rare

tics a

Received in revised form

Keywords:Air-powered engineTemperature dropThrottling effect

of aenc

problem. To predict temperature drops during the operation, rstly, the thermodynamic model of

calculation model of the throttling effect for dynamic temperature analysis is proposed. Further-

risis an

discharges breathable air at low temperature and low pressure, sothe operation of the APE can achieve zero carbon emissions, whichis signicant for air pollution abatement in some industrial country,especially in China.

due to the airsoorer power per-ven result in icewhich is a prob-Yung Huang bothre decrease in theresulted in lowercreasing the inlet

temperature is an effective way to enhance the engines powerand efciency, and Zhai also studied the optimization of the APEsinlet tube for the best heat transfer performance [16]. But fewstudies established complete heat transfer models of the APEsystem, as well as methods to calculate the air temperature dropsat critical locations, especially the high pressure reducing valveand the exhaust port of the APE.

Several studies provided references for the temperature analysisof the compressed air system. The JouleeThomson coefcient was

* Corresponding author. Tel.: 86 15810834177.

Contents lists availab

Ener

.e ls

Energy 68 (2014) 877e885E-mail address: yesoyou@163.com (Y. Shi).a kind of medium, is considered to have superior energy storagedensity to other media like batteries. It can be easily obtained fromthe power generation process of renewable energies, such as solarenergy, wind energy and tidal energy [3e6]. The air-powered en-gine (APE) is driven by compressed air. As a lightweight, non-polluting and safe power device, it can be applied to trans-portation, general aviation and small-scale power generation in thefuture. Especially, serving as engines of motor vehicles, the APE

still in developing stage. Low temperatureexpansion and the local throttling can lead to pformance of the compressed air, and can eblockings at critical locations of the APE systemlem that cannot be ignored. Liu Hao and Chihmentioned that there was a certain temperatucylinder of the APE and higher inlet pressurestemperatures [14,15]. Zhai Xin reported that init has become increasingly important to nd environmentallyfriendly energies to replace fossil fuels [1,2]. The compressed air, as

designs on kinds of APEs [9e13]. But generally limited by prob-lems of low working efciency and low temperature, the APE isDynamic modelHeat exchange

1. Introduction

Under the background of energy chttp://dx.doi.org/10.1016/j.energy.2014.02.1020360-5442/ 2014 Elsevier Ltd. All rights reserved.more, a complete dynamic model of the APE system is established, by considering models mentionedabove and models of the pressure tank and the supply pipeline as well. The models feasibility on thetemperature drop analysis is veried by comparing with corresponding experiments. Simulation of apractical APE system is carried out. Under specic parameter settings, temperature drops of criticallocations in the system are predicted. On this basis, the supply system of compressed air is modiedand a principle structure of the heat exchange system for the APE system is proposed. Theanalysis results in this paper can provide a theoretical support for the design of the heat exchangesystem.

2014 Elsevier Ltd. All rights reserved.

d environmental issue,

In the past 10 years, MDI, a French company of air-poweredvehicles, developed a series of APEs [7,8]. In China, some univer-sities and companies have also been conducting researches andAvailable online 21 March 201418 February 2014Accepted 27 February 2014

the APE and a calculation method for equivalent air temperatures at intake and exhaust ports aredescribed. The cooling mechanism of the pressure-reducing process is analyzed. Then a simpliedDynamic heat transfer model for tempeexchange system design of the air-powe

Qiyue Xu, Maolin Cai, Yan Shi*

School of Automation Science and Electrical Engineering, Beijing University of Aeronau

a r t i c l e i n f o

Article history:Received 19 November 2013

a b s t r a c t

In the operation processcritical locations can inu

journal homepage: wwwture drop analysis and heatd engine system

nd Astronautics, Beijing 100191, China

n air-powered engine (APE) system, temperature drops happening ine the engines performance negatively, and even lead to the ice blocking

le at ScienceDirect

gy

evier .com/locate/energy

used to calculate the temperature changes of the throttling [17e20]. The real gas effect on temperature was considered in the dis-charging process of high pressure vessels [21,22]. But most of thesemethods need to solve transcendental equations which have largecalculation complexity, thus they are not suitable for modeling ofdynamic pneumatic systems.

This paper focuses on the temperature drop analysis of the APEsystem. Dynamic air temperature calculation methods of locationsincluding the APEs ports and the throttling pressure reducer areproposed. Then the complete dynamic heat transfer model of theAPE system can be established for temperature analyses and thedesign of the heat transfer system. Results of this paper are helpfulto predict temperature drops in the practical APE system, to pre-vent the ice blocking problem and to improve the performance ofthe APE system.

2. Introduction of the APE system

2.1. Working principle of the APE

For a piston-type APE, the compressed air expands in the cyl-

cycle.

where G1 dm1/df, G2 dm2/df, G dm/df, m is the mass of the

Q. Xu et al. / Energy 688782.2. Locations of the temperature drop

This paper only concerns the piston-type APE. However, for anykind of APE, the compressed air goes through it will cool due to theexpansion power process.

