Abstract
Cooling losses in internal combustion engines are usually evaluated by using empirical equations such as Woschni’s and Eichelberg’s. Those equations provide heat transfer coefficients at combustion chamber wall surfaces and were originally derived from experimental results. However, those proposers carried outs temperature measurement only at several points in limited areas of combustion chamber walls. In this study, instantaneous surface temperature Tw [K] was measured at 100 points on all the combustion chamber wall surfaces (i.e. the piston, cylinder head, cylinder liner, intake valve and exhaust valve) of a naturally aspirated D.I. diesel engine so that instantaneous local heat transfer coefficients and the instantaneous global heat transfer coefficient could be obtained more accurately.
In the instantaneous temperature measurement, the authors used originally developed thinfilm thermocouples of which measurement error was minimized by choosing a proper body material corresponding to the material of the measured part. Instantaneous local heat flux q [W/m2] was obtained by heat transfer analysis with Tw imposed as the boundary condition. Then, the instantaneous local heat transfer coefficient αg [W/(m2・K)] at each point could be evaluated with Tw, q and the instantaneous gas temperature Tg [K]: αg = q / (Tg – Tw) Tg is mass-averaged gas temperature calculated from in-cylinder pressure p [Pa]. The global heat transfer coefficient αgm [W/(m2・K)] over all the combustion chamber wall surfaces was obtained using area weighted average of αg. The global heat transfer coefficient αgm evaluated by the present approach was compared with those calculated from Woschni’s and Eichelberg’s equations. As a result, the αgm by Woschni’s equation agreed well with the present αgm in the period of rise while the αgm by Woschni’s equation was lower than the present αgm in the compression stroke and higher in the combustion and expansion strokes. The present time-averaged heat transfer coefficient αgm was lower than those calculated from Woschni’s and Eichelberg’s equations between 800rpm and 1600rpm of engine speeds.
Keywords: heat transfer coefficient, D.I. diesel engine, combustion chamber wall, instantaneous temperature, instantaneous heat flux