Determination of the variable temperature fields in the cylinder head of a spark ignition engine.

Pinca--bretotean, Camelia ; Josan, Ana

Abstract: This paper aims to experimentally determine the variable temperature fields on the surface and in the superficial layer of the cylinder head of a spark ignition engine, Dacia brand, 1.6L. Highlighting the variable temperature fields is done by diagrams used to conduct the research on the action of the thermal stress producing thermal fatigue. Determination of temperature fields in the cylinder head will enable decisions on improving the heat transfer, in order to reduce the thermal stresses.. The need to optimize the thermal regime of the engine derives from the fact that having a high thermal regime has adverse consequences on the operation of the parts belonging of the engine combustion chamber.

Key words: temperature, chart, heat, cyclic, variable

1. INTRODUCTION

The machine parts, components of the automotive engines, participate in the work cycle, characterized by varying temperature. They take an important share of the heat developed in the combustion chamber and send it to the coolant. Depending on the amount of heat produced by the engine operation, on the thermal resistances and on the opportunities to reduce the heat and the coolant, temperature fields are creating in certain areas of these machine parts, (Botean, 2005; Nagy et al., 2010). The temperature afferent to these fields varies greatly over time, and it is transmitted in the form of oscillations on the surface of the machine parts that participate in the heat transfer inside the engine combustion chamber. In accordance with the performance of the engine working cycle, the cyclic variation in temperature has a speed of the order of seconds or even tenths of a second. These temperature fields generate the occurrence of cyclic thermal stresses overlapping the mechanical tensions. But, the thermal stress values can be higher than the mechanical ones, and can lead to cracks on the surface and in the surface layer of the parts, specific to the thermal fatigue cracking phenomenon. This phenomenon is especially pronounced to the engines operating in very different thermal regimes, and where the part lifetime is higher. The analysis of research on heat transfer in the cylinder head of the internal combustion engines showed that the number of papers published until now is relatively small, without reaching satisfactory results yet, although the engine heat transfer has been studied long enough, (Botean, 2005; Euamoto et al., 1987, Gardynski, 1995). The most studied machine parts, components of the internal combustion engines, are the pistons, valves and cylinders, (Gardynski, 1995). Likewise, there is a customization of the experimental studies on the types of engines, thus trying to improve the heat transfer for reducing the thermal stresses which are the main stresses responsible for the thermal fatigue cracks, (Botean, 2005; Nagy et al., 2010). The need to optimize the engine thermal regime derives from the fact that having a high thermal regime has adverse consequences on the functioning of the parts belonging of the engine combustion chamber, (Botean, 2005). This requires the correlation of the heat transmitted through the surface of the parts with the engine output, which is the main factor which the thermal stresses level depends on. This paper aims at presenting the experimental determination of the principle of variable temperature on the surface and in the superficial layer of the cylinder head of a spark ignition engine, Dacia brand, 1.6L. Highlighting fields is done by varying the temperature charts that allow maximum temperatures in the cylinder head knowledge. This will allow decisions about optimizing heat transfer for thermal applications of these pieces. Determination of temperature fields in the cylinder head will enable decisions on improving the heat transfer, in order to reduce the thermal stresses, (Euamoto et al., 1987). The importance of this paper derives also from the fact that the heat transfer in internal combustion engines is a complex process, characterized by a large number of quantities involved, which varies over time and space, making it difficult to calculate the heat on an engine operating cycle. Also, the theoretical study of the heat transfer raises questions on specifying the boundary conditions and assessing the heat that passes through the machine parts involved, as share developed in the combustion chamber. The need to optimize the thermal regime of the engine derives from the fact that having a high thermal regime has adverse consequences on the operation of the parts belonging of the engine combustion chamber. This requires the correlation of the heat transmitted through the surface parts with the engine output, which is the main factor on which the thermal stresses level depends on, (Botean, 2005).

2. THE VARIABLE TEMPERATURE FIELD FORMATION MECHANISM

During the engine operation, in its cylinder head occur temperature differences, [1]. The area where these temperature variations are relatively high is the bridge between the intake and the exhaust valve holes. In this area, radial temperature gradients occur (parallel to the cylinder axis), and the centre of the bridge between the valves is subject to cyclical variations in temperature, fig. 1.

[FIGURE 1 OMITTED]

The center deck of valves--the zone is located in section 1--the highest temperature occurs at the end of the combustion process. The highest temperature difference at the end of admission is between points 1 and 2, and between points 1 and 3 the difference still reaches the highest value at the end of the admission, but less than that between points 1 and 2. Temperature oscillation amplitude gets lower to reaching the cooling fluid. The highest amplitude of these oscillations occurs during the engine operation in overload, (Botean, 2005). Following this analysis, it was established the area where the experimental measurements are going to be made.

