# Combustion and Emission Characteristics of a Biodiesel-Hydrogen Dual-Fuel Engine
## 3 Results
In the tests, the limit value was exceeded by 0.5% only once for the 11% hydrogen share. Figure 6b shows a different nature of changes and repeatability of normalized heat release. It can be stated here that the share of hydrogen does not significantly affect the start of combustion. Figure 6 shows the uncertainty intervals for determining of the unrepeatability of IMEP, as well. As shown in Figure 4, it does not change the ignition delay time. From the example at Qnorm curves it can be seen that with the increase of the hydrogen content, the instability in the developed combustion process phase increases. Combustion processes with a low heat release rate will be less favorable to NOx emissions but will be a source of soot emissions. A lower value of heat release rate cause also a lower combustion temperature. A high rate of heat release, accompanied by high temperature in the combustion chamber promote an increase in NOx emission and decrease in soot emission. By analyzing the repeatability of engine work cycles, the distribution of IMEP values was presented. The results are presented in Figure 7.  
The presented distributions show that up to 5% of hydrogen, IMEP values are centered around the average value in a relatively narrow range. A similar relationship results from the COVIMEP analysis where for this range of H2 share the value of this parameter changed in the range of 1%. A significant increase in COVIMEP was observed for larger H2 shares. Analyzing the histograms of the IMEP distribution it can be said that with increase in hydrogen energetic share an equalization of distribution occurs with less visible dominant value of IMEP. This was due to prolonged combustion cycles that have low IMEP values. Exhaust emissions including soot emissions were measured in a biodiesel and hydrogen engine tests. For industrial engines such as those used in the tests, exhaust emissions are reported in relation to kWh. The obtained results show that in the tested engine, the increase in the energy share of hydrogen was followed by an increase in specific emission of total hydrocarbons (THC). Although the addition of hydrogen improves the quality of combustion of biodiesel hydrocarbons, the increase in THC emissions may be due to the increasing cycle-by-cycle variation in which the oxidation process may not take place completely. The highest increase in THC emissions was for 38% of H2 and was higher than obtained by combustion of biodiesel by 26%. Larger differences in emissions were recorded for NOx. Along with the increase in the H2 share, nitrogen oxide emissions increased in the whole range. The similar nature of the influence of hydrogen on the emission of nitrogen oxides is confirmed by literature data [26; 27]. For the combustion of pure biodiesel, the specific emission of NOx was 3.92 g/kWh and for the 38% hydrogen share it increased to 10.4 g/kWh, i.e., there was an increase of over 2.5 times. A way to reduce nitrogen oxide emissions, as the literature results show, is to use exhaust gas recirculation [19]. Figure 7: Distribution of indicated mean effective pressure (IMEP) for different energy shares of hydrogen. Regarding CO and CO\({}_{2}\) emissions, according to the assumptions, their emission decreased with an increase in the share of H\({}_{2}\), because the hydrocarbon fuel was replaced by a fuel without carbon in its structure. This fact is reflected in CO\({}_{2}\) emissions. The decreasing CO emission is also the effect of the described phenomenon but also because the hydrogen accelerates the combustion process, increases the phase of kinetic combustion and thus creates better conditions for complete and total combustion. The share of 38% hydrogen caused a more than 2.4-times decrease in specific CO emissions. Soot emission is a very important factor in allowing a compression ignition engine to be used. The combustion chamber of the compression-ignition engine has very rich and very lean fuel zones. Fuel-rich zones are responsible for soot emissions. Lack of oxygen promotes the formation of solid particles.