Study of System Integral Energy Efficiency of a Hybrid Pneumatic

applied sciences

Article Study of System Integral Energy Eﬃciency of a Hybrid Pneumatic Power System

Po-Tuan Chen 1,2 , Duong Dinh Nghia 3, Cheng-Jung Yang 4,* and K. David Huang 3,* 1 Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan; [email protected] 2 Center of Atomic Initiative for New Materials, National Taiwan University, Taipei 10617, Taiwan 3 Department of Vehicle Engineering, National Taipei University of Technology, Taipei 10608, Taiwan; [email protected] 4 Department of Mechanical Engineering, National Pingtung University of Science and Technology, Pingtung 91201, Taiwan * Correspondence: [email protected] (C.-J.Y.); [email protected] (K.D.H.); Tel.: +886-8-7703202 (ext. 7012) (C.-J.Y.); +886-2-2771-2171 (ext. 3676) (K.D.H.) Received: 14 April 2019; Accepted: 31 May 2019; Published: 6 June 2019

Abstract: Currently, in the field of vehicle engineering, researchers are focused on finding a new type of high-efficiency vehicle. Based on the conventional powertrain system, the hybrid pneumatic power system (HPPS) is a type of hybrid system that focuses on compressed air power instead of the electrochemical energy conversion of a battery. This study concentrates on the integral efficiency of the HPPS. The HPPS was operated under different conditions of internal combustion engine speed, fuel consumption, compressor speed, pressure in air tank, cross-sectional area, and air motor efficiency. Based on this, the best operating condition of the HPPS was defined, and the integral efficiency of the HPPS was measured under this condition. The experimental results show that the system integral efficiency can reach up to 45.3%. It is higher than 28% when using individual internal combustion engines. In addition, the HPPS could reduce fuel consumption by 38% during its best performance.

Keywords: vehicle engineering; hybrid pneumatic power system; hybrid system; fuel consumption

1. Introduction In recent years, in addition to the technical advancement of the internal combustion engine (ICE), a significant amount of research effort has been given to designing a new type of vehicle that utilizes the ICE, called the hybrid electric vehicle (HEV); it uses both an electric motor and an ICE to generate power for propulsion. The major advantages of the HEV are increased energy efficiency and a reduced greenhouse effect compared with conventional vehicles. The new technology has overcome several challenges of HEV development, such as system integration [1], difficulty in recharging the battery in a short time [2,3], matching of the best power conditions [4], and plausible alternative energy sources [5–7]. However, product design and product sustainability are crucial to the success of HEVs in the future [8–10]. Combined with reductions in energy and environmental pollution for future use, air-powered vehicles present several superiorities [11,12]. First, only air is emitted during operation; therefore, production of compressed air appears to be the only possible pollution source. Also, the cost of air power is much lower than that of fossil fuels, such as in the case of electric, battery, and fuel-cell vehicles. Finally, the container for the compressed air can be designed such that it can be assembled and disassembled conveniently. The vehicle can be refilled with power faster when compared to electric vehicles in particular. Dimitrova and Maréchal demonstrated that the pneumatic driveline was up to 67% lighter than the lightest competing hybrids, and that fuel consumption could be reduced from 35% to 30% [13].

Appl. Sci. 2019, 9, 2333; doi:10.3390/app9112333 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2333 2 of 15

The concept of the pneumatic car was proposed in the early 20th century. Broderick [14] obtained a patent named “Combined internal combustion and compressed air engine”. In this concept, the compressed air is used to ignite and start an ICE or vehicle brakes. Compressed air is produced from two of four engine cylinders while the other two cylinders perform their typical functions. The concept of applying a multicylinder engine to generate compressed air was proposed by Ochel et al. [15]. This concept is similar to that of Broderick’s patent. The only difference is that Ochel’s concept was intended for driving pneumatic tools such as spray guns and drill hammers. Brown [16] filed for a patent that utilized a compressor with an electric motor to generate compressed air to drive an air engine. Ueno [17] proposed an invention wherein ICE cylinders were used for compressing air and subsequently allow this air to drive a vehicle. Ueno’s invention could also perform regenerative braking, which was not feasible in Broderick’s invention. Moyers [18] defined the pneumatic hybrid, which is currently available. He utilized supercharging to store gas into a pressure tank such that the pressure inside the tank was greater than the outside environment. In 1998, Motor Development International, SA (MDI), first introduced air vehicles for commercial purposes. Currently, MDI can support single- to six-seater minibuses. In collaboration with MDI, Tata Motors released an air-powered vehicle named AIRPOD in 2012. The Korean company Energine created a pneumatic hybrid electric vehicle in 2005. The engine of this vehicle operated with an electric motor to generate the power source. The Lucksky Group from China displayed an airbus at the Beijing International High-Tech Expo in 2015. Additionally, several research institutes and car companies worldwide have recently focused on air-powered vehicle development [19]. In addition to purely air-powered vehicles, a part of the research is related to the combination of the systems with conventional internal combustion engine. Fang et al. showed that the pneumatic power system uses heat of cooling water to optimize the performance of air engine. It can increase power by 38.22% and efficiency from 26.18% to 41.22%, and save 50% of fan power [20]. Sasa Trajkovic et al. showed that this research focuses on using waste power when a vehicle is decelerated. After that, this power will drive the wheel when the vehicle is accelerated. The results show that this solution can reduce 58% fuel consumption [21]. A system of hybrid pneumatic drivelines was developed by EXELIS [22]. The system exhibits enhanced fuel efficiency and reduced air pollution than hydraulic and electric hybrids. Huang et al. [23,24] designed a hybrid pneumatic power system that combined exhaust flow and compressed air flow. This concept was adopted from the ICE to be in parallel with the air compressor such that the air can be directly compressed into a storage tank for energy storage. Additionally, high-temperature waste gas from the ICE is merged with the compressed air using a merger pipe to improve the output flow power. Energy management is emphasized in this study to obtain the highest system integral efficiency of a hybrid pneumatic power system (HPPS). In our HPPS, the crankshaft of the ICE is connected to the air compressor. The air compressor operates at many conditions of ICE, such as power and speed at the outlet ICE. By controlling the ICE, the characteristic of compressed air flow at the outlet compressor will change. It means that an ICE, a compressor, and an air tank have a binding relationship. Finally, the integral efficiency of the HPPS is measured at the best operating condition of the HPPS.

