air and helium shows that the maximum power output is obtained at 286 rpm for air and at 417 rpm for helium. As seen in Fig. 10, depending on the engine speed the power output is increased at a certain level. A decrease in the power is seen after that speed, due to insufficient heat transfer rate.
5. Conclusions In this study, a small Stirling engine with 98 cc total swept volume was manufactured and performance tests were performed. The test engine has reached a maximum power output of 30.7 W at 437 rpm engine speed, at the heater temperature of 1000 °C and 3.5 bars charge pressure using helium as working fluid. An increase trend was observed in engine torque with the increase of charge pressure up to 3 bar then it started to decrease. With an increase in the heater temperature, the power output and torque of the engine was increased. For the same heater temperature, higher power output was obtained with helium due to higher thermal conductivity compared to air. However, the use of air as the working fluid in Stirling engines has great advantages, such as abundant, readily available and free of charge except for compression cost. References
Fig. 9. Variation of power output versus charge pressure.
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