THE THERMAL FACTOR REDUCTION OF THE PISTON IN THE INTERNAL COMBUSTION ENGINE BY THE METHOD OF MICRO-ARC
OXIDATION OF THE HEAD
Asiyа Kamilevna Subaeva1, Khokhlov Alexey Leonidovich2
1The branch of Kazan (Volga region) Federal University " in Chistopol, 422980, Chistopol, Studencheskaya str, 15
2Federal State Budgetary Educational Institution of Higher Education Ulyanovsk State Agrarian University named after P.A. Stolypin
432017, Ulyanovsk, bul'var Novyj Venec,1 subaeva.ak@mail.ru
ABSTRACT
The way of the thermal factor reduction of the piston by the method of micro-arc oxidation of the head is considered. At practically constant temperature in the cold-producing medium and the geometrical dimensions of the piston head the estimation of their thermal factor can be narrowed down to the determination of the magnitude of the average specific heat flux rate through the wall. This magnitude will depend on the average resultant temperature of the exhaust gases in one cycle, on the time average heat transfer coefficient from gases to the wall, and on the wall temperature, or on the temperature difference in the wall, on the thermal conductivity of the material and on the wall thickness. Summarize, the oxide film is formed on the piston head during the micro-arc oxidation process creates a thermal barrier between the aluminum of the piston and the working gases inside the cylinder, which will increase the heat flux in the combustion chamber and decrease the density of the heat flow through the piston into the base chamber . In this case, the heat removal in engine lubrication system from the back of the piston and the cylinder is decreased proportionally to the temperature difference. It has been theoretically established and experimentally confirmed that the formation of an 25 ... 30 µm thick oxidized layer on the head of the pistons allows to reduce their thermal factors by 15 - 16%, and by taking into account the design features of the cylinder-piston group and engine behaviors, to increase its technical and economic performances.
Keywords: internal-combustion engine, piston, piston head, micro-arc oxidation, oxidized layer, thermal factor.
INTRODUCTION
The thermal factor level of the parts of the cylinder-piston group (CPG) can be characterized by the temperature fields and the fields of temperature stresses. However, their availability from experiments and, especially, by means of a calculation for each engine and its various operating conditions, is connected with technical and methodological difficulties, therefore it is practically impossible to determine the fields under service conditions. [1].
METHODS
Under service conditions, it is often resorted to the thermal factor estimation in the engine by the temperature of the exhaust gases tг due to the fact that it synchronously follows the change in the engine behavior and can easily be measured. However, tг does not always reliably indicate the temperature change in the parts of the engine CPG. Considering that the maximum temperature of the cooling surface is the characteristic of the thermal factor in the piston head, then for approximate estimation of the probable change in its thermal factor, it is possible to use a method based on the determination of the heat flux rate through the head, which magnitude is variable over the heat transfer surfaces of the CPG parts.
For the purposes of simplicity, it is possible to consider the concepts of the average specific heat flux rate separately for the piston top near the combustion chamber (Fig. 1).
Figure 1 – The distribution graph of the heat flux rate through the piston head RESULTS
At practically constant temperature in the cold-producing medium and the geometrical dimensions of the piston head the estimation of their thermal factor can be narrowed down to the determination of the magnitude of the average specific heat flux rate through the wall q. This magnitude will depend on the average resultant temperature of the exhaust gases in one cycle tг, on the time average heat transfer coefficient from gases to the wall αг, and on the wall temperature tгсг, or on the temperature difference in the wall (Δt = tгсг – tгск), on the thermal conductivity of the material λ and on the wall thickness δ. In this connection the comparison of two expressions for the determination of the heat flux rate makes it possible to calculate some average value of the wall temperature on the hot side, if we take the specific values of the magnitudes αг, tг , tгск:
( ) ( ),
г г гсг гсг гск
q a t t λ t t
= ⋅ − = δ ⋅ −
(1)
According to Eichelberg's formula, the heat-flux density passing through the piston qп depends on the rate speed, the injection rate, the air parameters in the cylinder inlet, the excess-air coefficient and the coefficient of fullness [2]:
( )
0,5 н,
n s
q B n p
а
= ⋅ ⋅ ⋅ η
(2)
where n - is the engine speed, min-1; ps - is the boost pressure, MPa; ηн - is the coefficient of fullness ; α - is the excess-air coefficient ; B - is the coefficient, depending on the design and engine condition.