2. Materials and Methods

2.1. Experimental Setup of Our HPPS The framework of our HPPS is shown in Figure1. This system contains an internal combustion engine (ICE) (1) that is used to drive an air compressor (2), a one-way valve (3) that controls the air flow to a high-pressure air storage tank (4), a control valve (5) that adjusts air flow to a merger pipe (6), and an air motor (7) that obtains air flow from both the ICE and air storage tank. The torque sensor (8), pressure sensor (P), flow sensor (F), and temperature sensor (T) are connected with an A/D transformer and a computer (11) to measure the system specification. Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 15

sensor (8), pressure sensor (P), flow sensor (F), and temperature sensor (T) are connected with an A/D Appl.transformer Sci. 2019, 9, and 2333 a computer (11) to measure the system specification. 3 of 15

Figure 1. The framework of our hybrid pneumatic power system (HPPS). Figure 1. The framework of our hybrid pneumatic power system (HPPS). The ICE drives a compressor to charge an air tank. The compressed air flow and exhaust flow are The ICE drives a compressor to charge an air tank. The compressed air flow and exhaust flow then mixed at merger pipe. The merger flow is controlled by a stepper motor, which adjusts the merger are then mixed at merger pipe. The merger flow is controlled by a stepper motor, which adjusts the positionmerger to position create a ventureto create geometry. a venture By geometry. combination Byof combination both flows, theof mergerboth flows, pipe enhancesthe merger the pipe kinetic energyenhances of the the compressed kinetic energy air flow. of the At compressed the output ofair the flow. merger At the pipe, output with of high the merger pressure pipe, and with temperature, high thepressure merger flowand drivestemperature, an air motor.the merger This notflow only drives saves an the air chemical motor. energyThis not from only the saves fuel, butthe alsochemical reduces airenergy pollution. from the fuel, but also reduces air pollution.

2.1.1.2.1.1. ICE ICE TheThe specifications specifications of of the the ICE ICE (SYM (SYM Corporation) Corporation) are are one one cylinder, cylinder, four four strokes, strokes, and 125andcc 125 exhaust cc volume.exhaust The volume. maximum The maximum output is output 12PS/7500 is 12PS rpm,/7500 the rpm, maximum the maximum torque is torque 1.2 kg ism 1.2/6500 kg∙m/6500 rpm, the rpm, fuel is · gasoline,the fuel theis gasoline, fuel feed the system fuel feed is a system carburetor, is a carb anduretor, the cooling and the mode cooling is airmode cooled. is air Itcooled. reaches It reaches the sweet spotthe of sweet brake spot specific of brake fuel consumptionspecific fuel consumption (BSFC) by 5000 (BSFC) rpm atby the 5000 free-load rpm at condition. the free-load Therefore, condition. the air compressorTherefore, canthe utilizeair compressor a stable loadcan utilize after the a stable ICE is load ignited, after thereby the ICE maintaining is ignited, thereby the ICE maintaining operating at a fixedthe rotationalICE operating speed. at a fixed rotational speed.

2.1.2.2.1.2. Air Air Compressor Compressor and and AirAir TankTank TheThe compressor compressor used used in in our our system system is anis an oil-injected, oil-injected, rotary-screw rotary-screw compressor. compressor. It has It has a power a power range ofrange up to 11of kW,up to maximum 11 kW, maximum acceleration acceleration of 8500 rpm, of maximum8500 rpm, flowmaximum rate of 1.2flow m 3rate/m, of and 1.2 functions m3/m, and under a pressurefunctions of under 15 bar. a An pressure air storage of 15 tank bar. is usedAn air for stor storingage tank compressed is used air for to storing maintain compressed a stable compressed air to flow,maintain reduce a pressure stable compressed fluctuations, flow, and allowreduce for pres thesure braking fluctuations, components and to allow perform for smoothly. the braking In our HPPS,components the tank to capacity perform is 110smoothly. L, with In compressed our HPPS, air th pressuree tank capacity that can is reach 110 upL, with to 15 compressed bar. air pressure that can reach up to 15 bar. 2.1.3. Energy Merger Pipe 2.1.3. Energy Merger Pipe The major advantage of the HPPS is the energy merger pipe, which has been evaluated and analyzedThe thoroughly major advantage in previous of the studies HPPS is [24 the]. Inenergy this pipe, merger the pipe, exhaust which pipe has diameter, been evaluated compressed and air analyzed thoroughly in previous studies [24]. In this pipe, the exhaust pipe diameter, compressed air pipe diameter, and common pipe diameter were all kept at 34 mm. The angle between the compressed air pipe and exhaust pipe was set to 30◦. The role of the energy merger pipe is important owing to its mixing capability that is responsible for generating merger flow. In particular, the cross-sectional area at the merging region of the pipe directly affects the recycling efficiency of the exhaust-gas energy. Appl. Sci. 2019, 9, 2333 4 of 15

From Ref. [25], information on high-pressure compressed airflow and high-temperature exhaust-gas flow under different cross-sectional areas can been found.

2.1.4. Air Motor The function of an air motor is to convert ﬂuid power to drive the shaft. The pressure and ﬂow rate are converted into torque and speed, respectively. An air motor (T30-M, TDI Tech Development) was utilized in our HPPS, whose maximum power was approximately 35PS/26 kW, maximum inlet pressure was approximately 150 Psig, and the maximum speed was higher than 4000 rpm.

2.2. Data Acquisition To run the system and to measure the signal from the sensors, this study used a data collector (USB-6218, National Instruments) and LabVIEW software [26].

2.3. Eﬃciency

2.3.1. ICE Heat Efficiency Thermal efficiency is defined as the percentage of energy that can be transferred from combustion to mechanical work. The base value of combustion efficiency ranged from 0.95 to 0.98 when the engine was operating. For one engine cycle in one cylinder, the added heat [27] is calculated as follows:

Qin = m f QHVηc (1)

For steady state: . . Qin = m f QHVηc (2) and the thermal eﬃciency is:

. . W W W η f ηt = = . = . = (3) Qin m Q ηc Qin f HVηc

. . where W is the work of one cycle, W is the power, m f is the mass of fuel of one cycle, and m f is the mass ﬂow rate of fuel; the gasoline density is 0.71–0.77 kg/L, QHV is the heating value of fuel: 47.3 MJ/kg, and η f is the fuel convention eﬃciency.

