Incylinder heat transfer and friction analysis in pressurised motored compression ignition engine

Abstract

An experimental study was performed to investigate the total mechanical friction and in-cylinder heat transfer from a pressurised motored engine. The mechanical friction study was done on a four cylinder engine, whereas heat transfer experiments were conducted on a single cylinder version of the same engine model as the four cylinder, converted in the same project. The pressurised motored setup was modified from its conventional configuration to allow the engine to be run on gases other than air. Argon and its mixtures with air were used as the working gas. This method made it possible to test the motored engine at peak bulk gas temperatures up to 1200 , around 600 higher than what is expected from conventional motoring using air. An investigation of several engine metrics in relation to the working gas was carried out. It was found that the location of peak in-cylinder pressure and location of peak bulk gas temperature are advanced from TDC by a magnitude that is directly proportional to the peak bulk gas temperature. The summation of the losses of heat and blow-by were found to show a linear increase with an increase in the peak bulk gas temperature, whereas the losses associated with the pumping of the gas showed a decrease with using gases of higher ratios of specific heats. The mechanical friction of the pressurised motored engine was found to be insensitive to the bulk gas temperature, and its effect on the combustion chamber surface temperature. It was hypothesised that this result is probably due to the gas pressure phasing with crank angle of the pressurised motored engine, which exhibits the peak gas pressure load very close to TDC where the piston lateral thrust is small. With the devised setup, the thermal load on the motored engine could be studied independently from the gas pressure load, however phasing of gas pressure with crank angle could not be changed. For the study of the transient component of heat flux from the combustion chamber, two fast response thermocouples of the eroding type were installed; one at the valvebridge, and another at the cylinder periphery in the squish region. The methods used for converting the temporal surface temperature measurements to heat flux were the traditional Fourier method and the Impulse Response method. The Impulse Response method was applied using basis functions obtained from a two-dimensional finite element study of the eroding thermocouple. This analysis allowed an evaluation of the heat flow that occurs in three different types of eroding thermocouples based on Zirconia, Stainless Steel and Aluminium. It was found that significant twodimensional effects occur at the surface of the Aluminium and Zirconia based thermocouples, whereas only minute two-dimensional heat transfer was noted for the Stainless Steel based thermocouple. A parametric in-cylinder heat flux study was conducted, where it was found that the transient component of heat flux increases with an increase in engine speed and peak in-cylinder pressure. An increase in the transient component of heat flux was also noted with using gases of higher ratios of specific heats; however it was found that the increase in heat flux experienced with the range of gases tested was not as large as initially expected. Results from this experimental session were compared to existing zero-dimensional and one-dimensional heat flux models. It was found that zero-dimensional, one-zone models of the Annand and Woschni type are able to determine the cycle-averaged total heat flux, but unable to predict the temporal heat flux curve. On the other hand, one-dimensional heat flux models derived from the one-dimensional unsteady energy equation in the boundary layer showed very good correlation with the experimental heat flux in terms of the temporal variation, but underpredicted the magnitude.