# Combustion and Emission Characteristics of a Biodiesel-Hydrogen Dual-Fuel Engine
## 4 Conclusions  
The paper presents the results of the assessment of the combustion process, combustion stability and exhaust emissions in a dual-fuel diesel engine fueled with biodiesel and hydrogen. During the research the part of biodiesel was replaced with hydrogen up to 38% of its energetic share to obtain constant load of the engine. The engine was powered by hydrogen through the electronic fuel injection system to the intake manifold and biodiesel was injected directly into the combustion chamber. Based on the results obtained, the following conclusions were drawn regarding the evaluation of the combustion process:  
* An increase in the share of hydrogen causes an increase in the maximum combustion pressure value;
* Increase in H\({}_{2}\) energetic share cause a reduction in the diffusion combustion phase in favor of an increase in the kinetic phase;
* For the 38% hydrogen share, the second peak on the HRR curve was higher than the first peak of combustion which is typical for engines;
* The addition of hydrogen moves the combustion process closer to the isochoric one and this brings the engine closer to the spark ignition engine combustion;
* The share of hydrogen practically did not affect the engine ignition delay time, while the duration of combustion decreases. With 38% hydrogen, the duration of combustion decreased by 25% compared to the biodiesel engine;
* The addition of hydrogen improving the thermal efficiency of the engine, the maximum value was obtained for a 21% hydrogen share;
* Instability of biodiesel and hydrogen fueled engine operation is within the allowable range for industrial engines (COV\({}_{\text{IMEP}}<5\%\)).  
Conclusion regarding exhaust emissions:  
* With the increase of hydrogen energy share, there was an increase in specific THC emission. The highest increase in THC emissions was for 38% H\({}_{2}\) and it was higher than that obtained for biodiesel fueled engine by 26%;
* H\({}_{2}\) contributed to an increase in nitrogen oxide emissions in the entire range of its share, for 38% hydrogen share an increase in specific NO\({}_{\text{x}}\) emission of over 2.5 times was noted;
* The share of hydrogen caused a decrease in CO and CO\({}_{2}\) emissions, which is normal, for a 38% share of hydrogen there was a 2.4 times decrease in specific CO emissions;
* The share of hydrogen has a very beneficial effect on soot emission because its 2% energy share has caused a reduction of its emission by over two times. With 38% hydrogen content, soot emission was only 70 mg/m\({}^{3}\) and was over 25 times lower than when biodiesel was combusted.  
The research shows that co-combustion of biodiesel with hydrogen gives great benefits both in terms of improving the engine's operational parameters as, e.g., an increase in engine efficiency and also has a positive effect on the emission of certain exhaust gas components and very significantly reduces soot emissions. For normal engine operation, the hydrogen content should not be greater than 10% due to high pressure increases and the risk of knocking.  
Conceptualization, W.T., A.J. and K.G.-R.; data curation, W.T., A.J. and K.G.-R.; formal analysis, W.T.; investigation, W.T., A.J. and K.G.-R.; methodology, W.T. and A.J.; writing--original draft, W.T.; writing--review and editing, W.T. and K.G.-R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Science and Higher Education of Poland from the funds dedicated to scientific research No. BS/PB 1-100-3010/2019/P.  
The authors declare no conflicts of interest.