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Hybrid pneumatic-power system which recycles exhaust gas of an internal-combustion engine
APPLIED ENERGY
Applied Energy 82 (2005) 117–132
www.elsevier.com/locate/apenergy
Hybrid pneumatic-power system which recycles exhaust gas of an internal-combustion engine K. David Huang a, Sheng-Chung Tzeng b,*, Wei-Ping Ma c, Wei-Chuan Chang a a
c
Graduate Institute of Vehicular Engineering, Dayeh University, Changhua 515, Taiwan, ROC b Department of Mechanical Engineering, Chien Kuo Institue of Technology University, Changhua 500, Taiwan, ROC Department of Information Management, Lan Yang Institute of Technology, Ilan 261, Taiwan, ROC Accepted 11 October 2004 Available online 25 December 2004
Abstract The hybrid pneumatic power system (HPPS) proposed in this research replaces the batteryÕs electric-chemical energy with flow work and optimizes the management and utilization of the energy. This power system is able to keep the internal-combustion engine working at its optimal condition and turn its waste energy into effective mechanical energy and so enhance the thermal efficiency of the whole system. Using computer simulation software ITI-SIM, this study simulates the overall dynamic characteristics of the system in accordance with the regulated running-vehicle test-mode ECE47, and, with experimental verification and analysis, proves that this system can meet the requirements of the standard running-car mode. As for recycling the waste energy, the experimental results show that this design could offset the shortcomings of the low-density of pneumatic power and so effectively enhance the efficiency of the whole system. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Hybrid pneumatic-power system; Waste-energy recycling; EngineÕs thermal-efficiency
*
Corresponding author. Tel.: +886 4 7111 111x3132; fax: +886 4 7357 193. E-mail address: [email protected] (S.-C. Tzeng).
0306-2619/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2004.10.006
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Nomenclature A M n P P1 P2 Q R Ra Rr T T1 Va VB W Ww Z DP g la lr q x
front projective area (m2) inlet flow-rate of pneumatic motor (m3/s) polytropic exponent pressure (bar) air-compressorÕs inlet-pressure (bar) air-compressorÕs outlet-pressure (bar) inlet mass-flow rate of compressed air (kg/s) ideal-gas constant (kJ/kg K) air-flow resistance (N) rolling resistance (N) torque (Nm) air-compressorÕs inlet-temperature (K) headwind velocity (m/s) capacity of storage tank (m3) output work (kW) vehicleÕs weight (kg) cycling number per hour of compressor pressure difference across pneumatic motor (bar) theoretical efficiency (%) coefficient of air-flow resistance coefficient of rolling resistance air-flow density (kg/ m3) angular velocity (rad/s)
1. Preface Because of pressures from both the energy supply and environmental protection, vehicle technology now emphasizes enhancing efficiency and the quality of energy consumption. New technologies of the internal-combustion engine have been developed and put to use on a daily basis; however, to reach the required speeds and loads, internal-combustion engines have to accelerate or decelerate frequently, and fail consistently to work at the optimal state of minimum fuel-consumption and pollution. This leads not only to a waste of energy, but also to severe problems with exhaust gases. This results in the energy efficiencies of vehicles equipped with internalcombustion engines being as low as 15% [1]. Great efforts have been devoted to the research and development of vehicles of high efficiency and low pollution, including electric vehicles, fuel cell vehicles, and hybrid electric vehicles. As for electric vehicles, there are many limitations, such as low charging capacity of batteries, slow charging speeds, inaccurate indications of residual electricity, and insufficient infrastructures. As for fuel-cell vehicles, the public is dubious about the safety and many technical difficulties need to be resolved before their widespread practical application. As
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for petrol–electric hybrid vehicles, no matter whether they are arranged in tandem type or parallel type, they still use electric-chemical energy, which brings about various shortcomings of battery technology. Furthermore, so far, there is no hybrid electric engine capable of keeping the internal-combustion engine working at its optimized state and the waste gas generated by internal-combustion engines fails to be utilized properly. All these result in low energy-efficiency of the system. Chau and Wong [2] emphasized the issue of energy expenditure with respect to environmental protection. They believe a petroleum crisis might occur in the year of 2010, which will put an end to cheap petroleum and give rise to fierce competition in developing vehicles using replaceable energy. So far, there are three options, namely, electric vehicles, hybrid electric vehicles, and fuel-cell vehicles. Morita [3] analyzed the developing modes of the power systems of future vehicles. They reviewed todayÕs research aims at developing zero-pollution vehicles and green engines. The electric vehicle (EV) is the typical product of zero-pollution vehicles, while greener engines include hybrid electric systems, the internal-combustion engine with gas direct injection and the fuel cell. A bi-energy engine using petrol and compressed air designed by Guy Negre has drawn much investment. The Moteur Development International (MDI) has been established: it is dedicated to the innovation and development of mono-energy compressed-air engines. In the African Expo, MDI exhibited its compressed-air vehicle, E.Volution [4], powered by compressed air stored in a 300 L tank, which is positioned at the bottom of the vehicle. The vehicle weights about 700 kg and can travel 200 km at the speed of 96 km/h. It takes about 4 h to charge the air store by an air compressor. However, if the charging is accomplished via a high-pressure air-charging station, the charging process just needs 3 min. In the Paris International Automobile Exhibition of October 2002, City Cat, a small car using a compressed-air-powered engine was introduced. The traveling capacity of the pneumatic vehicle is now able to meet the needs of urban drivers. The University of Northern Texas and the University of Washington [5] are jointly working on the liquid nitrogen-powered vehicles. The working mechanism is that the liquid nitrogen evaporates and expands quasi-isothermally at normal temperature and consequently generates power. Plummer et al. [6] proposed that the ideal efficiency of the power cycle of the quasi-isothermal expansion of liquid nitrogen might reach 85%. For example, a two-cylinder engine can generate 15 kW power and 190 Nm torque with an expansion pressure of 8 MPa and rotating speed of 850 rpm. Applying this system on a common vehicle, 200 litres of liquid nitrogen is capable of driving the car for 140 km. Ordonez [7] put forward the idea of a recycled power system using liquid nitrogen. The main problem of this system is the short traveling distance achieved, since its energy density is quite limited. Thermodynamically, changing OrdonezÕs original design to the renewable Brayton cycle, the effective power-supplying time could be 10 times longer but the fuel economy increased considerably. Maghoub and Craighead [8] came up with an idea of controlling the gas inlet and outlet of the piston-type pneumatic motor to meet the requirements of driven components by either a PID or H-bridge control method. Pandian et al. [9] pointed out that, compared with
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the electrical motor, the time constant, which is decided by the ratio of inertia and output torque, of the pneumatic motor is relatively small. This means the pneumatic motor can be activated and stopped smoothly and the ratio of output torque to weight is higher. Therefore, the pneumatic motor is better than the electrical motor in terms of power and torque performance and is regarded as a high-powered rotating engine. Tokhi et al. [10] also pointed out that, compared with the electrical motor, the pneumatic motor has such advantages as (i) easily obtainable power source, (ii) large activating torque, and (iii) easy control of rotating speed and torque: it only requires an air-flow controller and a pressure controller. Apart from these advantages, the pneumatic motor does not suffer from damage caused by overloading or heat generated by friction. Chang and Nishi [11] considered a low-pressure vane pneumatic motor as the driver of mechanical devices. This pneumatic motor has a pair of rotating vanes, switching valves and power-output axle. The equations of its static and dynamic performances have been deduced from experimental results. So far, all the applications of internal-combustion engines fail to keep them working at their optimized condition after start-up. The waste energy of the internal-combustion engine has not yet been properly utilized, which results in a low thermal efficiency and BSFC. To effectively overcome these shortcomings, the hybrid pneumatic power-system proposed by this article involves the replacement of the batteryÕs electric-chemical energy by flow work. This power system can keep the engine working at its optimized condition and so effectively reduce the fuel consumption and emissions of waste gases from the internal-combustion engine. Furthermore, this system can utilize the waste energy of the internal-combustion engine and greatly enhance the overall efficiency.
