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Heat release modelling of a range extender scroll engine
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Energy Procedia 158 Energy Procedia 00(2019) (2017)2039–2045 000–000 www.elsevier.com/locate/procedia
10th th
International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10 International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
Heat release modelling of a range extender scroll engine Heat release modellingSymposium of a range extender engine The 15th International on District Heatingscroll and Cooling Simon Emhardtaa, Guohong Tiana,a,*, John Chewaa Emhardt , Guohong Tian John Chew Assessing Simon the feasibility of using the*, heat demand-outdoor a
Department of Mechanical Engineering Sciences, University of Surrey, Guildford, Surrey, GU2 7XH, UK
Departmentfunction of Mechanical Engineering University ofdistrict Surrey, Guildford, Surrey,demand GU2 7XH, UK forecast temperature for a Sciences, long-term heat a
Abstract a,b,c Abstract I. Andrić *, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc The proposed study aims to evaluate the feasibility of a range extender scroll engine by means of an analytical heat release rate a IN+ Center for Innovation, Technology Policy Research - Instituto Superior Av. Rovisco of Pais 1049-001 Lisbon, Portugal The proposed to bevaluate the and feasibility of a an range extender scrollTécnico, engine an1,analytical heat release analysis. This study novelaims engine technology is comprising upstream compressor andbya means downstream scroll expander whichrate is Veolia Recherche & Innovation,an291 Avenue Dreyfous Daniel, 78520 Limay, Francescroll expander which is analysis. This novel engine technology is comprising upstream compressor and a downstream mechanically cconnected and drives the compressor. Hence, the compression and expansion processes of the air-fuel mixture are Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France mechanically connected the compressor. Hence, the compression and expansion processes the air-fuel mixture are decoupled which enablesand the drives possibility to apply higher expansion than compression ratios resulting in aofscroll engine combustion decoupled which enables apply higher compression ratiosinresulting in power a scrolloutput engineand combustion process being similar to the the possibility Miller or to Aktinson cycle.expansion The scrollthan engine performance terms of thermal process being similar to the Miller or cycle. The scroll engine in corresponding terms of power output and thermal efficiency was accordingly evaluated forAktinson two compression ratios of 8.2:1 andperformance 10.1:1 and six expansion ratios in the efficiency was to accordingly for twothat compression ratios of 8.2:1 and for six the corresponding expansion in the range of 8.2:1 17.8:1. It evaluated has been proven high compression ratios and are 10.1:1 beneficial power output as more ratios fuel can be Abstract range of 8.2:1 to 17.8:1. It has been proven that high compression ratios are beneficial for the power output as more fuel can introduced into the expander part. At a constant compression ratio, the power output increased for a rising expansion ratio but be at introduced into expander part. At a constant compression the power output for a rising expansion ratioand but an at the expense of athe reduced power density. Theaddressed evaluation a peak value a compression ratiofor of 10.1:1 District heating networks are commonly inrevealed the ratio, literature as oneof of44.48kW theincreased mostat effective solutions decreasing the the expense of a reduced power density. The evaluation revealed a peak value of 44.48kW at a compression ratio of 10.1:1 and an expansion ratio of 17.8:1. A more thorough expansion process due to the implementation of a Miller/Aktinson cycle resulted in a greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat expansion ratio A more thorough expansion to the implementation a Miller/Aktinson cycle resulted in a significantly increasing thermal efficiency for a rising ratio of due expansion to policies, compression ratio reaching a peak value of 43.11%. sales. Due to of the17.8:1. changed climate conditions and process building renovation heatof demand in the future could decrease, significantly thermal efficiency prolonging increasing the investment return period. for a rising ratio of expansion to compression ratio reaching a peak value of 43.11%. Copyright © 2018 Elsevier Ltd. All rights main scope of this paper isby toElsevier assessreserved. the feasibility of using the heat demand – outdoor temperature function for heat demand ©The 2019 The Authors. Published Ltd. Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Lisbon the scientific committee theas 10tha International Conference forecast. The access districtarticle of Alvalade, in (Portugal), was of used case study. The district on is Applied consistedEnergy of 665 This is an open under thelocated CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) th International Conference on Applied Energy Selection and peer-review under responsibility of the scientific committee of the 10 (ICAE2018). buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and threeEnergy. district Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied (ICAE2018). renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were Keywords: Range Hybrid vehicle drivetrain; Scrollmodel, expander; Miller/Aktinson cycle;and Internal combustion compared withextender; results from a dynamic heat demand previously developed validated by theengine authors. Keywords: Range extender; Hybrid vehicle drivetrain; Scroll expander; Miller/Aktinson cycle; Internal combustion engine The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation 1.scenarios, Introduction the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). 1.The Introduction value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease theelectric number vehicle of heating hours of systems 22-139h are during the heating seasoninterest (depending on the combination of weather and Hybridinand drivetrain currently attracting for the integration in road vehicles Hybrid and vehicleof drivetrain systems are currently attracting interest for the in(depending roadproduced vehicles scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade on the torenovation contribute toelectric the reduction global carbon dioxide (CO2) emissions by burning lessintegration fossil fuels. The tocoupled contribute to theofThe reduction of global carbon (CO2) emissions burning less fossil fuels. produced scenarios). suggested could bedioxide used to(IC) modify the is function parameters for the scenarios considered, and mechanical power the values traditional internal combustion engine onlyby used to drive a generator in aThe series hybrid mechanical of the traditional internal combustion only used drive a the generator in aor series hybrid improvedrivetrain thepower accuracy of heat demand estimations. vehicle system. The generated electricity is (IC) thenengine eitherisutilised to to recharge batteries is directly
