Trasnfernia de Calor en Pistones

8/12/2019 Trasnfernia de Calor en Pistones

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20 H. W. WU and C. P. Chiu

perature. Regarding investigations on the temperature of

pistons in automotive engines with experimental measurement,

there have been many reports (Furuhama and

Enomoto, 1973; Kruggle, Otto, 1971; Procknow, 1978; Yoshida,

1983; Takahisa et al, 1983). Furuhama (1973) and

Yoshida (1983) have made the most pertinent experiments for

our use among the investigators cited. They have studied the

correlations between engine speed, equivalence ratio and

temperature distribution of the piston with different engines

and different operating conditions. In recent years, finite

element method has been used to calculate piston temperature

(Procknow, 1978; Takahisa, et al, 1983; Chin Hsiu Li,

1982 ; Watlance, et al, 1983). Of the investigators cited above,

none of them have addressed the idea of the piston thermal

system considered as one part of the engine energy system.

We, in our studies, have developed a piston thermal system

model from which numerical piston temperature values for

gasoline engines can be computed (C. P. Chiu, et al, 1984). In

this paper, the gas pressure, gas temperature and gas-side

heat transfer coefficients were obtained from thermodynamics

cycle simulation by considering the balance of

the heat flow rate over the full cycle against the heat flow

rate through the metal components to modify the assumed

metal walls temperatures. Furthermore, the temperature

distribution of the piston was analyzed by an axi-symmetrical


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finite element conduction model under various engine speeds

and torque operating conditions. In addition, we address the

variations of piston temperature with change in gas-side heat

transfer coefficients and gas temperature respectively. One

important use of thermodynamics cycle simulation is in

conjunction with experiments where cylinder pressure was

recorded as a function of crank angle over a wide range of

engine operating conditions. For verification, the temperatures

of pistons were experimentally measured with thermocouples.

A special linkage mechanism was designed to connect

the thermocouple lead wires to the recorder.

2. CYCLE SIMULATION FOR

AN ENGINE

General heat transfer equation

The heat transfer model used for each of the five surfaces is

taken to be equivalent to one dimensional.

Q,.= Tg- Tc

1 + X~ + 1

(3)

hg denotes the heat transfer coefficient between the burned

gas and walls which can be estimated by a number of empirical

correlations (Eichelberg, 1939; Annand, 1963; Woschni,

1967). The empirical relation used here is proposed by Annand

(1963), but with the radiation effect additionally

incorporated. The resulting relationship is:

Ko ~-~' ( T ~o.m-b, (pCoV~D~b


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hg = 2.4 10 . . . . [)--- \T~o/ \ ~-~ -/

+a.a(l_e_~L) T#-T 4

Tw- T

(4)

where

Pr=0.7, b=0.75

Ko- 0. 0301W/m" ~

T0- 366.7~

a - Stefan- Boltzman constant

= 5.678 10-~W/m 2- ~

=0.9 for head and piston

- 0.25 for sleeve

A -designates emitted soot number per unit path

length

=2.185"p'f/l+f

L =bore, for the sleeve

-instantaneous distance from piston to head, for

head and piston.

ho is the heat transfer coefficient of coolant, Dittus and

Boelter correlation (Dittus and Boelter, 1930) is adopted for

this study since it considers turbulent forced convection heat

transfer.

ho=0.023 (Re)OS (p~)o.4 (5)

2.1 Calculation of Gas Temperature and Heat

Transfer Coefficients

This analysis is based on the concept that the piston thermal


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system is considered to be one part of the whole energy

engine system. So the current cycle simulation is intended to

obtain gas temperature and gas-side heat transfer coefficient

for the predicted piston temperature and temperature distribution

in the piston. Throughout the cycle, the cylinder is

treated as a variable volume plenum, spatially uniform in

pressure. The gas components are subjected to obey the ideal

gas laws. The engine system is subdivided in three parts,

engine cylinder, intake and exhaust ports. The fundamental

equations may be written as follows.

Conservation of mass

Equivalence ratio

Equivalence ratio is defined as the ratio of actual fuel-air

ratio to stoichiometric fuel-air ratio. Neglecting blowby, the

equation for equivalence ratio (Borman, et al, 1972) is

Mobo ;t/ MzF~ MEEE My \ -

F= l+foJo ~l~fT+1~77+~) dt

Mo rt/ Me Me \

1+/o Jo 1 77+1 7;) dt

(6)

The mass ratio of intake and exhaust process

The process is considered to be isentropic steady flow.

M~ APo RoTo k-1 ~-) -(Po) ] (7)

M=M,.-Mo~, (1) Simplification of the above equations results in the derivative

Conservation of energy--apply to an open transient system

d(MU) zQ-PV. +M,..-Mou, Ho~i (2)


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dt J

P 9u M ~? aR P aU~

7"=B A c)P M V ~ OF R 3F

t,

~U F OU P C

~?T 9P T A

(8)

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