2. Theoretical analysis This research is about a HPPS that integrates an internal-combustion engine, a compressor, a pneumatic motor and an exhaust gas-recycling device. The energy transformation parameters have been modularized to explore the relations between the componentsÕ transient responses, operating variables and the control parameters under various operation modes. ITI-SIM software, which is capable of analyzing the dynamic behavior of the system of any composition, is used to establish the systemÕs dynamic model. 2.1. Internal-combustion engine The model of the internal-combustion engine in the ITI-SIM is a four-stroke with a rigid crankshaft. It can operate in accordance with the input-torque curve and take into consideration the specific phenomena in the combustion and compression processes. The individual torque-curve of each cylinder can be superposed above the average output phase. The required input parameters include normal power, normal speed, and the number of cylinders in order to be able to calculate the torque applied to the crankshaft. The calculation basis for this model is the cylinderÕs torque curve;
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hence the torques generated by combustion and compression are also taken into consideration. When the input signals are set, the basic parameter configuration should refer to the actual measurements of the vehicleÕs internal-combustion engine BSFC and set the output operation mode of the internal-combustion engine at 3.41 kW/ 4000 rpm, which are the optima. 2.2. Air compressor The air compressor used in this hybrid pneumatic power system employs an auger type pressure-controlling valve, which applies a stable load to the internal-combustion engine and allows it to operate at a set rotational speed. The fuel efficiency can be improved and such pollutants as CO, CO2, HC, and NOx can reach the minimum emission values if the rotating speed is set at the optimized BSFC area. Similarly, problems with air admission, exhaustion, fuel injection, ignition, lubrication, cooling and noise could be met with an optimized design aiming at a specific operation mode, which in turn greatly reduces the design difficulty, time, and cost. The basic equation of the model is as follows: " # ðn1Þ nRT 1 P2 n W ¼ 1 : ð1Þ n1 P1 2.3. Air tank This is an indepensible component of the pneumatic power-system. It stores and regulates energy, keeps a stable supply of compressed air, reduces the pressure surges, and enables the braking component to operate smoothly. Besides, its volume and weight are closely related with the cruising mileage and the activation times of the internal-combustion engine. The basic equation to calculate the volume of the air tank is as follows: 15QP : ð2Þ DPZ This model can simulate any future possible developments. Such factors as improving the efficiencies of the compressor as well as the pneumatic motor, under different conditions, could be simulated via adjusting the parameters in the model. VB ¼
2.4. Pneumatic motor Due to the lack of a theoretical formula for the simulation of the pneumatic motor modelÕs behaviour, the power output characteristic used refers to the performance curves of the pneumatic motor (6AM-ARV-55) made by GAST and then introduced into the ITI-SIM motor model. The energy flows of the isothermal process and adiabatic process cannot be used fully. Accordingly, the theoretical net efficiency g can be calculated by the following equation:
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g¼
xT : DPM
ð3Þ
2.5. The vehicle in the environment The running resistance in this simulation arises from wheel rolling and air flow. The former is caused by deformation of tires and road surface, and the resistance generated by tires sliding on the ground. The rolling resistance is quite complicated and the simplified expression is as follows: Rr ¼ lr W w :
ð4Þ
The air resistance coefficient la varies in accordance with the shape of vehicle body and the air resistance coefficient Ra changes accordingly. Generally, the air resistance can be expressed as follows: q Ra ¼ la AV 2a : ð5Þ 2 Since the air resistance is related with vehicleÕs windward fac¸ade project area and the driving force is affected by the design of the tires, appropriate coefficients must be adopted in the computational simulation to reflect the real conditions. The coefficients adopted here are the same as those used commonly in vehicle design. Refer to Table 1 for the basic parameter configuration of the ITI-SIM model.