vehicle drivetrain system.motor. The In generated electricity is then either utilised the batteries is directly transferred to the electric other words, the main objective is to extendtotherecharge vehicle range as soon or as the battery ©depleted. 2017 Theto Authors. Published Ltd. known transferred the electric motor. InElsevier other also words, the main objective is to vehicles. extend theConventional vehicle rangereciprocating as soon as theengines, battery is Series hybrids are by therefore as range extender under responsibility of the Scientific Committee of Theextender 15th International on District Heating engines, and isPeer-review depleted. Series hybrids are therefore also known as range vehicles. Symposium Conventional reciprocating Cooling. * Corresponding author. Tel.: +44 (0) 1483 68 9283 * Corresponding author. Tel.: +44 (0) 1483 68 9283 E-mail address: [email protected] Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected]
1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility the scientific Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 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/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.470
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rotary wankel engines, micro turbines or fuel cells have been proven to be suitable propulsion systems for these range extender engines [1-4]. The proposed study of a range extender scroll engine aims to evaluate the feasibility of the concept by means of heat release modelling techniques. This novel type of engine has great potential advantages over its competitors. It is consisting of symmetric expansion chambers made of scroll profiles to make use of the characteristics of a scroll expander such as a high efficiency, more compact design, a low flow rate and low level of noise and vibration as a result of low rotational speeds and fewer moving parts. Further advantages may be the high reliability and the easiness and low cost to manufacture [5-8]. The aforementioned advantages make scroll engines suitable for Plug-in Hybrid Electric Vehicle range extender applications in the power range of 20-50 kW. This new engine concept could meet the needs for a range extender. In particular, the cost and simplicity need, the NVH (Noise, Vibration and Harshness) characteristics, and the compactness and lightweight need. In contrast sealing and cooling needs and the complex scroll geometry may disadvantage this novel engine technology. The constant wall thickness scroll expander geometry is the simplest and most popular form [9-12]. In the current study, the expander component of the scroll engine is accordingly formed by means of constant wall thickness scroll profiles which were generated with the help of a geometrical scroll expander model [13]. An analytical heat release rate analysis has been subsequently applied to predict the scroll engine performance characteristics in terms of power output and thermal efficiency. 2. Range extender scroll engine concept The range extender scroll engine is consisting of an upstream compressor and a downstream scroll expander made of constant wall thicknesses. The scroll profiles of the range extender scroll engine were generated with the help of a scroll expander geometrical model developed in MATLAB by Guo (2016) [13]. 6 different scroll expander geometries with expansion ratios of 8.2:1, 10.1:1, 12:1, 14:1, 15.9:1 and 17.8:1 were created respectively. The scroll tips were created with the help of the dual arc tip design.
Fig.1: Schematic cross-sectional view of the expander part of the range extender scroll engine (used drawings from [14,15])
Fig.2: P-V diagram of the scroll engine combustion process based on the Miller/Aktinson cycle [16]
The schematic cross-sectional view of the expander part of the range extender scroll engine is shown in Fig. (1). The working principle of the range extender scroll engine is the following. A mixture of air and fuel is compressed by a compressor at stoichiometric ratio and at two different compression ratios of 8.2:1 and 10.1:1 to high pressures of 15.41bar and 20.25bar respectively. The compressed mixture is then introduced into the expander which is mechanically connected and drives the compressor. The ignition of the air-fuel mixture with two spark plugs takes place as soon as the expander suction port is closed (ϴ=0º) in order to start the combustion process. The two spark plugs are integrated into two expansion chambers as illustrated in Fig. (1). A connected generator is driven by the produced mechanical movement of the orbiting scroll as a result of the combustion process. The exhaust gas is passed and cleaned by a catalytic converter after the expansion is finished. The engine operation (1-2-3-4M/A-5-1) is based