At room temperature, the real gases except hydrogen, heliumand neon cool upon expansion by the JouleeThomson (JeT) effect.Therefore when the compressed air discharges from a container oris forced through a pressure-reducing valve it has temperatureinder, pushing the piston to output shaft power. Its operation isshown in Fig. 1: in the suction power stroke the compressed airenters the cylinder through the intake valve, driving the pistondownward. Then the intake valve closes after a specic crankangle while the compressed air expands to push the piston downand output work. When the piston is near the bottom dead centerthe exhaust valve opens so that the air with residual pressuredischarges under the impetus of the piston. After the pistonmoves back to the top dead center, the APE completes a workFig. 1. Working cycle of the APE.air in the cylinder, Cv is the constant volume specic heat, ct is theheat transfer coefcient, Ah(f), Ta are the total heat transfer areaand the temperature of the internal walls, respectively, here Ta isassumed to be equal to the room temperature, T is the tempera-ture of the air in the cylinder, h1, h2 are the specic enthalpies ofthe air ow in and out of the cylinder, respectively, m1, m2 are themass of the air ow in and out, respectively, V is the instant vol-ume of the cylinder, u is the specic internal energy and f is thecrank angle.

The change rate of the volume is described by:

dVdf

p8D2S

264sin f lsin fcos f

1 l2sin2 fq

375 (2)

where D is the diameter of the cylinder, S is the stroke of the piston,l is the crank ratio.

The total heat transfer area is:

Ah p

2D2 p

2DS1 cosf 1

l

1

1 l2sin2f

q (3)

3.1.2. Continuity equationThe intake and exhaust airow rate of the APE can be calculateddrops. In the APE system, apart from the cylinder of the APE, thehigh pressure tank and the pressure reducer are typical locationswhere the air temperatures are predicted to decrease. Though thecompressed air experiences dry processing, it cannot be guaranteed100% without water. Thus when the temperature of air is lowerthan the dew point, the locations mentioned above are consider-ably possible to have ice blockings. In the following sections,thermodynamic models of these locations are established foranalysis.

3. Thermodynamic model of the APE and calculation methodof the equivalent air temperatures

3.1. Thermodynamic model of the APE

The model based on the crank angle was built at rst. In themodeling process, the following assumptions are made.

(1) The compressed air is ideal, which means specic heat u andspecic enthalpy h are only related to the temperature.

(2) The air in the cylinder is uniform during the thermodynamicprocess.

(3) The air ow in and out of the cylinder is considered as quasi-steady, one-dimension and isentropic.

(4) The airs kinetic energy and potential energy are ignored.(5) There is no leak during the working cycle.

3.1.1. Energy equationThe energy equation can be given in the form of temperature

differential equation:

dTdf

1mCv

ctAhfTa T h1G1 h2G2 p

dVdf

uG

(1)

(2014) 877e885as follows:

especially the latter are important locations in the temperatureanalysis.

However, the temperature of the air going through the ports istime varying. For the intake port, when the intake valve is openthus the port can be regarded as a part of the cylinder, the airtemperature at the port changes with the temperature in the cyl-inder. When the valve is closed, the air temperature at the port can

2 1 kk 1

2 1 kk 1

(4)

y 68 (2014) 877e885 879where u is the angular speed of the crank, k is the adiabaticexponent of the air, Rg is the gas constant, pi, Ti are the upstreampressure and temperature, respectively, po is the downstreampressure, A(f) is the effective sectional area of the intake or exhaustvalve:

Af pSvfDv sin2avSf=2cosav (5)

where Dv is the valve diameter, av is the valve contact angel, Sv(f) isthe motion of the valve lift which is expressed as:

Svf i"Hffafo

315H

ffafo

46H

ffafo

5#(6)

where i is the drive ratio of the rocker,H is the cam lift, fo is the camrise angle, fa is the advance angle. The motion of the valve fall canbe described in the symmetrical form.

3.1.3. State equationIdeal air meets the equation of the state:

pV mRgT (7)

where Rg is the gas constant of air [J (kg K)1].

3.1.4. Torque equationIn addition, the dynamic model of the APE is based on the dif-

ferential equations of the crankshaft which relate the crank anglewith the time:8>>>>>:dfdt

u

dudt

Mp Ms Mn Mi MeICr

(8)

where u is the angular velocity, t is the time, Mp is the drivingtorque generated by the expansion of the compressed air, Ms is an

Gi

8>>>>>>>>>>>>>>>>>:

1uA

0@f1Api

k

RgTi

2

k 1k 1k 1

vuuut;

popi

k

1uA

0@f1Api

2k

k 11

RgTi

"popi

2k

popi

k 1k#vuuut ; po

pi>

k

Q. Xu et al. / Energinstant stating torque,Mn is a tunable load torque,Mi (i 1, 2) is thereal time torque caused by the reaction force of the intake orexhaust valve, Me is the torque due to the friction loss and themechanical efciency and ICr is the total moment of inertia ofrotating parts on the crankshaft. These torques analytical expres-sions are not provided since they have no effect on the heat transfersystem.