3. Testing method To determine the performance of the internal-combustion engine, the pneumatic power system and the waste-energy recycling device, a series tests and analyses was conducted and the results compared with those of previous simulations and calculations. The test platform, as seen in Fig. 1, consists of an internal-combustion engine, a compressor, a unidirectional pressure-controlling valve, a junction station, a pneumatic motor, a magnetic particle trig, a torque meter, a
Table 1 Values of parameters used in the driving simulation Parameter
Value
MotorcycleÕs weight RiderÕs weight Headwind speed Radius of the rear wheel Windward area of the rider Windward area of the motorcycle Coefficient of wind resistance Air density
200 kg 70 kg 5 m/s 0.2 m 0.3 m2 0.4 m2 0.69 Ns/kg 1.22 kg/m3
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Fig. 1. Testing system.
tachometer, an air-flow meter, pressure sensors, temperature sensors and a signal amplifier. The data measured by these instruments are analyzed and compared with the results of previous simulations to prove the feasibility of the system and the correctness of the simulation. 3.1. Measuring instruments The test platform uses a TP-10KMCB tachometer manufactured by KYOWA to measure the output torque of the engine: combined with the signal amplifier and selector manufactured by HIOKI, it can observe the change of torque continually. After adjustment, 1 V corresponds with 29.5 Nm. The output signal is presumed linear. The measurement of rotating speed requires the use of a microcomputer rotating-speed and linear-speed indicator, and a controller manufactured by POUNDFUL. Combined with the photo rotating sensor manufactured by TAKENAKA, it can work as a rotating-speed recorder. The error in the response is below 100 ls and the outside random light resistance is below 3000 lux. The measurement systems for temperature and pressure use sensors with measuring ranges of 0 ! 1000 °C and 0 ! 16 bar, respectively. Each signal is relayed by GENIE software. The airflow measurement and recording system uses a flow meter with a temperature-compensation function and has a short response time and a high sensitivity at low flows.
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Table 2 Relevant experimental parameter settings Component
Setting term
Value
Internal-combustion engine Internal-combustion engine Auger type air-compressor Junction station (compressed-air end)
Throttle opening ratio Output rotating speed Outlet pressure control Flow rate control
80 % 4000 rpm 6 bar 0.007 kg/s
3.2. Configuration of relevant parameters The tested system mainly includes an internal-combustion engine, an air compressor, an air tank, a pneumatic motor and a waste-energy recycling device. Since the internal adjustment and setting of each major component affects the performance of the whole system, the configuration in the test follows that in the simulation. The configuration of relevant parameters in the experiments is shown in Table 2. 3.3. Testing process Prior to each test, set the open angle of the throttle valve and the outlet pressure controller of the air compressor, then check the testing instruments and the positions of measuring points. Then start the engine and keep it at its optimum condition. Then start the stability test. Record the response data of all components until the measurement stops 120 s later. In the waste-energy recycling device test, in order to accurately measure the exhaust temperature and the convergence effect, warm up the internal-combustion engine for 2 min before the test. Then restart the engine and record the temperature and flow rate of air before and after the convergence until the measurement stops 140 s later.
4. Test results and discussion Each test was conducted twice with the same instruments, method and relevant configuration. The results are as follows: 4.1. The best operating-point of the internal-combustion engine The internal-combustion engine used in this system is a 125 cc four-stroke engine. The performance test was conducted using an engine dynamometer made by SCHENCK. The maximum power output at full-throttle is 5.48 kW at 7750 rpm and the maximum torque is 8.16 Nm at 4000 rpm. The power performance curve serves as important referential basis for system adaptation. Since the best operative point of this system is benchmarked by fuel transformation efficiency, it is not the point of maximum power or maximum torque, but rather the lowest point of the braking fuel-consumption rate. As shown, the best operative point occurs when
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the throttle turn-down ratio is 80%. At the optimal operation point, the output power is 3.41 kW, the output torque is 8.14 Nm and the braking fuel-consumption is 273 g/kWh at 4000 rpm. The system is kept operating at its optimized fuel transformation efficiency (see Fig. 2). So far, the engineÕs exhaust-gas has not yet been taken into consideration. Generally, the lowest point of the braking fuel-consumption rate of an internal-combustion engine is also its lowest exhaust point. Measures of exhaust quantity are shown in Figs. 3 and 4; the released CO, at the engineÕs best operative point decreases dramatically; the release of HC is also significantly lower than that for the normal operative state. It is concluded that, though the lowest point of the engineÕs braking fuel-consumption rate is not the lowest point with respect to emitting CO and HC, the latter tend to be close to their respective minimum values. 4.2. Analysis of the systemÕs stability When the testing instruments are ready, the internal-combustion engine is started and the throttle turn-down ratio moved from zero to 80%. The test result is shown in Fig. 5: after working for 17 s, the internal-combustion engine can maintain a rotational speed of about 4000 rpm with a discrepancy of 30 rpm. This test result proves that this system can keep the engine at its best operative status. Repeated tests also show that the errors and the overall tendency are quite close, which also increases the credibility of the tests.
Fig. 2. Braking fuel-consumption of a 125 cc four-stroke engine.
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Eimssino CO (%)
8 Throttle operating 40% Throttle operating 60% Throttle operating 80% Throttle operating 100%
6
4
2
0
3000
4000
5000
6000
7000
8000
Engine speed (rpm) Fig. 3. Content of CO in engineÕs exhaust.
400 Throttle operating 40% Throttle operating 60% Throttle operating 80% Throttle operating 100%
Emission HC (ppm)
350 300 250 200 150 100 50 0 3000
4000
5000
6000
7000
8000
Engine speed (rpm) Fig. 4. Content of HC in engineÕs exhaust.
As seen in Fig. 6, when the internal-combustion engine working at 4000 rpm after having been ignited for 17 s, the torque delivered to the air compressor also becomes stable and maintains a stable load on the internal-combustion engine. The output pressure-controlling valve of the air compressor can keep the rotor of the compressor bearing stable. The compressorÕs load actually fluctuates by ±0.3 Nm. Two factors cause this phenomenon. One is the pulse from the pressure-controlling valve will bring about the change of pressure and load borne by the air compressor. The other is the noise interference caused by shocks from the testing platform. The outlet pressure of the air compressor is set at 4.4 bar (see Fig. 7) in the test, i.e. 2.2 bar less than that in the simulation. This is because several measuring instruments are installed, resulting in the dramatic increase of the rotating inertia and friction resistance in the tests. This leads to a loss of power transferred from the engine to the air compressor. If the outlet pressure were set above 4.4 bar, the axial load of
Engine-output rotational speed (rpm)
K.D. Huang et al. / Applied Energy 82 (2005) 117–132 5000
4000
3000 Experiment 1 Experiment 2 2000
1000
0 0
20
40
60
80
100
120
140
Time (s)
Torque of compressor load (Nm)
Fig. 5. Stability analysis in the rotating-speed test of the internal-combustion engine.
7 6 5 4 Experiment 1 Experiment 2
3 2
error = 0.015 1 0 0
20
40
60
80
100
120
140
Time (s)
Output pressure of compressor (bar)
Fig. 6. Stability analysis in the axial-load test of the air compressor.