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on the Miller or Aktinson cycle as shown in Fig. (2). The scroll engine provides the opportunity of applying different compression and expansion ratios since the compression and expansion processes takes place at different locations. This in turn can lead to a more thorough expansion and can generate additional power without the combustion of additional fuel. Hence, a higher efficiency can be achieved with the scroll engine cycle which is similar to Miller/Aktinson cycle. Lu et al. [17] also carried out a feasibility study of a scroll type rotary gasoline internal combustion engine but employed in a Humphrey, Otto and Brayton cycle to compare its performances. In other words, the main differences to the concept presented in this report are the applied thermodynamic cycle and the performance analysis method. 3. Methodology The following section describes the theoretical procedure including all principles and basic equations which are necessary to carry out an analytical heat release rate analysis provided by Haywood (1988) [18]. The working principle and performance of the scroll engine can be thereby evaluated and assessed in terms of thermal efficiency and power output to verify this novel engine technology. First of all, the compression process in the upstream located compressor was assumed to be isentropic. The compressor inlet pressure and temperature were set to 300K and 1bar. The pressure and temperature at the compressor outlet were calculated for two compression ratios (8.2:1 and 10.1:1) and utilised as initial conditions for the expander. Secondly, the compression process of the air-fuel (A/F) mixture was based on a stoichiometric A/F ratio of 14.7:1. The amount of the total heat introduced into the expander was determined by using the following equation 𝑄𝑄𝑖𝑖𝑖𝑖 = 𝑚𝑚𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 × 𝐿𝐿𝐿𝐿𝐿𝐿
(1)
where 𝑄𝑄𝑖𝑖𝑖𝑖 defines the total heat input and 𝑚𝑚𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 represents the mass of the fuel. 𝐿𝐿𝐿𝐿𝐿𝐿 is defined as the lower heating value commonly known as the net calorific value. A lower heating value of 44.5 MJ/kg was used to carry out the calculation. Thirdly, the mass fraction burned profile (𝑥𝑥𝑏𝑏 ) for the combustion process of the scroll engine was defined by applying Eqs. (2) which describes the Wiebe function 𝜃𝜃 − 𝜃𝜃0 𝑚𝑚+1 𝑥𝑥𝑏𝑏 = 1 − exp [−𝑎𝑎 ( ) ] ∆𝜃𝜃
(2)
where 𝜃𝜃 is denoted as the orbiting angle of the moving scroll and 𝜃𝜃0 (0º) as the orbiting starting angle when the airfuel mixture is ignited by the spark plugs. The duration of the combustion process ∆𝜃𝜃 was specified to 24º. The typical values for the parameters for a and m are a=5 and m=2 [18]. The heat release change rate during the combustion process was subsequently calculated by multiplying the derivatives of the Wiebe function with the amount of the total heat introduced to the expander. The heat release change rate reads
𝑑𝑑𝑑𝑑 𝛾𝛾 𝑑𝑑𝑑𝑑 1 𝑑𝑑𝑑𝑑 = 𝑝𝑝 + 𝑉𝑉 𝑑𝑑𝑑𝑑 𝛾𝛾 − 1 𝑑𝑑𝑑𝑑 𝛾𝛾 − 1 𝑑𝑑𝑑𝑑
(3)
with the heat release rate 𝑑𝑑𝑑𝑑 , the volume change 𝑑𝑑𝑑𝑑 and the pressure change 𝑑𝑑𝑑𝑑. The heat capacity ratio 𝛾𝛾 was equals to 1.3. The volume was computed with the help of the geometrical model developed by Guo [13]. The pressure distribution in the expansion chamber of the scroll engine during the combustion process was then determined based on Eqs. (3). The heat transfer and leakage flows were neglected to simplify the heat release calculations. Additionally, the expansion process of the burned air-fuel mixture was assumed to be isentropic as soon as the combustion process was finished. Finally, the produced work output of the scroll engine expander can be obtained by using Eqs. (4) 𝑊𝑊𝑒𝑒𝑒𝑒𝑒𝑒 = ∫ 𝑝𝑝𝑝𝑝𝑝𝑝
(4)
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The work consumption to drive the compressor, which is mechanically connected with the expander, and is assumed to be isentropic, can be calculated by means of Eqs. (5) 𝛾𝛾−1 𝛾𝛾 𝑝𝑝2 𝛾𝛾 𝑊𝑊𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝑅𝑅𝑇𝑇1 [( ) − 1] (5) 𝛾𝛾 − 1 𝑝𝑝1 The thermal efficiency of the range extender scroll engine can be calculated by Eqs. (6) which can be expressed as
4. Results and discussion
𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 =
𝑊𝑊𝑒𝑒𝑒𝑒𝑒𝑒 − 𝑊𝑊𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑄𝑄𝑖𝑖𝑖𝑖
(6)
4.1 Scroll engine performance analysis The evaluation of the scroll engine performance predicted by means of a heat release rate analysis is discussed in the subsequent section. The first sub-section is about the calculation of the heat release rates for two compression ratios of 8.2:1 and 10.1:1 based on the Wiebe function. The heat release rates were then utilised to determine the total pressure distributions in the working chambers during the combustion and expansion process for six different expansion ratios of 8.2:1, 10.1:1, 12:1, 14:1, 15.9:1 and 17.8:1. In other words, the compression ratios were kept constant whereas the expansion ratios were varied to carry out the analytical investigation. The discussion about the influence of increasing ratios of expansion to compression ratio (ER/CR) on the scroll engine performance in terms of power output and thermal efficiency was included into the second sub-section. 4.1.1 Heat release rates and total pressure distributions The heat release rates for two compression ratios (CR) of 8.2:1 and 10.1:1 which are based on the Wiebe function are plotted over the crank angle as illustrated in Fig. (3). The analytical calculation revealed uniform distributions of the heat release rates with peak values of 71.4J/deg and 88.1J/deg at a crank angle of 13º. Whereas the ignition of the air-fuel mixture was specified to be over at a crank angle of 24º, it can be noted that the entire combustion process was finished at a crank angle of 31º. This is due to the fact that no heat was released any more. Fig. (4) visualises the total pressure distributions in the combustion and expansion chambers of the scroll engine over the crank angle. The evaluation revealed peak values of 94.74bar and 118.04bar for the total pressures at a crank angle of 24º. This specific angle is representative for the end of the ignition of the air-fuel mixture by means of the spark plugs. Fig. (4) also shows an exponential decrease of the total pressures in the combustion and expansion chambers for an increasing crank angle. It can be seen that total pressures of 6.92bar and 8.62bar were reached at the end of Fig. 3: Heat release rates for two compression ratios of 8.2:1 and 10.1:1 based on the expansion process using the highest the Wiebe function expansion ratio of 17.8:1. In theory, it would be possible to further expand the burned air-fuel mixture to atmospheric pressure. But the expansion process is limited
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due to the geometrical constraints of the scroll expander geometry. Furthermore, the lower part of Fig. (4) depicts how the expansion chamber volume increased for increasing expansion ratios.
