# Combustion and Emission Characteristics of a Biodiesel-Hydrogen Dual-Fuel Engine
## 1 Introduction  
Compression ignition engines have been criticized for some time because of their emission of toxic exhaust gas components. However, thanks to new technologies, it is possible to significantly reduce the harmful impact of these engines on the natural environment by keeping an emission of exhaust gases at a very low level [1, 2]. In recent years, many compression-ignition engines have been implementing a dual-fuel combustion strategy using alternative fuels [3, 4, 5]. In the first solutions, natural gas was used as an alternative fuel, and diesel fuel was only a source of ignition for the gas-air mixture [4]. Such engines allowed to significantly reduce soot emissions from exhaust gas by eliminating the diffusion combustion phase. In other solutions, alcohols were used as complementary fuel to diesel fuel [6, 7, 8]. Alcohols due to the oxygen content in their structure greatly reduce soot emissions. Evaporating alcohol in the intake manifold and during the part of compression stroke reduces the temperature of the fresh charge and thus reduces nitrogen oxide emissions. There are proposals to completely eliminate fossil fuel to power a diesel engine. The growing share of fuels from renewable sources makes it possible to replace diesel by biodiesel. Biodiesel is also co-combusted with other alternative liquid fuels as alcohols or gaseous fuels such as biogas, compressed natural gas (CNG) and hydrogen. This gives the opportunity to change the combustion process and thus to improve the engine operating parameters and exhaust gas emissions. The results indicate that the use of CNG in a dual-fuel engine provides opportunities to reduce nitrogen oxides NOx and soot simultaneously [9]. Soot elimination from a compression-ignition engine can also be done by use of catalytic converters that affect combustion chemistry and the efficiency of this process [10]. Very interesting seems tobe hydrogen as a fuel without carbon in its structure and thus not generating carbon dioxide in the combustion process [11]. A hydrogen cannot be used alone as a fuel for the compression-ignition engine due to the high value of the auto-ignition temperature [12; 13]. In engine applications, it is usually co-combusted with other fuels such as diesel. Biodiesel from various raw materials is used in research. In [14], pomegranate seed oil biodiesel (POB) was used, which was co-combusted with hydrogen in a compression-ignition engine. It was found that compared to diesel, this POB combustion negatively affected engine performance and specific fuel consumption. These parameters as well as exhaust gas emissions have been improved after the addition of hydrogen. A reduction in carbon monoxide (CO) emissions was obtained, while a slight increase in NOx emissions was found. Biodiesel has a higher cetane number compared to diesel. In some applications this may cause pre-ignition. This property can be changed by using diesel blends instead of biodiesel alone. In [15], a mixture of diesel and biodiesel (B10-10% is biodiesel and 90% and B20-20% is biodiesel and 80%) was used, which was co-combusted with hydrogen. In both cases, there was also a decrease in engine performance and an increase in carbon dioxide (CO2) and NOx emissions with reduced CO emissions. It turns out that the biggest disadvantage of using hydrogen for co-combustion with biodiesel is the increase in nitrogen oxide emissions due to the higher temperature of combustion in the engine's cylinder [16; 17]. However, for small and medium loads, replacing diesel with biodiesel and co-combustion it with hydrogen with a large share of exhaust gas recirculation (EGR) contributes to reducing NOx emissions and keeping a constant level of soot emissions [18]. The share of hydrogen when combusting biodiesel reduces the combustion time and has a positive effect on engine efficiency. In a paper [19] are presented effects of replacing diesel fuel with waste cooking oil biodiesel and operating the engine at higher EGR rates.