A Hybrid Pneumatic Power System (HPPS) has been developed for several years with the major aim of reducing the vehicle fuel consumption, environment pollution and enhancing the vehicle performance as well. Comparing with the conventional hybrid system, HPPS replaces the battery's electrochemical energy with a high-pressure air storage tank and enables the internal combustion engine (ICE) to function at its sweet spot. Besides, the HPPS, which effectively merges both the high-pressure air flow from the storage tank and the recycled exhaust flow from the ICE, thereby increases the thermal efficiency of the ICE and transforms the merged flow energy into mechanical energy using a high-efficiency turbine. This paper focuses on the major research process into HPPSs, including overall dynamic simulation and experimental validation. By using the simulation tool ITI-Sim, this research demonstrates an experiment which can be operated precisely according to the requirements of various driving conditions under which a car actually runs on the road in accordance with the regulated running vehicle test mode. HPPS is expected to increase the performance of the entire system from 15% to 39%, and is likely to replace the traditional system in the coming years.
The development of the vehicle industry and conventional engines has gained many important achievements, especially at the end of the twentieth and beginning of the twenty-first century. However, such rapid development of the industry, with an increasing number of vehicles in not only developed countries but also developing ones, such as India, China, Vietnam, and so on, has resulted in many negative effects on modern society as well as on human lives. For instance, global warming and air pollution as well as climbing fuel prices have recently become serious problems. More and more stringent emissions and fuel consumption regulations are stimulating an interest in the development of safe, clean, and high efficiency transportations, including Electric Vehicles (EVs), Hybrid Electric Vehicles (HEVs), and Fuel-cell Electric Vehicles (FEVs), which offer the best possibilities for use of new energy sources [
Some other new technologies have been developed to overcome the above disadvantages. During the Paris International Vehicle Fair held in October 2002, a minitype pneumatic power vehicle driven by compressed air in lieu of fuel attracted the attention of the world, which indicated
However, from the perspective of researchers, up to now neither a zero-pollution vehicle nor a green engine can currently enable the ICE to operate stably at its optimal operating point. In addition, heat from exhaust gases from engines has still not been used effectively or researched without being verified carefully. As indicated in [
In order to solve the aforementioned problems, a HPPS is proposed, which is integrated with an ICE, an air compressor, a high-pressure air storage tank, an energy merger pipe, and a high-efficiency turbine. This system stores the flow energy instead of storing a battery’s electrochemical energy. Furthermore, it can recycle the exhaust gas energy and make the ICE operate at an optimal state of maximum efficiency. According to the energy estimated calculation as shown in Figure
Losses of traditional ICE engine [
As illustrated in Figure
The concept of HPPS.
The recycling capabilities of the exhaust gas depend significantly, however, on the flow energy merger in the system. In the HPPS operating process, a more highly pressured air flow from the air-tank side will obviously lead to the static pressure in the merging region of the merger pipe being too high, which prevents the exhaust gas flowing out. In some cases, the exhaust gas cannot flow out as well as the engine is affected in a negative way by high-pressure air from the air tank. Thus, to avoid unexpected phenomena, the cross sectional area at the merger-pipe device will be adjusted by the actuator E-motor, which is described more clearly in [
With the aim of absorbing waste heat from the ICE in order to increase the flow energy significantly, this system will not only improve the thermal efficiency of the engine by recycling its waste heat, but also reduce the exhaust emissions of the engine. In the scope of current research, the regeneration system upon vehicle deceleration were not taken into account. Besides, the engine is started with a conventional electric starter, which is not to be shown in the concept
The system uses the 125 cc, one cylinder, four-stroke engine manufactured by SYM Corporation. According to the producer, the engine reaches the lowest brake specific fuel consumption (BSFC) by 5000 rpm, 4.76 PS, which is the sweet spot of this engine. Throughout the experiment and simulation, the engine always works at this optimal condition. This results in an increase in the economic efficiency of energy consumption as well as a significant decrease in exhaust gases such as CO, CO2, HC, and PM. The brake horse power of the engine can be calculated by the following equation:
The pneumatic power system analyses and simulates a two-state reciprocating compressor. The outstanding operating point of such a compressor in comparison with the screw compressor is the capacity of the former to provide high pressure in a short period of time. This is reasonable for providing sufficient energy for the air storage tank under any condition. The work done on the compressor shaft under polytropic condition can be calculated as follows:
The volumetric efficiency of the reciprocating compressor is a function of the first state geometry, although for the simulation purposes, the separate states can be separately analyzed. It is given by:
The air tank in HPPS should be atleast large enough to hold all the air delivered by the compressor. Received size of the tank can be calculated as follows: (with considering the volumetric efficiency and for constant delivery compressor)
The outstanding overall efficiency of air turbine inside HPPS, compared to other type of motor such as electric motor and so on, is one of the advantage points of this system, which can reach an optimal efficiency of up to 78% [
Due to the lack of a theoretical formula for the simulation of the air turbine model’s behavior, the simulation is essentially based on the performance curve provided by GAST Manufacturing (6AM-ARV-55) as in Figures
Air-Turbine Power Characteristic.
