COMPUTATIONAL AND EXPERIMENTAL DETERMINATION OF ENERGY LOSS OF THE OPERATING FLUID IN THE INTAKE SYSTEM OF THE AUTOMOBILE PISTON PNEUMATIC ENGINE USING THE EXERGY METHOD

The application exergy method of the thermodynamic analysis for determining the energy loss of the operating fluid in the intake system of the automobile piston pneumatic engine using experimental data of its engine test rigs has been considered. The analysis of the performed exergy calculational and experimental research on the evaluation of energy loss in the intake system of the engine for current modes of its operation has been presented. The sample of a piston pneumatic engine for a combined automobile power unit created at the ICE Department of KhNAHU requires further improving and developing especially such components and systems as those connected to the burn process. A compressed air intake system is one of them. Compressed air is not only some working fluid for a pneumatic engine but the energy source for its intake system. The energy loss factor like reliability and durability is referred to the performance and efficiency factors. The object of the study is a pneumatic engine created by converting a gasoline four-cylinder four-stroke ICE at the ICE Department of KhNAHU. The experimental research was carried out by laboratory engine test rigs of the pneumatic engine with speed characterization provided that two thermodynamic parameters of compressed air at the intake were kept constant for each characteristic: pressure ps = idem and temperature Тs = 293 K. In each test mode all external parameters of the pneumatic engine were recorded and indicator diagrams of the first cylinder were taken. Each speed characteristic consisted of 6–8 modes ps= idem and Тs= 293 К = idem with changing the rotational rate of the crankshaft n, rpm, from minimally stable (about n=200 rpm) to maximum possible n = 1000 ± 50 rpm It should be noted that there is an operating fluid pressure drop in the intake system chamber due to available inconvertibilities: throttling in the test port, hydraulic loss along the intake port as a result of fluid friction, swirls and other gas-dynamic phenomena. The exergy loss of the operating fluid enthalpy in the intake system Dвп=Евп-Енапwith the intake pressure рs = 0.5 MPais 13.3 kW, with the intake pressure рs = 0.7 MPa is 15.0 kW, with the intake pressure рs = 0.9 MPa is 16.9 kW, with the intake pressure рs = 1.1 MPa is 19.7 kW.


Introduction
The sample of a piston pneumatic engine for a combined automobile power unit created at the ICE Department of KhNAHU requires further improving and developing especially such components and systems as those connected to the burn process. A compressed air intake system is one of them. Compressed air is not only some working fluid for a pneumatic engine but the energy source for its intake system. The energy loss factor like reliability and durability is referred to the performance and efficiency factors.
The method of determining the energy loss of the compressed air in the intake system of the pneumatic engine using the experimental data [1] and well-known exergy method [2,3] has been considered in the paper.

Purpose and Tasks
The purpose of this computational and experimental study is to develop a technique for applying the well-known exergy method to determine the energy loss of the operating fluid of the piston pneumatic engine in the intake system and the energy loss of operating fluid (compressed air) as its enthalpy exergy loss in the intake system of the piston pneumatic fourcylinder engine with intake operating pressures p s = 0.5-1.1 MPa at the modes of the maximum possible efficient power N e max at given pressure levels p s according to speed characteristics p s = idem with in take constant temperature Т s = 293 K.

Analysis of Publications
Nowadays there are many publications devoted to the fundamentals of the exergy analysis and its application in various fields of technology. Last decade of the twentieth century the work on creation of a mathematical apparatus of exergy analysis in solving optimization problems in the field of improving currentequipment and designing new equipment and technologywas completed. The fundamentals of exergy analysis are mostly generalized and fully described in the works of V. M. Brodyansky [2], G. N. Kostenko [3], A. R. Kotin [4], G. D. Rem [5]. The issues of the exergy method application in the field of heat engineering are studied by A. I. Andryushchenko [6 and others].
Almost all the studies on the exergy method published since 1964 are referred to the exergy applications in heat engineering, chemical technology, metallurgy, cryogenic engineering and economics. The discussions about the necessity of this section in thermodynamics and correctness of its main principles were left in past. In the 21 st century the exergy method got known to many specialists of various fields of technology. They considered it as a convenient tool for solving their specific tasks. The main advantages of the exergy method of thermodynamic analysis are its simplicity and universality [2]. It is possible to determine both absolute and relative values of energy loss due to inconvertibilities for any process or its part using the exergy balance.
So in the recently published work [7] the simplicity and accuracy of determining the energy loss of the operating fluid in case of its inconvertible leakages using the exergy method were presented. This paper shows determiningthe dependence of the exergy characteristic -enthalpy exergy loss due to external conditions: pressure and temperature of the supplied compressed air at the intakeof the power unit p s and T s were found using experimental data and theoretical dependences between the burn process parameters.