3. EXPERIMENTAL MEASUREMENTS

The determination of the variable temperature fields is experimentally realized by fitting thermocouples in the wall of the Dacia engine cylinder head, in the area of the intake and exhaust valves, fig. 2.

[FIGURE 2 OMITTED]

For this purpose, the engine cylinder head has been properly prepared by detecting the areas where the temperature difference is relatively large. In this area, we made channels through which some thermocouples were inserted. The positioning of the channels for fitting the thermocouples is shown in fig. 3.

[FIGURE 3 OMITTED]

The thermocouples used are of K type, chromel (+), alumel (-) alumel, and the measurement field are -50 ... 900[degrees]C, (Euamoto et al., 1987).

[FIGURE 4 OMITTED]

We used three thermocouples, fitted near the intake and exhaust galleries, and in the area of the valves bridge, fig.4. Through these temperature diagrams, we are going to determine the level and evolution of the thermal stresses mainly responsible for the occurrence of the thermal fatigue. These thermocouples are connected to a data acquisition system which allows recording data during the engine operation, (Grigorescu et al., 2010). The software enables the system to store the experimental data as databases, from where they are processed and convened into temperature charts. The level and evolution of the thermal stresses caused by the temperature fields are determined by the diagrams recorded, representing the cyclical variations of the variable temperature fields in the wall of the engine cylinder head, respectively from the area of the bridge located between the intake and exhaust valves. Diagram obtained experimentally observed the existence of the temperature peaks, fig.5. Knowledge of these peaks allows optimizing heat transfer temperature in this piece.

[FIGURE 5 OMITTED]

Optimization of heat transfer decreases the thermal stress level, the main responsible for the phenomenon of thermal fatigue. On the other hand, these experiments are used to study the resistance at thermal fatigue of the cylinder head of a spark ignition engine. The findings may open the way for creating materials with high resistance to thermal fatigue, allowing the development of high performance parts fitting in the automotive engines.

4. RESULTS

Experimental results allow: knowing the limits of variation temperature internal combustion engine cylinder head in order to study heat transfer in the body engine; a high temperature has adverse consequences on the functioning parts that separate the engine combustion chamber. This requires correlation of surface heat transmitted through the power output of engine parts is the main factor that depends on the heat strain.

5. CONCLUSIONS

The experimentally obtained temperature charts confirms the existence of variable temperature fields in the area between the intake and exhaust valves of the spark ignition engine cylinder head. The profile of these charts confirms the rapid variation in time of the temperature, which is transmitted as oscillations on the surface of the cylinder head. The temperature variation chart shows that its rate of change is of the order of seconds, consistent with the engine working cycle. The temperature charts allow the evaluation of the variable temperature fields and the determination of the associated thermal stresses. Their knowledge allows us to make a comparison with those determined by numerical methods, commonly used in the design of the machine parts fitted in the internal combustion engines.

6. REFERENCES

Botean, A (2005). The study of the thermo-mechanical, stresses spark ignition engine using modern method of study, Research rapport, Td2 cod CNCSIS 230, Bucharest

Euamoto, Y.; Fumhama, S.; Takai, S.(1987). Heat transfer on the ceramic combustion chamber wall of diesel engine, Institute of Technology, Japan, 1987

Gardynski, L.(1995). Investigation of temperature fluctuations of diesel engine piston, Technical University of Lublin, Poland, pp.108-116

Grigorescu, C.; M.; Moraru, S.; A. & Badea, M. (2010). Smart Data Acquisition Software Used in Industrial Monitoring Systems, Annals of DAAAM for 2010 & Proceedings of the 21st International DAAAM Symposium, 20-23rd October 2010, Zadar, Croatia, ISSN 1726-9679, ISBN 978-3-901509-73-5, Katalinic, B. (Ed.), pp. 0103-0104, Published by DAAAM International Vienna, Vienna

Nagi, M.; Carabas, I. D.; Ilies, P. & Ghita, E. (2010). Research Regarding Thermal and Hydrodynamic Performance Levels of Surfaces with Sinuous Fins for Automotive Heat Exchangers, Annals of DAAAM for 2010 & Proceedings of the 21st International DAAAM Symposium, 20-23rd October 2010, Zadar, Croatia, ISSN 1726-9679, ISBN 978-3-901509-73-5, Katalinic, B. (Ed.), pp. 0459-0460, Published by DAAAM International Vienna, Vienna