Abstract
Heat loss is one of the main causes of energy losses in modern direct injection diesel engines. This heat loss of the engine occurs during combustion, mainly due to the heat transfer between the impinging spray flame and the piston cavity wall. It is of more critical in small size engines. In order to decrease heat transfer, we need to examine the phenomenon of heat transfer through the combustion chamber walls more fully. To achieve this, we investigated the effects of flame impingement on transient heat flux to the wall. By using a constant volume vessel with a fixed impingement wall, the surface heat flux of the wall at the locations of spray flame impingement was measured with three thin film thermocouple heat flux sensors. The combined effect of impingement distance and injection pressure on the heat transfer was investigated parametrically. The results showed that an increase of injection pressure with longer impinging distance led to an increase in the heat transfer coefficient, which had a dominant effect on local heat flux compared with local temperature distribution. Moreover, we confirmed that the relation between Nusselt number and Reynolds number is a useful measure for describing the heat transfer phenomena in diesel combustion.
Highlights
Despite increasing efficiency and improved fuel economy, heat loss still remains as a major factor, which contributes to substantial amount of energy loss in small size direct injection diesel engines
The portion which was attributed to spray motion reached around 30% at injection pressure of 180 MPa
Local heat transfer coefficients were calculated by using measured heat flux and temperature in both combustion and non-combustion conditions
Summary
Despite increasing efficiency and improved fuel economy, heat loss still remains as a major factor, which contributes to substantial amount of energy loss in small size direct injection diesel engines. Heat loss reduction, which affects engine efficiency, has been of a challenging task. This heat loss occurs in various ways such as cooling loss, exhaust loss, and friction loss. Of these, cooling heat loss through the combustion chamber wall is the most significant one. A full understanding of the heat loss mechanism from combustion gas to cylinder wall would be needed to maximize thermal efficiency in the design of future engines.
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