3.2. Equivalent air temperatures at intake and exhaust ports

Due to the throttling of the valve and the air expansion in thecylinder, the air temperatures of the intake and exhaust ports arepredicted to be low. And blocking of the ports seriously affects theperformance of the APE. Hence intake and exhaust ports of the APEbe set equal to the upstream temperature Tin in the inlet pipe. As tothe exhaust port, the air temperature at the port is closer to theroom temperature when the exhaust valve is closed. And in theexhaust process, due to the airow from the cylinder, the air tem-perature is regarded as equal to the temperature in the cylinder.

In addition, the intake and exhaust ports are both very short andthe airow quickly in the operation. Thus, for the convenience ofanalysis, it is assumed that the inner walls of the ports have no heattransfer with the air.

The state of the valves can be judged exactly by monitoring themass of the air in the cylinder. Under certain parameter settings,Fig. 2 shows curves of the cylinder temperature andmass simulatedduring a period, and angles (f1, f2, f3 and f4) which mark theintake and exhaust phases. Thus in a relatively short period of time,the equivalent air temperature at the intake port can be expressedby:

Tine

Z f2f1

TdfZ f10

TindfZ 360f2

Tindf

360(9)

where dm/df> 0, if f (f1, f2).And the equivalent air temperature at the exhaust port is:Fig. 2. Curves of the cylinder temperature and mass simulated during a period.

vTv p pv Tv

y 68potential energy, which has an inconvenient impact on thetemperature change during the rapid throttling process of awhere R is the gas constant [J (mol K)1], so that for an ideal gasmj 0. Its temperature remains constant during the throttlingprocess. But the ideal gas state equation ignores the molecularTexe

Z f4f3

TdfZ f30

TadfZ 360f4

Tadf

360(10)

where dm/df> 0, if f (f3, f4).

4. Simplied model of throttling effect for dynamictemperature analysis

In high pressure pneumatic systems, temperature effectsgenerated by the throttling cannot be ignored. The followinganalysis shows the greater the pressure difference is, the moresignicant the temperature change becomes. The pressure-reducing valve is an important component of the APE system,and it is also a main location of low temperature due to thethrottling. Because the state of the source air is in changing duringAPEs operation, the air pressure and temperature before thereducing valve is not constant. So it is necessary to establish adynamic model of the pressure reducer for temperature dropanalysis.

The temperature change of the throttling process is explained bythe JeT effect:

The throttling process proceeds very quickly so that the processcan be considered adiabatic. On this basis the enthalpy of a gasremains constant before and after the throttling:

hv1 hv2 (11)

where hv1 and hv2 denote the molar enthalpies (Jmol1) of the gasbefore and after the throttling, respectively.

The molar enthalpy of a real gas can be described in terms of thegass state parameter as:

dhv CpdTv "Vm Tv

vVmvTv

p

#dpv (12)

where Cp is the molar heat capacity at constant pressure[J (mol K)1], Tv, Vm and pv are the temperature, molar volume(m3mol1) and pressure of the gas, respectively.

In the isenthalpic process dhv 0, therefore it can be obtainedfrom Eq. (12):

dTv 1Cp

"Tv

vVmvTv

p Vm

#dpv (13)

Then the JeT coefcient mj is dened in terms of the partialderivative of Tv with respect to pv at constant enthalpy:

mj vTvvpv

h"1Cp

"Tv

vVmvTv

p Vm

##(14)

It can be derived from the equation of the state of ideal gas:

vVm

R Vm (15)

Q. Xu et al. / Energ880real gas of high pressure. The real gas Van der Waals stateequation is:expression. In addition, a correction term d(pv1 pv2)/pv1pv2 isadded considering the inuence of the initial air pressure on pv aV2m

!Vm b RTv (16)

where a is a measure of the attraction between the particles(Pam6mol2), b is a correction term considering the gas molecularvolume (m3mol1). For air, values of a and b are 0.1361 and0.0000367, respectively. Then the temperature change of thecompressed air due to the throttling can be calculated based on Eqs.(11)e(16). It should be noted that the temperature of a real gas mayeither increase or decrease, depending not only on the type of thegas but also on the initial state before expansion. This phenomenoncan be reected by the sign of mj. For high pressure air at roomtemperature mj is always positive during the expansion process[23]. Since vpv < 0, thus vTv must be negative.

However, numerical methods of solving transcendental equa-tion like Eq. (14) are complex and time-consuming. Researcherssuch as Luo and Yuan [21,24] tried to calculate the JeT coefcientbased on the compressibility factor but faced similar problems. Andin dynamic pneumatic systems, calculation considering the throt-tling effect is even impracticable. In this paper, a simplied calcu-lation method of the throttling effect for the dynamic temperatureanalysis is proposed.

On both sides of Eq. (16), solving the partial derivative of Vmwith respect to Tv at constant pressure yields:

vVmvTv

p RV

3m

pvV3m aVm 2ab(17)

Substituting Eq. (17) into Eq. (14) yields:

mj 1Cp$pvbV3m 2aV2m 3abVm

pvV3m aVm 2ab(18)

To avoid integral calculations, equivalent values of pv and Vm areused to replace their real time values during the throttling process.The equivalent pressure is the algebraic average of the pressuresbefore (pv1) and after (pv2) throttling:

pv pv1 pv2

2(19)

where pv2 is also equal to the set pressure of the reducer pvs.The equivalent molar volume is calculated by substituting Eq.