5
4
3
Experiment 1 Experiment 2
2 error = 0.003
1
0 0
20
40
60
80
100
120
140
Time (s) Fig. 7. Stability analysis in the outlet pressure-controlling valve test of the air compressor.
127
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the air compressor would exceed the engineÕs best operative status point. The measurements show that the outlet pressure becomes stable after having worked for 15 s. This is very influential on the internal-combustion engine and the air compressor. The results of adjusting this device would decide whether the air compressor can maintain a stable axial load and whether the internal-combustion engine can be maintained at its best operative status. To make full use of the thermal efficiency of the internal-combustion engine, the waste-energy generated by the internal-combustion engine needs to be recycled and utilized. After having warmed up for 2 min, the internal-combustion engine is reignited and kept operating at best status after 17 s. The temperatures measured at the exhaust are shown in Fig. 8. The measuring points are as in Fig. 9. Though there is a difference between the two tests, the exhaust temperature in both tends to become stable at 510 °C after the internal-combustion engine has been ignited for 115 s. The exhaust heat needs to be sent to the junction station to mix with the compressed air. The test results, shown in Fig. 10, indicate that the temperature can be maintained at about 86 ! 88 °C when only exhaust heat is supplied to the junction station. After being mixed with compressed air, whose temperature is 40 °C and the flow rate is about 0.007 kg/s, the actual measurements are shown in Fig. 11, i.e. the temperature is now about 60 °C. The overall thermal efficiency would be dramatically enhanced if this device were used in the system. 4.3. Comparison between theory and test results This system proves the correctness of the simulation. The simulation and test results after igniting the internal-combustion engine are shown in Fig. 12. The best operative status in these can be reached, respectively, after 5 and 17 s. The test results show a relatively high amplitude at 20 and 45 s, which can be taken as the noise generated by vibration of the test platform. The output rotating-speedÕs amplitude of the internal-combustion engine in the simulation is bigger than the actual measured value. It also takes less time to reach a stable status. These differences are caused by the
Exhaust temperature (˚C)
550 500 450 400 Experiment 1 Experiment 2
350 300
error = 0.030
250 200 0
20
40
60
80
100
120
140
160
Time (s) Fig. 8. Stability analysis of the exhaust-temperature test of the internal-combustion engine.
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129
Fig. 9. Locations of the systemÕs behavioural sensors.
Mixer temperature (°C)
90 88 86 84 82 Experiment 1 Experiment 2
80 78
error = 0.004
76 0
20
40
60
80
100
120
140
160
Time (s)
Fig. 10. Stability analysis of the temperature test of a junction station (before the convergence).
installation of the tachometer and its overall positioning brings about an inertia bigger than what is assumed in the simulation. The rotating speed changes more slowly when the rotating inertia is bigger. As seen in Fig. 13, the torque transferred from the internal-combustion engine to the air compressor, when the load of air compressor tends to be stable, will reach a stable output, respectively, after 5 and 17 s in the simulation and test results, respectively. However, there is still a difference of about 0.5 Nm between the simulation and test results. This is because the large friction increased by the overall test platform causes a loss of output torque. Its driving efficiency is also not as good as that in the simulation. The actual rotating inertia is a far cry from that in the simulation. These result in a gap of 12 s in the time needed for the engineÕs rotation to become stable.
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Mixer temperature (°C)
70 65 60 55 Experiment 1 Experiment 2
50 45
error = 0.020
40 0
20
40
60
80
100
120
140
160
Time (s)
Fig. 11. Stability analysis of the temperature test of a junction station (after the convergence).
Engine output (rpm)
6000 5000 4000 3000
Simulation Experiment
2000 1000 0 0
20
40
60
80
100
120
Time (s)
Fig. 12. Comparison of the simulation and test results for the internal-combustion engineÕs rotatingspeed.
Torqueof compressor load (Nm)
7 6 5 4 Simulation Experiment
3 2 1 0 0
20
40
60
80
100
120
Time (s)
Fig. 13. Comparison of the simulation and test results for the axial load of the air compressor.
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Previous research shows that the matching of the 125 cc four-stroke internal-combustion engine with an auger-type air-compressor is exempt from the unstable rotating speed and the changing load caused by the air compressorÕs impulse effect. The auger-type air-compressor can exert a stable load on the internal-combustion engine and keep it operating at its best status. In terms of recycling waste energy, the internal-combustion engine and the air compressor are both exothermic. The temperature at the head of the four-stroke engineÕs exhaust tube is the highest. Initial tests also prove that the systemÕs overall flow work and efficiency would be greatly enhanced if the waste energy of the internalcombustion engine were mixed with that of the compressed air. 5. Conclusions The power-generation system in this study uses high pressure and high temperature air-flows. Compared with traditional mechanical energy, it could not only reduce friction loss, but also keep the internal-combustion engine working optimally. The waste energy can be recycled and utilized to decrease the pollution and fuel consumption. All these are what the ‘‘green’’ engine desires. In this system, the air compressor would exert a stable load on the internal-combustion engine after it is ignited. This enables the engine to work in an optimized state of low fuel-consumption and low pollution. Currently, either hybrid vehicles or electrical vehicles still cannot charge sufficient amounts of electricity. The exhaust gas of the internal-combustion engine can be merged with the compressed air to increase the pressure and temperature, and eventually lead to a higher work output, which can be converted into effective mechanical energy and enhance the thermal efficiency of the internal-combustion engine. Compared with the electrical motor, the pneumatic motor is free of damage caused by overloading and the fear of burnout. Its time constant, which is determined by the ratio of inertia and output force, is relatively small. Therefore, it can be smoothly started and stopped. As for output power, it can control the output rotating direction, torque and rotating speed via controlling valves. This system features good scalability. Flywheel energy-storing devices and braking energy recycling devices can be installed to offset the shortcomings of low energy density to enhance the overall efficiencies of the systems.
References [1] Nobuo I. Analysis of fuel economy and advanced systems of hybrid vehicles. J Soc Automotive Eng Jpn 1997;20:3–11. [2] Chau KT, Wong YS. Overview of power management in hybrid electric-vehicles. Energ Convers Manage 2002;45:1953–68.
