## 1 Introduction
Addition of the LPG in diesel fuel can accomplish a good spray atomization and contributes the fuel-air mixing process, however, the high proportion of LPG in the blends may induce engine knock or combustion noise [23; 24; 25; 26]. [PERSON] et al. [23] studied comparison of the LPG and diesel fuels in diesel engines. The LPG in the liquid phase and diesel fuel were transferred to the high-pressure pump as a mixture. LPG-diesel fuel mixture injected into cylinder between 180 and 260 bar pressure by a common injector. They performed experiments with %100 diesel, %10 and %30 LPG ratios. They observed that engine power and torque remained the same level in used fuel ratios under constant 1800 rpm speed. The best CO, NO\({}_{\rm x}\), and smoke emissions were achieved using %30 LPG ratio. On the other hand, the best HC emission was obtained with %100 diesel fuel. [PERSON] et al. [24] investigated combustion and emission characteristics of diesel/LPG dual fuel in a compression ignition engine. They mixed diesel fuel and liquid phase LPG with 10%, 20%, 30%, and 40% ratios and injected with a common injector. The tests were carried out at engine speeds of 1500 rpm and 2000 rpm under between 15% and 90% engine loads. In all load and cycling conditions, it was observed that the cylinder pressure decreased while the LPG ratio increased. In terms of emissions, NO\({}_{\rm x}\) decreased, and HC increased at both engine speeds and at all loads when the LPG ratio was increased. The main reason is that the cylinder gas temperature is lower for blended fuel operation at the low engine load with the increase of LPG mass flow rate, and the more aromatic hydrocarbons in the LPG content, which are too stable to burn out entirely. On the other hand, a good spray can reduce blended fuel close to the cylinder chamber wall, thus HC emissions greatly reduces. There was not a great change in CO emission with pure diesel fuel operation. Smoke emissions decreased gradually, and best smoke emission was achieved by using 40% LPG ratio. [PERSON] et al. [26] studied the effect of diesel and diesel-propane blends on fuel injection timing in a single cylinder compression ignition engine. Propane rate, maximum heat release rate, premixed heat release, maximum cylinder gas temperature and NO\({}_{\rm x}\) emissions increased for the same engine speed, engine load, and injection advance while total combustion time, CO, HC, and smoke emissions reduced. The objective of this study is to observe the effect of diesel/LPG dual fuel, on engine performance and exhaust emissions of a DI small diesel engine at constant engine speed and different loads. For this purpose, a conventional small diesel engine was converted to direct injection diesel/LPG dual fuel engine. The test engine was operated at constant 3000 rpm speed and different engine loads changing from 500 to 1500 W. For each fuel blend and engine load, the impact of LPG direct injection on a single cylinder diesel engine was investigated on engine performance (effective efficiency, fuel consumption, BSFC, EGT) and emissions (NO\({}_{\text{X}}\), HC, CO, smoke). Although many studies of LPG/diesel dual fuel have been found in the literature, these studies usually show that LPG is injected into the intake manifold and sent into the cylinder together with the intake air. It is believed that this study could help fill the gap in the literature about the direct injection of LPG on diesel engines.