(19) into Eq. (16):

Vm f Tv1;pv (20)

where Tv1 is the air temperature before throttling. Using compu-tational tools, the only expression of real value of Vm can be ob-tained by solving Eq. (16). But the expression is too long to bedescribed here.

Substituting Eqs. (19) and (20) into Eq. (18) yields the equivalentaverage JeT coefcient mj. Then the air temperature after throttlingis described in form of the following expression:

Tv2 Tv1 mjpv1 pv2 (21)To compensate the errors caused by the differential calculation

and the equivalent treatments, the coefcient mj is amended asamj b and a quadratic term g(pv1 pv2)2 is added to the

(2014) 877e885the temperature drop. Eventually, the air temperature after throt-tling is:

Tv2Tv1pv1pv2amjb

gpv1pv22d

pv1pv2pv1pv2

(22)

where a, b, g and d are empirical coefcients. They can be obtainedand veried through the nonlinear tting of a large amount ofrelated data.

The following comparison shows that it can be applied in awiderange of pressure from 30MPa to 1 MPa. The data in related articles[21e26] are used. Here, corresponding values of a, b, g and d are

corresponding results are also shown in Table 2.

5. Heat transfer models of the air tank and the pipe

transfer is:

Table 1Values of the empirical coefcients.

a b 105/K Pa1 g 1014/K Pa2 d 106/K Pa3.869 4.9654 1.6 1.3

Q. Xu et al. / Energy 68The air tank and the pipe are also basic components of the APEsystem. Unlike the throttling process, temperature changes in thesemodels are relatively slow, and the convective heat transfer isconsidered in the temperature differential equation. So the air isconsidered to be ideal to simplify the calculation. The ideal gas stateequations and continuity equations of the air tank and the pipe aresimilar as in Section 3.

5.1. Model of the air tank

The temperature differential equation deduced from the energyequation is expressed by:

Table 2Typical values of the throttling temperature drops calculated by different methodswhen Tv1 298 K.

Method Pv2/MPa

22 15 10 5 3 1

Pv1 12 MPaSePeK 3.2 13.2 17.9 23.1PeR 3.8 15.4 21.1 27.3As it can be seen, most of the results calculated by DTE methodare within the range of the above two methods results. Thereforethe DTE method is reliable in accuracy in the pressure range from30 MPa to 1 MPa compared to other methods of throttling effect.And the working pressure of the practical APEs air tank is withinthis range. In addition, due to the reduced integral operation theDTEs calculating speed is superior to other methods, thus it issuitable for simulation of dynamic models of the APE system.shown in Table 1.Table 2 shows typical values of the throttling temperature drop

calculated by SeReK equation and PeR equation in Ref. [21]. Thedynamic calculation method used in this paper is denoted by DTE(dynamic calculation method of throttling cooling effect) and itsDTE 3.3 15.3 22.5 31.2

Pv1 20 MPaSePeK 5.9 14.1 25.1 30.4 36.2PeR 6.9 16.4 29.3 35.6 42.6DTE 6.3 14.5 25.6 31.2 37.2

Pv1 30 MPaSePeK 5.4 13.7 22.5 34.4 40.2 46.5PeR 6.6 16.2 26.4 40.5 47.4 55.1DTE 8.9 19.3 28.3 39.1 46.8 48.9cs laHs Nus (25)

where ls is the thermal conductivity of air, Hs is the height of thetank. The Nusselt number is calculated by:

Nus CsGrsPrn (26)

where Pr is the Prandtl number of air, the Grashof number isdened as:

Grs gjTws TsjH3s

Tmn2a(27)

where g is the gravitational acceleration, na is the kinematic vis-cosity of air, Tm is the reference temperature which can be denedas the average of Tws and Ts. Values of the empirical coefcients canrefer to reference [27]:

8>:

Cs 0:59;n 0:25; Grs0;3 109

Cs 0:0292;n 0:39; Grs3 109;2 1010

Cs 0:11;n 0:333; Grs2 1010;N (28)

In addition, the radiation heat transfer of the tanks inner wallQs2 is calculated by:

dQs2 AssT4ws T4s

(29)

where is the radiation emissivity, s is the StefaneBoltzmannconstant.

5.2. Model of the pipe

The temperature differential equation of the pipe is:

dTpdf

1mpCv

dQpdf

hpedmpedf hpadmpadf

updmpdf

(30)

Denitions of the parameters in Eq. (30) can refer to those inEqs. (1) and (23). The forced convective heat transfer coefcient ofthe pipe is calculated by:

cp laDp Nup (31)dTsdf

1msCv

dQsdf

hsdmsdf usdmsdf

(23)

where ms, hs, us, Ts are the mass, the specic enthalpy, the specicinternal energy and the temperature of the air in the tank,respectively. The heat absorbed by the air in the tank Qs includestwo parts which are Qs1 and Qs2. Qs1 is from the natural convectiveheat transfer with the inner wall of the tank:

dQs1 csAsTws Ts (24)

where As is the heat transfer area, Tws is the temperature of thetanks inner wall which is assumed to be equal to theroom temperature. The coefcient of natural convective heat

(2014) 877e885 881where Dp is the inner diameter of the pipe, and the Nusseltnumber is:

accurately. For ease of measurement and analysis of the data, at

Fig. 4. Phtograph of the verication APE system and its test bench.

