RESEARCH OF ROTARY PISTON ENGINE USE IN TRANSPORT POWER PLANTS

A schematic diagram of a transport hybrid power plant using a new design RPE-4.4/1.75 rotary piston air engine is proposed. Its external speed characteristic is determined, according to which the maximum engine power is 8.75 kW at 850 rpm and the maximum torque is 127.54 N∙m at 400 rpm. For various gears and speeds, all the components of the power balance were determined and the dynamic characteristic of the hybrid car was obtained when operated on an air engine. According to the dependences of the power balance, the total traction force from the rotary piston air engine on the driving wheels is 5 kN. The performance of acceleration of a hybrid car while working on an air engine is estimated, namely, the dependences of acceleration, time, and acceleration path are obtained. In urban traffic, the required time to accelerate the car to a speed of 60 km/h is 15.2 s and the path is 173 m. The possible drive range of the hybrid car on compressed air without additional recharging is analyzed. On one cylinder with compressed air with a volume of 100 liters, an initial pressure of 35 MPa, and a final pressure of 2 MPa, the hybrid car can travel about 26 km.


INTRODUCTION
Urbanization of cities leads to a continuous and rapid increase in the number of public and private transport vehicles. This, in turn, leads to the problem of environmental pollution, especially in the central parts of cities.
The environmental degradation is largely affected by the steady increase in the average age of vehicles, the poor technical condition, and the use of old cars. In addition, the limited carriageway with a high traffic flow leads to a decrease in the speed of vehicles or even large traffic jams. In such conditions, a conventional internal combustion engine (ICE) is quite inefficient and displays increased emissions of toxic elements with exhaust gases. Thus, environmental pollution from vehicle emissions and the consequent deterioration in the health of the population have become one of the most acute problems of a modern city.

RECENT RESEARCH AND PUBLICATION ANALYSIS
The problem of air pollution in large cities can be partially solved by increasing the percentage of public electric transport in urban transportation, prohibiting the use of vehicles with internal combustion engines in city centers, and increasing environmental taxes and fees. However, all these measures are not able to fully solve the environmental problem of air pollution and ensure the necessary traffic flow.
One of the possible solutions to the problem may be the use of environmentally friendly vehicles, for example, electric cars [1][2][3][4] or hybrid cars [5][6][7][8][9]. A significant number of publications on the use of compressed air energy in transport plants report on the demand and relevance of this area of research. Converting ordinary internal combustion engines into air engines, during vehicles mass production, is economically unprofitable for a number of reasons. The use of air engines, mass-produced for industry, also has its drawbacks, primarily associated with the specifics of their purpose, conditions, and modes of operation.
The development and creation of a new reliable and efficient air engine that meets the specifics and satisfies all operating conditions on the vehicle is more appropriate. First of all, an air engine for a transport power plant must fulfil the following requirements: -to have minimum weight and dimensions relative to its power; -to provide minimum compressed air consumption; -to be reversible and at the same time have the same efficiency; -to efficiently operate at a wide range of engine inlet pressures and various rotations; -to ensure normal operation at various ambient temperatures (from minus to plus); -to have a minimum dead volume and the ability to control operating modes by changing the degree of filling; and -to be reliable, resistant to overloads, easy to operate, and cheap to manufacture and repair.
Research of rotary piston engine use in transport power plants 167 The developed RPE-4.4/1.75 rotary piston engine of a fundamentally new design ( Fig. 1) fulfills all the specified requirements and can be used as part of a transport power plant, and studying its characteristics and operating modes as part of a power plant is a very important and urgent task [24,25]. The engine housing contains a central rotor with radial pair-opposed cylinders. In the cylinders, pistons move back and forth, which are interconnected by floating pins and rigid links, wherein, the interconnected links form a hinged quadrangle, in the center of which a regulating cam is installed. Turning the regulating cam allows you to change the degree of filling of the working cylinder, which ensures the regulation of the engine operating modes. Gas exchange in the engine is provided by inlet and outlet openings made in the housing, which are closed and opened when the rotor turns.
The main advantages of a rotary piston engine are its simple, compact, and reliable spool design for regulating the phases of gas exchange, which makes it possible to regulate operating modes by changing the degree of filling. Due to the uniform placement of the cylinders, the engine has high balance, no vibration, and also starts in any position. Due to the combination of the design of the rotary and piston engines, the rotary piston engine, in comparison with rotary ones, has better sealing of the working space, and compared to the piston ones, there is no relative dead volume. Better sealing of the working cylinder minimizes compressed air losses, and the absence of relative dead volume improves efficiency. The ratio of piston stroke to cylinder bore is 0.4, which provides low piston speeds, compactness, and minimum relative weight. Therefore, by reducing the piston speed, the pressure loss at the inlet and the back pressure at the outlet are reduced. In addition, according to experimental studies, the rotary piston engine shows reliable operation at low temperatures of the exhaust air [26].
The aim of this research is to develop a hybrid transport power plant based on a rotary piston air engine and determine its operational characteristics.

O. Mytrofanov, A. Proskurin, A. Poznanskyi
The main tasks of the research include the following: -development of a schematic diagram of a transport hybrid power plant using a new design rotary piston air engine; -determination of the external speed characteristics of a rotary piston air engine as part of a power plant; -determination of power balance and dynamic characteristics of a hybrid car using an air engine; and -evaluation of acceleration performance of a hybrid car using an air engine.
To implement the task of developing a schematic diagram of a transport hybrid power plant using a new design rotary piston air engine, system analysis and generalization are performed. A systematic analysis of the various schemes of hybrid transport systems allows us to evaluate their advantages and disadvantages, as well as highlight a more suitable scheme of a power plant in which it is possible to use an air engine with high efficiency.
Determination of the characteristics of a hybrid power plant with an air engine as well as evaluation of the effectiveness of using a new design rotary piston engine are carried out by mathematical modeling based on preliminary experimental research of a prototype [25,26].
The object of the research is a hybrid transport power plant with a new design of a rotary piston air engine. The subject of the research is the operational characteristics of a hybrid car when operating on an air engine.

RESEARCH RESULTS
In existing hybrid transport power plants, three main schemes are used, namely, serial, parallel, and mixed (serial-parallel).
When using a serial scheme, the internal combustion engine (ICE) is combined with an electric current generator, which, in turn, powers the traction engines or additionally charges the batteries. During the period of insufficient power supply from the ICE and generator, the traction engine is additionally powered by storage batteries. In the event of excess energy, the batteries are charged. The advantages of this scheme are that the ICE operates in a constant mode of minimum fuel consumption. In addition, the serial scheme provides ease of control, there are no transmission units, and there is a possibility of different layout of the power plant. The main disadvantage of the serial scheme is that, due to the repeated conversion of energy (from mechanical to electrical, and then from electrical to mechanical), such power plants have low efficiency values.
When using a parallel scheme, the torque from ICE through the transmission is transmitted to the drive wheels of the vehicle. In the modes of excess power of ICE, energy is directed through a special power take-off system to charge the batteries. In the modes of insufficient power, the accumulated energy is used to power the elements of the electric transmission. The advantages of such a scheme are a higher level of efficiency and the possibility of using one electric machine. Accordingly, the disadvantages of the parallel scheme are the complication of the transmission, the operation of ICE at various modes of fuel consumption, and the complications of the control of the power plant.
When using a mixed ICE scheme, the electric current generator and the traction engine are interconnected by means of a planetary gear. In this case, ICE operates in a constant mode of minimum fuel consumption, and the speed of the transmission outlet shaft is controlled by a traction engine. Accordingly, the advantage of this scheme is high efficiency values. And the disadvantage is a significant complication of transmission design and power plant control system.
Based on the advantages and disadvantages of various schemes, the parallel scheme of a hybrid transport power plant is a kind of optimal compromise between the efficiency of energy conversion and the complexity of the design, and is also the most suitable for using an air engine. Fig. 2 shows the project of a parallel diagram of a hybrid transport power plant using compressed air energy. In this scheme, ICE and the air engine are interconnected using a drive axle transmission. This interconnection of the two engines makes it possible to increase the efficiency of energy transfer from the internal combustion engine to the wheels of the car (compared to the serial scheme of transport hybrid plants); however, it somewhat complicates the transmission and control of the power plant as a whole. Also, the use of a parallel scheme makes it possible to use the heat of the exhaust gases of ICE for heating compressed air before expansion, as well as the excess energy of the internal combustion engine to power the high-pressure compressor for pumping the cylinders.
In the internal combustion engine operating modes (for example, when the car is moving outside the city), the torque from engine 1 is transmitted through drive axle transmission 2, viscous coupling 3, and rear wheel gear 4 to rear drive axle wheels 5; this is, in fact, a classic scheme of a rear-wheel drive car.
When the car is moving in a traffic flow, a rotary piston air engine 12 is used to reduce environmental pollution. Compressed air, used in a rotary piston engine, is stored in cylinders 9. The transport power plant has two cylinders made of carbon fiber with a Kevlar sheath, which can significantly reduce their weight. The capacity of each cylinder is 100 liters, and the storage pressure is 35 MPa. Refueling from an external source of compressed air is carried out through valve 7. To increase the effective performance of the rotary piston engine, it is possible to preheat the compressed air from the exhaust gases of the internal combustion engine in a special heat exchanger 10. The necessary air pressure in the receiver of the rotary piston engine is provided with gear 13 with electromagnetic control. The gearbox maintains a constant working value of air pressure and, if necessary, can increase it to provide an increase in engine torque.
After throttling and expansion in the working cylinder of a rotary piston engine, the air is cooled. In summer, the air exhausted in the engine is partially or completely directed to the heat exchanger 16 of the car air conditioning system. Regulation of the operation of the air conditioning system of the car is carried out using bypass electromagnetic valves 14 and 15.
-compressed air; -exhaust air; -ICE exhaust gases Also, the proposed scheme of the power plant provides for the possibility of recharging the supply cylinders of compressed air using high-pressure compressor 17, which operates from ICE in excess power modes. At above average vehicle speeds, ICE transfers part of the energy to the wheels and another part to the high-pressure compressor. In addition, in braking modes, the inertial mass of the vehicle is used to power the compressor. The compressor operating modes are consistent with the overall control system of the power plant. The parameters of the rotary piston engine of the transport power plant should provide the necessary dynamics and speed of the car in urban traffic. When determining the effective indicators and speed characteristics of a rotary piston engine, it is necessary to establish the value of the maximum vehicle speed that the engine must provide. According to the traffic laws in inhabited localities, the movement of vehicles is limited to a speed of 50 km/h. Therefore, taking into account a certain margin of speed, we take the maximum vehicle speed when the rotary piston engine is 60 km/h. The main technical characteristics of the hybrid car are shown in Table 1. Therefore, as the project of a possible hybrid vehicle is proposed, the main parameters for the calculation were selected based on similar vehicles and their characteristics [27][28][29][30][31][32]. It is also worth clarifying that in Table 1, the car curb weight is the total mass of the vehicle with all equipment (including the pneumatic system and the air motor), all the necessary operating consumables, and a full tank of fuel, excluding passengers and cargo. The value of the gear ratio of the gearbox corresponds to the VAZ 1111 car, and the gear ratio of the main gear was selected taking into account the torque of the rotary piston engine, provided that starting off was ensured using only an air engine. In Fig. 3, the external speed characteristic of a rotary piston air engine in urban traffic with a maximum permissible speed of 60 km/h is shown.
To build the dynamic characteristics of a hybrid car for all gears and speeds, it is necessary to determine the components of the power balance equation, which are determined by the known dependencies [33,34] as follows: , where Pk is the tractive effort on driving wheels of the car; Pf is the total road resistance (which takes into account the coefficient of rolling resistance and the total car weight); Pw is the air resistance force (which takes into account the coefficient of air resistance and the frontal area of the car); and Pj is the acceleration resistance. The tractive effort on driving wheels of the car is determined by the formula , (2) where Мk is the current torque value and ηtr is the transmission efficiency of the corresponding gear.
In the calculations, it is assumed that the vehicle is moving on a horizontal surface, that is, there is no scope for the resistance to increase when determining the force of the total road resistance. Then, the force of the total road resistance is determined by the formula: , (3) where mа is the vehicle gross weight; g is the acceleration of gravity; f0 is the rolling resistance coefficient; and v is the vehicle speed.
The total weight of a passenger car in accordance with [33] is determined as follows: , where n1 is the number of passengers.
The value of the rolling resistance coefficient was chosen as 0.01, which corresponds to the conditions of the asphalt pavement in good condition [34].
The force of air resistance is determined by the formula: , (4) where Cx is the drag coefficient (for a passenger car, it is 0.26...0.38); ρ is the air density, kg/m 3 (ρ = 1.225 kg/m 3 ); and Fa is the frontal area of a car, m 2 .
The value of the frontal area of a car is determined from the drawing. In the absence of a drawing, for an approximate calculation of the frontal area of the car, the following dependence is used: .
(5) The value of the acceleration resistance is defined as the difference between the tractive effort and the sum of the resistance to movement. It is advisable to plot the power balance and dynamic characteristics of a hybrid car as a function of speed, which is directly related to the speed of the rotary piston engine, the gear ratio of the corresponding gearbox and final drive, and also the static radius of the car's wheel [33,34]: where ω is the angular speed of rotation of the air engine rotor, s -1 .
If we express angular velocity ω in terms of revolutions of rotary piston engine n and change dimensions from m/s to km/h, the speed equation will take the following form: , km/h . The dynamic characteristic is the dependence of dynamic factors in different gears on vehicle speed. Therefore, the dynamic factor is determined by the formula: .
(8) The results of calculation of the traction-speed characteristics of a hybrid vehicle are presented in Table 2, and the dynamic factor results are presented in Table 3.
The dynamics of acceleration of a hybrid car with an air engine is dependent on the change in acceleration, time, and acceleration distance from the car speed.