2.3.2. Compressor Efficiency The definition of isentropic efficiency in a compressor is the ratio of work input to the isentropic process, to the work input to the actual process between the same inlet and outlet pressures.

Isentropic compressor work Ws ηC = = (4) Actual compressor work Wa where ηC is the isentropic eﬃciency; Ws is the isentropic compressor work; Wa is the actual compressor work; Wa and Ws can be determined from the energy balance of the pumps. During an enthalpy change, energy associated with the gas ﬂowing through the compressor are negligible. The energy balance of the compressor becomes:

. . W = m(h h ) (5) − 2 − 1 The isentropic eﬃciency of a compressor becomes:

ηc=e(h2s h1)/(h2a h1) (6) − − Appl. Sci. 2019, 9, 2333 5 of 15

In the HPPS, Ws equals the output of ICE–W (work of one cycle). Wa equals the energy stored in the air tank. This energy can be calculated by the theory of storage thermodynamics. For a process from an initial condition A to a final condition B, with constant absolute temperature T = TA = TB. Additionally, if the outside of the container maintains a constant pressure that equals the initial pressure PA, the external pressure will reduce the available energy. The equation to calculate the energy stored in an air tank is as follows: PA PA Wa = W = PAPBln + (VA VB)PA = PAVAln + (PB PA)VB (7) PB − PB − 2 where W is the energy stored in the air tank (Watt), PA is the absolute pressure in the air tank (N/m ), 3 2 VA is the volume of the air tank (m ), PB is the absolute pressure at state B (N/m ), and VB is the volume at state B. (m3).

2.3.3. Air Motor Efficiency The inlet and outlet pressures are fixed for an adiabatic turbine in a steady flow process. Therefore, the turbine is an isentropic process between the inlet and outlet pressures under an idealized process. Hence, the ratio of the actual work outlet of the turbine to the work output of the turbine can be defined as the isentropic efficiency, if the turbine is under an isentropic process.

Actual turbine work W η = = at (8) Isentropic turbine work Wst

The value of Wat and Wst can be acquire form the energy balance of the turbine. Otherwise, the eﬃciency of the turbine is calculated based on the following equation [24]:

ωT η = (9) m∆P where η is the theoretical efficiency (%); ω is angular velocity (rad/s); T is the torque (Nm); ∆P is the pressure difference across the air turbine (bar); and m is the inlet flow rate of the air turbine.

2.3.4. System Efficiency Herein, we present the theoretical calculation of integral efficiency. The ICE efficiency, compressor efficiency, and air motor efficiency have been defined. We apply the Bernoulli equation to the Y junction, and the energy at inlet and outlet of merger pipe is measured as:

Qin = Qloss + Qout (10) where Qin is the energy of flow at the merger inlet (Q1 (compressed airflow inlet) and Q2 (exhaust-gas flow inlet)); Qout is the energy of flow at the merger outlet (Q3); and Qloss is the energy loses in pipe; we assume that Qloss = 0. Equation (10) becomes: Q1 + Q2 = Q3 (11) Base on the energy being released when the fuel is burnt, Equation (11) can eb rewritten as:

ηtηCW + ηexhaustW = Q3 (12) where ηt denotes the ICE efficiency; ηC denotes the compressor efficiency; and ηexhaust denotes the efficiency loses in the exhaust gas of ICE. Following the construction of the system (Figure1), the integral e fficiency can be calculated as:

ηt HPPSW = Q3η ηt HPPSW = (ηtηCW + ηexhaustW)η (13) − ⇔ − Appl. Sci. 2019, 9, 2333 6 of 15 where η denotes the air motor efficiency. In the HPPS, system efficiency is known as fuel efficiency during operation. It is the efficiency with which the potential energy of the fuel is converted into its kinetic energy. The overall fuel efficiency varies depending on the equipment, especially for fossil fuels in combustion power plants. Fuel economy is the energy efficiency of a particular system in the HPPS, and is related to engine, transmission, and compressor efficiencies. Fuel consumption allows for a more accurate measurement of the HPPS performance, owing to the fact that fuel consumption exhibits a linear relationship whereas fuel economy results in a distorted improvement of efficiency. Here, we can define the HPPS efficiency:

Wout ηt HPPS = (14) − Qin.ηtranmission where ηt HPPS denotes HPPS eﬃciency and Wout denotes the outlet power of the air motor; Qin is the − energy including fuel consumption; ηtranmission is the eﬃciency of the mechanical setup on the HPPS.

ηtranmission = ηchain tranmissionηbelt tranmissionηroller (15) where ηchain tranmission is the chain transmission efficiency; ηbelt tranmission is the belt transmission efficiency; ηroller is the efficiency of the roller.

3. Results When our HPPS is running, ICE connects with the compressor to create compressed air. It means the ICE operates with no free load. In this case, the ICE characteristic depends on the volume of the butterfly valve (intake throttle) of ICE. This valve controls the intake flow of the ICE. It will change from 0◦ at the closing position to 90◦ at the maximum opening position. Following Figure1, before taking the data from the HPPS, the ICE was warmed up for 2 min without connecting to the compressor. When it was warmed up, the ICE was directly connected to the compressor. The compressed air after the compressor was stored in the air tank. Following Ref. [24], the cross-section area was set at an optimal position of approximately 40% by adjusting the electric motor. The compressed air flow was adjusted by an electric valve.

