Development of rotary Wankel devices for hybrid automotive applications

Energy Conversion and Management 202 (2019) 112159

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Development of rotary Wankel devices for hybrid automotive applications Ghada A. Sadiq a b

a,b,⁎

b

, Raya Al-Dadah , Saad Mahmoud

T

b

Mustansiriya University, Baghdad, Iraq School of Engineering, University of Birmingham, Birmingham, UK

A R T I C LE I N FO

A B S T R A C T

Keywords: Wankel compressor and expander CAES Thermal analysis Hybrid vehicles

Several attempts have been made to reduce greenhouse gas emissions (GHG) employing hybridization. Compressed air-electrical hybrid system is a promising technology which can be used to reduce the environmental impact of the automotive industry. In cities and urban areas, vehicle’s kinetic energy through braking can be converted to pressurized air which can be stored in a storage tank for reuse to operate the vehicle during starting, cruising, and acceleration. In this study, an efficient expander and compressor are developed using a Wankel device for the compressed air-electric hybrid vehicle. Volumetric Wankel rotary expander and compressor are essential devices for power generation in compressed air energy storage (CAES) system and compressed air hybrid engine powered by the braking energy system leading to a significant decrease in GHG emissions. Compared with conventional expanders and compressors, Wankel expander and compressor have significant advantages due to its compactness, lower vibration, noise, and cost. Different parametric studies have been carried out with different vehicle initial speeds (80, 60, 40) km/h and weights (500, 750, 1000) kg, various braking times (2, 3, 5, 7) s, and finally three different tank sizes (20, 35, 50) l using mathematical modelling MATLAB software. The maximum system energy-saving efficiency achieved was around 77% and instantaneous system efficiency of about 85% at an initial vehicle speed of 80 km/h, braking time of 3 s, and vehicle weight of 750 kg and compressed air tank of 35 l.

1. Introduction The automotive industry has seen rapid developments over the past decade to address global concerns regarding the high risk of air pollution and decreasing fossil fuel recourses. Worldwide, the transportation sector consists of around 800 million passenger cars, which consume a significant amount of energy [1,2]. The primary power source of conventional vehicles is the internal combustion engine, which has a maximum efficiency of 45% for turbocharged direct injection (DI) diesel engines [2,3]. The vehicle’s kinetic energy wasted during braking can be recovered to reduce fuel consumption and CO2 emissions [1,2]. In 2010, the United State (US) Environmental Protection Agency (EPA) reported that there is an increase in the greenhouse gas (GHG) emissions from the human activity of (14% in the US and 26% in the world) since 1990. This increase leads to a rise in global earth temperature, which in turn results in many environmental disasters. Therefore, many scientific organisations recommended a significant reduction of 50% to 80% in GHG emissions below 1990 level by 2050 to prevent any risks to humans from climate change-related to a temperature increase of at least 2 °C. The study shows that 30% of GHG emissions in the United States are from the transportation sector, which is the second-largest ⁎

source of GHG emissions. The majority of GHG emissions of transportation come from light-duty vehicles, with a percentage of almost 60%. From 2009 to 2035, freight transportation GHG is expected to grow three times as fast as GHG from passenger vehicles [2,4]. Automobiles development with heat engines is one of the most significant achievements of modern technology. However, the automotive industry has developed rapidly, and a large number of vehicles in use around the world. Vehicles have caused and are still causing serious problems for society and human life. Degradation in air quality, global warming, and a reduction in oil resources are becoming significant threats to human beings. To control emissions and fuel consumption, development of clean, safe, and high-efficiency transportation are stimulating interest in using hybrid technologies. It has been acknowledged that electric, hybrid electric, and fuel cell-powered drive train technologies are promising solutions to the problem of transportation in the future [2,5]. Therefore, hybrid electric, electric battery, hydrogen, and compressed air vehicles are green environmentally friendly vehicles which are powered by alternative fuels and advanced technologies [2,6]. Pneumatic hybrid vehicles follow the same principle as hybrid electric ones. They operate two energy sources, fuel and compressed

Corresponding author. E-mail address: [email protected] (G.A. Sadiq).

https://doi.org/10.1016/j.enconman.2019.112159 Received 24 June 2019; Received in revised form 7 October 2019; Accepted 8 October 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

Energy Conversion and Management 202 (2019) 112159

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Nomenclature

Greek symbols

e h K.E m ṁ Mv N R p P PR r s t T v W x y

α θ ν η

eccentricity (mm) enthalpy (kJ/kg) kinetic energy (J) mass (kg) mass flow rate (kg/s) Mass of vehicle (kg) shaft speed per second (rpm) Constant gas (kJ/kg.k) pressure (N/m2) Power (kW) Pressure ratio (–) rotor radius (mm) entropy (kJ/kg.K) time (s) Temperature (K) volume (m3) work (J) coordinates in the x direction coordinates in the y direction

angle (degree) rotation angle (degree) rotor angle (degree) Efficiency (%)