Object of Study
The object of the study is a piston pneumatic engine with specifications presented in Table 1. The considered pneumatic engine was created at the ICE Department of KhNAHU by converting a gasoline four-cylinder four-stroke ICE. The experience of this conversion is described in detail in [1].
In accordance with the aboveformulated purposethe specific object of the study in the experiment and calculations is the intake system of the pneumatic engineprototype sample. Each cylinder of the engine has the same intake system through which the process of filling the cylinders with the incoming charge of compressed air is carried out.
Adiabatic exponent of dry air k -1.40 Adiabatic exponent of humid air, relative humidity φ=100 % [9] k p -1.32 This process is not adjustable. It is performed by a structurally specified period of filling the cylinder with the incoming charge of compressed air. The process of filling lasts up to 58 degrees of crankshaft turn: three degrees before-top dead centre (TDC) and 55 degrees after TDC.
The cylinder in take system includesan in take port of circular cross-section with a diameter of 20 mm and a length of 800 mm, two test ports of the same cross-section: the first is on the outer cylindrical rotor surface of the spool-type air distributor and the second is on the inner cylindrical stator surface at the beginning of the intake port. Test ports are partially or fully matched during the filling. Their combined flow area varies from zero to a maximum that is equal to the area of an open port.
The volume of the internal chamber of the intake system alongside with the object above the piston space at the top dead centre is socalled dead volume that increases the flow density of compressed air ɡ е , kg / (kW/h).

Experimental Techniques
The experimental study was carried out by laboratory engine test rigs of the pneumatic engine with speed characterization provided that two thermodynamic parameters of compressed air at the intake were kept constant for each characteristic: pressure p s = idem and temperature Т s = 293 K. In each test mode all external parameters of the pneumatic engine were recorded and indicator diagrams of the first cylinder were taken. Each speed characteristic consisted of 6-8 modes p s = idem and Т s = 293 К = idem with changing the rotational rate of the crankshaft n, rpm, from minimally stable (about n=200 rpm) to maximum possible n = 1000 ± 50 rpm. The method of pneumatic engine testing to take the speed characteristics of the pneumatic engine is described in detail in [1].The whole experiment comprised four speed characteristics for intake pressures of the compressed air (being the subject of the greatest scientific and practical interest), p s = 0.5; 0.7; 0.9 and 1.1 MPa, at constant temperature Т s = 293 K.
Then using the obtained experimental data we constructed the dependences graphs of the efficient power N e , kW, on the rotational speed of the shaftn, rpm and determined the maxima of the functions N e (n) -the modes of the indicated power of the pneumatic engine for the corresponding pressure level p s at the temperature Т s , corresponding to the ambient temperature Т о.с = 293 K.
The operating fluid energy loss in the intake system of the pneumatic engine was determined only for these four modes N e max mentioned above (being of exceptional interest).