3.1. Exhaust Pressure and Temperature of the ICE The exhaust pressure was measured by the pressure sensor, in the case of different volumes, in the butterfly valve (intake throttle) of the ICE, as shown in Figure2. The horizontal axis (load percentage) is the volume of the butterfly valve (intake throttle) of the ICE. It changed from 0◦ at the closing position to 90◦ at the maximum position. The vertical axis (exhaust pressure) is relative pressure (not including the atmosphere). Figure2 shows that the exhaust pressure depended on the intake of the ICE and was linear. When the volume was increased from 0% to 90%, the exhaust pressure rose regularly from 14 to 34 Psig. The exhaust temperature was measured by the temperature sensor, in the case of different volumes, in the butterfly valve (intake throttle) of the ICE, as shown in Figure3. The horizontal axis (load percentage) is the volume of the butterfly valve (intake throttle) of the ICE. It changed from 0◦ at the closing position to 90◦ at the maximum position. The vertical axis shows the exhaust temperature in ◦C. Following Figure3, we can see that the exhaust temperature increased linearly with the intake throttle of ICE. When the intake valve of the ICE changed from 0% to 30%, the temperature of the exhaust rose slightly from 100 ◦C to 170 ◦C. After that, from 30% to 90% of the volume of intake throttle, the temperature increased from 170 ◦C to 450 ◦C, at a faster rate than before. Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 15

Fuel economy is the energy efficiency of a particular system in the HPPS, and is related to engine, transmission, and compressor efficiencies. Fuel consumption allows for a more accurate measurement of the HPPS performance, owing to the fact that fuel consumption exhibits a linear relationship whereas fuel economy results in a distorted improvement of efficiency. Here, we can define the HPPS efficiency:

𝑊 𝜂 = (14) 𝑄.𝜂

where 𝜂 denotes HPPS efficiency and 𝑊 denotes the outlet power of the air motor; 𝑄 is the energy including fuel consumption; 𝜂 is the efficiency of the mechanical setup on the HPPS.

𝜂 =𝜂 𝜂 𝜂 (15)

where 𝜂 is the chain transmission efficiency; 𝜂 is the belt transmission efficiency; 𝜂 is the efficiency of the roller.

3. Results When our HPPS is running, ICE connects with the compressor to create compressed air. It means the ICE operates with no free load. In this case, the ICE characteristic depends on the volume of the butterfly valve (intake throttle) of ICE. This valve controls the intake flow of the ICE. It will change from 0° at the closing position to 90° at the maximum opening position. Following Figure 1, before taking the data from the HPPS, the ICE was warmed up for 2 min without connecting to the compressor. When it was warmed up, the ICE was directly connected to the compressor. The compressed air after the compressor was stored in the air tank. Following Ref. [24], the cross-section area was set at an optimal position of approximately 40% by adjusting the electric motor. The compressed air flow was adjusted by an electric valve.

Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 15

Figure 2. Relationship between %load and exhaust pressure of ICE.

The exhaust temperature was measured by the temperature sensor, in the case of different volumes, in the butterfly valve (intake throttle) of the ICE, as shown in Figure 3. The horizontal axis (load percentage) is the volume of the butterfly valve (intake throttle) of the ICE. It changed from 0° at the closing position to 90° at the maximum position. The vertical axis shows the exhaust temperature in °C. Following Figure 3, we can see that the exhaust temperature increased linearly with the intake throttle of ICE. When the intake valve of the ICE changed from 0% to 30%, the temperature of the exhaust rose slightly from 100 °C to 170 °C. After that, from 30% to 90% of the volume of intake throttle, the temperature increased from 170 °C to 450 °C, at a faster rate than before. Figure 2. Relationship between %load and exhaust pressure of ICE.

FigureFigure 3. Relationship3. Relationship between between %load %load and and exhaust exhaust temperature. temperature. 3.2. Fuel Consumption and Heat Efficiency of ICE 3.2. Fuel Consumption and Heat Efficiency of ICE As theAs ICE the drivesICE drives the compressor, the compressor, we recorded we recor fuelded consumptionfuel consumption following following the timethe time and pressureand in airpressure tank, at in many air tank, speeds. at many The speeds. horizontal The horizontal axis (rpm) axis is(rpm) the combustionis the combustion engine engine speed speed at at the the crank shaftcrank for a shaft period for ofa period 1 min. of The 1 min. vertical The vertical axis (fuel axis (fuel consumption, consumption, g/kWh) g/kWh) is is the the value value of gasoline gasoline for whichfor the which ICE the spent ICE 1 spent kilowatt 1 kilowatt in 1 h. in Figure 1 h. Figure4 estimates 4 estimates a relationship a relationship between between engine engine speed speed andand fuel consumption,fuel consumption, where the where engine the engine speed speed ranges ranges from from 3000 3000 to 4250 to 4250 rpm. rpm. It It shows shows clearly clearly that when when the enginethe speed engine was speed approximately was approximately 3800 3800 rpm, rpm, the the lowest lowest fuel fuel consumption consumption was approximately approximately 316.9 316.9 g. At ang. engine At an engine speed speed of 3000 of to3000 3800 to 3800 rpm, rpm, the the fuel fuel consumption consumption slightly slightly reduced from from 350 350 to to316.9 316.9 g. g. Moreover, from 3800 to 4500 rpm, the fuel consumption was regularly increased from 316.9 to Moreover, from 3800 to 4500 rpm, the fuel consumption was regularly increased from 316.9 to nearly nearly 330 g. From this result, we know that when the ICE drives the compressor, an engine speed of 330 g. From this result, we know that when the ICE drives the compressor, an engine speed of 3500 rpm 3500 rpm to 4000 rpm is the best position to conserve fuel. to 4000 rpm is the best position to conserve fuel. The heat efficiency of the ICE was measured by the ratio of power at the crank shaft (which is recorded by the torque sensor) and the chemical power of the fuel. The horizontal axis (rpm) is the combustion engine speed at the crank shaft for a period of 1 min. The vertical axis (heat Efficiency) is percentage (%). Figure5 shows the relationship between engine speed and heat e fficiency; this is important information required to run the ICE. When it drove the compressor, from 3500 to 4000 rpm, the ICE had high heat efficiency, from 26% to 28%. Before and after that, the efficiency was reduced.

Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 15

Figure 2. Relationship between %load and exhaust pressure of ICE.

Figure 3. Relationship between %load and exhaust temperature.