Table 4Operating parameters of the test bench.

Parameter Value

Table 3Major structural parameters of the verication prototype.

Parameter Value

Bore stroke 0.052 0.050 mCylinders initial volume 0.00003 m3

Crank ratio 0.263Intake advance angle 10

Intake duration angle 85

Exhaust advance angle 10

Exhaust duration angle 120

Q. Xu et al. / Energy 68 (2014) 877e885882present, a low pressure air system with highest supply pressure of1 MPa was set up for the verication prototype. Its schematicstructure can refer to Fig. 3. The compressed air discharges from anair tank (0.6/1.3S-II, Haikong vessel Co. LTD, Qingdao, China), goesthrough the pipeline and a pressure reducer (AC40B-04G-TV, SMCCorporation, Tokyo, Japan), and then supplies to the APE. And adedicated test bench for the APEwas designed and built tomeasurethe APEs operating parameters and performances, including cyl-inder pressure, rotate speed, power and torque of the APE as well astemperatures of the air at intake and exhaust ports, as shown inFig. 4. Associate with this article, the air temperatures weremonitored by installing two PT100 temperature transmitters (withrange of measurement 50 C to 50 C) close to the intake andexhaust ports. And a pressure transducer (JYB-KO-MAG1, COLLI-HIGH, Beijing, China) was installed at the inlet pipe of the APE toNup 0:023R0:8e P0:4rTrTwp

0:5 (32)where Twp is the temperature of the pipes inner wall which isassumed to be equal to the room temperature, Tr is the referencetemperature which can be dened as the average of Twp and Tp, theReynolds number is calculated by:

Re Dpupna

(33)

here up is the ow velocity of the compressed air in the pipe.Themodels mentioned aboveweremodularized with all control

variables parameterized, so that the dynamic simulation model ofthe APE system can be established by connecting these models.

6. Experimental study on the APE system

To verify the feasibility of the APE, a prototype was modiedfrom a single cylinder piston-type internal combustion(IC) engineby transforming its valve system. Major structural parameters ofthe prototype are shown in Table 3.

The temperature change during the operation of a high pressuresystem is very fast, thus, it is not easy to measure the temperaturesFig. 3. Principle diagram of the dynamic simulation model of the verication APEsystem.measure inlet pressure. In the experiments, each time the air tankwas charged by a compressor (UD11A-10C, United OSD, Shanghai,China) until the air pressure reached 1 MPa. Then the compressorwas turned off and the switch valve of the tank was opened tosupply compressed air to the APE. Several tests were done atdifferent inlet pressures. And the systems operating parametersare shown in Table 4.

Meanwhile, using the models mentioned above, a dynamicsimulation model of this APE systemwas built to verify its accuracyon temperature drops analysis. Its principle diagram is shown inFig. 3. Parameters of the model are set as in Tables 3 and 4. Thesimulation system was running until the air pressure in the tankwas lower than the set pressure of the reducer.

Although it is not related to the temperature analysis, to brieyillustrate the validity of the APEs thermodynamic model, theaverage powers simulated when the set speed is 500r/min arecompared with the measured data, as can be seen in Table 5. Withthe simulated powers as references, the average deviation is about4.1% which is an acceptable error.

During running processes of the dynamic system model, airtemperatures in the tank (Ts), at the export of the pipeline (Tp) andat the export of the reducer (Tv) were monitored. Fig. 5 shows thesimulated temperature curves when the set pressure of the reduceris 0.7 MPa. It should be noted that the actual values of Tp and Tv areuctuant. For convenience of observation, they are averaged. As theair pressure in the tank drops, the air temperature in the tankcontinuously decreases in the discharging process, but with smallamplitude. Due to the heat exchange with the pipeline, the airtemperature before the reducer approximately restores to the roomVolume of the tank 600 LInitial pressure in the tank 1.0 MPaLength and diameter of the pipeline 4F0.01 mInlet pressure 0.4e0.9 MPaRoom temperature 309 KSet rotate speed 500 rmin1

Table 5Simulated powers and measured powers at different inlet pressures.

Inlet pressure/MPa 0.4 0.5 0.6 0.7 0.8 0.9

Power simulated/W 106 163 219 275 331 387Power measured/W 97 160 205 265 323 380Deviation/W 9 3 14 10 8 7

exhaust port. But the temperature differences standard deviation is

to prevent the ice blocking.

Fig. 5. Curves of ps Ts, Tp and Tv during the running process of the dynamic modelwhen the inlet pressure is 0.7 MPa.

Fig. 7. Schematic diagram of a practical APE system.

Q. Xu et al. / Energy 68 (2014) 877e885 883temperature. Although the pressure difference is relatively small,the throttling cooling effect of the pressure reducer still can beobserved. And as the upstream pressure in the tank drops, thetemperature decrease amplitude reduces.