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[3] Morita K. Automotive power source in 21st century. J Soc Automotive Eng Jpn 2002;24:3–7. [4] MDI (Moteur Developpment International), Available from:
Applied Energy 82 (2005) 117–132
www.elsevier.com/locate/apenergy
Hybrid pneumatic-power system which recycles exhaust gas of an internal-combustion engine K. David Huang a, Sheng-Chung Tzeng b,*, Wei-Ping Ma c, Wei-Chuan Chang a a
c
Graduate Institute of Vehicular Engineering, Dayeh University, Changhua 515, Taiwan, ROC b Department of Mechanical Engineering, Chien Kuo Institue of Technology University, Changhua 500, Taiwan, ROC Department of Information Management, Lan Yang Institute of Technology, Ilan 261, Taiwan, ROC Accepted 11 October 2004 Available online 25 December 2004
Abstract The hybrid pneumatic power system (HPPS) proposed in this research replaces the batteryÕs electric-chemical energy with flow work and optimizes the management and utilization of the energy. This power system is able to keep the internal-combustion engine working at its optimal condition and turn its waste energy into effective mechanical energy and so enhance the thermal efficiency of the whole system. Using computer simulation software ITI-SIM, this study simulates the overall dynamic characteristics of the system in accordance with the regulated running-vehicle test-mode ECE47, and, with experimental verification and analysis, proves that this system can meet the requirements of the standard running-car mode. As for recycling the waste energy, the experimental results show that this design could offset the shortcomings of the low-density of pneumatic power and so effectively enhance the efficiency of the whole system. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Hybrid pneumatic-power system; Waste-energy recycling; EngineÕs thermal-efficiency
*
Corresponding author. Tel.: +886 4 7111 111x3132; fax: +886 4 7357 193. E-mail address: [email protected] (S.-C. Tzeng).
0306-2619/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2004.10.006
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K.D. Huang et al. / Applied Energy 82 (2005) 117–132
Nomenclature A M n P P1 P2 Q R Ra Rr T T1 Va VB W Ww Z DP g la lr q x
front projective area (m2) inlet flow-rate of pneumatic motor (m3/s) polytropic exponent pressure (bar) air-compressorÕs inlet-pressure (bar) air-compressorÕs outlet-pressure (bar) inlet mass-flow rate of compressed air (kg/s) ideal-gas constant (kJ/kg K) air-flow resistance (N) rolling resistance (N) torque (Nm) air-compressorÕs inlet-temperature (K) headwind velocity (m/s) capacity of storage tank (m3) output work (kW) vehicleÕs weight (kg) cycling number per hour of compressor pressure difference across pneumatic motor (bar) theoretical efficiency (%) coefficient of air-flow resistance coefficient of rolling resistance air-flow density (kg/ m3) angular velocity (rad/s)
1. Preface Because of pressures from both the energy supply and environmental protection, vehicle technology now emphasizes enhancing efficiency and the quality of energy consumption. New technologies of the internal-combustion engine have been developed and put to use on a daily basis; however, to reach the required speeds and loads, internal-combustion engines have to accelerate or decelerate frequently, and fail consistently to work at the optimal state of minimum fuel-consumption and pollution. This leads not only to a waste of energy, but also to severe problems with exhaust gases. This results in the energy efficiencies of vehicles equipped with internalcombustion engines being as low as 15% [1]. Great efforts have been devoted to the research and development of vehicles of high efficiency and low pollution, including electric vehicles, fuel cell vehicles, and hybrid electric vehicles. As for electric vehicles, there are many limitations, such as low charging capacity of batteries, slow charging speeds, inaccurate indications of residual electricity, and insufficient infrastructures. As for fuel-cell vehicles, the public is dubious about the safety and many technical difficulties need to be resolved before their widespread practical application. As
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for petrol–electric hybrid vehicles, no matter whether they are arranged in tandem type or parallel type, they still use electric-chemical energy, which brings about various shortcomings of battery technology. Furthermore, so far, there is no hybrid electric engine capable of keeping the internal-combustion engine working at its optimized state and the waste gas generated by internal-combustion engines fails to be utilized properly. All these result in low energy-efficiency of the system. Chau and Wong [2] emphasized the issue of energy expenditure with respect to environmental protection. They believe a petroleum crisis might occur in the year of 2010, which will put an end to cheap petroleum and give rise to fierce competition in developing vehicles using replaceable energy. So far, there are three options, namely, electric vehicles, hybrid electric vehicles, and fuel-cell vehicles. Morita [3] analyzed the developing modes of the power systems of future vehicles. They reviewed todayÕs research aims at developing zero-pollution vehicles and green engines. The electric vehicle (EV) is the typical product of zero-pollution vehicles, while greener engines include hybrid electric systems, the internal-combustion engine with gas direct injection and the fuel cell. A bi-energy engine using petrol and compressed air designed by Guy Negre has drawn much investment. The Moteur Development International (MDI) has been established: it is dedicated to the innovation and development of mono-energy compressed-air engines. In the African Expo, MDI exhibited its compressed-air vehicle, E.Volution [4], powered by compressed air stored in a 300 L tank, which is positioned at the bottom of the vehicle. The vehicle weights about 700 kg and can travel 200 km at the speed of 96 km/h. It takes about 4 h to charge the air store by an air compressor. However, if the charging is accomplished via a high-pressure air-charging station, the charging process just needs 3 min. In the Paris International Automobile Exhibition of October 2002, City Cat, a small car using a compressed-air-powered engine was introduced. The traveling capacity of the pneumatic vehicle is now able to meet the needs of urban drivers. The University of Northern Texas and the University of Washington [5] are jointly working on the liquid nitrogen-powered vehicles. The working mechanism is that the liquid nitrogen evaporates and expands quasi-isothermally at normal temperature and consequently generates power. Plummer et al. [6] proposed that the ideal efficiency of the power cycle of the quasi-isothermal expansion of liquid nitrogen might reach 85%. For example, a two-cylinder engine can generate 15 kW power and 190 Nm torque with an expansion pressure of 8 MPa and rotating speed of 850 rpm. Applying this system on a common vehicle, 200 litres of liquid nitrogen is capable of driving the car for 140 km. Ordonez [7] put forward the idea of a recycled power system using liquid nitrogen. The main problem of this system is the short traveling distance achieved, since its energy density is quite limited. Thermodynamically, changing OrdonezÕs original design to the renewable Brayton cycle, the effective power-supplying time could be 10 times longer but the fuel economy increased considerably. Maghoub and Craighead [8] came up with an idea of controlling the gas inlet and outlet of the piston-type pneumatic motor to meet the requirements of driven components by either a PID or H-bridge control method. Pandian et al. [9] pointed out that, compared with
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the electrical motor, the time constant, which is decided by the ratio of inertia and output torque, of the pneumatic motor is relatively small. This means the pneumatic motor can be activated and stopped smoothly and the ratio of output torque to weight is higher. Therefore, the pneumatic motor is better than the electrical motor in terms of power and torque performance and is regarded as a high-powered rotating engine. Tokhi et al. [10] also pointed out that, compared with the electrical motor, the pneumatic motor has such advantages as (i) easily obtainable power source, (ii) large activating torque, and (iii) easy control of rotating speed and torque: it only requires an air-flow controller and a pressure controller. Apart from these advantages, the pneumatic motor does not suffer from damage caused by overloading or heat generated by friction. Chang and Nishi [11] considered a low-pressure vane pneumatic motor as the driver of mechanical devices. This pneumatic motor has a pair of rotating vanes, switching valves and power-output axle. The equations of its static and dynamic performances have been deduced from experimental results. So far, all the applications of internal-combustion engines fail to keep them working at their optimized condition after start-up. The waste energy of the internal-combustion engine has not yet been properly utilized, which results in a low thermal efficiency and BSFC. To effectively overcome these shortcomings, the hybrid pneumatic power-system proposed by this article involves the replacement of the batteryÕs electric-chemical energy by flow work. This power system can keep the engine working at its optimized condition and so effectively reduce the fuel consumption and emissions of waste gases from the internal-combustion engine. Furthermore, this system can utilize the waste energy of the internal-combustion engine and greatly enhance the overall efficiency.