3.2. Fuel Consumption and Heat Efficiency of ICE As the ICE drives the compressor, we recorded fuel consumption following the time and pressure in air tank, at many speeds. The horizontal axis (rpm) is the combustion engine speed at the crank shaft for a period of 1 min. The vertical axis (fuel consumption, g/kWh) is the value of gasoline for which the ICE spent 1 kilowatt in 1 h. Figure 4 estimates a relationship between engine speed and fuel consumption, where the engine speed ranges from 3000 to 4250 rpm. It shows clearly that when the engine speed was approximately 3800 rpm, the lowest fuel consumption was approximately 316.9 g. At an engine speed of 3000 to 3800 rpm, the fuel consumption slightly reduced from 350 to 316.9 g. Moreover, from 3800 to 4500 rpm, the fuel consumption was regularly increased from 316.9 to nearly 330 g. From this result, we know that when the ICE drives the compressor, an engine speed of Appl. Sci. 2019, 9, 2333 8 of 15 Appl.3500 Sci. rpm 2019 to, 9 4000, x FOR rpm PEER is REVIEWthe best position to conserve fuel. 8 of 15

Appl. Sci. 2019Figure, 9, x FOR 4. Fuel PEER consumption REVIEW of combustion engine when it connects with the compressor. 8 of 15

The heatFigure efficiency 4. Fuel consumption of the ICE wasof combustion measured engine by the when ratio it connectsof power with at the the crankcompressor. shaft (which is recorded by the torque sensor) and the chemical power of the fuel. The horizontal axis (rpm) is the combustionThe heat engine efficiency speed of at the the ICE crank was shaft measured for a period by the of ratio 1 min. of Thepower vertical at the axis crank (heat shaft Efficiency) (which is percentagerecorded by (%). the Figuretorque sensor)5 shows and the therelationship chemical powerbetween of enginethe fuel. speed The horizontaland heat efficiency; axis (rpm) this is the is importantcombustion information engine speed required at the crank to run shaft the forICE. a periodWhen ofit drove1 min. theThe compressor, vertical axis from(heat Efficiency)3500 to 4000 is rpm,percentage the ICE (%). had Figure high 5heat shows efficiency, the relationship from 26% between to 28%. engineBefore andspeed after and that, heat the efficiency; efficiency this was is reduced.important information required to run the ICE. When it drove the compressor, from 3500 to 4000 rpm, the ICE had high heat efficiency, from 26% to 28%. Before and after that, the efficiency was reduced. Figure 4. Fuel consumption of combustion engine when it connects with the compressor.

Figure 5. Heat efficiency of combustion engine when it connects with the compressor. Figure 5. Heat efficiency of combustion engine when it connects with the compressor. 3.3. CompressorFigure Efficiency 5. Heat andefficiency Flow Enof combustionergy Storing engine of Air whenStorage it connects Tank with the compressor. 3.3. Compressor Efficiency and Flow Energy Storing of Air Storage Tank The efficiency of the compressor is the ratio of inlet power (at the driver shaft) and power of 3.3. Compressor Efficiency and Flow Energy Storing of Air Storage Tank compressedThe effi ciencyair (which of the is stored compressor in the air is thetank). ratio On of our inlet HPPS, power the (atcompressor the driver was shaft) driven and by power an ICE. of compressedThis meansThe efficiency that air (which at every of the is speed stored compressor of in the the ICE, airis the tank).the ratiocompre On of our ssorinlet HPPS, will power have the (at compressor a correspondingthe driver was shaft) driven efficiency and by power an value. ICE. of ThisThecompressed meansexperimental that air (which at everyresults is speed stored in Figure of in the the 6 ICE, airshows tank). the compressorthat On from our HPPS,3700 will to havethe 4250 compressor a correspondingrpm, the was compressor driven efficiency by had an value. ICE. the ThehighestThis experimental means efficiency that at results everyof approximately inspeed Figure of 6the shows 70%.ICE, thattheAt compre<3700 from 3700 rpm,ssor to willthe 4250 haveefficiency rpm, a thecorresponding compressorof the compressor efficiency had the reduced highest value. eslightlyThefficiency experimental when of approximately the engineresults speedin 70%. Figure was At < 63700decreased. shows rpm, that the Wh efromffienciency the 3700 ICE of to thespeed 4250 compressor wasrpm, higher the reduced compressor than 4250 slightly rpm,had when the theefficiencyhighest engine efficiency of speed the compressor was of approximately decreased. went When down 70%. the considerably. At ICE <3700 speed rpm, was higherthe efficiency than 4250 of rpm,the compressor the efficiency reduced of the compressorslightly when went the downengine considerably. speed was decreased. When the ICE speed was higher than 4250 rpm, the efficiency of the compressor went down considerably.

Figure 6. Compressor efficiency when it driven by a combustion engine. Figure 6. Compressor efficiency when it driven by a combustion engine. Figure7 shows the pressure value of the air storage tank at di fferent ICE speeds. When the ICE was operatingFigure 7 shows at aFigure high the speed, 6.pressure Compressor the value energy efficiency of storage the air when flowstorage it ofdriven the tank air by at storagea differentcombustion tank ICE increasedengine. speeds. rapidly.When the This ICE is becausewas operating when theat a ICEhigh speed speed, increases, the energy the storage compressed flow of air the supplied air storage to the tank air increased storage tank rapidly. by the This air compressoris becauseFigure when 7 increases shows the theICE as pressure well.speed For increases, value instance, of thethe the compressedair pressure storage tank of air the suppliedat airdifferent storage to theICE tank air speeds. reachedstorage When tank 120 the Psigby ICEthe at approximatelyairwas compressor operating at 150increases a high s when speed, as thewell. the ICE For energy speed instance, wasstorage the 4250 pressureflow rpm. of the The of airthe experimental storage air storage tank resulttank increased reached indicates rapidly. 120 that Psig This the at approximatelyis because when 150 the s ICEwhen speed the ICEincreases, speed thewas compressed 4250 rpm. Theair suppliedexperimental to the result air storage indicates tank that by the air compressor increases as well. For instance, the pressure of the air storage tank reached 120 Psig at approximately 150 s when the ICE speed was 4250 rpm. The experimental result indicates that the Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 15

Appl.flow Sci. energy 2019, 9 ,can 2333x FOR be PEERfilled REVIEW quickly into the air storage tank in our HPPS. Additionally, the compressed9 of 15 air flow stored in the air storage tank can be used at all times. flow energy can be filled quickly into the air storage tank in our HPPS. Additionally, the compressed flowair flow energy stored can in be the filled air storag quicklye tank into thecan airbe storageused at tankall times. in our HPPS. Additionally, the compressed air flow stored in the air storage tank can be used at all times.

