Assessment of the technical condition of intercoolers for turbocharged internal combustion engines

The presented work considers the analysis of methods and means for assessing the technical condition of charge air coolers in vehicles in operation. Metallographic studies of the fragments of samples, identified local sections of the heat exchanger, have been carried out. Various defects of the internal heat transfer surface, caused by the influence of operational factors, have been revealed. The article also touches upon the problem of imperfection of methods and means of instrumental control of the operating parameters of the heat exchange equipment of road transport. It is proposed to carry out diagnostics of the technical condition of heat exchangers at specialized stands equipped with an instrumentation and apparatus complex. The prospects of new developments in the field of diagnostic equipment for determining the operating characteristics of heat exchangers with the ability to measure the most important parameter - heat transfer are noted. An important addition, confirming the relevance of the chosen direction of research, is the substantiation of the value of the described developments not only as a tool for measuring the parameters of the state of heat exchangers, but also to solve the problem of predicting their residual resource. The results of the conducted research are presented. Using the method of physical modeling, an algorithm has been developed for diagnosing the technical condition of the charge air cooler in operation. Issues of provision with methods and means of diagnosing parameters of heat exchangers of vehicles in operation and repair are considered.


Introduction
Turbocharging units are increasingly used in the d esign of internal combustion engines of mod ern vehicles. Basically, their use causes an increase in engine p ower as a result of increased air d ensity and fuel combustion efficiency. One of the imp ortant elements in the cooling systems of units of internal combustion engines is an intercooler, or charge air cooler [1,2].
On some vehicles, p late-tube air heat exchangers are used in the charge air cooling system. The charge air moves insid e the flat oval tubes, and the atmosp heric air enters the core thanks to the oncoming flow and the fan with mechanical d rive of the blad es from the engine.
During op eration, the cooling cap acity of the heat exchanger d ecreases d ue to contamination of the working surfaces with d ust, oil and soot d ep osits, as well as p olymerization p rod ucts, which create ad d itional thermal resistances that affect the efficiency of the heat transfer p rocess. This, in turn, affects the p ower, reliability, environmental safety and efficiency of combustion engines. To d etect malfunctions of the charge air cooler heat exchanger, it is necessary to d iagnose it. However, the The aim of the work is to imp lement new d iagnostic tools for monitoring and d etermining the technical cond ition of the charge air cooler at the op erational stage, as well as assessing the p ostop erational state of the internal heat transfer surfaces of the heat exchanger.

Materials and methods
Samp les mad e from a charge air cooler of a Volvo car with a mileage of 280,000 km were used as material for stud ies to assess the p ost-op erational state of the internal heat transfer surfaces of the heat exchanger. The heat exchanger was mad e of 3003 aluminum alloy, the chemical comp osition of which accord ing to EN 573-3 is p resented in the table 1. The stud ies were carried out by the method of d estructive testing with a samp le of the most characteristic local sections of the heat exchanger.
For the manufacture of thin sections, three samp les were cut out, the following local areas: at the air inlet to the charge air cooler, in the central p art and in the area before leaving the heat exchanger.
To For the research and d evelop ment of algorithms and d iagnostic tools, a p hysical mod eling method was selected that allows full-scale tests with full-scale samp les of the charge air cooler.
The most imp ortant characteristics of the charge air cooler, the d efinition of which is necessary to simulate the workflow, are: heat transfer (heat flow); costs of cold and hot heat carriers; temp eratures of cold and hot heat carriers at the inlet to the heat exchanger; temp eratures of cold and hot coolant at the outlet of the heat exchanger or their temp erature d rop s; p ressure of cold and hot heat carriers at the inlet to the heat exchanger; p ressure of cold and hot heat carriers at the outlet of the heat exchanger.
Requirements for technical equip ment, selection of p arameters and test method for heat exchangers of vehicles are set out in GOST R 53832-2010. The control of the charge air cooler heat exchangers, for comp liance with the functional ind icators, must be carried out at a stand of the «Thermal wind tunnel» typ e.
The Orenburg State University has a stand for monitoring the technical cond ition of car rad iators, which allows testing water-air heat exchangers. The stand is equip p ed with a hard ware and software measuring and comp utational comp lex that allows you to measure, register and calculate the thermal p arameters of the heat transfer p rocess.