Fig. 4: Total pressure distribution in the combustion and expansion chambers of the scroll engine (upper figure) and volume distribution of the expansion chambers over the crank angle (bottom figure)
4.1.2 Influence of increasing ratios of expansion to compression ratio on the scroll engine performance Fig. (5) shows the results of the influence of increasing ratios of expansion to compresssion ratio (ER/CR) on the scroll engine performance in terms of power output. The rotational speed was set to 3000 rpm in order to determine the power output which is also dependent on the compressor consumption and the heat input to the expander of the range extender engine. The studies revealed an increasing power output for increasing ratios of ER/CR but at the expense of a reduced power density. Specifically, 22.59kW, 26.81kW, 30.09kW, 32.75kW, 34.97kW and 36.88kW for a compression ratio of 8.2:1 and increasing expansion ratios. The highest power output of 44.48kW was achieved for a compression ratio of 10.1:1 and an expansion ratio of 17.8:1. These results indicate the importance of the compression ratio as more fuel can be introduced into the expander with higher compression ratios. It can be also said that higher expansion ratios are resulting in a more thorough expansion of the air-fuel mixture in the expansion chambers. Furthermore, it should be noted that the combustion process in the scroll engine is taking place in two chambers for every revolution of the orbiting scroll. In contrast to conventional Otto cycle engines in which two revolutions are necessary. Hence, the produced power output of the scroll engine is supposed to be four times higher while the displacement is the same. The performance evaluation in terms of thermal efficiencies for the scroll engine itself and the scroll engine driven by a mechanically connected compressor is presented in Fig. (6). It can be determined from the figure that the thermal efficiency of the scroll engine itself is significantly increasing for
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increasing ratios of ER/CR. Specifically from 37.25% up to 53.96% for the compression ratio of 8.2:1 and from 42.62% up to 54.5% for the compression ratio of 10.1:1. The same applies to the overall thermal efficiency which is rising from 26.41% to 43.11% and 30.26% to 42.15% respectively. In other words, this proves that a more thorough
Fig. 5: The influence of increasing ratios of ER/CR on the scroll engine performance in terms of power output
Fig. 6: The influence of increasing ratios of ER/CR on the thermal efficiency of the scroll engine with and without a compressor
expansion can be achieved by using the range extender scroll engine in a Miller/Aktinson cycle compared to conventional Otto cycle engines where the compression and expansion ratio are equal due to geometrical constraints in its cylinders. It is possible to generate additional power without the combustion of an additional air-fuel mixture which makes this novel engine technology more efficient and more advantageous compared to its competitors. The scroll engine may have the ability to further enhance the power output and efficiency in plug-in hybrid electric vehicle range extender. It has also the opportunity of being integrated in applications such as unmanned aerial vehicles in which a high power to weight ratio is necessary. 5. Conclusions This paper provides an analytical heat release rate analysis of a range extender scroll engine. Two compression ratios of 8.2:1 and 10.1:1 and six expansion ratios in the range of 8.2:1 and 17.8:1 were used to evaluate the scroll engine performance in terms of power output and thermal efficiency. The compression ratio was found to play an important role as more fuel can be introduced into the expander part with higher compression ratios. At the same compression ratio, the power output increased for increasing expansion ratios but at the expense of a reduced power density. The highest power output of 44.48kW was produced for a compression ratio of 10.1:1 and an expansion ratio of 17.8:1. Furthermore, it was identified that the thermal efficiency was significantly enhanced for increasing ratios of expansion to compression ratio reaching a peak value of 43.11%. Hence, it was proven that a more thorough expansion was achieved by employing the scroll engine in a Miller/Aktinson cycle instead of the conventional Otto cycle. Further investigations using more sophisticated heat release models, CFD tools and variable wall thickness designs should be conducted. These expander geometries enable the possibility of increasing the built-in volume ratio while keeping the scroll turns constant in comparison to conventional expander designs. Moreover, the integration of oil cooling applications can be realised. References [1] Lotus Engineering. http://www.greencarcongress.com/2009/09/lotus-range-extender.html (date of access: 08/05/2018) [2] Audi AG. https://www.caranddriver.com/features/how-audi-hybrids-could-keep-the-wankel-rotary-alive-feature (date of access: 08/05/2018)
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[3] BMW. https://en.wikipedia.org/wiki/BMW_i3 (date of access: 08/05/2018) [4] MAHLE Powertrain. http://www.mahle-powertrain.com/en/experience/mahle-compact-range-extender-engine/ (date of access: 08/05/2018) [5] S. Quolilin, M. VanDenBroek, S. Declaye, P. Dewallef, V. Lemort. Techno-economic survey of organic Rankine cycle (ORC) systems. Renewable and Sustainable Energy Reviews 22, pp. 168-186, 2013. [6] G. Qiu, H. Liu, S. Riffat. Expanders for micro-CHP systems with organic Rankine cycle. Applied Thermal Engineering 31, pp. 3301-3307, 2011. [7] V. Lemort, S. Quoilin, C. Cuevas, J. Lebrun. Testing and modeling a scroll expander integrated into an organic Rankine cycle. Applied Thermal Engineering 29, pp. 3094-3102, 2009. [8] J. Bao and L. Zhao. A review of working fluid and expander selections for organic Rankine cycle. Renewable and Sustainable Energy Reviews 24, pp. 325-342, 2013. [9] P. Song, M. Wei, L. Shi, S. N. Danish, C. Ma. A review of scroll expanders for organic Rankine cycle systems. Applied Thermal Engineering 75, pp. 54-64, 2015. [10] Y.-Q. Feng, T.-C. Hung, S.-L. Wu, C.-H. Lin, B.-X. Li, K.-C. Huang, J. Qin. Operation characteristic of a R123-based organic Rankine cycle depending on working fluid mass flow rates and heat source temperatures. Energy Conversion and Management 131, pp. 55-68, 2017. [11] S.-C. Yang, T.-C. Hung, Y.-Q. Feng, C.-J. Wu, K.-W. Wong, K.-C. Huang. Experimental investigation on a 3kW organic Rankine cycle for low-grade waste heat under different operation parameters. Applied Thermal Engineering 113, pp. 756-764, 2017. [12] S. Declaye, S. Quoilin, L. Guillaume, V. Lemort. Experimental study on an open-drive scroll expander integrated into an ORC (organic Rankine cycle) system with R245fa as working fluid. Energy 55, pp. 173-183, 2013. [13] H. Guo. Design and manufacture of a scroll expander. Project report, Department of Mechanical Eng. Science, University of Surrey, 2016. [14] H.-A. Li, C.-H. Yeh, C.-H. Wei, G.D. Tarng. Scroll-rotation prevention assembly of scroll compressor, U.S. patent no. 8,814,545 B2, August 26, 2014 [15] Combustion in Compression Ignition Engines Essential features. Online available at: mechanicalinventions.blogspot.co.uk/ 2013/08/ combustion-in-compression-ignition.html. 2013. (date of access: 08/05/2018) [16] J. Zhao. Research and application of over-expansion cycle (Aktinson and Miller) engines – A review, Applied Energy 185, pp. 300-319, 2017 [17] Y. Lu, A.P. Roskilly, X. Yu, L. Jiang, L. Chen. Technical feasibility study of scroll-type rotary gasoline engine: A compact and efficient smallscale Humphrey cycle engine, Applied Energy 221, pp. 67-74, 2018 [18] J.B. Heywood. Internal Combustion Engine Fundamentals, 1988
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10th th
International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10 International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
Heat release modelling of a range extender scroll engine Heat release modellingSymposium of a range extender engine The 15th International on District Heatingscroll and Cooling Simon Emhardtaa, Guohong Tiana,a,*, John Chewaa Emhardt , Guohong Tian John Chew Assessing Simon the feasibility of using the*, heat demand-outdoor a
Department of Mechanical Engineering Sciences, University of Surrey, Guildford, Surrey, GU2 7XH, UK
Departmentfunction of Mechanical Engineering University ofdistrict Surrey, Guildford, Surrey,demand GU2 7XH, UK forecast temperature for a Sciences, long-term heat a
Abstract a,b,c Abstract I. Andrić *, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc The proposed study aims to evaluate the feasibility of a range extender scroll engine by means of an analytical heat release rate a IN+ Center for Innovation, Technology Policy Research - Instituto Superior Av. Rovisco of Pais 1049-001 Lisbon, Portugal The proposed to bevaluate the and feasibility of a an range extender scrollTécnico, engine an1,analytical heat release analysis. This study novelaims engine technology is comprising upstream compressor andbya means downstream scroll expander whichrate is Veolia Recherche & Innovation,an291 Avenue Dreyfous Daniel, 78520 Limay, Francescroll expander which is analysis. This novel engine technology is comprising upstream compressor and a downstream mechanically cconnected and drives the compressor. Hence, the compression and expansion processes of the air-fuel mixture are Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France mechanically connected the compressor. Hence, the compression and expansion processes the air-fuel mixture are decoupled which enablesand the drives possibility to apply higher expansion than compression ratios resulting in aofscroll engine combustion decoupled which enables apply higher compression ratiosinresulting in power a scrolloutput engineand combustion process being similar to the the possibility Miller or to Aktinson cycle.expansion The scrollthan engine performance terms of thermal process being similar to the Miller or cycle. The scroll engine in corresponding terms of power output and thermal efficiency was accordingly evaluated forAktinson two compression ratios of 8.2:1 andperformance 10.1:1 and six expansion ratios in the efficiency was to accordingly for twothat compression ratios of 8.2:1 and for six the corresponding expansion in the range of 8.2:1 17.8:1. It evaluated has been proven high compression ratios and are 10.1:1 beneficial power output as more ratios fuel can be Abstract range of 8.2:1 to 17.8:1. It has been proven that high compression ratios are beneficial for the power output as more fuel can introduced into the expander part. At a constant compression ratio, the power output increased for a rising expansion ratio but be at introduced into expander part. At a constant compression the power output for a rising expansion ratioand but an at the expense of athe reduced power density. Theaddressed evaluation a peak value a compression ratiofor of 10.1:1 District heating networks are commonly inrevealed the ratio, literature as oneof of44.48kW theincreased mostat effective solutions decreasing the the expense of a reduced power density. The evaluation revealed a peak value of 44.48kW at a compression ratio of 10.1:1 and an expansion ratio of 17.8:1. A more thorough expansion process due to the implementation of a Miller/Aktinson cycle resulted in a greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat expansion ratio A more thorough expansion to the implementation a Miller/Aktinson cycle resulted in a significantly increasing thermal efficiency for a rising ratio of due expansion to policies, compression ratio reaching a peak value of 43.11%. sales. Due to of the17.8:1. changed climate conditions and process building renovation heatof demand in the future could decrease, significantly thermal efficiency prolonging increasing the investment return period. for a rising ratio of expansion to compression ratio reaching a peak value of 43.11%. Copyright © 2018 Elsevier Ltd. All rights main scope of this paper isby toElsevier assessreserved. the feasibility of using the heat demand – outdoor temperature function for heat demand ©The 2019 The Authors. Published Ltd. Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Lisbon the scientific committee theas 10tha International Conference forecast. The access districtarticle of Alvalade, in (Portugal), was of used case study. The district on is Applied consistedEnergy of 665 This is an open under thelocated CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) th International Conference on Applied Energy Selection and peer-review under responsibility of the scientific committee of the 10 (ICAE2018). buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and threeEnergy. district Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied (ICAE2018). renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were Keywords: Range Hybrid vehicle drivetrain; Scrollmodel, expander; Miller/Aktinson cycle;and Internal combustion compared withextender; results from a dynamic heat demand previously developed validated by theengine authors. Keywords: Range extender; Hybrid vehicle drivetrain; Scroll expander; Miller/Aktinson cycle; Internal combustion engine The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation 1.scenarios, Introduction the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). 1.The Introduction value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease theelectric number vehicle of heating hours of systems 22-139h are during the heating seasoninterest (depending on the combination of weather and Hybridinand drivetrain currently attracting for the integration in road vehicles Hybrid and vehicleof drivetrain systems are currently attracting interest for the in(depending roadproduced vehicles scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade on the torenovation contribute toelectric the reduction global carbon dioxide (CO2) emissions by burning lessintegration fossil fuels. The tocoupled contribute to theofThe reduction of global carbon (CO2) emissions burning less fossil fuels. produced scenarios). suggested could bedioxide used to(IC) modify the is function parameters for the scenarios considered, and mechanical power the values traditional internal combustion engine onlyby used to drive a generator in aThe series hybrid mechanical of the traditional internal combustion only used drive a the generator in aor series hybrid improvedrivetrain thepower accuracy of heat demand estimations. vehicle system. The generated electricity is (IC) thenengine eitherisutilised to to recharge batteries is directly
vehicle drivetrain system.motor. The In generated electricity is then either utilised the batteries is directly transferred to the electric other words, the main objective is to extendtotherecharge vehicle range as soon or as the battery ©depleted. 2017 Theto Authors. Published Ltd. known transferred the electric motor. InElsevier other also words, the main objective is to vehicles. extend theConventional vehicle rangereciprocating as soon as theengines, battery is Series hybrids are by therefore as range extender under responsibility of the Scientific Committee of Theextender 15th International on District Heating engines, and isPeer-review depleted. Series hybrids are therefore also known as range vehicles. Symposium Conventional reciprocating Cooling. * Corresponding author. Tel.: +44 (0) 1483 68 9283 * Corresponding author. Tel.: +44 (0) 1483 68 9283 E-mail address: [email protected] Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected]
1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility the scientific Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 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/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.470
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rotary wankel engines, micro turbines or fuel cells have been proven to be suitable propulsion systems for these range extender engines [1-4]. The proposed study of a range extender scroll engine aims to evaluate the feasibility of the concept by means of heat release modelling techniques. This novel type of engine has great potential advantages over its competitors. It is consisting of symmetric expansion chambers made of scroll profiles to make use of the characteristics of a scroll expander such as a high efficiency, more compact design, a low flow rate and low level of noise and vibration as a result of low rotational speeds and fewer moving parts. Further advantages may be the high reliability and the easiness and low cost to manufacture [5-8]. The aforementioned advantages make scroll engines suitable for Plug-in Hybrid Electric Vehicle range extender applications in the power range of 20-50 kW. This new engine concept could meet the needs for a range extender. In particular, the cost and simplicity need, the NVH (Noise, Vibration and Harshness) characteristics, and the compactness and lightweight need. In contrast sealing and cooling needs and the complex scroll geometry may disadvantage this novel engine technology. The constant wall thickness scroll expander geometry is the simplest and most popular form [9-12]. In the current study, the expander component of the scroll engine is accordingly formed by means of constant wall thickness scroll profiles which were generated with the help of a geometrical scroll expander model [13]. An analytical heat release rate analysis has been subsequently applied to predict the scroll engine performance characteristics in terms of power output and thermal efficiency. 2. Range extender scroll engine concept The range extender scroll engine is consisting of an upstream compressor and a downstream scroll expander made of constant wall thicknesses. The scroll profiles of the range extender scroll engine were generated with the help of a scroll expander geometrical model developed in MATLAB by Guo (2016) [13]. 6 different scroll expander geometries with expansion ratios of 8.2:1, 10.1:1, 12:1, 14:1, 15.9:1 and 17.8:1 were created respectively. The scroll tips were created with the help of the dual arc tip design.
Fig.1: Schematic cross-sectional view of the expander part of the range extender scroll engine (used drawings from [14,15])
Fig.2: P-V diagram of the scroll engine combustion process based on the Miller/Aktinson cycle [16]
The schematic cross-sectional view of the expander part of the range extender scroll engine is shown in Fig. (1). The working principle of the range extender scroll engine is the following. A mixture of air and fuel is compressed by a compressor at stoichiometric ratio and at two different compression ratios of 8.2:1 and 10.1:1 to high pressures of 15.41bar and 20.25bar respectively. The compressed mixture is then introduced into the expander which is mechanically connected and drives the compressor. The ignition of the air-fuel mixture with two spark plugs takes place as soon as the expander suction port is closed (ϴ=0º) in order to start the combustion process. The two spark plugs are integrated into two expansion chambers as illustrated in Fig. (1). A connected generator is driven by the produced mechanical movement of the orbiting scroll as a result of the combustion process. The exhaust gas is passed and cleaned by a catalytic converter after the expansion is finished. The engine operation (1-2-3-4M/A-5-1) is based
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on the Miller or Aktinson cycle as shown in Fig. (2). The scroll engine provides the opportunity of applying different compression and expansion ratios since the compression and expansion processes takes place at different locations. This in turn can lead to a more thorough expansion and can generate additional power without the combustion of additional fuel. Hence, a higher efficiency can be achieved with the scroll engine cycle which is similar to Miller/Aktinson cycle. Lu et al. [17] also carried out a feasibility study of a scroll type rotary gasoline internal combustion engine but employed in a Humphrey, Otto and Brayton cycle to compare its performances. In other words, the main differences to the concept presented in this report are the applied thermodynamic cycle and the performance analysis method. 3. Methodology The following section describes the theoretical procedure including all principles and basic equations which are necessary to carry out an analytical heat release rate analysis provided by Haywood (1988) [18]. The working principle and performance of the scroll engine can be thereby evaluated and assessed in terms of thermal efficiency and power output to verify this novel engine technology. First of all, the compression process in the upstream located compressor was assumed to be isentropic. The compressor inlet pressure and temperature were set to 300K and 1bar. The pressure and temperature at the compressor outlet were calculated for two compression ratios (8.2:1 and 10.1:1) and utilised as initial conditions for the expander. Secondly, the compression process of the air-fuel (A/F) mixture was based on a stoichiometric A/F ratio of 14.7:1. The amount of the total heat introduced into the expander was determined by using the following equation 𝑄𝑄𝑖𝑖𝑖𝑖 = 𝑚𝑚𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 × 𝐿𝐿𝐿𝐿𝐿𝐿
(1)
where 𝑄𝑄𝑖𝑖𝑖𝑖 defines the total heat input and 𝑚𝑚𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 represents the mass of the fuel. 𝐿𝐿𝐿𝐿𝐿𝐿 is defined as the lower heating value commonly known as the net calorific value. A lower heating value of 44.5 MJ/kg was used to carry out the calculation. Thirdly, the mass fraction burned profile (𝑥𝑥𝑏𝑏 ) for the combustion process of the scroll engine was defined by applying Eqs. (2) which describes the Wiebe function 𝜃𝜃 − 𝜃𝜃0 𝑚𝑚+1 𝑥𝑥𝑏𝑏 = 1 − exp [−𝑎𝑎 ( ) ] ∆𝜃𝜃
(2)
where 𝜃𝜃 is denoted as the orbiting angle of the moving scroll and 𝜃𝜃0 (0º) as the orbiting starting angle when the airfuel mixture is ignited by the spark plugs. The duration of the combustion process ∆𝜃𝜃 was specified to 24º. The typical values for the parameters for a and m are a=5 and m=2 [18]. The heat release change rate during the combustion process was subsequently calculated by multiplying the derivatives of the Wiebe function with the amount of the total heat introduced to the expander. The heat release change rate reads
𝑑𝑑𝑑𝑑 𝛾𝛾 𝑑𝑑𝑑𝑑 1 𝑑𝑑𝑑𝑑 = 𝑝𝑝 + 𝑉𝑉 𝑑𝑑𝑑𝑑 𝛾𝛾 − 1 𝑑𝑑𝑑𝑑 𝛾𝛾 − 1 𝑑𝑑𝑑𝑑
(3)
with the heat release rate 𝑑𝑑𝑑𝑑 , the volume change 𝑑𝑑𝑑𝑑 and the pressure change 𝑑𝑑𝑑𝑑. The heat capacity ratio 𝛾𝛾 was equals to 1.3. The volume was computed with the help of the geometrical model developed by Guo [13]. The pressure distribution in the expansion chamber of the scroll engine during the combustion process was then determined based on Eqs. (3). The heat transfer and leakage flows were neglected to simplify the heat release calculations. Additionally, the expansion process of the burned air-fuel mixture was assumed to be isentropic as soon as the combustion process was finished. Finally, the produced work output of the scroll engine expander can be obtained by using Eqs. (4) 𝑊𝑊𝑒𝑒𝑒𝑒𝑒𝑒 = ∫ 𝑝𝑝𝑝𝑝𝑝𝑝
(4)
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The work consumption to drive the compressor, which is mechanically connected with the expander, and is assumed to be isentropic, can be calculated by means of Eqs. (5) 𝛾𝛾−1 𝛾𝛾 𝑝𝑝2 𝛾𝛾 𝑊𝑊𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝑅𝑅𝑇𝑇1 [( ) − 1] (5) 𝛾𝛾 − 1 𝑝𝑝1 The thermal efficiency of the range extender scroll engine can be calculated by Eqs. (6) which can be expressed as
4. Results and discussion
𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 =
𝑊𝑊𝑒𝑒𝑒𝑒𝑒𝑒 − 𝑊𝑊𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑄𝑄𝑖𝑖𝑖𝑖
(6)
4.1 Scroll engine performance analysis The evaluation of the scroll engine performance predicted by means of a heat release rate analysis is discussed in the subsequent section. The first sub-section is about the calculation of the heat release rates for two compression ratios of 8.2:1 and 10.1:1 based on the Wiebe function. The heat release rates were then utilised to determine the total pressure distributions in the working chambers during the combustion and expansion process for six different expansion ratios of 8.2:1, 10.1:1, 12:1, 14:1, 15.9:1 and 17.8:1. In other words, the compression ratios were kept constant whereas the expansion ratios were varied to carry out the analytical investigation. The discussion about the influence of increasing ratios of expansion to compression ratio (ER/CR) on the scroll engine performance in terms of power output and thermal efficiency was included into the second sub-section. 4.1.1 Heat release rates and total pressure distributions The heat release rates for two compression ratios (CR) of 8.2:1 and 10.1:1 which are based on the Wiebe function are plotted over the crank angle as illustrated in Fig. (3). The analytical calculation revealed uniform distributions of the heat release rates with peak values of 71.4J/deg and 88.1J/deg at a crank angle of 13º. Whereas the ignition of the air-fuel mixture was specified to be over at a crank angle of 24º, it can be noted that the entire combustion process was finished at a crank angle of 31º. This is due to the fact that no heat was released any more. Fig. (4) visualises the total pressure distributions in the combustion and expansion chambers of the scroll engine over the crank angle. The evaluation revealed peak values of 94.74bar and 118.04bar for the total pressures at a crank angle of 24º. This specific angle is representative for the end of the ignition of the air-fuel mixture by means of the spark plugs. Fig. (4) also shows an exponential decrease of the total pressures in the combustion and expansion chambers for an increasing crank angle. It can be seen that total pressures of 6.92bar and 8.62bar were reached at the end of Fig. 3: Heat release rates for two compression ratios of 8.2:1 and 10.1:1 based on the expansion process using the highest the Wiebe function expansion ratio of 17.8:1. In theory, it would be possible to further expand the burned air-fuel mixture to atmospheric pressure. But the expansion process is limited
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due to the geometrical constraints of the scroll expander geometry. Furthermore, the lower part of Fig. (4) depicts how the expansion chamber volume increased for increasing expansion ratios.
Fig. 4: Total pressure distribution in the combustion and expansion chambers of the scroll engine (upper figure) and volume distribution of the expansion chambers over the crank angle (bottom figure)
4.1.2 Influence of increasing ratios of expansion to compression ratio on the scroll engine performance Fig. (5) shows the results of the influence of increasing ratios of expansion to compresssion ratio (ER/CR) on the scroll engine performance in terms of power output. The rotational speed was set to 3000 rpm in order to determine the power output which is also dependent on the compressor consumption and the heat input to the expander of the range extender engine. The studies revealed an increasing power output for increasing ratios of ER/CR but at the expense of a reduced power density. Specifically, 22.59kW, 26.81kW, 30.09kW, 32.75kW, 34.97kW and 36.88kW for a compression ratio of 8.2:1 and increasing expansion ratios. The highest power output of 44.48kW was achieved for a compression ratio of 10.1:1 and an expansion ratio of 17.8:1. These results indicate the importance of the compression ratio as more fuel can be introduced into the expander with higher compression ratios. It can be also said that higher expansion ratios are resulting in a more thorough expansion of the air-fuel mixture in the expansion chambers. Furthermore, it should be noted that the combustion process in the scroll engine is taking place in two chambers for every revolution of the orbiting scroll. In contrast to conventional Otto cycle engines in which two revolutions are necessary. Hence, the produced power output of the scroll engine is supposed to be four times higher while the displacement is the same. The performance evaluation in terms of thermal efficiencies for the scroll engine itself and the scroll engine driven by a mechanically connected compressor is presented in Fig. (6). It can be determined from the figure that the thermal efficiency of the scroll engine itself is significantly increasing for
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increasing ratios of ER/CR. Specifically from 37.25% up to 53.96% for the compression ratio of 8.2:1 and from 42.62% up to 54.5% for the compression ratio of 10.1:1. The same applies to the overall thermal efficiency which is rising from 26.41% to 43.11% and 30.26% to 42.15% respectively. In other words, this proves that a more thorough
Fig. 5: The influence of increasing ratios of ER/CR on the scroll engine performance in terms of power output
Fig. 6: The influence of increasing ratios of ER/CR on the thermal efficiency of the scroll engine with and without a compressor
expansion can be achieved by using the range extender scroll engine in a Miller/Aktinson cycle compared to conventional Otto cycle engines where the compression and expansion ratio are equal due to geometrical constraints in its cylinders. It is possible to generate additional power without the combustion of an additional air-fuel mixture which makes this novel engine technology more efficient and more advantageous compared to its competitors. The scroll engine may have the ability to further enhance the power output and efficiency in plug-in hybrid electric vehicle range extender. It has also the opportunity of being integrated in applications such as unmanned aerial vehicles in which a high power to weight ratio is necessary. 5. Conclusions This paper provides an analytical heat release rate analysis of a range extender scroll engine. Two compression ratios of 8.2:1 and 10.1:1 and six expansion ratios in the range of 8.2:1 and 17.8:1 were used to evaluate the scroll engine performance in terms of power output and thermal efficiency. The compression ratio was found to play an important role as more fuel can be introduced into the expander part with higher compression ratios. At the same compression ratio, the power output increased for increasing expansion ratios but at the expense of a reduced power density. The highest power output of 44.48kW was produced for a compression ratio of 10.1:1 and an expansion ratio of 17.8:1. Furthermore, it was identified that the thermal efficiency was significantly enhanced for increasing ratios of expansion to compression ratio reaching a peak value of 43.11%. Hence, it was proven that a more thorough expansion was achieved by employing the scroll engine in a Miller/Aktinson cycle instead of the conventional Otto cycle. Further investigations using more sophisticated heat release models, CFD tools and variable wall thickness designs should be conducted. These expander geometries enable the possibility of increasing the built-in volume ratio while keeping the scroll turns constant in comparison to conventional expander designs. Moreover, the integration of oil cooling applications can be realised. References [1] Lotus Engineering. http://www.greencarcongress.com/2009/09/lotus-range-extender.html (date of access: 08/05/2018) [2] Audi AG. https://www.caranddriver.com/features/how-audi-hybrids-could-keep-the-wankel-rotary-alive-feature (date of access: 08/05/2018)
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