Figure 7. Pressure of the air storage tank at different internal combustion engine (ICE) speeds. Figure 7. Pressure of the air storage tank at different internal combustion engine (ICE) speeds. 3.4. FreeFigure Load Condition7. Pressure and of the Efficiency air storage of Air tank Motor at different internal combustion engine (ICE) speeds. 3.4. Free Load Condition and Efficiency of Air Motor The air motor performance in cases of with and without exhaust from the ICE are presented. 3.4. FreeThe Load air motorCondition performance and Efficiency in casesof Air ofMotor with and without exhaust from the ICE are presented. This is very important to determine the best position working area and the efficiency of our HPPS. In ThisThe is very air importantmotor performance to determine in cases the best of with position and workingwithout exhaust area and from the ethefficiency ICE are of presented. our HPPS. this case, the air motor ran at the free load condition with and without exhaust from the ICE. The InThis this is very case, important the air motor to determine ran at the the free best load positi conditionon working with area and and without the efficiency exhaust of from our HPPS. the ICE. In value of the load cell was 0. The ICE and compressor were run at the sweet spot condition, with an Thethis valuecase, the ofthe air loadmotor cell ran was at 0.the The free ICE load and cond compressorition with were and run without at the exhaust sweet spot from condition, the ICE.with The ICE speed of 3800 rpm. The exhaust pressure was 27 Psi, and the temperature was approximately 350 anvalue ICE of speed the load of 3800 cell rpm.was 0. The The exhaust ICE and pressure compressor was 27 were Psi, run and at the the temperature sweet spot wascondition, approximately with an °C. Following Figure 8, at the free load condition and outlet of the air tank pressure was lower than 350ICE◦ speedC. Following of 3800 Figurerpm. The8, at exhaust the free pressure load condition was 27 and Psi, outlet and the of thetemperature air tank pressure was approximately was lower than 350 4 bar, the air motor speed rose rapidly from 0 to approximately 2200 rpm without the exhaust 4°C. bar, Following the air motor Figure speed 8, at rose the rapidlyfree load from condition 0 to approximately and outlet of 2200 the rpm air tank without pressure the exhaust was lower condition than condition and from 0 to 2500 rpm with the exhaust condition, when the outlet air tank pressure was and4 bar, from the 0 air to 2500motor rpm speed with rose the exhaustrapidly condition,from 0 to whenapproximately the outlet air2200 tank rpm pressure without was the increased exhaust increased from 0 to 4 bar. After that, the air motor speed increased from approximately 2200 to 2750 fromcondition 0 to 4 and bar. from After 0 that, to 2500 the airrpm motor with speedthe exhaust increased condition, from approximately when the outlet 2200 air to tank 2750 pressure rpm without was rpm without the exhaust and from approximately 2500 to 2900 rpm in the exhaust condition, when theincreased exhaust from and 0 fromto 4 bar. approximately After that, the 2500 air to mo 2900tor rpmspeed in increased the exhaust from condition, approximately when the 2200 outlet to 2750 air the outlet air tank pressure was raised from 4 to 8 bar. tankrpm pressurewithout the was exhaust raised fromand from 4 to 8 approximately bar. 2500 to 2900 rpm in the exhaust condition, when the outlet air tank pressure was raised from 4 to 8 bar.

Figure 8. Relationship between air motor speed and inlet pressure in case of with and without exhaust Figure 8. Relationship between air motor speed and inlet pressure in case of with and without exhaust at the free load condition. at the free load condition. Figure 8. Relationship between air motor speed and inlet pressure in case of with and without exhaust at the free load condition. Appl. Sci. 2019, 9, 2333 10 of 15 Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 15

InIn thisthis test,test, wewe measuredmeasured thethe eefficiencyfficiency ofof thethe airair motormotor underunder twotwo conditionsconditions forfor thethe inletinlet airair motor,motor, with with and and without without exhaust exhaust from from the the ICE. ICE. In In Figure Figure9, the9, the lower lower line line chart chart shows shows the the e ffi efficiencyciency of theof the air air motor motor without without the the exhaust exhaust inlet inlet condition, condition, and and the the higher higher line line chart chart measures measures the the effi efficiencyciency of theof the air motorair motor with with exhaust exhaust at the at inlet the condition.inlet condit Withion. the With exhaust the exhaust at a high at temperature a high temperature and pressure and forpressure the inlet for condition, the inlet condition, the efficiency the efficiency of the air motorof the air increased motor increased to approximately to approximately 30% at a pressure30% at a lowerpressure than lower 4 bar than for the 4 outletbar for air the tank. outlet Without air tank the. exhaustWithout at the the exhaust inlet condition, at the inlet the e fficondition,ciency of airthe motorefficiency dropped of air rapidly motor from dropped approximately rapidly from 38% atappr 4 baroximately of the outlet 38% air at tank 4 bar pressure, of the tooutlet 0% at air 1.5 tank bar ofpressure, the outlet to air0% tankat 1.5 pressure. bar of the With outlet the air exhaust tank pr atessure. the inlet With condition, the exhaust the e ffiat ciencythe inlet of condition, the air motor the reducedefficiency significantly of the air motor from reduced approximately significantly 62% at fr 4om bar approximately of outlet air tank 62% pressure, at 4 bar toof 0%outlet at 0air bar tank of outletpressure, air tankto 0% pressure. at 0 bar of From outlet 4 to air 8 bar,tank in pressure. the no-exhaust From 4 condition,to 8 bar, in the the e no-exhaustfficiency of condition, the air motor the increasedefficiency slightly of the air from motor 38% increas to 48%.ed However,slightly from in the 38% exhaust to 48%. condition, However, the in ethefficiency exhaust of condition, the air motor the roseefficiency considerably of the air from motor 62% rose to 72%. considerably from 62% to 72%.

