1. Introduction

Nowadays, the environmental regulations of the exhaust emission from internal combustion engines are more rigorous every year. Despite the high progress of the new types of combustion processes such homogeneous charge compression ignition (HCCI), controlled auto-ignition (CAI) [1], ATAC [2] and other or applying of different complicated fuel injection systems, the emission of the combustion products of the hydrocarbon fuel is still high, particularly in lower engine loads. Only small energy of fuel (about 25–45%) depending on the engine type is

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

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 combustion engine works only outside the city.

maximum speed of around 38.2 km/h and a distance up to 5 km equipped with two 9 l bottles

Modern Pneumatic and Combustion Hybrid Engines http://dx.doi.org/10.5772/intechopen.69689 131

Currently across the world, there are realized several projects of road vehicles with pneumatic drive, including project developed by Motor Development International (MDI) [16], and sold under the name of the Indian company TATA under the name Tata Air Car Mini Cat [17]. This engine operates in a four-stroke cycle and outside air is drawn into the compression chamber and compressed to 20 bar. At top dead centre (TDC), this air reaches 400C, and at that point, air from the storage tank is injected into the combustion chamber. The compressed air is stored in carbon fibre tanks at 30 MPa. Recently, the company Peugeot has developed a drive internal combustion engine with a hydraulic system driven by compressed air under the name Peugeot 2008 Hybrid Air [18]. This solution continues to include the concept of the engine powered only with air or the two drive units. The proposed solution is also unique in the world because it includes a combustion engine and air in one drive unit, providing a compact whole drive without having to install a complicated powertrain unit as the current hybrid structures.

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 tempera-

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.

filled by air under pressure of 25 MPa.

2. Principles of work of pneumatic engine

2.1. Why air compressed two-stroke engine?

ture inside the cylinder is low even in TDC.

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

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 maximum speed of around 38.2 km/h and a distance up to 5 km equipped with two 9 l bottles filled by air under pressure of 25 MPa.

Currently across the world, there are realized several projects of road vehicles with pneumatic drive, including project developed by Motor Development International (MDI) [16], and sold under the name of the Indian company TATA under the name Tata Air Car Mini Cat [17]. This engine operates in a four-stroke cycle and outside air is drawn into the compression chamber and compressed to 20 bar. At top dead centre (TDC), this air reaches 400C, and at that point, air from the storage tank is injected into the combustion chamber. The compressed air is stored in carbon fibre tanks at 30 MPa. Recently, the company Peugeot has developed a drive internal combustion engine with a hydraulic system driven by compressed air under the name Peugeot 2008 Hybrid Air [18]. This solution continues to include the concept of the engine powered only with air or the two drive units. The proposed solution is also unique in the world because it includes a combustion engine and air in one drive unit, providing a compact whole drive without having to install a complicated powertrain unit as the current hybrid structures.