Subscript/superscript h r min max

housing rotor minimum maximum

Acronyms AM CB CPC EPA GHG ORC US

Air motor Compression braking Compound parabolic collectors Environmental Protection Agency Greenhouse gas Organic Rankine Cycle United State

expander and compressor, air motor, and turbine [2]. However, there is limited published work using Wankel geometry in the hybrid vehicle. Improving the design and performance of the rotary Wankel engine has received significant attention in research work since its invention. Various studies were carried out to simulate and optimise the Wankel engine of combustion chambers using different fuels like petrol [2,9,10]. Experimental and CFD simulation of the Wankel engine was investigated by [2,9] using isooctane fuel in the simulation while ROZ95 fuel was used during the bench testing. The software Converge from Convergent Science was selected for CFD simulation. Results showed that the combustion in the CFD simulation happens faster than in the experimental test bench related to an increasing level of pressure in the combustion chamber and the different fuels used. The power obtained from the test engine was 35 kW at 6000 rpm. Yamada and Moriyoshi [2,10] analysed the flow and combustion inside a small rotary engine to develop a propulsive helicopter. ANSYS-FLUENT 6.3 was used for a displacement of 30 cc, rotor radius of 32.5 mm, an eccentricity of 5 mm and rotor thickness of 35 mm. The Wankel rotary engine produced a maximum target power of 4.3 kW at 13000 rpm using gasoline fuel. Jeng et al. [2,11] performed a numerical simulation of the performance of the Wankel engine with leakage through the apex seal, different Fuels using methane CH4 and octane C8H18 and recess sizes. Two-dimensional was used, and compressible gas, transient and periodic flow and combustion computation were applied using Fluent CFD software. Results showed that the highest pressure and indicated powers were about 54 bars and 49.3 kW with octane while 22 bar and 10 kW with methane at 7000 rpm. CFD Code AVL-Fire v7.x and v8.x were used to simulate one rotor side with its moving mesh capability and presenting the intake and compression stroke [2,12]. The k-ε turbulence model, as the most widely used in practical engineering applications compiled with CFD code, was used to simulate the complex movement of the rotor. Predict the fluid flow parameters such as pressure, temperature, velocity, volume changes, and combustion variables using diesel fuel and hydrogen mixture formation. Hydrogenenriched gasoline was studied experimentally in a rotary Wankel engine to improve the performance of a lean-burn spark-ignition rotary engine. The thermal efficiency and the power output were enhanced when adding hydrogen to gasoline in the engine. With the increase of hydrogen fraction, results also showed that carbon monoxide

air, selectively to propel the car. These engines have different operation modes such as compression braking (CB), air motor (AM), starting up, supercharged, and conventional [2,7]. Many investigations have been carried out to study the electric hybrid vehicle and pneumatic hybrid engine in various configurations. Also, compressed air hybrid engine, i.e., combined either electric motor or internal combustion engine with pressurized air has received considerable attention in recent years [1,2]. Such technology will become more efficient when braking waste energy is recovered to produce pressurized air. In the cities and urban areas, the kinetic energy from stopping the vehicle motion during braking can be converted to compressed air stored in the tank and reused to power the engine. Energy storage technologies absorb energy to save it and then release it to supply energy or power services when required. For decades, scientists and engineers have been interested in compressed air as a source of energy which is used in various applications in general, and as a non-polluting fuel in compressed air vehicles. Compressed air has a high energy density, fast filling, low cost, low toxicity, long service life, and it's favourable when compared to batteries [2,6]. To meet the standard level of transport emissions, the essential elements required by improving the fuel economy and reduce emissions from fuel combustion engines. The compressed air hybrid engine converts vehicle kinetic energy during braking into compressed air to operate the vehicle in acceleration using the same principle of electric hybrid engines. Fazeli et al. [2,8] developed an air hybrid concept where an IC reciprocating engine works as a combustion engine, air compressor, and air motor. The pressurised air is stored in a tank when it works as a compressor, which later is used in the engine when it works as an air motor. Their results showed that a significant reduction in exhaust emissions and fuel consumption was achieved. Highlighting the potential of air hybrid engine in providing better efficiency with less complexity, weight, and cost compared with a hybrid electric motor [2,8]. The development of compressed air hybrid vehicles requires a significant effort to develop optimal designs. The system includes compression and expansion devices connected with an air storage tank to recover, store, and reuse the braking energy. Therefore, the development of highly efficient compressor and the expander is an essential factor to increase regenerative braking efficiency. Different studies were conducted to develop a pneumatic hybrid vehicle using piston 2