Figure 9. Relationship between eﬃciency of air motor and inlet pressure for the with and without Figure 9. Relationship between efficiency of air motor and inlet pressure for the with and without exhaust cases. exhaust cases. 3.5. Driving Cycle 3.5. Driving Cycle This study applies the emissions standards of Economic Commission for Europe (ECE 40). AccordingThis study to the ECEapplies 40 drivingthe emissions cycle requirement, standards of we Economic must control Commission the electric for valve Europe at the (ECE air tank 40). outletAccording and control to the ECE the brake 40 driving power cycle to follow requirement, the outlet we of must the system. control The the valueelectric of valve the electric at the valveair tank is shownoutlet and in Figure control 10 .the Figure brake 11 power depicts to thefollow experimental the outlet resultsof the system. of our HPPS The value when of it ranthe electric based on valve the ECEis shown 40 standard. in Figure Our 10.HPPS Figure cycles 11 depicts can follow the experi the drivingmental cycleresults closely. of our There HPPS are when some it minorran based errors on thatthe ECE occurred 40 standard. with the Our HPPS HPPS speed; cycles this can is because follow the responsedriving cycle of the closely. electric There valve are at thesome air minor tank outleterrors wasthat notoccurred fast enough. with the HPPS speed; this is because the response of the electric valve at the air tank outlet was not fast enough. Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 15 Appl. Sci. 2019, 9, 2333 11 of 15 Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 15

Figure 10. Relationship between the volume of the electric valve and driving time of the emissions Figure 10. Relationship between the volume of the electric valve and driving time of the emissions Figure 10. Relationshipstandards between of the Economic volume Commission of the electric for valve Europe and (ECEdriving 40). time of the emissions standards of Economic Commission for Europe (ECE 40). standards of Economic Commission for Europe (ECE 40).

Figure 11. Experimental results of HPPS driving cycle. Figure 11. Experimental results of HPPS driving cycle. 4. Discussion 4. Discussion 4. Discussion 4.1. HPPS Performance 4.1. HPPS Performance 4.1. HPPSBased Performance on Figures 4–6, we compared three specifications of the HPPS performance at di fferent ICE speeds.Based on The Figures specifications 4–6, we werecompared fuel consumption, three specifications ICE heat of e thefficiency, HPPS and performance compressor at edifferentfficiency. Based on Figures 4–6, we compared three specifications of the HPPS performance at different BaseICE speeds. on this The comparison, specifications we were determined fuel consumptio the bestn, working ICE heat area efficiency, for the and HPPS compressor with high efficiency. ICE and ICE speeds. The specifications were fuel consumption, ICE heat efficiency, and compressor efficiency. compressorBase on this ecomparison,fficiency, and we low determined fuel consumption. the best working The relationship area for the between HPPS thewith ICE high speed, ICE fueland Base on this comparison, we determined the best working area for the HPPS with high ICE and consumption,compressor efficiency, ICE efficiency, and low and fuel compressor consumption. efficiency The is illustratedrelationship in between Figure 12 the. ICE speed, fuel compressor efficiency, and low fuel consumption. The relationship between the ICE speed, fuel consumption,Figure 12 ICEdepicts efficiency, that when and compressor the ICE speed efficiency was in is theillustrated range of in 3500–4000Figure 12. rpm, the ICE and consumption, ICE efficiency, and compressor efficiency is illustrated in Figure 12. compressor demonstrated the best performance in relationship with each other. At ICE speeds <3500 rpm, the system performance reduced as a result of the decreased ICE heat efficiency, compressor efficiency, and increased fuel consumption. At ICE speeds >4000 rpm, the compressor efficiency reduced slightly. Additionally, the ICE efficiency decreased and fuel consumption increased. Therefore, the ICE and compressor did not demonstrate a good performance in relationship with each other. Hence, to obtain the best performance of the HPPS, we need to determine the best position for air motor performance. As can be seen in Figure9, when the pressure of the compressed air flow was in the range of 6–8 bar, the air motor had the highest efficiency for both cases, with or without exhaust form ICE. Appl. Sci. 2019, 9, 2333 12 of 15 Appl. Sci. 2019, 9, x FOR PEER REVIEW 12 of 15

Figure 12. Relationship between the ICE speed, fuel consumption, ICE efficiency,and compressor efficiency. Figure 12. Relationship between the ICE speed, fuel consumption, ICE efficiency, and compressor 4.2. HPPSefficiency. Efficiency From Equation (13), the integral efficiency can be calculated as: Figure 12 depicts that when the ICE speed was in the range of 3500–4000 rpm, the ICE and compressor demonstrated the best performance in relationship with each other. At ICE speeds <3500 ηt HPPSW = Q3η ηt HPPSW = (ηtηCW + ηexhaustW)η, rpm, the system performance− reduced as⇔ a result− of the decreased ICE heat efficiency, compressor where,efficiency, the bestand workingincreased area fuel of consumption. HPPS (Figure 12At), ηICE(air speeds motor e>4000fficiency) rpm, is the 0.719; compressorηt (ICE effi efficiencyciency) is reduced slightly. Additionally, the ICE efficiency decreased and fuel consumption increased. 0.28; ηC (compressor efficiency) is 0.7. Therefore,According the ICE to Ref. and [ 28compressor], the exhaust did heatnot lossdemonstr can beate determined a good performance to be approximately in relationship 45% of with the each other. Hence, to obtain the best performance of the HPPS, we need to determine the best position total net power. Substitute the values of η, ηt, ηC, and ηexhaust in Equation (13): for air motor performance. As can be seen in Figure 9, when the pressure of the compressed air flow was in the range of η6–8t HPPS bar,= the(η tairηC motor+ ηexhaust had)η the= ( hi0.28ghest0.7 efficiency+ 0.45) 0.72for both0.465 cases, with or without − ∗ ∗ ≈ exhaust form ICE. Based on the experimental results of the ICE efficiency in Figure5, ICE fuel consumption in Figure4.2. HPPS4, compressor Efficiency e fficiency in Figure6, air motor e fficiency in Figure9, and Ref. [ 24], we determined the integral efficiency of the HPPS. The system was run at five different ICE speeds (3000, 3250, 3600, From Equation (13), the integral efficiency can be calculated as: 3800, and 4250 rpm). When the ICE is running,𝜂 compressed𝑊=Q𝜂 air ⟺ is𝜂 created𝑊=(𝜂 and charged𝜂𝑊+𝜂 into the𝑊)𝜂 storage, air tank. In case of the cross-section area set at the best position [24], the control electric valve was used to control where, the best working area of HPPS (Figure 12), 𝜂(air motor efficiency) is 0.719; 𝜂 (ICE efficiency) the pressure and flow rate of the air tank outlet. Then, this air mixes with the exhaust from the ICE. is 0.28; 𝜂 (compressor efficiency) is 0.7. For different ICE speeds, the HPPS will have different stable operations for the air motor. Based on According to Ref. [28], the exhaust heat loss can be determined to be approximately 45% of the this, the input and output powers of the HPPS were measured, which are summarized in Table1. total net power. Substitute the values of 𝜂, 𝜂, 𝜂, and 𝜂 in Equation (13): By applying Equation (14), we obtained the efficiency of the HPPS. The experimental result of the HPPS integral efficiency𝜂 is= shown(𝜂𝜂 in+𝜂 Figure 13)𝜂=. (0.28 ∗ 0.7 + 0.45) ∗ 0.72 ≈ 0.465 Based on the experimental results of the ICE efficiency in Figure 5, ICE fuel consumption in Figure 4, compressor efficiency in Figure 6, air motor efficiency in Figure 9, and Ref. [24], we determined the integral efficiency of the HPPS. The system was run at five different ICE speeds (3000, 3250, 3600, 3800, and 4250 rpm). When the ICE is running, compressed air is created and charged into the storage air tank. In case of the cross-section area set at the best position [24], the control electric valve was used to control the pressure and flow rate of the air tank outlet. Then, this air mixes with the exhaust from the ICE. For different ICE speeds, the HPPS will have different stable operations for the air motor. Based on this, the input and output powers of the HPPS were measured, which are summarized in Table 1.