2. Theoretical analysis This research is about a HPPS that integrates an internal-combustion engine, a compressor, a pneumatic motor and an exhaust gas-recycling device. The energy transformation parameters have been modularized to explore the relations between the componentsÕ transient responses, operating variables and the control parameters under various operation modes. ITI-SIM software, which is capable of analyzing the dynamic behavior of the system of any composition, is used to establish the systemÕs dynamic model. 2.1. Internal-combustion engine The model of the internal-combustion engine in the ITI-SIM is a four-stroke with a rigid crankshaft. It can operate in accordance with the input-torque curve and take into consideration the specific phenomena in the combustion and compression processes. The individual torque-curve of each cylinder can be superposed above the average output phase. The required input parameters include normal power, normal speed, and the number of cylinders in order to be able to calculate the torque applied to the crankshaft. The calculation basis for this model is the cylinderÕs torque curve;
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hence the torques generated by combustion and compression are also taken into consideration. When the input signals are set, the basic parameter configuration should refer to the actual measurements of the vehicleÕs internal-combustion engine BSFC and set the output operation mode of the internal-combustion engine at 3.41 kW/ 4000 rpm, which are the optima. 2.2. Air compressor The air compressor used in this hybrid pneumatic power system employs an auger type pressure-controlling valve, which applies a stable load to the internal-combustion engine and allows it to operate at a set rotational speed. The fuel efficiency can be improved and such pollutants as CO, CO2, HC, and NOx can reach the minimum emission values if the rotating speed is set at the optimized BSFC area. Similarly, problems with air admission, exhaustion, fuel injection, ignition, lubrication, cooling and noise could be met with an optimized design aiming at a specific operation mode, which in turn greatly reduces the design difficulty, time, and cost. The basic equation of the model is as follows: " # ðn1Þ nRT 1 P2 n W ¼ 1 : ð1Þ n1 P1 2.3. Air tank This is an indepensible component of the pneumatic power-system. It stores and regulates energy, keeps a stable supply of compressed air, reduces the pressure surges, and enables the braking component to operate smoothly. Besides, its volume and weight are closely related with the cruising mileage and the activation times of the internal-combustion engine. The basic equation to calculate the volume of the air tank is as follows: 15QP : ð2Þ DPZ This model can simulate any future possible developments. Such factors as improving the efficiencies of the compressor as well as the pneumatic motor, under different conditions, could be simulated via adjusting the parameters in the model. VB ¼
2.4. Pneumatic motor Due to the lack of a theoretical formula for the simulation of the pneumatic motor modelÕs behaviour, the power output characteristic used refers to the performance curves of the pneumatic motor (6AM-ARV-55) made by GAST and then introduced into the ITI-SIM motor model. The energy flows of the isothermal process and adiabatic process cannot be used fully. Accordingly, the theoretical net efficiency g can be calculated by the following equation:
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g¼
xT : DPM
ð3Þ
2.5. The vehicle in the environment The running resistance in this simulation arises from wheel rolling and air flow. The former is caused by deformation of tires and road surface, and the resistance generated by tires sliding on the ground. The rolling resistance is quite complicated and the simplified expression is as follows: Rr ¼ lr W w :
ð4Þ
The air resistance coefficient la varies in accordance with the shape of vehicle body and the air resistance coefficient Ra changes accordingly. Generally, the air resistance can be expressed as follows: q Ra ¼ la AV 2a : ð5Þ 2 Since the air resistance is related with vehicleÕs windward fac¸ade project area and the driving force is affected by the design of the tires, appropriate coefficients must be adopted in the computational simulation to reflect the real conditions. The coefficients adopted here are the same as those used commonly in vehicle design. Refer to Table 1 for the basic parameter configuration of the ITI-SIM model.
