**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 [1–4].

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 [5–13]. 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 [14–19]. 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 [19–22]. 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 [23–26]. 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 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.

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 NOx 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 NOx 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 NOx 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. NOx 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

fuel consumption were achieved with 25% LPG ratio. In terms of exhaust emissions, NOx and HC emissions decreased with all LPG ratios while CO and CO2 emissions increased.

Lata et al. [32], investigated the influence of hydrogen and LPG addition on the efficiency and emissions of a dual-fuel diesel engine. They showed that the efficiency was increased with the usage of LPG at high loads while HC, NOx, and smoke emissions were reduced. They observed a serious knocking in the test engine at the 70% LPG ratio. The best engine performance was obtained at 40% LPG ratio.

Mirgal et al. [33] studied on the diesel/LPG dual fuel engine. The gas phase LPG fuel was delivered to the intake manifold of a single-cylinder diesel engine. The experiments were carried out at 50% engine load and constant 1500 rpm engine speed. Experimental results were recorded at 35%, 67%, 73% and 90% LPG fuel ratio approximately. As the LPG fuel ratio increased, NOx emissions decreased, and HC emissions increased regularly. Additionally, CO emissions increased at first and then decreased slightly. It was seen that there was a slight decrease in the cylinder pressure due to the increase in the LPG fuel ratio.

LPG in the liquid phase is mixed with diesel fuel and delivered to the high-pressure pump when the liquid phase LPG fuel is used in diesel engines. The mixture of liquid LPG and diesel fuel is injected into the cylinder with the diesel injector at high pressure. The liquid phase LPG can change to the gas phase easily when it is injected into the cylinder because of the low boiling point of LPG. The quick evaporation of the LPG in diesel/LPG blend can improve the atomization of the fuel spray. The increase of LPG content in the fuel blend will decrease the cetane number of diesel/LPG blend, and this will lead to the increase of ignition delay. In addition, the latent heat of evaporation and the Lower Heating Value (LHV) of diesel/LPG blend give a slight increase in ignition delay. 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 [23–26].

Cao 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, NOx, and smoke emissions were achieved using %30 LPG ratio. On the other hand, the best HC emission was obtained with %100 diesel fuel.

Qi 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, NOx 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 grea<sup>t</sup> change in CO emission with pure diesel fuel operation. Smoke emissions decreased gradually, and best smoke emission was achieved by using 40% LPG ratio.

Ma 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 NOx 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 (NOx, 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.
