## 1 Introduction  
Because of the rapidly increasing human population around the world, mechanization and energy requirements have increased in many fields such as transportation, agriculture, electric generation and heavy industry. The diesel engine has a very high utilization rate in those fields due to producing high power at low cost when compared with other engine types in the same size range. Depending on the widespread use of diesel engines, the essential research titles related to the diesel engines are improving the performance and reducing the harmful emissions while the fuel consumption is decreasing. Because of exhausting lifetimes of fossil fuels and tightening emissions standards around the world, developing eco-friendly fuels and fuel systems for diesel engines has been keeping up its importance. In diesel engines, there are many studies to improve engine performance and reduce harmful emissions by using alternative fuels [1; 2; 3; 4]. The primary fuel used in diesel engines is diesel as well as many liquid or gaseous fuels are used as alternative fuels. Biodiesel produced from various sources such as vegetable oil, animal fat, waste plastics and waste cooking oils, Tire Derived Fuel (TDF) obtained from waste tires, and various alcohol mixtures are preferred alternative liquid fuels [5; 6; 7; 8; 9; 10; 11; 12; 13]. In addition to the liquid fuels, the gas fuels such as hydrogen, Compressed Natural Gas (CNG), Diesel Methyl Ester (DME), biogas and LPG can be used in diesel engines [14; 15; 16; 17; 18; 19]. Liquefied Petroleum Gas (LPG) and CNG have currently easiest accessibility and usability among the gas fuels. LPG fuel can be used as gas or liquid phase in diesel engines. In the gas phase, it is fumigated in the air intake and the LPG-air mixture is formed in the intake manifold [19; 20; 21; 22]. When LPG is the liquid phase, it mixes with diesel fuel under higher pressure than 0.5 MPa. Liquefied LPG is mixed with diesel fuel and pressurized by the high-pressure pump. The high-pressure pump delivers diesel/LPG blends to the injector [23; 24; 25; 26]. The liquid phase LPG is injected either as LPG-diesel mixture by a single injector or separately by a second injector [27]. In diesel engines operating with LPG in the gas phase, the vapored LPG is taken into the cylinder with the intake air and LPG-air mixture is compressed like in a conventional diesel engine. The LPG-air mixture does not auto-ignite because of its high self-ignition temperature. A small amount of diesel fuel called pilot is injected for ignition of LPG-air mixture. The pilot diesel fuel, which is injected by the conventional diesel injection equipment, normally contributes only a small fraction of the engine output power [28]. LPG usage in the gas phase has been extensively studied. It leads to better engine performance, low particulate and smoke emissions [20; 28; 29]. [PERSON] [19] investigated the effect of diesel/LPG dual fuel in diesel engine on performance and emissions. They designed gas adjustment valve system in order to deliver the LPG with 30% rate to the intake manifold. Experimental results showed that the engine power, engine torque, and specific fuel consumption were improved with dual fuel run. As a result, the dual fuel operation when compared with the single operation, engine moment and power were increased 5.8%, and NOx emission and k factor were decreased 5.9% and 1/9 respectively. Additionally, they showed that CO2 emissions were lower than single fuel mode because CO emissions could not be converted to CO2 in dual fuel mode. [PERSON] et al. [20] studied the performance and emissions of a direct injection diesel engine operated on 100% butane LPG. They added di-tertiary-butyl peroxide (DTBP) and aliphatic hydrocarbon (AHC) to the LPG fuel in order to enhance the cetane number. A stable diesel engine operation in wide engine load range was possible with the cetane improved LPG. A few different LPG blended fuels were obtained by changing the concentration of DTBP and AHC.