3. Testing method To determine the performance of the internal-combustion engine, the pneumatic power system and the waste-energy recycling device, a series tests and analyses was conducted and the results compared with those of previous simulations and calculations. The test platform, as seen in Fig. 1, consists of an internal-combustion engine, a compressor, a unidirectional pressure-controlling valve, a junction station, a pneumatic motor, a magnetic particle trig, a torque meter, a
Table 1 Values of parameters used in the driving simulation Parameter
Value
MotorcycleÕs weight RiderÕs weight Headwind speed Radius of the rear wheel Windward area of the rider Windward area of the motorcycle Coefficient of wind resistance Air density
200 kg 70 kg 5 m/s 0.2 m 0.3 m2 0.4 m2 0.69 Ns/kg 1.22 kg/m3
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Fig. 1. Testing system.
tachometer, an air-flow meter, pressure sensors, temperature sensors and a signal amplifier. The data measured by these instruments are analyzed and compared with the results of previous simulations to prove the feasibility of the system and the correctness of the simulation. 3.1. Measuring instruments The test platform uses a TP-10KMCB tachometer manufactured by KYOWA to measure the output torque of the engine: combined with the signal amplifier and selector manufactured by HIOKI, it can observe the change of torque continually. After adjustment, 1 V corresponds with 29.5 Nm. The output signal is presumed linear. The measurement of rotating speed requires the use of a microcomputer rotating-speed and linear-speed indicator, and a controller manufactured by POUNDFUL. Combined with the photo rotating sensor manufactured by TAKENAKA, it can work as a rotating-speed recorder. The error in the response is below 100 ls and the outside random light resistance is below 3000 lux. The measurement systems for temperature and pressure use sensors with measuring ranges of 0 ! 1000 °C and 0 ! 16 bar, respectively. Each signal is relayed by GENIE software. The airflow measurement and recording system uses a flow meter with a temperature-compensation function and has a short response time and a high sensitivity at low flows.
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Table 2 Relevant experimental parameter settings Component
Setting term
Value
Internal-combustion engine Internal-combustion engine Auger type air-compressor Junction station (compressed-air end)
Throttle opening ratio Output rotating speed Outlet pressure control Flow rate control
80 % 4000 rpm 6 bar 0.007 kg/s
3.2. Configuration of relevant parameters The tested system mainly includes an internal-combustion engine, an air compressor, an air tank, a pneumatic motor and a waste-energy recycling device. Since the internal adjustment and setting of each major component affects the performance of the whole system, the configuration in the test follows that in the simulation. The configuration of relevant parameters in the experiments is shown in Table 2. 3.3. Testing process Prior to each test, set the open angle of the throttle valve and the outlet pressure controller of the air compressor, then check the testing instruments and the positions of measuring points. Then start the engine and keep it at its optimum condition. Then start the stability test. Record the response data of all components until the measurement stops 120 s later. In the waste-energy recycling device test, in order to accurately measure the exhaust temperature and the convergence effect, warm up the internal-combustion engine for 2 min before the test. Then restart the engine and record the temperature and flow rate of air before and after the convergence until the measurement stops 140 s later.
4. Test results and discussion Each test was conducted twice with the same instruments, method and relevant configuration. The results are as follows: 4.1. The best operating-point of the internal-combustion engine The internal-combustion engine used in this system is a 125 cc four-stroke engine. The performance test was conducted using an engine dynamometer made by SCHENCK. The maximum power output at full-throttle is 5.48 kW at 7750 rpm and the maximum torque is 8.16 Nm at 4000 rpm. The power performance curve serves as important referential basis for system adaptation. Since the best operative point of this system is benchmarked by fuel transformation efficiency, it is not the point of maximum power or maximum torque, but rather the lowest point of the braking fuel-consumption rate. As shown, the best operative point occurs when
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the throttle turn-down ratio is 80%. At the optimal operation point, the output power is 3.41 kW, the output torque is 8.14 Nm and the braking fuel-consumption is 273 g/kWh at 4000 rpm. The system is kept operating at its optimized fuel transformation efficiency (see Fig. 2). So far, the engineÕs exhaust-gas has not yet been taken into consideration. Generally, the lowest point of the braking fuel-consumption rate of an internal-combustion engine is also its lowest exhaust point. Measures of exhaust quantity are shown in Figs. 3 and 4; the released CO, at the engineÕs best operative point decreases dramatically; the release of HC is also significantly lower than that for the normal operative state. It is concluded that, though the lowest point of the engineÕs braking fuel-consumption rate is not the lowest point with respect to emitting CO and HC, the latter tend to be close to their respective minimum values. 4.2. Analysis of the systemÕs stability When the testing instruments are ready, the internal-combustion engine is started and the throttle turn-down ratio moved from zero to 80%. The test result is shown in Fig. 5: after working for 17 s, the internal-combustion engine can maintain a rotational speed of about 4000 rpm with a discrepancy of 30 rpm. This test result proves that this system can keep the engine at its best operative status. Repeated tests also show that the errors and the overall tendency are quite close, which also increases the credibility of the tests.
Fig. 2. Braking fuel-consumption of a 125 cc four-stroke engine.
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Eimssino CO (%)
8 Throttle operating 40% Throttle operating 60% Throttle operating 80% Throttle operating 100%
6
4
2
0
3000
4000
5000
6000
7000
8000
Engine speed (rpm) Fig. 3. Content of CO in engineÕs exhaust.
400 Throttle operating 40% Throttle operating 60% Throttle operating 80% Throttle operating 100%
Emission HC (ppm)
350 300 250 200 150 100 50 0 3000
4000
5000
6000
7000
8000
Engine speed (rpm) Fig. 4. Content of HC in engineÕs exhaust.
As seen in Fig. 6, when the internal-combustion engine working at 4000 rpm after having been ignited for 17 s, the torque delivered to the air compressor also becomes stable and maintains a stable load on the internal-combustion engine. The output pressure-controlling valve of the air compressor can keep the rotor of the compressor bearing stable. The compressorÕs load actually fluctuates by ±0.3 Nm. Two factors cause this phenomenon. One is the pulse from the pressure-controlling valve will bring about the change of pressure and load borne by the air compressor. The other is the noise interference caused by shocks from the testing platform. The outlet pressure of the air compressor is set at 4.4 bar (see Fig. 7) in the test, i.e. 2.2 bar less than that in the simulation. This is because several measuring instruments are installed, resulting in the dramatic increase of the rotating inertia and friction resistance in the tests. This leads to a loss of power transferred from the engine to the air compressor. If the outlet pressure were set above 4.4 bar, the axial load of
Engine-output rotational speed (rpm)
K.D. Huang et al. / Applied Energy 82 (2005) 117–132 5000
4000
3000 Experiment 1 Experiment 2 2000
1000
0 0
20
40
60
80
100
120
140
Time (s)
Torque of compressor load (Nm)
Fig. 5. Stability analysis in the rotating-speed test of the internal-combustion engine.
7 6 5 4 Experiment 1 Experiment 2
3 2
error = 0.015 1 0 0
20
40
60
80
100
120
140
Time (s)
Output pressure of compressor (bar)
Fig. 6. Stability analysis in the axial-load test of the air compressor.