Results of the Computational and Experimental Study and Their Analysis
Using the Exergy Method The main results of this experimental and computational study are presented in Table 2 and fig. 1. They are the results of the second stage of the study, the starting points for which are the results of the first stage devoted to the determining the operating fluid energy loss at the very beginning of the working chamber of the pneumatic engine-in the spool-type air distributor due to leakage of incoming compressed air D ym , kW [7].
Thus the starting point for the second stage of the study is the level of exergy of the operating fluid enthalpy at the beginning of the intake process equal to the enthalpy exergy of Е s , kW, supplied to the engine, minus the energy loss for leakage.
(1) After that we defined the level of exergy of the operating fluid enthalpy at the end of the intake, i.e. at the end of the filling process, Е нап , kW, that was dependant on the thermodynamic parameters of the air condition at that moment, primarily on its pressure p нап and temperature Т нап .
It should be noted that there is an operating fluid pressure drop in the intake system chamber due to available inconvertibilities: throttling in the test port, hydraulic loss along the intake port as a result of fluid friction, swirls and other gasdynamic phenomena.
The analysis of the obtained indicator diagrams during the engine test rigs allowed to determine the cylinder pressure values while filling. Table 2 shows the pressure values in the working chamber at various levels of compressed air intake pressure p s for maximum power modes according to speed characteristics. The thermodynamic parameters (enthalpy and entropy) were determined according to the level of air pressure and temperatureusing tables [8]. The performance was calculated as the specific enthalpy exergy е нап , kJ/kg, and the mass for the gas flow Е нап , kW.
It should be noted thatchangingtemperature of the operating fluid while filling provided that the compressed air is supplied into the engine at ambient temperature Т s = Т о.с = 293 K may be neglectedon the basis of little heat exchange with the environment due to the absence of temperature difference and extremely short filling process (milliseconds), absence of expansion and contraction processes, and present throttling process occurred with a constant gas enthalpycan cause as known [10] a Joule-Thompson effect not exceeding the temperature drop by more than 1 K, i. e., within the accuracy of temperature measuring.
All this allows us to calculate the exergy of the operating fluid enthalpy at the end of the filling process according to its pressureр нап , MPa and temperature Т нап = Т о.с. = 293 К taking into account known dependences [2 и 5]: -specific exergy (2) -air flow at the end of the filling process where G полез = G -G ут , kg/s -mass second compressed airflowefficiently used; G, kg/scompressed air flow supplied into the pneumatic engine; G ут , kg/s -experimentally found mass flow of compressed air for leakage inthe given pneumatic engine, depending on its parameters at the intake; h нап , h о.с. , kJ/kg -air specific enthalpy respectively for the given pressure р нап and temperature Т нап , and for ambient parameters р о.с and Т о.с ; Т о.с , К -ambient temperature; S нап , S о.с. ; kJ/(kg K) -air specific entropy respectively in terms of р нап , Т нап and in terms of р о.с and Т о.с The exergy loss of the operating fluid enthalpy in the intake system is equal to the difference in its exergy levels at the beginning and at the end of the process  1. Diagrams of absolute D вп , kW, and relative D вп , % changes from Е s , exergy loss of the operating fluid enthalpy due to reducing gasdynamic loss of the pressure in the intake system during the filling while operating the pneumaticengine at maximum power N e max with speed characteristics dependant on the level of pressure p s , MPa Fig. 1 represents the increase of the energy loss of the operating fluid in the form of its enthalpy exergy loss in proportion to the increase of the supplied compressed air pressure at the engine intake p s not linearly, but by some increasing intensity. This is quite logical and is explained by the fact that with an increase in the pressure p s of the crankshaft rotational speed n, at which the maximum power N e is achieved, and shifted towards higher frequencies.
Definitely this issue, first, l eads to increasing the hydraulic resistance in the intake and exhaust air ports.

Conclusion
A technique for applying the well-known exergy method to determine the energy loss of the operating fluid of the piston pneumatic engine in the intake system with intake operating pressures p s = 0.5-1.1 MPa at the modes of the maximum possible efficient power N e max at given pressure levels p s according to speed characteristics p s = idem withintake constant temperature Т s = 293 K has been developed.
The scientific significance of this work is the development of a technique for applying the well-known exergy method to determining the energy loss of the operating fluid of the piston pneumatic engine in the intake system. The practical and scientific significance of this work is the following: for a particular type of piston automobile pneumatic engine with a spool-type air distribution the level and patterns of changes in the energy loss of the operating fluid in the form of the enthalpy exergy loss in the intake system depending on the compressed air supplied pressure at the intake in the most important operating range of р s = 0.5 -1.1 MPa at constant air temperature equal to ambient temperature T s = T o.c = 293 K have been determined.
The level of energy loss of the operating fluid (compressed air) in the form of its enthalpy exergy loss in the intake system of a fourcylinder piston pneumatic engine while its operating with various intake pressures are: with intake pressure р s = 0.5 MPa is 13.3 kW (116.5 % of Е s ), with intake pressure р s = 0.7 MPa is 15.0 kW (135.3 % of Е s ), with intake pressure р s = 0.9 MPa is 16.9 kW (150.7 % of Е s ), with intake pressure р s = 1.1 MPa is 19.7 kW (155.6 % of Е s ).