**1. Introduction**

It is a well-known fact that a large amount of the international transport of various goods and materials is carried out by the sea. However, it is also necessary to emphasize such reality that the sea transport is characterized by a high fuel consumption as well as by production of a large volume of the harmful gaseous emissions. There is often neglected a relevant fact that the ship transport causes more serious and more dangerous air pollution than the standard motor vehicles. With regard to these negative circumstances, the International Maritime Organization (IMO) determined the new internationally valid regulations in order to reduce the increasing amount of the most dangerous air pollutants, namely the sulphur oxides and the nitrogen oxides.

The diesel engines installed in the trans-continental ships are burning the heavy fuel oils (HFO) both in the main driving engines and in the auxiliary diesel engines. This kind of fuel produces a large amount of the harmful gaseous emissions. In the view of the above mentioned facts, it is necessary to look for other solutions, such as biodiesel. Biodiesel is a renewable natural fuel source with only a minimum content of sulphur and aromatic hydrocarbons, as well as it is characterised by a high value of the cetane number and biodegradability [1–3]. At the same time, it does not require constructional modification of the diesel engine itself.

Various studies were elaborated in order to investigate the real environmental impacts of the gaseous emissions caused by biodiesel fuel. The results gained from the performed studies show a positive fact that the values of sulfur, CO emissions, unburned hydrocarbons, and particulate matter were significantly reduced in the exhaust gases compared to the standard diesel fuels. Biodiesel is soluble in normal diesel oil and for that reason it can be combined with diesel oil in any proportion. All the studies concerning application of biodiesel fuels were focused only on diesel engines working in motorcars [4–10]. However, it is also necessary to investigate gaseous emissions which are emitted from the auxiliary diesel engines installed in ships as diesel-powered electric generators. Another considerable problem is a lack of the scientific research studies describing the relation between the NO2 and NO emissions that are caused by combustion of the fuel mixture "biodiesel–diesel oil" [11–14].

The main task of the performed research was to analyse the influence of various mixtures of biodiesel and diesel fuels on the combustion process and on the NOX emissions in the case of diesel combustion engines that are installed in the ships.

### **2. Experimental Engine and Measuring Specifications**

### *2.1. Experimental Engine and Diesel Mixtures*

The testing experimental engine, which was applied during realisation of the experiments, is an auxiliary six-cylinder diesel engine (Figure 1). The technical data of this engine are presented in Table 1. The individual diesel fuel mixtures were created by mixing of the ULSDF (Ultra Low Sulphur Diesel Fuel) with the biodiesel using various mixing ratios. The tested engine was connected to a test stand [15–17].

**Figure 1.** Experimental engine.


**Table 1.** Technical data of the engine.

The ULSDF, which was used during testing, contained less than 10 ppm of sulfur. Parameters of the applied biodiesel are in accordance with the European standard EN 14214 (Table 2).


**Table 2.** The ULSDF and biodiesel specifications.

The tested fuels were obtained by mixing of the ULSDF with the biodiesel, applying various mixing ratio values, namely, 0%, 30%, 70%, and 100%, using the following marking: BU (0:100), BU (30:70), BU (70:30), and BU (100:0), respectively, whereby the letter B means biodiesel and the letter U means the ULSDF. There were also tested diesel engine fuels with other ratios between the biodiesel and the ULSDF, but with regard to the main goal of this study, the above-mentioned fuel mixing ratios were the most suitable. The first type of the tested fuel was the pure ULSDF and the last tested fuel was the pure diesel oil. The fuel consumption of the tested engine was measured for each one of the fuels. The operational conditions during the measuring process are summarized in Table 3.


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