5
4
3
Experiment 1 Experiment 2
2 error = 0.003
1
0 0
20
40
60
80
100
120
140
Time (s) Fig. 7. Stability analysis in the outlet pressure-controlling valve test of the air compressor.
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the air compressor would exceed the engineÕs best operative status point. The measurements show that the outlet pressure becomes stable after having worked for 15 s. This is very influential on the internal-combustion engine and the air compressor. The results of adjusting this device would decide whether the air compressor can maintain a stable axial load and whether the internal-combustion engine can be maintained at its best operative status. To make full use of the thermal efficiency of the internal-combustion engine, the waste-energy generated by the internal-combustion engine needs to be recycled and utilized. After having warmed up for 2 min, the internal-combustion engine is reignited and kept operating at best status after 17 s. The temperatures measured at the exhaust are shown in Fig. 8. The measuring points are as in Fig. 9. Though there is a difference between the two tests, the exhaust temperature in both tends to become stable at 510 °C after the internal-combustion engine has been ignited for 115 s. The exhaust heat needs to be sent to the junction station to mix with the compressed air. The test results, shown in Fig. 10, indicate that the temperature can be maintained at about 86 ! 88 °C when only exhaust heat is supplied to the junction station. After being mixed with compressed air, whose temperature is 40 °C and the flow rate is about 0.007 kg/s, the actual measurements are shown in Fig. 11, i.e. the temperature is now about 60 °C. The overall thermal efficiency would be dramatically enhanced if this device were used in the system. 4.3. Comparison between theory and test results This system proves the correctness of the simulation. The simulation and test results after igniting the internal-combustion engine are shown in Fig. 12. The best operative status in these can be reached, respectively, after 5 and 17 s. The test results show a relatively high amplitude at 20 and 45 s, which can be taken as the noise generated by vibration of the test platform. The output rotating-speedÕs amplitude of the internal-combustion engine in the simulation is bigger than the actual measured value. It also takes less time to reach a stable status. These differences are caused by the
Exhaust temperature (˚C)
550 500 450 400 Experiment 1 Experiment 2
350 300
error = 0.030
250 200 0
20
40
60
80
100
120
140
160
Time (s) Fig. 8. Stability analysis of the exhaust-temperature test of the internal-combustion engine.
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Fig. 9. Locations of the systemÕs behavioural sensors.
Mixer temperature (°C)
90 88 86 84 82 Experiment 1 Experiment 2
80 78
error = 0.004
76 0
20
40
60
80
100
120
140
160
Time (s)
Fig. 10. Stability analysis of the temperature test of a junction station (before the convergence).
installation of the tachometer and its overall positioning brings about an inertia bigger than what is assumed in the simulation. The rotating speed changes more slowly when the rotating inertia is bigger. As seen in Fig. 13, the torque transferred from the internal-combustion engine to the air compressor, when the load of air compressor tends to be stable, will reach a stable output, respectively, after 5 and 17 s in the simulation and test results, respectively. However, there is still a difference of about 0.5 Nm between the simulation and test results. This is because the large friction increased by the overall test platform causes a loss of output torque. Its driving efficiency is also not as good as that in the simulation. The actual rotating inertia is a far cry from that in the simulation. These result in a gap of 12 s in the time needed for the engineÕs rotation to become stable.
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Mixer temperature (°C)
70 65 60 55 Experiment 1 Experiment 2
50 45
error = 0.020
40 0
20
40
60
80
100
120
140
160
Time (s)
Fig. 11. Stability analysis of the temperature test of a junction station (after the convergence).
Engine output (rpm)
6000 5000 4000 3000
Simulation Experiment
2000 1000 0 0
20
40
60
80
100
120
Time (s)
Fig. 12. Comparison of the simulation and test results for the internal-combustion engineÕs rotatingspeed.
Torqueof compressor load (Nm)
7 6 5 4 Simulation Experiment
3 2 1 0 0
20
40
60
80
100
120
Time (s)
Fig. 13. Comparison of the simulation and test results for the axial load of the air compressor.
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Previous research shows that the matching of the 125 cc four-stroke internal-combustion engine with an auger-type air-compressor is exempt from the unstable rotating speed and the changing load caused by the air compressorÕs impulse effect. The auger-type air-compressor can exert a stable load on the internal-combustion engine and keep it operating at its best status. In terms of recycling waste energy, the internal-combustion engine and the air compressor are both exothermic. The temperature at the head of the four-stroke engineÕs exhaust tube is the highest. Initial tests also prove that the systemÕs overall flow work and efficiency would be greatly enhanced if the waste energy of the internalcombustion engine were mixed with that of the compressed air. 5. Conclusions The power-generation system in this study uses high pressure and high temperature air-flows. Compared with traditional mechanical energy, it could not only reduce friction loss, but also keep the internal-combustion engine working optimally. The waste energy can be recycled and utilized to decrease the pollution and fuel consumption. All these are what the ‘‘green’’ engine desires. In this system, the air compressor would exert a stable load on the internal-combustion engine after it is ignited. This enables the engine to work in an optimized state of low fuel-consumption and low pollution. Currently, either hybrid vehicles or electrical vehicles still cannot charge sufficient amounts of electricity. The exhaust gas of the internal-combustion engine can be merged with the compressed air to increase the pressure and temperature, and eventually lead to a higher work output, which can be converted into effective mechanical energy and enhance the thermal efficiency of the internal-combustion engine. Compared with the electrical motor, the pneumatic motor is free of damage caused by overloading and the fear of burnout. Its time constant, which is determined by the ratio of inertia and output force, is relatively small. Therefore, it can be smoothly started and stopped. As for output power, it can control the output rotating direction, torque and rotating speed via controlling valves. This system features good scalability. Flywheel energy-storing devices and braking energy recycling devices can be installed to offset the shortcomings of low energy density to enhance the overall efficiencies of the systems.
References [1] Nobuo I. Analysis of fuel economy and advanced systems of hybrid vehicles. J Soc Automotive Eng Jpn 1997;20:3–11. [2] Chau KT, Wong YS. Overview of power management in hybrid electric-vehicles. Energ Convers Manage 2002;45:1953–68.
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[3] Morita K. Automotive power source in 21st century. J Soc Automotive Eng Jpn 2002;24:3–7. [4] MDI (Moteur Developpment International), Available from: