2. Principles of work of pneumatic engine

## 2.1. Why air compressed two-stroke engine?

transformed into mechanical power. The application of alternative energy sources and alternative driving system is need instead of those based on fossil fuels. However, the main environmental problem takes place in big cities with transportation vehicles, where only the fossil fuels are used. Recently, the hybrid systems and fuel cell system are considered for future transportation means. Until now the electricity is produced mostly in many countries by burning fossil fuels. It is connected with production of CO2 and emission of the toxic components of exhaust gases. The electric vehicles have small possibility to drive a long distance. Up to now, the highest distance for such vehicles reaches maximum 150 km at medium speed and load, but real distance is up to 100 km. For that reason, an additional source power for generating an electric energy or driving source is still required. Many works are concerned on range-extender vehicles with a piston engine driving the electrical generator that charges the batteries. The current from batteries is delivered to the electric engine connected with a driving gearbox that transmits power to the wheels. The combus-

The alternative proposition of power source is to apply the air energy stored in the tank at high pressure. The idea of air-powered engines is known from many years. Already in the nineteenth century, were given concepts of such an engine. In 1847, Mr Parsey invented the air-compressed locomotive and after many years in 1896, the conception of Porter's pneumatic locomotive appeared [3]. The idea of pneumatic engines for transportation was revived again at the end of twentieth century. Many scientific and research works on pneumatic piston engines were carried

A car using energy stored in compressed air produced by a compressor has been suggested as an environmentally friendly vehicle in the future by Creutzig et al. [7, 8]. They analysed the thermodynamic efficiency of a compressed air car powered by a pneumatic engine and consider the merits of compressed air versus chemical storage of potential energy. Many proposals of applying the air piston engines were presented by researchers from Asia [9, 10]: for application in transportation. The researchers presented theoretical studies on engines of a typical small-scale passenger car, which are used for the analyses, and the comparison is based on the shaft work, cooling, efficiency and energy density. They found that optimization of the internal-combustion and recycling of the exhaust energy can increase the vehicle's efficiency from an original 15 to 33%, an overall increase of 18%. A hybrid pneumatic system with recirculation of exhaust gases was proposed by Huang et al. [11]. Huang et al [12] carried out a modification of four-stroke engine for operation in two-stroke engine, which was fed with compressed air. Their study presents an experimental investigation on a piston engine driven by compressed air. The compressed air engine was a modified 100 cm<sup>3</sup> internal combustion engine obtained from a motorcycle manufacturer. The experimental and theoretical analysis of a compressed air four-stroke engine was conducted by Chinese researchers, Yu and Cai [13]. The results show that the prototype of such an engine has a good economic performance under low speed and when the supply pressure is 2 MPa. Many works concern to application of the air engine in motorcycles [14], particularly in regions where motorcycle is a main transportation source. A prototype was built with a fuzzy logic speed controller and tested on the real road. Another prototype of motorcycle air engine with a capacity of 100 cm<sup>3</sup> was built by Wang et al. [15]. The motorcycle installed with the compressed air engine can operate at a

tion engine works only outside the city.

130 Improvement Trends for Internal Combustion Engines

out across the world in the past few years [4–6].

The energy of the air pressure is delivered to the engine in strictly defined period in order to force the piston in the cylinder of almost standard engine. The work cycle follows only when piston moves down. For that case, the best solution is applying of the two-stroke engine, which performs the real work for every rotation of the crankshaft. The two-stroke engine with port timing is cheaper and simply designed compared to the four-stroke engine of the same capacity. Theoretically, the two-stroke engine gives two times higher power than the fourstroke engine, and a direct fuel injection can fulfil environmental requirements [19]. The energy of the compressed air is converted during the expansion process on the mechanical work. The temperature of air stored in the tank is the same as the ambient temperature, thus the energy depends only on the pressure. The temperature can be increased by heating of the air transferred to the cylinder and thus the energy delivered to the cylinder is higher. However, the thermal losses during opening the exhaust port are also higher. The heat exchange with cylinder walls is smaller than in the classic IC two-stroke engine, because the charge temperature inside the cylinder is low even in TDC.

The pneumatic engine works until the pressure in the tank is high enough to fill the cylinder. The value of torque depends on the air mass delivered from the tank trough valve to the cylinder. One of the most important factors influencing the work of the pneumatic engine is valve timing and a value of the air pressure. The pneumatic engine enables the driving of the vehicle with real-zero emission without any combustion process. The vehicle mobility can be increased by adding an additional heat source in order to deliver higher energy to the cylinder. The pneumatic two-stroke engine together with electric engine will fulfil the future environmental requirements. The experimental set-up of the pneumatic engine has been carried in whole across the world and some vehicles appeared for testing on the road.

#### 2.2. Operation of pneumatic engine

The work performed by the pneumatic engine depends on the pressure difference between higher and lower heat source. The air expansion process is shown in Figure 1 from pressure p<sup>1</sup> to pressure p<sup>2</sup> with temperatures T<sup>1</sup> and T2, respectively [20]. The thermodynamic process between point 1 and point 2 is non-isentropic process, and the work l<sup>s</sup> has lower value than the isentropic process [21]. In order to obtain the higher power during one work cycle, the higher pressure of the higher heat source (tank) is required. If temperature of the air in the tank has value near the ambient temperature T<sup>1</sup> then temperature of the expanded air T<sup>2</sup> has lower temperature than ambient temperature. But, in a real pneumatic engine, the air injection takes place at the maximum value of compressed air delivered to the cylinder during the intake process. Therefore, the temperature T<sup>1</sup> has higher value than ambient temperature.

The engine is filled only by the air at high pressure when the piston is at TDC. The pneumatic engine can be simply done by modification of the design of the classic two-stroke engine. The engine does not require the inlet port delivering the air to the crankcase. The crankcase has a vent that causes only small compression of the air. The oiling of the bearings and the cylinder surface is ensured by a small oil pump or by oil drop valve in a close cycle. The schematic idea of the pneumatic two-stroke engine and timing of valve and port opening are shown in Figure 2. Only one exhaust port is used for the gas exchange in the cylinder. The engine has an injector or pneumatic valve controlled by the electronic unit. The bottle of certain volume contains the air at high pressure. The pressure of stored air in the bottle or tank (about 300 bar) is reduced by a pressure regulator to smaller injection pressure about 20–30 bar. The pressure is controlled by the sensor and the air is delivered by the pipe of small diameter (about 5–8 mm) to the valve. The air volumetric flow rate through the valve is rather high in comparison to the liquid fuel injection. The use of the electromagnetic stem valve requires high voltage and high electric power. For that case, the electromagnetic pneumatic valve used in industry is better solution. The air flow control should enable the high pressure in the cylinder after top dead centre (ATDC), and on the other hand, the opening of the pneumatic valve lasts very short (about 40–60 CA) and due to this reason, the natural frequency of the moving elements

in the valve should be high. The engine is equipped with a muffler for damping the air outflow from the cylinder after opening the exhaust port. Depending on the rotational speed and load, the air injection period begins several degree of CA before TDC. The expansion follows after

Modern Pneumatic and Combustion Hybrid Engines http://dx.doi.org/10.5772/intechopen.69689 133

The engine power is controlled only by change of the valve timing. The friction losses, compression stroke, pumping losses in the crankcase and outflow energy decrease the total engine

The mathematical model of the pneumatic engine was carried out to determine the engine performance at different control parameters. Calculation of the air mass delivered to the cylinder by determination of velocity and density of the air in inlet duct in front of the pneumatic valve enables assessment of engine work time at given tank volume and initial pressure. The air was treated as semi-perfect gas, where the specific heat ratio was calculated every time step [22].

The air thermodynamic parameters in the pipes and ducts were determined at assumption of unsteady gas flow from the three hyperbolic nonlinear partial differential equations: mass, momentum and energy balance. The system of the equations was solved by using the Lax-Harten-Leer scheme [23] based on Godunov's method. The engine parameters were

3. Modelling of physical processes in piston pneumatic engine

the air injection and it lasts until the exhaust port opens.

Figure 2. Diagram of two-stroke pneumatic engine and engine timing.

efficiency.

3.1. Mass balance

Figure 1. Non-isentropic work during air expansion.

Figure 2. Diagram of two-stroke pneumatic engine and engine timing.

in the valve should be high. The engine is equipped with a muffler for damping the air outflow from the cylinder after opening the exhaust port. Depending on the rotational speed and load, the air injection period begins several degree of CA before TDC. The expansion follows after the air injection and it lasts until the exhaust port opens.

The engine power is controlled only by change of the valve timing. The friction losses, compression stroke, pumping losses in the crankcase and outflow energy decrease the total engine efficiency.