Air-Turbine Torque Characteristic.
According to the analysis of Wong in Theory of Ground Vehicle [
For an individual wheel, the rolling resistance on the wheel is defined as
The friction coefficient and slip characteristic between tire and road depend significantly on the road condition of dry, wet and so on, which have been defined in [
Developing simulation models for Hybrid Pneumatic Power System implies the inclusion of the different physical components of the power-train, ranging from mechanics, for instance the internal combustion engine and automotive components, to pneumatics such as compressor, pneumatic air tank, gas turbine, or even the consideration of the involved control systems. For all of these fields there exist different established modeling approaches. From the point of view of system simulation, three core approaches can be distinguished:
A structurally motivated partitioning of the overall system finally leads to a separation into physical elements. The interconnection between the objects is noncausal and is defined by the balance equations for potential and flow quantities of the respective physics. This way of modeling preserves the physical relationships between the elements in the model structure. Any researcher familiar with the system will immediately understand the layout of such a model, and will know where parameters are applied and where the physical result quantities are found.
In signal modeling the behavior of the system under study is implemented into a block structure. The blocks communicate to each other in a strictly causal (directed) manner and, consequently, there are clearly assigned inputs and outputs on the blocks. Signal modeling is the method of choice for mapping control systems.
The function of subsystems and components in a system can also be described directly by differential equations. If a simulation environment does not provide a predefined element covering a particular physical relationship, this approach is the straightforward way to include its functionality into the simulation model
While starting the simulation of the entire HPPS, the research group realized that objects which need to be simulated including all three characteristics above and, therefore, the software ITI Sim, which has the capacity to apply the best fit to each component of the system [
As can be seen in Appendix
The model meets the design’s technical requirement of fixing the turning speed of the ICE at the sweet spot, reaching the lowest brake specific fuel consumption and also the optimal operating point of engine. Here, the control signal is transferred for the gas factor at two conditions: no injection or full load. The feedback signal for the model is provided by the pressure sensor of the air storage tank model. Models of the air compressor and turbine are also indicated. Because ITI provides insufficient models for the air compressor and turbine, their specific characteristics are divided into different objects and analyzed separately. Residues in the air compressor as well as the turbine are analyzed in the friction model. Vehicles as well as driving resistances are illustrated, including vehicle mass, rolling resistance, and aerodynamic resistance as well as the wheels-ground contact between vehicle tire and road. The fundamental parameters of this system are illustrated in Tables
Assumed values of the pertinent parameters to various components.
Parameter | Value |
---|---|
Optimal mode of the ICE | 3.5 kW |
5000 rpm | |
Rotational inertia of flywheel | 0.5 kg/m2 |
Ideal air condition (ambient pressure and temperature) | 1 bar @ |
Frictional resistance of air compressor | 2 Nm |
Mechanical efficiency of air compressor | 80% |
Volume of storage tank | 105 liters |
Efficiency of air turbine | 75% |
Frictional resistance of air turbine | 2 Nm |
Allowable exhaust-gas backpressure | 0.5 bar |
The parameters of the simulated motor vehicles.
Parameter | Value |
---|---|
Motorcycle’s mass | 140 kg |
Rider’s weight | 70 kg |
Headwind speed | 5 m/s |
Radius of the rear wheel | 0.2 m |
Windward area of the rider | 0.3 m2 |
Windward area of the Motorcycle | 0.4 m2 |
Coefficient of wind resistance | 0.69 Ns/kg |
Air density | 1.22 kg/m3 |
In order to validate the accuracy of the simulation, the study of HPPS experiment has been conducted. As illustrated in Figure
The Experiment parameters.
Parameter | Value |
---|---|
Gasoline engine (SYM) | 125 cc, 4-stroke |
Max input of air compressor | 10 PS |
Max output of turbine | 15 PS |
Load cell | 25 Nm 3600 rpm |
Electric motor (force, itinerary) | 30 N, 30 mm |
Torque sensor | Kyowa 0–50 Nm |
Air tank | 105 liters |
Temperature sensor | 0–1000 C |
Pressure sensor | 0–10 bar |
Flow meter sensor | 0–60 m/s |
Schematic layout of the experimental setup.
Temperature sensors, pressure sensors, and torque sensors are installed in reasonable positions, as can be seen in Figure
The signal from the torque sensor is similarly digitalized and communicated through the PC and Comsoft 3 software. In the experiment, ICE always works well at the optimal speed of up to 5000 rpm. All the experiments above are carried out three times in order to minimize errors as well as obtain results that are as accurate as possible. For instance, in the dynamic load test, when the air compressor starts to operate, the test takes place repeatedly from three to five times in order to find the right time at which the engine works well under sufficient load.