Then the equivalent air temperatures at intake and exhaustports were calculated when the simulation time is around 120 s, ascan be seen in Fig. 6 The intake and exhaust air temperatures of theexperiments were also measured at about 2 min after the start ofeach test, and are shown in the gure for comparison.

As can be seen, after running the same period of time, themeasured intake air temperatures have a satisfying agreement withthe calculated values, as the maximum difference between them is1.1 K. While the exhaust air temperatures are consistently lowerthan the measured data. The average temperature deviation is5.25 K. That is caused by the deviation between the sensormounting position and the actual calculating position of theFig. 6. Intake and exhaust air temperatures at different pressures when the runningtime is about 2 min.7. Simulation for temperature analysis of the practical APEsystem

Serving as the engine of vehicles, the practical APE has its inletpressure higher than 1 MPa so that it can output enough power.And sufcient compressed air is needed to meet the demand of acertain mileage. Thus the practical APE systems air source is a highpressure system, as shown in Fig. 7. Besides the high pressure tanksgroup, in order to stabilize the intake ow, a low pressure buffertank and a reducer is added before the inlet of the APE. Designparameters of the practical APE system including the correspond-ing APEs major structures are shown in Table 6. A dynamic systemmodel was built according to the principle in Fig. 7. Air pipes beforethe high pressure reducer and after the low pressure reducer areabout 1.2 K, it shows the temperature change trend of the simu-lated data is consistent with the measured ones. Therefore, thedynamic model has a good reference value in estimating the tem-perature drops of the real APE system, and the calculation methodof equivalent air temperatures at intake and exhaust ports is useful.

From the results it also can be concluded that, as the inletpressure rises, the cooling effect of the throttling process reduces,thus, the temperature drop amplitude before the APEs air intakedecreases. But the expansion cooling effect in the cylinder in-creases, the exhaust air temperature gradually gets close to 0 C. Inthe practical high pressure APE system, even if the intake airtemperature is close to room temperature, high inlet pressures willlead to extremely low exhaust temperatures. So heat exchanger ofthe intake and exhaust system of the APE is an important directionTable 6Design parameters of the practical APE system.

Parameter Value

Volume of the tanks group 100 LInitial pressure in the tank 30 MPaLength and diameter of the pipeline 1F0.01 mVolume of the buffer tank 70 LSet pressure of the buffer tank 5.0 MPaInlet pressure 2.0 MPaRoom temperature 293 KBore Stroke 0.085 0.088 mCylinders initial volume 0.00008 m3

Crank ratio 0.3165Intake advance angle 10

Intake duration angle 60

Exhaust advance angle 30

Exhaust duration angle 190

Set rotate speed 1000 rmin1

ignored because they are relatively short, then the pipelines in thesystem model are simplied as one pipe model between the highpressure reducer and the buffer tank.

In the simulation process, the room temperature was set as293 K. Thus, temperatures of all inner walls in the system modelwere ideally assumed to be equal to 293 K. The APE model wasrunning until the pressure in the buffer tank was lower than 5MPa.In the case of rotate speed 1000 rmin1, the engines output powerreaches 5 kW. During the operation, air temperatures in the highpressure tank (Tsh), at the export of the high pressure reducer (Tvh),at the export of the buffer tank (Tsl) and at the export of the lowpressure reducer (Tvl) were monitored and are shown in Fig. 8.

As can be seen, Tsh drops dramatically, at the end of the opera-tion its lowest value reaches 258 K. Due to the large pressure dif-ference, the instant temperature drop after the reducer is severe,especially in the initial stage of the operation. Though the tem-perature recovers slowly after 150 s, the highest value of Tvh islower than 257 K. Sustained low temperature makes the reducersoutlet very easy to have ice blocking. Heat exchanges with thepipeline and the buffer tank is effective so that Tsl is higher than280 K at most of the time. But after the throttling of the low

low temperature of the inlet air also affects the APEs power outputperformance.

Fig. 9. Structure diagram of the heat exchange system for the APE system. (1) Tanksgroup; (2) supply pipeline; (3) medium pipeline; (4, 10) heat exchange pipe; (5) pump;(6, 12) transmissions; (7) low pressure reducer; (8) buffer tank; (9) cylinder head; (11)energy recovery device; and (13) radiator assembly.

Q. Xu et al. / Energy 68 (2014) 877e885884pressure reducer, Tvl which is equal to the inlet temperature dropsto lower than 270 K. At about 300 s, the inlet air temperature is265.8 K. Then according to Eqs. (9) and (10), the calculated equiv-alent air temperatures at intake and exhaust ports of the APE are268.8 K and 225.6 K, respectively. It shows the very bad tempera-ture condition at the exhaust port. Furthermore, the pneumaticpower of the air Pa [28] is calculated by the following equation:

Pa dmindt RgTinlnpinpa

(34)

here min, Tin and pin are the mass, temperature and pressure of theair in the inlet pipe, respectively. Since the inlet air temperature is10% lower than the room temperature, in the case of same massow and pressure, the inlet compressed air loses 10% availableenergy due to the temperature drop.