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advantages compared to conventional reciprocating engines such as higher power per unit weight and lower noise and vibration. Research using Wankel engine for hybrid powertrain was also carried out by Butti and Delle [2,25] using two types of parallel and series electric hybrid engine. 1000 kg passenger car has been equipped with 11 kW parallel hybrid systems with a continuously variable transmission. The average power provided by the Wankel engine was 2.8 kW at 3000 rpm during the urban cycle and 9 kW at 5000 rpm during the extra-urban cycle. The Pb-acid battery was used as specific energy of 32 Wh/kg. The peak specific power was about 120 W/kg, and to cover 80 km range, 450 kg battery pack was necessary. Therefore a considerable reduction of the vehicle weight can be achieved with a hybrid propulsion system by using a small 9 kW Wankel engine. Another 6000 kg minibus has been equipped with a 20 kW motor for a series hybrid system to cover a range of 80 km, 600 kg battery pack was required. Besides, the Wankel geometry has also been employed as a compressor [2,27,28], as a pump [2,29] and also as an expansion device [2,30,31]. Rotary Wankel expander type is one of the volumetric expander devices. It converts the energy of the compressed working fluid to power output through an eccentric motion between the rotor and its housing. Compared to other expanders, rotary Wankel expander has advantages of compactness, low noise, vibration, cost and the capability of producing two power pulses per revolution per stage [2,32]. Therefore, small-scale (1–100) kW Wankel expanders can be used in small scales power generation systems such as steam and organic Rankine cycle (ORC) [2,32,33]. Furthermore, the common disadvantages of the Wankel engine of the high specific consumption and emissions are eliminated when used as Wankel expander. Also, as a result of the low pressure and temperature values, the wear of the rotor sealing is considerably reduced [2,33]. Recently, many researchers have investigated the rotary Wankel device [2] as an expander [2,30–37]. Badr and co-workers examined the use of the Wankel expander to generate power employing Rankine cycle [2,31,32,34]. Badr et al. [2,32] developed a model to describe the performance of Mazda Wankel engines and Curtiss-Wright Wankel engines which are available commercially (r = 118.5, e = 17, b = 69) mm and (r = 131.4, e = 19.1, b = 76) mm respectively at different boiler pressures. The geometry was chosen to consider the design; two inlet ports were fixed on the periphery while two exhaust ports were situated on the side housing providing two pulses of power per revolution [2]. The operating conditions were held at constant condenser pressure of 1.25 bar, a various boiler pressure of (4–10) bar and inlet temperature of (200–350) °C. Results showed that optimal Mazda pressure ratio was (5–6) while (4.5–5.25) pressure ratio of Curtiss-Wright was produced with higher indicated power output compared with Mazda [2,31]. A power output range of (5–20) kW was obtained for the Mazda and Curtiss- Wright engines at a shaft speed of 3000 rpm. Badr et al. [2,31] also investigated the same design used by Mazda where the mechanism of intake valve was optimised employing a computer-aided-design method [2,31]. Also, a discussion of expander material and lubrication was reported. The mass flow rate and power output of the Mazda Wankel expander of 0.12 kg/s and 16.8 kW were indicated, respectively, at boiler and condenser pressures of 6 and 1.25 bar. In [2,34] a comparison between the Wankel expander performance and helical-screw, rotary vane and turbines expanders were carried out. The study showed the advantages of employing the Wankel geometry as an expander, which includes compactness, low noise, vibration, and cost [2]. Even though, the most suitable devices are Wankel expander and helical-screw. Screw expanders remain to suffer from some problems such as the cost of the speed control equipment and reduction gearboxes. Antonelli et al. [2,33,35–37] were investigated Wankel expander’s performance using steam and different Organic Rankine Cycle (ORC) working fluids. A theoretical investigation of a rotary Wankel expander utilising renewable heat sources and driven by steam for a small power plant was carried out by Antonelli et al. [2,37]. The study revealed a

and hydrocarbon emissions were decreased, whereas nitrogen oxide emissions grew [2,13]. The same group even [2,14] experimentally investigated a rotary Wankel engine fuelled with hydrogen-enriched ethanol at full load and ultra-lean conditions at a constant speed of 3000 rpm. Results showed that the combustion process was enhanced with hydrogen enrichment level, and the thermal efficiency was improved with the increase of hydrogen fraction and decreasing the brakespecific energy consumption. Ji et al. [15] have been investigated the effect of dual-spark plug arrangements on ignition and combustion processes of a gasoline Wankel rotary engine with hydrogen direct-injection enrichment. 3D dynamic simulation model combining with the chemical kinetic mechanisms was studied using CONVERGE software and validated by the experimental data. Simulation results showed that a mainstream flow field formed during the end period of the compression stokes whose direction was identical to the rotor rotating direction. The peak pressure increased by 24.4% and corresponding crank position advanced by 9.2°CA when compared with the scheme of the closest duel-spark plug. Although nitric oxides emissions increased slightly, carbon monoxide emission notably was reduced by 57.6% versus the system of the farthest duel-spark plug. The combustion process of a hydrogen direct-injection stratified gasoline Wankel engine was investigated by Shi et al. [16]. The numerical simulation model (3D) was established and validated by the measured results. Results revealed that the average flow velocities were 18.8, 20.7, 23.6, and 25.4 m/s for the spark timings (ST) of 45, 35, 25, and crank angle degrees before top dead centre 15°CA BTDC respectively. Shi et al. [17] also studied the effects of hydrogen direct-injection angle and charged concentration on gasoline-hydrogen blending lean combustion in a Wankel engine. The work performed a numerical simulation model coupling with the kinetic mechanisms and validated among the experimental data. Results found that increasing the hydrogen charge concentration (HCC), the penetration of hydrogen injection is expanded and the area of the high-speed jet flow is increased. The jet-flow area for injection angle (IAs) of 45° or 135° is important than that of 90°. An efficient combustion performance may be performed in engineering application if the hydrogen IA is by the rotor rotating direction at lower HCC. The combined influence of hydrogen direct-injection pressure and nozzle diameter on lean combustion in a spark-ignited rotary engine was numerically examined by Shi et al. [18]. The connected influence of hydrogen injection pressure (HIP) and nozzle diameter (HND) was investigated in the critical injection state. With the increases of HIP and HND, further penetration and larger dispersion angle of jet-flow were performed. In practice operations, the combination of lower HIP and smaller HND left a positive influence on improving the combustion characteristics of Wankel engines was recommended. The Wankel engine performance was also studied the influence of apex seals of the rotor by many researchers [2,19,20]. Warren et al. [2,19] examined the effect of using spring-loaded apex seals instead of the designed rotor apexes for sealing the chambers. The method of design of the conventional rotary engines does not consider the profile of the apex seal and is restricted to epitrochoidal-based housing. The design of the apex seal profile was developed for theory and algorithm based on the deviationfunction (DF) method of conjugate pair design and generated the engine housing conjugating to the apex seal. Whereas some workers were investigated the effect of side ports in the performance of rotary Wankel engine and developed a micro Wankel internal combustion engine [2,21,22] and designed consideration of a Wankel engine [2,23]. Some studies have been conducted to investigate the Wankel engines as a part of motorised hybrid systems using an electric motor and as a range extender [2,24–26]. The hybrid engine, as a range-extending system, has been presented by Varnhagen et al. [2,24]. Series plug-in hybrid electric vehicles of different engine configurations and battery were modelled using Advanced Vehicle Simulator (ADVISOR). For well-towheel path simulations, the Wankel engine consumed less energy and emitted fewer greenhouse gases compared to the reciprocating engine during many driving cycles. The rotary engine showed several 3