As shown in Figure
Output Speed of the ICE.
In view of the air compressor, Figure
Torque load of the compressor.
Mass Flow Rate of the compressor.
Output temperature of the compressor.
As illustrated in Figure
Filling up speed of the air storage tank.
As can be seen in Figures
Output Air Turbine Torque.
Output Air Turbine Speed.
This part concentrates on evaluating the impact of important parameters throughout the experiment and operation, including: controlling the valve’s aperture for compressed air, or throttle, with apertures of 25%, 50%, 75%, and 100%; the aperture of the valve to the compressed air tank before the engine operation remains stable at 8 bars, 10 bars, and 12, bars (it is noticeable that the pressure of the air storage tank remains at 10 bar in the experiment); and vehicle velocity (due to the different aperture of the throttle and the different pressure in air storage tank)
Apparently, as can be seen in Figure
Velocity of the motor vehicles in various preset air storage tank’s pressure when throttle stays at 100%.
The main cause of this phenomenon is that we are unable to take full advantage of the heat energy from the engine. In addition, sometimes exhaust gas flows backward because of the impact of high pressure from the air storage tank. This is very nonbeneficial to the engine on a practical level. At a fixed level of pressure in the air storage tank, the throttle’s aperture gets smaller, resulting in a decrease in static pressure. Consequently, velocity increases considerably and energy is transformed from pressure energy into kinetic energy. This leads to an increase in static pressure in the CSA and an improvement in the circulation of exhaust gas flow as well as a combination of exhaust gas and compressed air that increases the regenerated exhaust gas energy. As a result, the whole system’s efficiency increases significantly.
As can be seen in Figures
Velocity of the motor vehicles in various throttles.
The decrease in pressure of air storage tank.
In brief, the decrease in energy is much greater than the decrease in the throttle aperture. So it is undoubted that the more the throttle aperture decreases, the greater the resistance force at the throttle, thus the whole system, increase. This results in the difference between the proportional increase or decrease in the throttle and velocity. Figure
Figure
Validation results of ICE speed.
Figure
Validation results of Compressor Torque Load.
The speed of reaching the pressure illustrated in Figure
Filling up the air storage tank.
The low variation in the experiment curve proves that provision of compressed air to the air storage tank is fast and stable, thanks to the stable operation of engine and air compressor. The time taken to fill the air storage tank for the simulation curve is 262.6 s shorter and tends to decrease because the air compressor stops providing compressed air accordant with the control signal. Furthermore, in a specific period of time, the pressure in the air storage tank in the simulation is always higher than in the experiment, which proves the greater stability of the model.
This research has analyzed the behavior of a HPPS. The construction of the system has not created new energy but optimized the use of the existing energy. From the Figures The internal combustion engine can be maintained within an optimal operating range, and the waste gas and heat can be recycled. Thus, the pollution and fuel consumption of the internal combustion engine can be minimized. Compared with an electric motor that may face the danger of overheating, the airturbine can provide overload protection, with a smaller time-constant and inertia/output ratio. Thus, it can be activated and stopped stably. Furthermore, in view of its power output, the control valve can be applied to control the torque and rotation speed. 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 in order to enhance the overall efficiencies of the systems. The experimental and simulation results show that a vehicle equipped with an HPPS could achieve efficiency approximately 35–39% higher than that of conventional vehicles.
Energy efficiency diagram of conventional system.
HPPS energy-efficiency calculation.
In the next step of research procedure, research group will be going to carry out the experiments with diesel engines with the purpose of investigation for the advantages of heat efficiency of the recycled exhaust gas energy. Besides, the suitable air turbines for the system will be found and evaluated for full practical experiments, in order to make the entire design of the prototype from all available components. In addition, the regeneration system will also be integated for the next HPPS generation.
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Based on the experimental result, exhaust-gas velocity, exhaust-gas temperature and exhaust-gas pressure have been identified.
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(See Figure
Brake horsepower of the engine (PS)
Engine speed (rpm)
Engine torque (Nm)
Ambient temperature (
Output pressure of compression (
Work done
Polytropic index
Ambient pressure (bar)
Output pressure of compression (bar)
Gas constant (J
Volumetric efficiency
Compressor factor
Relative clearance volume
Delivery pressures in the first stage
Intake pressures in the first stage
Compressibility factor at intake
Compressibility factor at discharge
Index of expansion
Receive size of air-storage-tank (l)
Volume capacity (m3/min)
Discharge pressure (bar)
Angular velocity (rad/s)
Pressure difference (bar)
Inlet flow-rate (m3/s)
Aerodynamic resistance (N)
Rolling resistance (N)
Mass density of the air (kg/m3)
Coefficient of aerodynamic resistance
Frontal area of the vehicle (m2)
Vehicle speed relative to the wind (m/s)
Coefficient of rolling resistance
Vehicle’s weight (kg)
Slip coefficient
Road condition coefficients.