The heat-flux density q from the gas to the piston surface in one cycle can also be determined from the equation [3]:
), (
г.ср п.срT
T T
q = α −
(3)where αт - is the heat transfer coefficient from the gas to the piston surface; Тг.ср - is the average temperature of the gas in the cylinder, K; Тп.ср - is the average temperature of the piston top surface, К.
In the equation (1), all the terms are known except for αт, for which determination there are no significant correspondences at present. However, the magnitude αт can be determined using the correspondence obtained by Eichelberg [2]:
,
3
С рТ
К
тТ
=
α
(4)where Ст - is the average piston speed, m/s; р - is the gas pressure in the cylinder, MPa; Т - is the temperature of the gases in the cylinder, K; K - is a coefficient of proportionality, depending on the thrust augmentation ratio of this engine and its design features (K = 2,1).
To determine the heat flux rate through a piston with an oxidized head, we shall consider the heat-transfer process as the heat transfer through the wall consisting of three layers.
What is more, the first layer of thickness δ1 is the oxide film, the second δ2 is the piston head, and the third δ3 is the layer of engine oil on the inner piston surface for its cooling (Fig. 2).
Figure 2 – The heat transfer through the piston with oxidized head Then the following statement is fulfilled:
> > > ,
стг о ов гск
t t t t
(5)where tстг - is the oxide film temperature on the gases side, K; tо - is the temperature of the piston head under the oxide film, K; tов - is the temperature of the piston head from the side of the crankcase under the lubricating oil layer, K; tгск - is the temperature of the outer layer of the lubricating oil, K.
Then the heat flux rate passing through each layer can be expressible as:
for the oxide film:
1
( ),
о о стг о
q λ t t
= δ ⋅ −
(6)
where λо - is the thermal conductivity of the oxide film, W / m2 · K; δ1 - is the oxide film thickness, mm.
2
( ),
д
д о ов
q λ t t
= δ ⋅ −
(7)
where λд - is the thermal conductivity of the piston head, W / m2 · К; δ2 - is the thickness of the piston head, mm.
for the oil layer:
3
( ),
м
м ов гск
q λ t t
= δ ⋅ −
(8)
where λм - is the thermal conductivity of the oil layer, W / m2 · K; δ3 - is the thickness of the oil layer, mm.
Solving equations (6-8) relating to the heat flux, we obtain:
1
,
стг о о
о
t t δ q
− = λ ⋅
(9)2
,
о ов д
д
t t δ q
− = λ ⋅
(10)3
.
ов гск м
м
t t δ q
− = λ ⋅
(11)Adding the expressions (9-11), we obtain:
1 2 3
.
стг гск
о д м
t t q δ δ δ
λ λ λ
⎛ ⎞
− = ⋅ ⎜ + + ⎟
⎝ ⎠
(12)
From this:
1 2 3
.
стг гск
о д м
t t
q δ δ δ
λ λ λ
= −
+ +
(13)
On the other hand, the heat flux density, that the gas gave the piston with an oxidized head, can be determined from the equation:
), (
гокс.ср покс.срокс
T T
q = α −
(14)where αокс - is the heat-transfer coefficient from the gas to the piston surface;
Т
срокс - is the average magnitude of the gas temperature in the cylinder, K;Т
п срокс. - is the average temperature over the piston surface with an oxidized head, K.Determining in an experimental way the average temperature of the head of the oxidized piston and the temperature of the gases in the combustion chamber by means of a calculation, it is possible to determine the coefficient of the heat transfer αокс for a piston with an oxidized head, and then the coefficient of proportionality Kокс.
Laboratory studies of the piston engine of the Ulyanovsk Motor Plant [4-10] have shown that the piston engine has an increased thermal resistance due to an an 25 ... 30 µm thick oxide film on the piston head and the temperature difference between the typical and the piston with an oxidized head rises by increasing piston heater and it is 36 °C at heat to 240 ° C (Fig. 3).
Figure 3 - The temperature change of typical and the oxidized piston head from the heating temperature Thus, oxidation of the pistons allows to reduce their thermal factors by 15…16%.
Due to increasing the thermal resistance of the oxidized piston head, the loss of heat to the engine lubrication system from the total heat is decreased, and the heat loss from the internal surfaces of the piston and the lower part of the cylinder will be nearly proportional to the temperature difference (Fig. 4).
Figure 4 – The distribution graph of the heat flux through the oxidized piston head
As the heat transfer in the combustion chamber is also carried out by radiation [11], it is possible to use a well-known method based on the Stefan-Boltzmann law. The calculated formula of the radiant component of the heat flux has the following form [12]:
4 4
100
г100
стг,
л пр s
T T
dQ = ε ⋅ С S ⋅ ⋅ ⎡ ⎢ ⎜ ⎛ ⎞ ⎟ − ⎜ ⎛ ⎟ ⎞ ⎥ ⎤ Fdt
⎝ ⎠ ⎝ ⎠
⎢ ⎥
⎣ ⎦
(15)where Сs - is the radiation coefficient of an absolutely blackbody, (Сs = 5,67 W / m2 · K4; Тг - is the instantaneous value of the gas temperature, K; Тстг - is the temperature of the oxidized layer, K; S - is the surface area of the radiation, m2; F - is the surface area of the piston, m2; t - is time, s; εпр - is the blackness reduced factor of the radiant and irradiated bodies.
It should be noted that when calculating the heat flux of the radiation through an oxidized piston head, the heat exchange process must be considered as heat exchange by radiation between bodies in the presence of a screen between them. An oxidized layer of the piston head will be as a screen. It follows from expression (15) that the density of the thermal flux of radiation Фго from gases of the combustion chamber to the oxidized layer of the piston head can be written as:
4 4
100г 100стг ,
го пр s
T T
Ф =
ε
⋅С S⋅ ⋅⎡⎢⎛⎜ ⎞⎟ −⎛⎜ ⎞⎟ ⎤⎥⎝ ⎠ ⎝ ⎠
⎢ ⎥
⎣ ⎦ (16)
The radiation flux from the oxidized layer to the piston head Фодп:
4 4
/ ,
100стг 100о
одп пр s
T T
Ф =ε ⋅С S⋅ ⋅⎡⎢⎛⎜ ⎞⎟ −⎛⎜ ⎞⎟ ⎤⎥
⎝ ⎠ ⎝ ⎠
⎢ ⎥
⎣ ⎦ (17)
where ε /пр - is the given coefficient of blackness of the oxidized layer and the piston head.
At the steady heat transfer
Ф
го= Ф
одп i.e.:4 4 4 4
/ .
100г 100стг 100стг 100о
пр s пр s
T T T T
С S С S
ε ⋅ ⋅ ⋅⎡⎢⎜⎛ ⎟⎞ −⎜⎛ ⎟⎞ ⎥⎤=ε ⋅ ⋅ ⋅⎢⎡⎜⎛ ⎞⎟ −⎜⎛ ⎟⎞ ⎥⎤
⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠
⎢ ⎥ ⎢ ⎥
⎣ ⎦ ⎣ ⎦ (18)
Then the heat transfer coefficient by the radiation of the oxidized layer can be defined as:
4 4
100 100 ,
ос s г стг
пос г ос
стг с
С T T
T T
α = ε ⋅− ⋅⎢⎣⎢⎡ε ⎜⎛⎝ ⎟⎞⎠ −ε ⋅⎜⎝⎛ ⎞⎟⎠ ⎦⎥⎥⎤ (19)
where εос - is the given coefficient of blackness of the oxidized layer; εг - is the given coefficient of gases blackness.
Then, the average temperature of the gases in the combustion chamber is defined as:
( ) ( )
/ //
/ /
// //
0,5( ) ,
2,31
г стг г стг
г cтг стг
г стг
г стг
T T T T
T T T
T T lg T T
− − −
= + +
⎛ − ⎞
⎜ − ⎟
⎝ ⎠ (20)
where
Т
стг/, Т
стг// is an initial and final temperature of the oxidized layer of the piston head, K;Т Т
г/,
г// isan initial and final temperature of gases, K.
SUMMARY
Summarize, the oxide film is formed on the piston head during the micro-arc oxidation process creates a thermal barrier between the aluminum of the piston and the working gases inside the cylinder, which will increase the heat flux in the combustion chamber and decrease the density of the heat flow through the piston into the base chamber . In this case, the heat removal in engine lubrication system from the back of the piston and the cylinder is decreased proportionally to the temperature difference.
CONCLUSIONS
For the foregoing reasons, it should be noted that the microarc oxidation of the piston head, as a result of which an an 25 ... 30 µm thick oxidized allows to reduce not only its thermal factors by 15 ... 16%, but by taking into account the design features of the cylinder-piston group and engine behaviors, to increase its technical and economic performances.
ACKNOWLEDGEMENTS
The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University.
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