Appl. Sci. 2019, 9, 2333 13 of 15 Appl. Sci. 2019, 9, x FOR PEER REVIEW 13 of 15

Table 1. Input and output specifications speciﬁcations of HPPS. Air Motor Speed Air Motor Torque Fuel Consumption ICEICE Speed Speed (rpm)Number Number Air of Motor Tests Speed Air Motor Torque Fuel Consumption of ICE (rpm) (Nm) of ICE (mL/min) (rpm) of Tests (rpm) (Nm) (mL/min) 1 11375 1375 9.25 9.25 12.46 12.46 2 21300 1300 9.20 9.20 12.41 12.41 30003000 3 31305 1305 9.00 9.00 12.72 12.72 4 41351 1351 9.30 9.30 12.15 12.15 5 51328 1328 9.22 9.22 12.35 12.35 1 1545 11.00 13.01 1 1545 11.00 13.01 2 1573 10.85 12.86 2 1573 10.85 12.86 3250 3 1515 10.90 12.96 3250 4 31525 1515 11.01 10.90 13.25 12.96 5 41539 1525 11.10 11.01 13.15 13.25 1 51765 1539 16.25 11.10 14.63 13.15 2 11800 1765 16.10 16.25 14.61 14.63 3600 3 21805 1800 16.15 16.10 14.51 14.61 4 1750 16.30 14.86 3600 3 1805 16.15 14.51 5 1755 16.31 14.76 4 1750 16.30 14.86 1 1885 16.75 14.79 2 51851 1755 16.35 16.31 15.00 14.76 3800 3 11825 1885 16.80 16.75 14.50 14.79 4 21856 1851 16.55 16.35 15.00 15.00 3800 5 31871 1825 16.50 16.80 14.90 14.50 1 41975 1856 17.00 16.55 18.10 15.00 2 2000 16.85 18.36 5 1871 16.50 14.90 4250 3 2001 17.01 17.95 1 1975 17.00 18.10 4 1951 17.20 17.87 5 21985 2000 16.80 16.85 18.24 18.36 4250 3 2001 17.01 17.95 By applying Equation (14),4 we obtained the 1951 efficiency of the HPPS. 17.20 The experimental 17.87 result of the HPPS integral efficiency is5 shown in Figure 198513. 16.80 18.24

Figure 13. HPPS integral eﬃciency. Figure 13. HPPS integral efficiency. Appl. Sci. 2019, 9, 2333 14 of 15

Figure 13 depicts that the HPPS integral efficiency was different for different ICE speeds. As can be seen in Figure 13, low speeds for the ICE were 3000 and 3250 rpm. The system’s integral efficiency was about 0.215 at 3000 rpm, and about 0.282 at 3250 rpm. At this position, based on Figures5 and6, the ICE efficiency was about 0.254 at 3000 rpm, and 0.26 at 3250 rpm. Additionally, the compressor efficiency was about 0.602 at 3000 rpm, and about 0.62 at 3250 rpm. Therefore, the compressed air in the tank has a much lower efficiency in comparison with the ICE efficiency. At low ICE speeds, the HPPS operated in a stable condition at low pressure, with low outlet torque and low air motor speed, as summarized in Table1. This is because of the low pressure of the outlet merger pipe, which was about 3.8 bars at 3000 rpm, and about 4.5 bars at 3250 rpm. Our HPPS had the highest efficiency at an ICE speed of about 3800 rpm, which averages to about 45.3%. The stable operation of the HPPS was observed at about 7 bars of the outlet merger pipe. Table1 lists the high outlet torque and high speed of the air motor. At ICE speeds of around 3800 rpm, such as 3600 and 4250 rpm, the HPPS had a high efficiency. However, this efficiency was still lesser than that at an ICE speed of 3800 rpm. This is because of the low efficiency of the ICE and compressor, and high fuel consumption.

5. Conclusions In this study, the experimental performance of our HPPS was analyzed and evaluated. The performance of the HPPS was optimized to the best working area. At this position, the HPPS performance in terms of fuel consumption was lowest, and power at the air motor outlet was highest. The comparison of different HPPS performances demonstrates the role of system operation conditions in system integral efficiency. This study determined the best performance area of the HPPS. At this position, the system integral efficiency was 45% with a fuel consumption conservation of 38%.

Author Contributions: Conceptualization, D.D.N.; methodology, D.D.N.; software, D.D.N.; validation, P.-T.C., D.D.N. and K.D.H.; formal analysis, D.D.N.; investigation, D.D.N.; resources, C.-J.Y.; data curation, D.D.N.; writing—original draft preparation, P.-T.C. and C.-J.Y.; writing—review and editing, P.-T.C. and C.-J.Y.; visualization, D.D.N.; supervision, C.-J.Y. and K.D.H. Funding: This research received no external funding. Acknowledgments: This manuscript was edited by Editage. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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