**1. Introduction**

Vehicle electrification is vigorously promoted to achieve net-zero CO2 emissions by 2050, considering one of the significant contributors of global CO2 emissions comes from road transport. However, even on the International Energy Agency (IEA's) most aggressive scenario toward future renewable society, around 40% of vehicles sold in 2030 worldwide are still predicted to be powered by internal combustion engines [1]. Therefore, in order to practically minimize the long-term CO2 emission, not only vehicle electrification but also thermal efficiency improvement of internal combustion engines is absolutely necessary.

As an effective remedy to improve the thermal efficiency of diesel engines, reduction of heat loss through the engine combustion chamber wall is known to have significant potential. Numerous researches have been done in this field, particularly

on improving the thermal efficiency of diesel engines. It was conducted using a single-cylinder diesel engine [2–5] as well as a wall insertion-type constant volume vessel (CVV) [6–8]. To enhance thermal efficiency in the design of future engines, complete knowledge of the heat loss pathway from combustion gas to cylinder wall is essential.

In order to elucidate the mechanism of the wall heat transfer during the diesel spray flame impingement, a series of parametric study of wall impinging diesel spray flame combining transient wall heat flux measurements and high-speed optical diagnostics was conducted detailed by authors [9–11]. The results indicate how various experimental conditions affect the spray/flame impingement behavior, with considerable heat loss resulting in some cases. Gas flame velocity, contact area, and temperature difference are important factors of affect substantial heat loss. Therefore, identifying local temperature distribution is most needed to clarify the temperature difference near the wall during the combustion period.

Regarding some experimental parameters study [12], combining higher injection pressure resulted in higher heat loss, which is naturally attributed to higher flame velocity impinging on the wall with increased heat transfer coefficient. Therefore, this study aims to investigate the local temperature distribution under different injection pressure conditions. Furthermore, it also attempts to investigate the soot emission (KL Factor) distribution to contribute to the realization of the carbon-neutral future society. We used the two-color method to observe the local temperature and soot emission distribution.