Through the analyses above it can be concluded: low tempera-ture locations in the system include the outlet of the high pressuretanks, outlets of the pressure reducers and the exhaust port of theAPE. In a vehiclemounted APE system, any of these locations havingice blocking can cause abnormal operation of the engine. And theFig. 8. Curves of Tsh, Tvh, Tsl, and Tvl during the running process of the dynamic model.8. Heat exchange system design for the APE system

A heat transfer system for the practical APE system is designedwith its structure diagram shown in Fig. 9. On the basis of theoriginal system shown in Fig. 7, a set of heat exchange pipe (4) withtotal length 4 m and diameter 0.006 m is added between the tanksgroup (1) and the high pressure reducer (11). Size of the pipe afterthe reducer (10) is changed into the same size as pipe (4). Positionsof the buffer tank (8) and the low pressure reducer (7) are swapped.Compared with the position order in Fig. 7, this measure can makethe temperature difference between the inlet air and the inner wallof the buffer tank (8) bigger, resulting in better heat transfer effectin the tank. For comparison, other parameters remain unchanged asshown in Table 6. Then the modied model was simulated. Airtemperature curves in the high pressure tanks (1) (Tsh), at theexport of the pipe (4) (Tph), at the export of the pipe (10) (Tpl) and atthe export of the buffer tank (8) (Tsl) are shown in Fig. 10. As can beseen, these improvements can make the air temperature in thesupply pipelines (2) more close to the room temperature. When allthe temperatures are higher than 273 K, no ice block will happenFig. 10. Curves of Tsh, Tph, Tpl, and Tsl during the running process of the dynamic model.

before the inlet of the APE. When the inlet air temperature is about282 K, the equivalent temperatures at intake and exhaust portsraise to 285 K and 230 K, respectively.

proposed simplied calculation method of throttling effect can alsobe applied to temperature analysis of other high pressure gassystems.

equation of state in calculating compressibility factor of natural Gas. Pipeline

air. J Fluids Eng 2006;128:402e5.

Q. Xu et al. / Energy 68 (2014) 877e885 885For piston-type engine, the intake and exhaust system locatedon the cylinder head (9), thus a heat exchange chamber is placed inthe cylinder head (9) so that a heat transfer medium can ow in itto prevent excessive cooling. The heat transfer medium is stored inan engine radiator assembly (13) which is composed of a radiatortank and a fan. The medium of low temperature absorbs heat fromthe atmosphere through the radiator and the temperature rises.Then the liquid is extracted by a pump (5) out of the tank. Throughthe medium pipelines (3), it ows through the cylinder head (9),heat exchange pipes (4), (10) and returns to the radiator tank (13).

In addition, the high pressure reducer is a major cause of energyloses in the pneumatic system. In the simulation of the high pres-sure APE system, the average air ow through the reducer is0.0425 kg/s. Calculated by Eq. (34), the pneumatic power loses dueto the reducing process is approximately 6.3 kW which is evenhigher than output power of the APE. Therefore the high pressurereducer is modied into a pressure reducing and energy recoverydevice (11). It is functionally combined by a high pressure rotarymotor and a reducer. When the APE is not running, it plays the roleof a pressure reducer. When there is mass ow in the pipeline, partof the energy can be recycled by the rotarymotor to drive the pump(5) and the radiator fan (13) through transmissions (6) and (12).Thus the heat exchange system can be running automaticallyduring operation of the engine without additional energyconsumption.

9. Conclusions

The thermodynamic model of the APE and a calculation methodfor equivalent air temperatures at intake and exhaust ports weredescribed. A simplied calculation method of throttling effect forthe dynamic temperature analysis was proposed. Models of the airtank and the pipe were considered. Then the dynamic heat transfermodel of the APE system was established to predict temperaturedrops in the operation process, to prevent the ice blocking problemand improve the performance of the APE system. Conclusions aresummarized as follows:

(1) The simplied calculation method of throttling effect for thedynamic temperature analysis is proved to be effective in apressure range from 30 MPa to 1 MPa.

(2) The dynamic system models feasibility on the temperaturedrop analysis is veried by comparing its simulated resultswith the measured data in corresponding experiments.

(3) The outlet of the high pressure tanks, outlets of the pressurereducers and the exhaust port of the APE are low tempera-ture locations during the operation of the system. Heatexchanging with the buffer tank and pipelines is effective torecover temperatures. Simulation results show that the heatexchange system can maintain the air temperatures in thesupply system above 0 C, preventing the ice blockingproblem before the inlet of the APE. In addition, the energyloss due to the high pressure-reducing process can be used todrive the heat exchange system.

The analyses of this paper can provide a theoretical basis for thedesign of the heat exchange system for the APE system. TheTech Equip 2009;3:004.[25] Liu X, Mei N, Ding S, Zhao J. Experimental investigation of JouleeThomson

effect for ammonia solution throttle process. J Therm Sci Technol 2009;4:015.[26] Mao W, Zhang L. A method study for calculating JouleeThompson coefcient.

Spec Oil Gas Reservoir 2002;5:013.[27] Yang S, Tao W. Heat Transfer. Beijing: Higher Education Press; 1998.[28] Cai ML, Kawashima K, Kagawa T. Power assessment of owing compressedAcknowledgements

The research work presented in this paper is nancially sup-ported by grant 51375028, 51205008 of the National Natural Sci-ence Foundation of China.

References

[1] Veziroglu TN, Sahin S. 21st centurys energy: hydrogen energy system. EnergyConvers Manag 2008;49:1820e31.

[2] Li Y, Chen H, Zhang X, Tan C, Ding Y. Renewable energy carriers: hydrogen orliquid air/nitrogen. Appl Therm Eng 2010;30:1985e90.

[3] Ibrahim H, Ilinca A, Perron J. Energy storage systems e characteristics andcomparisons. Renew Sustain Energy Rev 2008;12:1221e50.

[4] Lund H, Salgi G. The role of compressed air energy storage (CAES) in futuresustainable energy systems. Energy Convers Manag 2009;50:1172e9.

[5] Foley A, Daz Lobera I. Impacts of compressed air energy storage plant on anelectricity market with a large renewable energy portfolio. Energy 2013;57:85e94.

[6] Drury E, Denholm P, Sioshansi R. The value of compressed air energy storagein energy and reserve markets. Energy 2011;36:4959e73.

[7] Motor Development International (MDI) Home Page. Available online: http://www.mdi.lu/english/index.php [accessed 08.03.13].

[8] Motor Development International (MDI). Web Page. AIRPod. Available online:http://www.mdi.lu/english/airpod.php [accessed 27.08.12].

[9] Zuo CJ, Qian YJ, An D, Ouyang MG, Yang FY. Experimental study on air-powered engine. Chin J Mech Eng 2007;43:93e7.

[10] He W, Wu Y, Ma C, Ma G. Research of air powered engine system using two-stage single screw expander. Chin J Mech Eng 2010;46:139e43.

[11] Wang Y, Liu XY, Zhou Y. Design and performance analysis of a triangle rotarypiston air-powered engine. Mech Sci Technol Aerosp Eng 2009;28:950e4.

[12] Nie XH, Yu XL, Hu JQ, Chen PL. Inuence of inlet and outlet valves open timingon pneumatic engine and their optimal design. J Eng Des 2009;16:16e20.

[13] Xu QY, Cai ML, Yu QH, Fu XH. Multi-parameter and multi-objective optimi-zation method for air powered engine. J Beijing Univ Aeronaut Astronaut2013;39:803e7.

[14] Liu H. Research on air powered vehicle engine. PhD thesis. China: ZhejiangUniversity; 2004.

[15] Huang CY, Hu CK, Yu CJ, Sun CK. Experimental investigation on the perfor-mance of a compressed-air driven piston engine. Energies 2013;6:1731e45.

[16] Zhai X. Research on hybrid of compressed-air and fuel. PhD thesis. China:Zhejiang University; 2005.

[17] Cherkez R. Theoretical studies on the efciency of air conditioner based onpermeable thermoelectric converter. Appl Therm Eng 2012;38:7e13.

[18] Gong M, Wu J, Cheng Q, Sun Z, Liu J, Hu Q. Development of a 186 C cryo-genic preservation chamber based on a dual mixed-gases JouleeThomsonrefrigeration cycle. Appl Therm Eng 2012;36:188e92.

[19] Maytal BZ, Pfotenhauer JM. The JouleeThomson effect, its inversion and otherexpansions. In: Miniature JouleeThomson cryocooling. New York: Springer;2013. pp. 39e72.

[20] Luo Y, Wang X. Exergy analysis on throttle reduction efciency based on realgas equations. Energy 2010;35:181e7.

[21] Luo YX. A study of the real gas effects on high pressure pneumatics and thebasic theoretical and experimental researches of pressure reduction system.PhD thesis. China: Zhejiang University; 2011.

[22] Luo Y, Wang X, Ge Y. Real gas effects on charging and discharging processes ofhigh pressure pneumatics. Chin J Mech Eng 2013;26:61e8.

[23] Yin Y, Shen L, Fu J, Dai Y. Outlet temperature characteristics of pneumaticpressure-reducing valve for hydrogen vehicles. Chin J Constr Mach 2009;7:383e7.

[24] Yuan WM, He S, Yuan ZM, Liu C, Zhang HL. Numerical methods of using BWRS

Dynamic heat transfer model for temperature drop analysis and heat exchange system design of the air-powered engine system1 Introduction2 Introduction of the APE system2.1 Working principle of the APE2.2 Locations of the temperature drop

3 Thermodynamic model of the APE and calculation method of the equivalent air temperatures3.1 Thermodynamic model of the APE3.1.1 Energy equation3.1.2 Continuity equation3.1.3 State equation3.1.4 Torque equation

3.2 Equivalent air temperatures at intake and exhaust ports

4 Simplified model of throttling effect for dynamic temperature analysis5 Heat transfer models of the air tank and the pipe5.1 Model of the air tank5.2 Model of the pipe

6 Experimental study on the APE system7 Simulation for temperature analysis of the practical APE system8 Heat exchange system design for the APE system9 ConclusionsAcknowledgementsReferences