Abstract
In design of Internal Combustion Engines, the adequate estimation of heat transfer is of special importance. Heat transfer affects the performance, efficiency and emissions from engine. With a specified amount of fuel in the cylinder, increasing heat transfer to combustion chamber walls attenuates the bulk gas temperature and pressure and finally decreases the work.
The objective of this paper is to study the influence of various parameters such as compression ratio, equivalence ratio, spark timing, engine speed, inlet temperature mixture and swirl ratio as well as fuel type on heat transfer through chamber walls of an SI engine.
The KIVA-II multidimensional reacting flow modeling code is used to obtain the instantaneous local heat flux on the combustion chamber surfaces. To increase the accuracy of prediction, a new temperature wall function is implemented into the code.
In the KIVA-II program, the wall temperatures are assumed to be uniform. Therefore, to increase the accuracy it is necessary to determine the temperature distribution in the chamber surfaces (cylinder wall, head and piston). For this purpose, a Finite Element Heat Conduction (FEHC) code is developed and used in an iterative sequence with the KIVA-II code.
The original temperature wall function in KIVA caused the heat fluxes be underpredicted. With implementation of a new temperature wall function, that accounts for compressibility, the predicted heat loss values become more accurate.
In the next step, the effect of changing operational characteristics is investigated using this computational tool. According to results, an increase in compression ratio from 7.8 to 8.5 causes a decrease in heat loss to cooling system. Also, reducing equivalence ratio from 1.2 to 1.0 results in a decrease in heat loss. Retarding the spark timing also reduces the heat loss. Moreover, increasing the inlet mixture temperature and swirl ratio can increase the heat loss to chamber walls. Finally, the effect of changing fuel from gasoline to natural gas (methane) is investigated and influence of various parameters on heat loss through chamber walls of gas-fueled engine is also determined. By changing operational characteristics in gas-fueled engine, similar trends can be observed.