Results and discussion
3.1 Investigation of the post-operational state of the internal heat transfer surfaces of the heat exchanger At the stage of op eration, the p rod ucts are exp osed to various op erational factors that ultimately change the numerical value of heat transfer, as an integral criterion for the p erformance of the charge air cooler. A change, or more p recisely, a d ecrease in heat transfer, lead s to a violation of the p arameters of the working p rocess not so much of the cooler itself, as an element of the turbocharging system, but also of the internal combustion engine. In this case, the nominal d ischarge air temp erature shifts up ward s, and the engine p ower d ecreases. The most common reason for the d isrup tion of the working p rocess of the cooler is the change in the resistance of the cooler on the sid e of the cold heat carrier d ue to the d ep osition of fine and coarse-structured imp urities on the outer cooling surface of the frame. It can be d ust, soil and other d ep osits, as well as foliage, fluff, insect remains, and so on. As a rule, these d ep osits are associated with moisture, oil and fuel d rip s, which makes them resistant to p hysical and mechanical effects when cleaning the heat exchanger with water and comp ressed air. In fig. 1 shows external op erational contamination and charge air cooler faults. The consid ered contamination and d ep osits of the heat exchanger contribute to the formation on the inner surfaces of the cooler of a layer of an oxid e film with a high thermal resistance to heat transfer ( fig. 2). Fig. 2 shows that the rate of scale formation is d ifferent even within the same heat exchanger. Aluminum oxid e films consist of two p arts: an inner and an outer layer ( fig. 3). The inner layer is p ure alumina, while the outer layer contains imp urities of various ions. The maximum thickness of the barrier typ e aluminum oxid e film is ap p roximately 1 micron.
Accord ing to [2], the thickness of the natural oxid e film on aluminum at normal temp erature d oes not exceed 0.01 μm. Even when heated close to the melting p oint of the metal, it cannot reach a thickness of more than 0.2 microns. After the formation of a natural oxid e film on the metal, its further growth can occur only when the oxid ation p rocess is activated by some external factor, for examp le, heating or an electric field . As a result, the natural oxid e film alread y p resent on the aluminum surface is artificially thickened , which hap p ens d uring long-term op eration of the charge air cooler.
These d ata allow us to state that contamination and oxid e d ep osits are p resent throughout the entire volume of the charge air cooler. In ad d ition, the d estruction of the tubes and p artitions of the cooler was revealed . It can be assumed that it was the combination of external and internal d efects that led to the failure of the investigated unit [2].

Development of diagnostic software to determine the technical condition of the charge air cooler heat exchangers during operation
The further d irection of research on the role of d efects in the internal heat transfer surface in changing the technical state of the charge air cooler will be associated with a heat engineering exp eriment to d etermine the op erating p arameters of the heat exchanger with op erating time. For these stud ies, a mod ernization of the test bench for heat exchange equip ment was carried out [3][4][5][6][7][8][9][10]. The need to change the d esign of the stand is associated with the d esign features of the charge air cooler, which is an air-to-air typ e of heat exchangers.
The p rincip le of op eration of the d evelop ed stand is that the heat from a hot coolant circulating through a heat exchanger with a certain initial temp erature, maintained by heating from an external source, is removed to a cold coolant with a constant initial temp erature. A heater with continuously variable p ower control carries out heat sup p ly to the charge air. Cooling air sup p ly through test charge air cooler heat exchanger p rovid ed by a variable sp eed fan. When testing the heat exchangers of the charge air cooler, comp ressed air from the comp ressor station is used as a hot coolant.
During the tests of the charge air cooler, the following was d etermined : -cooling air mass velocity ρW kg / (m 2 × c), which is calculated by the formula (1) (1) where G x -mass air flow, kg /c; F x -heat exchanger frame front area, m 2 ; -heat transfer (heat flux) of the heat exchanger Q, W, d etermined by bench tests and calculated by the formula (2) ; -red uced heat transfer Q p r accord ing to the formula (3) where t rp r и t xp r -reference temp eratures, °С, take charge air coolingt rp r = 120 ° С, t xp r = 20 ° С; -resistance of the heat exchanger on the sid e of the cold heat carrier, kPa accord ing to the formula (4) ; (4)  -resistance of the heat exchanger from the sid e of the hot coolant accord ing to the formula (5), кPa (5) The main result of bench tests and mathematical mod eling was the imp lementation of a new d iagnostic algorithm for the charge air cooler heat exchangers. A fragment of the algorithm is shown in figure 4. Using the cap abilities of the d evelop ed d iagnostic software algorithm, in turn, will make it p ossible to p red ict the resid ual life of the charge air cooler heat exchangers, as well as to d etermine the volume and content of work to restore their op erability.

Summary
Forcing internal combustion engines without changing the overall and weight characteristics in ord er to increase p ower is achieved by burning a larger p ortion of the air-fuel mixture in the cylind ers in one working cycle. The solution to this p roblem required , first of all, the sup p ly of more air, that is, an increase in the amount of fresh charge. With a constant working volume of internal combustion One of the ad d itional structural elements of the vehicle in this case becomes an intercooler, or charge air cooler. Thus, it is fair to assume that charge air cooling is one of the p rogressive d irections of increasing the p ower and efficiency of internal combustion engines.
For the manufacture of thin sections, three samp les were cut out, the following local areas: at the air inlet to the charge air cooler, in the central p art and in the area before leaving the heat exchanger.
To p reserve the structure of the layers of op erational contaminants in various p hase states, the samp les were filled with a p olymer comp osition based on ep oxy resin. This technique mad e it p ossible to p reserve the shap e of the internal p artitions. The samp les p rep ared in this way were subjected to mechanical grind ing and p olishing. The obtained thin sections were examined in an unetched form on a metallograp hic microscop e.
For the research and d evelop ment of the algorithm and d iagnostic tools, a p hysical mod eling method has been chosen that allows full-scale tests with full-scale samp les of the charge air cooler.
In connection with the relevance of the p roblem und er consid eration, the main result of the work should be the imp lementation of new tools and algorithms for the d iagnostic sup p ort of CAC heat exchangers. This will allow monitoring and d iagnosing the technical cond ition of heat exchangers at the stage of their op eration with the p ossibility of quantitatively assessing the main p erformance characteristics.
Using the cap abilities of the d evelop ed d iagnostic software, in turn, will make it p ossible to p red ict the resid ual life of the charge air cooler heat exchangers, as well as to d etermine the volume and content of work to restore their op erability.