Therefore, the vehicle acceleration is determined by the known dependencies for all gears and speeds, using the corresponding values of the dynamic characteristics. The acceleration of a hybrid vehicle in various gears when driving on a horizontal section of the road is determined by the formula [34] , (9) where δ is the coefficient taking into account the rotating mass of a vehicle for different gears.
The moment of inertia of rotating parts of a vehicle is not known; hence, the value of the coefficient can be estimated using the equation: .
(10) Fig. 5 shows the dependences of changes in accelerations and reverse accelerations when moving on a rotary piston air engine.
The results of calculation of the acceleration of a hybrid vehicle are presented in Table 4. Table 2 0  To determine the necessary time and acceleration path of a hybrid car when driving on a rotary piston air engine, it is necessary to calculate certain integrals accordingly [33; 34]: In addition, the acceleration time can be obtained graphically by determining the area limited by the inverse acceleration curves 1/ji = f(V) (see Fig. 5, b), and the acceleration path by calculating the area of the dependence of the acceleration time t = f(V). The area under the curves is divided into a random number of sections. In the corresponding scale, the areas of sections Fti on the reverse acceleration graph are the acceleration time: , where is the scale-back acceleration factor; is the scale speed factor; and Fti is the corresponding section area. In the corresponding scale, the areas of sections Fsi on the acceleration time graph are the acceleration path: is the scale acceleration time factor and Fsi is the area of the corresponding acceleration time section. Graphs of time and acceleration path of a hybrid car depending on the speed of movement are shown in Fig. 6, and the calculation results are shown in Tables 5 and 6. Table 3 Results of calculation of the dynamic factor of a hybrid vehicle when operating on a rotary piston air engine in urban traffic conditions    Table 4 Results of calculation of the acceleration of a hybrid vehicle when operating on a rotary piston air engine in urban traffic conditions  The approximate driving duration of the hybrid car using only the rotary piston air engine at a speed of 60 km/h without parallel recharging on a single cylinder with 100 l compressed air, with a storage temperature of 20°C, and an initial pressure of 35 MPa and a final pressure of 2 MPa is about 26 km. The calculation results for the duration of the movement are rather approximate and conditional. The vehicle's power reserve was calculated based on the conditions of the vehicle's movement along a straight section of the road at a constant speed. In this case, the specific consumption of the working fluid of air engine and the possibility of discharging the cylinders to the minimum possible pressure from the point of view of ensuring the operation of the air engine (according to the data in Table 1) were taken into account. In addition, in the calculations, the acceleration period and the flow rate of the working fluid were not taken into account.
Heavy traffic, especially in central parts of large cities, leads to a decrease in the average vehicle speed. Thus, the average speed of passenger cars in conditions of average traffic intensity is about 40 km/h, and in peak traffic conditions, it can decrease to 12.5...15.0 km/h [35; 36]. Accordingly, in such conditions, conventional ICEs operate ineffectively and have high levels of harmful emissions. In such conditions of vehicle movement, it is efficient to use environmentally friendly rotary piston engines that provide the necessary dynamic characteristics of the vehicle, which is confirmed by the results of the calculations.

CONCLUSIONS
A parallel scheme of a transport hybrid power plant based on the RPE -4.4/1.75 rotary piston air engine with a power of 8.75 kW has been developed. The air engine provides a maximum car speed of up to 60 km/h, which is more necessary in urban traffic conditions. At the same time, the car's driving range on one cylinder with 100 l compressed air (initial pressure 35 MPa, final 2 MPa) is about 26 km. An external speed characteristic of a rotary piston air engine was obtained, according to which the maximum engine torque is 127.54 Nm at 400 rpm and the maximum power is 8.75 kW at 850 rpm. The components of the power balance and the dynamic factor of the hybrid car are determined when operating on a rotary piston air engine for various gears and speeds. In this case, the highest traction on the driving wheels is 5 kN. The dependences of acceleration, time, and acceleration path of a hybrid car when operating on an air engine to a speed of 60 km/h are obtained. Therefore, the required acceleration time of the car is 15.2 s and the path is 173 m, which is a sufficient indicator in urban traffic conditions.
In developed countries, the demand for and distribution of environmentally friendly vehicles of various designs are increasing steadily. Due to their advantages, hybrid compressed-air transport power plants have great economic potential and further development prospects. In contrast to convertible air engines, the new design rotary piston engine has a much simpler and more compact design, as well as a rather low specific consumption rate of compressed air, which makes it effective for use in transport power plants. However, it is worth noting that it is impossible to take into account all the possible features of using a new design rotary piston air engine as part of a transport power plant in a real-life vehicle operation. Therefore, further comprehensive full-scale road tests are required.