Abstract
Keywords D.I. diesel engine, deposit, combustion chamber walls, instantaneous temperature, instantaneous heat flux
Abstract One approach to realise higher efficiency of internal combustion engines is to decrease cooling loss. Useful guidelines for this are provided by clarifying the heat loss condition to the combustion chamber walls. The authors' group has worked on evaluation of the heat loss by measuring the instantaneous heat flux onto the combustion chamber walls using original thin-film thermocouples (TFTs). One of the major factors that affect the heat loss is deposit adhering to combustion chamber wall surfaces especially in D.I diesel engines where a large amount of soot is generated. It was reported that gradients of instantaneous temperature and heat flux fluctuation curves became less steep as a lot of deposit adhered onto the combustion chamber wall surfaces. However, details of deposit influence on the heat loss condition still remain unknown because there are too many things which affect a state of the deposit.
The authors group, as the first step, had carried out the experiment of long duration at constant engine operating conditions to investigate change of the instantaneous temperature and the instantaneous heat flux in the process of deposit adhesion. The measurement had been limited to the cavity side wall. In this study, the measurement was expanded to the piston cavity, the piston top surface and the cylinder head. Furthermore, after the 6 hour long operation, the engine was decomposed so that the state of deposit could be examined and close observation of deposit was made. The thickness of the deposit was measured on flat surfaces such as the piston top and the cavity bottom by a laser displacement meter.
As a result, the amount of deposit strongly depended on the location, and the instantaneous temperature and heat flux changed according to the amount of deposit at the measured point.
For example, in the curves of the instantaneous temperature at the piston cavity surface where much deposit was found due to the direct injection of the fuel, the timing of the maximum value was delayed, the gradient in ascent and descent phases became less steep and consequently the amplitude of the fluctuation decreased. The same tendency was also found in the instantaneous heat flux curves. On the other hand, at measured points on the piston top surface and the cylinder head where deposit was less than 30 micro-metres thick, no change in the curves of the instantaneous temperature and the instantaneous heat flux was observed throughout the experiment of the 6 hour duration except for a slight decrease of peak values.
Although remarkable change in instantaneous values was caused by deposit at some measured points, the mean heat flux over the total strokes hardly changed no matter how thick deposit was. It can be concluded that the total heat flowing into the combustion chamber walls in a cycle is little influenced by deposit.