Studying the Effect of the Flame Passage on the Convective Heat Transfer in a S.I. Engine

Abstract

Engine optimization requires a good understanding of the in-cylinder heat transfer since it affects the power output, engine efficiency and emissions of the engine. However little is known about the convective heat transfer inside the combustion chamber due to its complexity. To aid the understanding of the heat transfer phenomena in a Spark Ignition (SI) engine, accurate measurements of the local instantaneous heat flux are wanted. An improved understanding will lead to better heat transfer modelling, which will improve the accuracy of current simulation software. In this research, prototype thin film gauge (TFG) heat flux sensors are used to capture the transient in-cylinder heat flux within a Cooperative Fuel Research (CFR) engine. A two-zone temperature model is linked with the heat flux data. This allows the distinction between the convection coefficient in the unburned and burned zone. The experimental time-resolved convection coefficient is then calculated by deriving the moment of flame arrival. The convection coefficient contains all the information of the driving force of the convective heat transfer. The work focusses on the effect of the flame passage on the convective heat transfer. IntroductionA better understanding of the heat transfer phenomena inside the combustion chamber is key towards improving engine efficiency. The knowledge of the in-cylinder heat loss is gaining importance due to the trend of downsizing which increases the engine load and hence the heat loss. According to [1] up to 10% and according to [2] up to 15% of the fuel energy is lost due to in-cylinder heat transfer.Although an important process, recent heat transfer research is lacking. This can be attributed to the difficulty of performing accurate heat transfer measurements inside the combustion chamber. This environment is corrosive and characterized by high temperature and pressure oscillations demanding a robust and fast heat flux sensor.The main mode of heat transfer in an SI engine is convection. Due to the absence of soot particles no radiative heat transfer takes place except in diesel engines [1]. This means that the heat transfer from the working gases to the cylinder walls is mainly driven by the incylinder gas motion and the gas properties of the fluid. The convective heat transfer can be expressed through the convection coefficient (h) if the heat transfer is assumed to be quasi-steady. This coefficient can be calculated using Equation (1). (1)Where q is the measured heat flux, T w the measured wall temperature and T g the gas temperature. The wall temperature and the heat flux are measured and calculated using thin film Resistance Temperature Detectors (RTDs). These heat flux sensors have also been used in [1,3,4,5].The heat transfer models used in current engine simulation software are mostly based on the models of Annand [6] or Woschni [7]. These models have been developed for a one-zone combustion model. This means the gas temperature is a bulk averaged value. The use of a bulk value implies that the ef...