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considerable reduction of 25% in the specific consumption of the steam and rose in the thermal efficiency for the multistage compared the single-stage Wankel expanders [2]. Also, a theoretical model is developed to validate the experimental data employing the compressed air as a working fluid. Antonelli et al. [2,35] studied measured pressure, mass flow rate and the torque to compare the numerical simulation software AMESim solution with experimental data using compressed air as a working fluid. The experiments were carried out a range of speed and pressure of (500–1500) rpm and (3–5) bar respectively, the mechanism of intake and exhaust valves system was introduced to control the inlet and outlet flow of working fluid [2]. Results also reported that around 85% isentropic expansion efficiency was reached with pentane as a working fluid at a temperature of 80 °C. The Wankel expander was also studied by [2,36] using the organic fluid like a real two-phase fluid instead of a perfect gas at (1000–3000) rpm and inlet temperature (100–120) °C. The influence of admission and recompression grades on the expander performance was investigated by adjusting the time of opening and closing valves. The comparison between the numerical and experimental results showed a 10% maximum difference with an efficiency uncertainty of about 5%. Results showed that maximum isentropic efficiency of 95% for R600a compared to less than 90% for R152a at saturated temperature 110 °C and 1500 rpm. Investigation of rotary Wankel expander driven by compressed vapour generated by the heat collected using compound parabolic collectors (CPC) solar plant was analysed in [2,37]. Steady-state conditions were assumed using various working fluids to evaluate the power delivered at fixed expander speed. Working fluid saturation temperature was considered in the range of (80–130) °C with expander speed of 3000 rpm. The design point was selected during July month, which produced the highest working fluid mass flow rate. The computational fluid dynamics (CFD) modelling was used to investigate the performance of the rotary device using air as an ideal gas by Sadiq et al. [30]. Different Wankel expander size for single-stage and two-stage expanders with different configurations were studied at various operating conditions. Combining two Wankel expanders produced 8.52 kW compared with single-stage, which gave 4.75 kW power output at the same operating conditions. Also, maximum isentropic efficiency of 91% was achieved for two-stage compared to 87.25% for single-stage at an inlet pressure of 6 bar and inlet temperature of 400 K at 7500 rpm. The motivation of this study is to develop an efficient electriccompressed air hybrid vehicle using the Wankel compressor and expander. Compressed air expander is used to extend the range of an electric vehicle through charging the batteries while a compressor harnesses the energy generated during vehicle braking to produce compressed air.

xh = e cos3θ + r cosθ

(1)

yh = esin3θ + r sinθ

(2)

Equations for the rotor shape: 1

x r = r cos2ν +

2 3e 2 9e 2 (cos8ν − cos4ν ) ± e ⎛1 − 2 sin2 3ν ⎞ (cos5ν + cosν ) 2r r ⎝ ⎠ (3) ⎜

⎟

1

yr = r sin2ν +

2 3e 2 9e 2 (sin8ν − sin4ν ) ± e ⎛1 − 2 sin2 3ν ⎞ (cos5ν + cosν ) 2r r ⎝ ⎠ (4)

Where the intervals ν are: π 5π 11π 13π ν = ⎡ 2 , 6 ⎤, ⎡ 6 , 6 ⎤, ⎣ ⎦ ⎣ ⎦

⎜

⎡ ⎣

⎟

19π 21π , 6 ⎤ 6 ⎦

2.2. Mathmatical modelling using (matlab) A mathematical model was developed using MATLAB to solve the above-described equations. The model was used to create the preliminary design of the Wankel rotary machines geometry. Fig. 2 shows the flow chart of the Wankel device mathematical model [2]. The first step to calculate the area is to create a set of points for the housing with parametric Eqs. (1) and (2) in the x and y direction with different values. The same step is also repeated for the rotor for each of the three sectors with parametric Eqs. (3) and (4). The second step is to add the equation of rotation to move the rotor correctly and then set up an iteration process to show the full rotation. The third step is to obtain the intersection points of the housing with the apex seal by adding offset to the housing to allow for seal placement. It is carried out by creating equations of lines which start at a specified location on the inner circle (centre of the rotor). Then they pass through the corners of the rotor, after that they cross the housing at the end and start of the specified sector (this represented the direction of the seal) as shown in Fig. 3 [2]. The area can be calculated when the three corners intersection points of the rotor are identified. This area is restricted by the housing and rotor of each chamber separately. Now there are two angles known, (the angle for the corner points of the rotor) and the angle θ for the housing, by dividing the angles into small sections to generate quadrilaterals shapes as shown in Fig. 3 [2]. It is possible to calculate the area of each ‘pizza slice’ when all the x and y points of all the quadrilaterals corners are worked out by substituting the equivalent angle values into appropriate equations. Then, the total area of each chamber can be calculated by adding up all these slices. To attain a precise result of the definite geometry, the number of intervals between two intersection points needs to be raised to increase the number of quadrilaterals shaped. This produces a better presentation of the housing curves and the rotor leading to accurate estimation of the fluid area. The volume of chambers can be easily calculated by multiplying the rotor housing width to the final area of each chamber [2]. For Wankel expander thermodynamic analysis, the working fluid air is assumed to be an ideal gas and the expansion process proceeds as

2. Modelling of the Wankel device 2.1. Wankel geometry The rotary Wankel engine consists of the housing and two moving parts, the rotor and the eccentric output shaft. Two spur gears control the motion of the rotor, an external gear is fixed to the side housing, and the internal gear is fixed within the rotor to ensure the rotor tips maintain contact with the housing [2,38,39]. The geometry of the rotor housing and flanks are controlled principally by the radius r of the rotor and the eccentricity e of the output shaft. The eccentricity e and the generating radius r are the key dimensions in designing the Wankel rotary expander as shown in Fig. 1. The rotor has two simple motions; translation of the rotor centre along the eccentric shaft with radius e and rotating around its own centre. The rotor rotates one revolution around its centre when the output shaft completes three revolutions around the eccentric circle. Both the housing and the rotor are controlled by the main parameters e and r [2].

Fig. 1. Definitions of parameters of the geometry [2,30]. 4

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Fig. 2. Flowchart showing the major steps in the mathematical model [2].

4. Thermodynamic analysis of compressed air energy storage system

adiabatic from inlet operating conditions (pi and Ti). It is also assumed that the inlet ports open at minimum volume to a working chamber which extends over the rotor displacement angle while the exit ports open at maximum volume and close at the end of expansion to produce the power required at estimated isentropic efficiency [2].

In the compressed air system, the vehicle kinetic energy during braking is converted to pressurised air which is stored in the tank and reused to power the vehicle. The regenerative braking energy is converted into mechanical energy for shaft rotation to turn the compressor. The following analysis was carried out to determine the recovered energy from the braking system: Deceleration (Dec) can be calculated as the ratio between the initial vehicle velocity (V) over the braking time (t) [2].

3. Electric and compressed air hybrid engine Fig. 4 shows a schematic diagram of the electric and compressed air hybrid system to be used for automotive applications. In this system, the vehicle will be driven by two power generation systems, electric and compressed air working together using the power-split concept. The power to the wheel can be either electrical, which is the primary system or mechanical or both. The battery used in Chevrolet cars have the capacity of 24 kWh with a weight of 426 kg and can be used to cover 100 km [2]. In the proposed compressed air system, Wankel compressors fitted to the wheels are used to convert the braking energy to compressed air stored energy. The compressors are activated when the driver applies the brake pedal, storing the vehicle’s kinetic energy in the form of pressurized air in the tank. The energy stored in the tank can be used to drive the Wankel expander to generate power that can be used in the vehicle through the start-up of the engine, cruising, or acceleration. The system analysis concentrates on the compressed air cycle rather than the electric motor to demonstrate the advantages of using such a new system [2].

Dec =

V t

(5)

Braking distance (D) is the distance travelled through the deceleration.

D=

V2 2 × Dec

(6)

The kinetic energy (K.E) is the amount of energy achieved by the movement of the vehicle mass (Mv).

K ∙E =

1 Mv × V 2 2

(7)

The braking power (PB) is defined as the energy consumed or produced per unit of time.

Fig. 3. Shows the seals for three chambers and small suction to calculate the area [2]. 5

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Fig. 4. Electric – Compressed Air Hybrid Engine [2].

PB =

K ∙E t

consumption in adiabatic and reversible (isentropic) compression process to the actual work consumption during the compression process with the same pressure ratio PR as [2]:

(8)

4.1. Compressor thermal analysis Air behaves as an ideal gas in the compression process with the compressor isentropic efficiency ηcomp. The braking power is converted into mechanical energy turning the compressor to produced compressed air. Fig. 5 shows a schematic diagram of the compressor connected to the buffer tank, which acts as a storage vessel to accumulate the compressed air. Air enters the compressor at atmospheric condition p1, T1 and is compressed to p2, T2 [2]. The braking power PB input to the Wankel compressor is required to achieve a specified compression of the working fluid [2,40].

PB = ṁ comp (h2 − h1)

(9) Fig. 5. Thermodynamic model of the regenerative braking air compression [2].

The isentropic efficiency of a compressor relates the ideal work 6

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G.A. Sadiq, et al.

h2s − h1 h2 − h1

ηcomp =

(10)

The enthalpy h2s at compressor exit can be determined at s2s and p2.

s1 = s2s

(11)

p2 = PR × p1

(12)

The enthalpy and entropy (h1 & s1) at the compressor inlet can be determined at (p1, T1). Then the actual enthalpy (h2) at compressor exit is calculated as:

h2 = h1 + ((h2s − h1)/ ηcomp) ṁ comp =

(13) Fig. 6. Thermodynamic model for air expansion model [2].

PB (h2 − h1 )

(14)

vmax =

ṁ comp × R × T1 p1

60 ×⎛ ⎞ ⎝N⎠

pdisch − tank =

∫0

t

ṁ exp dt

(22)

mdisch − tank ∙R ∙Tt − tank Vtank

(23)

mdisch − tank = mt − tank −

If the inlet temperature T1 of the air is assumed to remain constant, the maximum volume of the air in the Wankel compressor can be estimated as:

P Tdisch − tank = Tt − tank ⎛ disch − tank ⎞ ⎝ Pt − tank ⎠

(15)

⎜

where;

k−1 k

⎟

(24)

where; N: Rotational speed (rpm) of the Wankel compressor. p1, T1: Atmospheric pressure and ambient temperature. vmax : Maximum volume of the air in the Wankel compressor.

pdisch − tank : discharging pressure from the tank (kpa). pt − tank : Maximum pressure in the tank (kpa). Tdisch − tank : discharging temperature from the tank (K). Tt − tank : Maximum temperature in the tank (K). mdisch − tank : discharging mass from the tank (kg). mt − tank : Cumulative mass in the tank (kg).

4.2. Air storage tank The storage tank is charged with air during the braking process, and the air is discharged when power is needed to drive the vehicle. The air storage tank is assumed to be adiabatic with constant volume. During the charging process, the total mass accumulated in the tank can be determined by integrating the mass flow rate throughout the charging time as [2]:

mch − tank = mi +

∫0

t

ṁ comp dt

4.3. Expander thermal analysis Fig. 6 consists of the storage tank with constant volume Vtank filled with compressed air with pressure (p_(t-tank)), temperature (T_(t-tank)) and mass (m_(t-tank)) respectively. The pressurised air is delivered to an expansion device (Wankel expander) to convert the compressed air to mechanical power to drive the vehicle [2].

(16)

The pressure of the air in the tank can be calculated using ideal gas law. The range of both pressures and temperatures are low; therefore, air behaves as an ideal gas. When pressures and temperatures are high, the real gas law should be applied.

pv = mRT

pch − tank

m ∙R ∙Ti = ch − tank Vtank

⎜

Pch − tank ⎞ Pi ⎠

mt − tank t

(18)

p4 = p1

(27)

(28)

Assuming isentropic expansion process where;

k−1 k

(29)

s4s = s3

(19)

The isentropic enthalpy h4s at expander outlet can be determined at s4s and p4. Then the actual enthalpy h4 at expander outlet can be calculated using an assumed value of expander isentropic efficiency defined in Eq. (30) [2].

(20)

ṁ ch : charging mass flow rate to the tank (kg/s). mi : Initial mass in the tank (kg). R: gas constant (J/chg.). Vtank: constant tank volume (m3). k: specific heat ratio

ηexpander =

h3 − h4 h3 − h4s

(30)

The expander power output is calculated as:

̇ = ṁ exp (h3 − h4 ) Pexp = Wexp

Cp Cv

(26)

The enthalpy h3 and entropy s3 of air delivered to the expander are determined at (p3 and T3).

where;

k=

T3 = Tt − tank ṁ expander =

⎟

pi = p1 ,Ti = T1

(25)

(17)

The temperature of air in the tank can be determined as:

Tch − tank = Ti ⎛ ⎝

p3 = p2 = pt − tank

(31)

(21)

The discharging process can be analysed using the cumulative mass, maximum pressure and temperature in the tank to determine the instantaneous mass of air in the tank, the pressure and temperature of the tank during the discharging process [2].

4.4. Energy storage efficiency The Wankel device performs as a compressor during regenerative braking mode in an air hybrid vehicle. The second law definition for 7

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Fig. 7. Comparison between Matlab and published paper Badr et al. [31] for three chambers of expander’s volume with rotor angle [2].

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Fig. 8. Comparison between the present study and published work [1,2].

