# Combustion and Emission Characteristics of a Biodiesel-Hydrogen Dual-Fuel Engine
## 3 Results
These two peaks or trends related to heat release are associated with the combustion of fuels with different reactivity, as well. Figure 4 shows the results of determining combustion stages. The first observation is the fact that the hydrogen content practically does not affect the ignition delay time in the engine. For the entire range of hydrogen share, ignition delay was constant. This was due to the high auto-ignition temperature of hydrogen and its participation in the mixture with air, despite the fact that it reduces the pressure of fresh charge before ignition, it does not affect the ignition delay. Another important stage of combustion is a time of 50% of mass fraction burn or 50% of heat release. This combustion stage is evaluated primarily in spark-ignition engines, where the combustion process is determined by the kinetic combustion phase. In the analyzed case, despite the fact that it is a compression-ignition engine, as already mentioned the addition of hydrogen moves the combustion system towards the isochoric process and this brings the engine closer in this respect to the spark-ignition one. The results obtained are particularly important for large hydrogen proportions, where the diffusion combustion phase is minimized. In the studied case, along with the increase in the H\({}_{2}\) share, the value of the 50% heat release angle approached TDC. According to the literature, the engine reaches its highest efficiency as 50% of heat is released about 7-10 deg after TDC. In the analyzed cases, for the engine powered by B100 50% of Q\({}_{\rm norm}\) there was 14.5 deg after TDC and for the engine powered with 38% hydrogen share it was only 5.5 deg after TDC. Analyzing the duration of combustion, it was found that with increasing hydrogen content in the combustion process, the duration of combustion decreases. With 38% hydrogen, the duration of combustion decreased from 69.2 deg for biodiesel to 55.7 deg. Shortening the combustion time has affect in reducing heat losses to the engine cylinder walls. This effect has been confirmed by increase in engine efficiency with the participation of hydrogen in the combustion process. Figure 5 shows the results of thermal efficiency for a biodiesel-hydrogen engine. It was found that for all analyzed hydrogen shares a higher thermal efficiency of the engine was obtained compared to the engine powered with biodiesel. The highest efficiency value was obtained for a 21% hydrogen  
Figure 4: Combustion stages: ignition delay, time of 50% of heat release and combustion duration. powered engine and it was 41%, which was 7% more than for a biodiesel engine. For the highest share of hydrogen, a slight decrease in efficiency was noted, which may be caused by too early ignition of fuels, which can be clearly seen in Figure 2. For this energetic share of hydrogen, a 50% of the heat released is already too close to TDC, which does not favor the conditions for obtaining the highest efficiency of the engine [25]. Figure 5 shows the uncertainty intervals for determining the thermal efficiency, as well. In the IC engine, the cycle-by-cycle variation of engine work cycles is an important problem. Analyzing the repeatability of engine cycles, the stability of the combustion process in the engine can be determined. The unrepeatability of IMEP (COVIMEP) was used as a parameter to determine the cycle-by-cycle variations. The COVIMEP is directly related to the investigated combustion stability. The COVIMEP was calculated based on set of IMEP values from 200 following work cycles of the test engine. The results of the analysis are shown in Figure 6.  
The analysis of the repeatability of IMEP shows that for the majority of analyzed cases for hydrogen co-combusted with biodiesel, the COVIMEP value was within the acceptable range for  
Figure 5: Thermal efficiency of test engine. Figure 6: COVIMEP (**a**) and normalized heat released for B100, BH:11 and BH:38 (**b**). industrial stationary engines. For industrial engines, COVIMEP below 5% is used as the limit value [25].