Compared to other engines of the same size, diesel engines are more economical in addition to their ability to generate high power. For this reason, they are widely used in many fields such as industry, agriculture, transportation, electricity generation. The increasing environmental concerns and diminishing oil resources led researchers to improve fuel consumption and emissions. In this context, the usage of Liquefied Petroleum Gas (LPG) fuel in diesel engines is one of the important research

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... ubjects that has been keeping up to date. This paper investigates the effects of LPG direct injection towards the end of air inlet period on engine emissions and performance characteristics. A four-stroke, air cooled, single cylinder diesel engine was modified to direct injection of LPG for diesel/LPG dual fuel operation. An Electronic Control Unit (ECU) was designed and used to adjust LPG injection timing and duration. LPG injection rates were selected as 30%, 50% and 70% on a mass base. The test engine was operated at 3000 rpm constant engine speed under varying load conditions. Throughout the experiments, it was observed that smoke density significantly reduced on the dual-fuel operation, compared to the pure diesel operation. Carbon Monoxide (CO) and Hydrocarbon (HC) emissions decreased by 30% and 20%, respectively. Brake Specific Fuel Consumption (BSFC) decreased by 8%. Nitrogen Oxide (NO x ) emissions increased by 6% while effective efficiency increased up to 1.25%. 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] . Ciniviz [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 NO x emission and k factor were decreased 5.9% and 1/9 respectively. Additionally, they showed that CO 2 emissions were lower than single fuel mode because CO emissions could not be converted to CO 2 in dual fuel mode. Alam 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. According to experimental results, LPG and only AHC blended fuels increased the NO x emission compared to diesel fuel operation. Experimental result showed that the thermal efficiency of LPG powered diesel engine was comparable to pure diesel fuel operation. In terms of exhaust emissions, the NO x and smoke could be considerably reduced with using the various blend of LPG, DTBP, and AHC. Saleh [21] focused on the effect of propane ratio changes in LPG content on emissions and performance in dual-fuel diesel engines. In the study, LPG with various propane contents was delivered to a diesel engine with EGR capability. The best engine efficiency was achieved with a 40% propane ratio. Depending on the LPG content, high butane ratio led to the decreasing of NO x emissions and high propane content caused the reduction of CO emission as well. In a mixture of 30% butane and 70% propane content, the engine performance remained at the same level as pure diesel fuel. NO x emissions were reduced about 27% at full load in 70% propane and 30% butane mixture. Rao et al. [30] conducted a performance evaluation of a diesel/LPG dual fuel engine. 10%, 20%, 30%, 40% and 50% of LPG were sent to intake manifold of the single-cylinder test engine. Experiments were carried out at constant 1500 rpm engine speed at different loads. The 50% LPG fuel ratio could only be used up to 40% of the engine load. In all LPG fuel mixture ratios, the effective efficiency had increased when it was compared with pure diesel fuel. They proved that smoke emission and specific fuel consumption were reduced gradually while the LPG ratio of fuel mixture was increasing. Ergenç and Koca [31] , studied the usage of LPG in the diesel engines experimentally. They used an LPG injector mounted in the intake manifold. The measurements were performed in 10%, 20% and 25% LPG ratios. The maximum improvements in engine power, engine torque, and specific