Fig. 9. Variation of tank air pressure with time for 2 ports and 4 ports [2]. Table 1 Input data of the braking system [2]. Parameter

Unit

Value

Compressor Rotor radius (r) Compressor eccentricity (e) Compressor width (b) Atmospheric Pressure Ambient temperature Compressor rotational speed (N) PR ηexpander

mm mm mm kPa K rpm – –

92 13 56 101 293 2000 20 0.8

ηcompressor

–

0.8

Vehicle mass (m) Vehicle speed Braking time Tank volume

kg km/hr s l

75 80 3 35

Δϑ = (h2 − h1) − To (s2 − s1)

(32)

Δϑ = (htank − 2 − htank − 1) − T1 (stank − 2 − stank − 1)

(33)

The second law efficiency of the regenerative braking process based on the definition for exergy can be calculated as:

ηreg =

ϑ2 − ϑ2 1 2

∙Mv (V12 − V22 )

(34)

The instantaneous system efficiency can be defined as the ratio of the instantaneous expander work to the energy stored in the tank which can be determined as:

ηinst − sys =

h3 (i) − h4 (i) htank − 2 − htank − 1

where;

htank − 2 @pt − tank &Tt − tank htank − 1 @p1 &T1 stank − 2 @pt − tank &Tt − tank stank − 1 @p1 &T1

efficiency is used to define the regenerative efficiency. However, the maximum useful work of a process which brings the system to equilibrium with the environment can be defined as exergy, or availability of a system and expressed by [1,2]. 9

(35)

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Fig. 10. Pressure of three chambers of Wankel device with the rotor angle [2].

Fig. 11. Effect of vehicle speed of charging phase – Tank air pressure [2].

Fig. 12. Effect of vehicle speed of charging phase (a) Tank air temperature (b) Tank air mass [2].

V1 and V2 are the vehicle’s initial and final velocity V1 = (80, 60, and 40) km/h V2 = 0.

5. Results of parametric study The mathematical model developed for Wankel expander was validated using published work. The comparison between the mathematical modelling (Matlab) and published work [31] of predicted expander’s volume for three chambers using the Mazda Wankel engine 10

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Fig. 13. Effect of vehicle speed of discharging phase for tank (a) air pressure (b) air temperature (c) air mass [2].

Fig. 14. Effect of vehicle speed of (a) Expander power (b) Instantaneous system efficiency [2].

with a lower time when using the Wankel compressor compared with piston compressor [2]. Fig. 9 compares the performance of the Wankel compressor with two ports (inlet and outlet) to the Wankel compressor with four ports (two inlets and two outlets). The air pressure in the tank reached to 15 bar through 4.5 s charging from four ports Wankel compressor. While the two ports Wankel compressor requires around 9 s highlighting the advantage of using four ports Wankel compressor, which doubles the output of compressed air [2]. The boundary conditions are presented in Table 1 for one parametric study and then compared with different cases. Fig. 10 demonstrates the pressure of the three chambers in the Wankel compressor versus the rotor angle to reach around 20 bar maximum value. Regenerative braking energy system was investigated by applying various vehicle initial speed and weight, several braking time and different tank size to reach to the optimal design of the hybrid vehicles. The parameters are used to determine vehicle deceleration and braking distance, which depends on both vehicle speed and braking time to stop the car. Also, it is necessary to know the kinetic energy to calculate the

(e = 17, r = 118.5, b = 69) mm [2,31] at various rotor angles showed good agreement as shown in Fig. 7 [2]. Compressed air hybrid engine is utilised in this study using the Wankel rotary concept. The braking energy is converted to mechanical energy driving the compressor to produce compressed air where a Wankel compressor is placed at each wheel. The compressor design is considered to be dependent on the required pressure of compressed air stored in the tank which is then used in the Wankel expander to power the engine [2]. The pressure in the storage tank was compared when using both the Wankel compressor and a piston compressor [1], as shown in Fig. 8. The specification of this comparison is a vehicle mass equal to 450 kg, storage tank size of 7.5 l, and a compression ratio of 10 for storage tank initial pressure of 1 bar. Also, it is assumed that during braking, the vehicle decelerates from 60 km/h and stops in about 25 s and the tank pressure builds up to 15 bar [2]. Good agreement was achieved from the comparison result between the Wankel compressor and piston compressor with a slight difference at the beginning due to the various configurations used (Wankel compressor and piston compressor). The pressure in the tank reached 15 bar 11

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Fig. 15. Effect of braking time of charging phase for tank (a) air pressure (b) air temperature (c) air mass [2].

Fig. 16. Effect of braking time of discharging phase for tank (a) air pressure (b) air temperature (c) air mass [2].

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Fig. 17. Effect of braking time of (a) Expander power (b) Instantaneous system efficiency [2].

Fig. 18. Effect of vehicle weight of charging phase for tank (a) air pressure (b) air temperature (c) air mass [2].

recovered only 68% and 47% of the vehicle’s kinetic energy at 60 km/h and 40 km/h respectively [2]. At the charging phase, Fig. 12(a, b) presents the temperature and the mass of the air in the tank for (80, 60, and 40) km/h. The energy stored in the tank can be achieved at the highest initial speed, as a result of an increase in the kinetic energy created and braking power generated, while the two other speeds need more time to reach a similar temperature of 650 K and 0.39 kg [2]. The discharging process of the air in the tank is shown in Fig. 13(a)–(c) for pressure, temperature and mass respectively. The pressure, temperature and mass flow rate of the air in the tank decreases to reach to atmospheric pressure and ambient temperature with around 2 s when the speed vehicle 40 km/h [2]. The maximum expander power output and instantaneous system efficiency can be achieved at vehicle initial speed of 80 km/h due to the largest energy stored in the tank and this energy can be used to operate the vehicle at staring up of the vehicle for the time required leading to significant reduction in the fuel consumption as shown in Fig. 14(a) and (b) [2].

braking power, which also depends on braking time, vehicle speed, and weight. The kinetic energy converts to compressed air stored in the tank to reuse it to power the vehicle.

5.1. Effect of vehicle speed For initial vehicle speed, three various rates of (80, 60, and 40) km/ h were selected to determine the amount of pressurised air stored in the tank for both charging and discharging mode of air. The expander power and efficiency and regenerative braking system were analysed. Fig. 11 presents the tank air pressure in the charging phase at different initial vehicle speed. The compressed air stored in the tank reached the pressure of 20 bar with less than 3 s for the initial vehicle speed of 80 km/h. While for speeds of 60 km/h and 40 km/h, more time is needed to charge the tank to reach 20 bar. In the charging phase, the energy storage in the tank is higher at an initial vehicle speed of 80 km/ h than other speeds. Therefore, the system energy-storing efficiency can be achieved to reach about 77% at 80 km/h based on equation (34) highlighting the effect of vehicle speed on the pressure and mass flow rate of air stored in the tank. The regenerative braking system can be 13

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Fig. 19. Effect of vehicle weight of discharging phase for tank (a) air pressure (b) air temperature (c) air mass [2].

Fig. 20. Effect of vehicle weight of (a) Expander power (b) Instantaneous system efficiency [2].

mass variation during the charging phase for the three different vehicle weights [2]. The tank air pressure, temperature and mass reached maximum values at vehicle weight 750 kg and 1000 kg of 20 bar, 650 K and 0.39 kg with the braking time of 3 s. However, the 500 kg vehicle weight only reached a pressure of 13.5 bars, temperature of 581 K and mass of 0.28 kg including lower stored energy [2]. Fig. 19(a–c) presents the vehicle weights of discharging phase for tank air pressure, temperature and mass. Fig. 20(a, b) shows the expander power and instantaneous system efficiency which are the same for vehicle weights 750 kg and 1000 kg while the energy stored in the tank is less at 500 kg [2].

5.2. Effect of braking time The same assumptions and operating conditions in Table 1 were used but with various braking time (2, 3, 5, 7) s. The regenerative braking efficiency is achieved to reach to about 77% because the air in the tank reached to the maximum pressure, temperature and mass but with different time as shown from Fig. 15(a–c) for charging phase [2]. Fig. 16(a–c) shows the effect of braking time (2 s, 3 s, 5 s, and 7 s) of the pressure , temperature and mass of the air in the tank for the discharging phase [2]. Fig. 17(a, b) presents the expander power output and the instantaneous system efficiency, it is correct that the power produced for 2 s braking time is the highest value due to the braking energy magnitude but this stored energy is reused with less 2 s while the energy consumed with longer time of around 6 s when the braking time is 7 s [2]. The instantaneous system can be achieved to reach of around 83%.

5.4. Effect of tank size Different tank sizes were used to show the performance of compressed air hybrid system (20, 35, and 50) l. The efficiency of regenerative energy storing is 44%, 77% and 75.7% at 20 l, 35 l and 50 l respectively. Fig. 21(a–c) shows the charging phase using the operating conditions shown in Table but with different tank sizes (20, 35 and 50) l [2]. Fig. 22(a–c) presents pressure, temperature, and mass of the air in

5.3. Effect of vehicle weight Various vehicle weights were investigated in this study, the braking regenerative efficiency were 74%, 77% and 58% at 500 kg, 750 kg and 1000 kg respectively. Fig. 18(a–c) shows pressure, temperature and 14

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Fig. 21. Effect of tank size of charging phase for tank (a) air pressure (b) air temperature (c) air mass [2].

Fig. 22. Effect of tank size of discharging phase for tank (a) air pressure (b) air temperature (c) air mass [2].

6. Conclusion

the tank using different tank size (20, 35, and 50) l through discharging phase [2]. Fig. 23(a, b) presents the expander power output and instantaneous system efficiency with time, the best state at 35 l [2].

Compressed air hybrid engine is a new technology which aims to reduce CHG emissions and keep the environment safe. In this work, the waste braking energy is converted to mechanical shaft rotational energy

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Fig. 23. Effect of tank size of (a) Expander power (b) Instantaneous system efficiency [2].

for driving Wankel compressor to produce compressed air to power the vehicle. The charging and discharging of air flow for the tank are the key factors of any design. To increase the energy storing capacity of the system, either the engine compression ratio or the tank volume should be increased, which are limited by the space available in the vehicle. Good agreement was achieved from the comparison between the compressed air system using the Wankel compressor and piston compressor. Different parametric studies have been carried out with different vehicle initial speeds and weights, various braking times, and finally, three various tank sizes. The maximum system energy storage efficiency achieved was around 77% and instantaneous system efficiency of about 85% at an initial vehicle speed of 80 km/h, braking time of 3 s, and vehicle weight of 750 kg and compressed air tank of 35 l. The results of this research highlight the potential of using Wankel devices (expander and compressor) in the development of compressed air – electric hybrid vehicles.

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