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Hydrogen as potential engine fuel can appear either as a single gas or as a component in processing gases e.g. syngas, hythane and coke gas. The research in this paper investigates impact of combustible mixture dilution on abnormal combustion called knock in the reciprocating internal combustion engine. Dilution can be realized by either exhaust gas recirculation (EGR) or making the combustible mixture lean. Novelty of this work is a new metrics defined as dilution ratio, which makes it possible to compare knock reduction caused by either EGR or leaning the air-gas mixture to the engine. Two gaseous fuels were investigated: hydrogen and coke gas with 65% hydrogen. Conclusion based on the proposed dilution ratio states that, for hydrogen as the fuel, applying EGR is more effective in knock reduction than making the mixture lean. It was found that EGR strategy in the hydrogen fueled engine can reduce knock intensity from initial 40 kPa–20 kPa, whereas by leaning the mixture to the same dilution ratio, the knock is reduced to approximately 28 kPa. With respect to coke gas, it is proved that both EGR and lean mixtures influence on knock reduction at the same strength. • Combustible mixture dilution impact on combustion knock intensity. • Dilution ratio as a metrics for both exhaust gas recirculation and lambda. • Engine knock reduced more effectively by applying exhaust gases recirculation. • The correlation between dilution ratio and knock intensity is almost linear. [ABSTRACT FROM AUTHOR]

The main objective of this study was to examine impact of hydrogen addition to the compression ignition engine fueled with either rapeseed methyl ester (RME) or 7% RME blended diesel fuel (RME7) on combustion phases and ignition delay as well as smoke and exhaust toxic emissions. Literature review shows in general, hydrogen in those cases is used in small amounts below lower flammability limits. Novelty of this work is in applying hydrogen at amounts up to 44% by energy as secondary fuel to the compression ignition engine. Results from experiments show that increase of hydrogen into the engine makes ignition delay shortened that also affects main combustion phase. In all tests the trends of exhaust HC and CO toxic emissions vs. hydrogen addition were negative. The trend of smokiness decreased steadily with increase of hydrogen. Amounts of hydrogen addition by energy share were limited to nearly 35% due to combustion knock occurring at nominal load. • Hydrogen as secondary fuel for the diesel engine. • Main combustion phase and ignition delay shorten with hydrogen addition. • Indicated thermal efficiency stays not changed remarkably. • Hydrogen addition decreases smokiness, unburnt hydrocarbons and carbon monoxide. • Knock occurred at hydrogen addition over 35% by energy. [ABSTRACT FROM AUTHOR]

Abstract: Combustion knock is abnormal combustion taking place in an internal combustion spark ignited engine. It might be particularly observed in the engine at the end of combustion when the air–fuel mixture residue can be self-ignited due to exceeding auto-ignition temperature of this mixture. However, while hydrogen is combusted the knock can also occur as a result of non-auto-ignited combustion events. Investigation on knock, presented in the manuscript, was conducted in a hydrogen fueled spark ignited single cylinder engine with variable compression ratio. To express in numbers intensity of the combustion knock the in-cylinder pressure pulsations were used as a credible metrics. On the basis of analysis of these pulsations the hydrogen knock was distinguished as light and heavy one depending on its origin. The light knock is generated by combustion instabilities, which are a source for generating pressure waves inside the engine cylinder. The heavy knock results from hydrogen auto-ignition at the end of combustion. Its intensity is several times higher in comparison to the light knock. These observations were additionally confirmed by analysis of heat release rate. Finally, the light and the heavy knock were characterized by average amplitude of the pulsations from the entire test series of hundreds and several thousands kPa, respectively. [Copyright &y& Elsevier]

Abstract: The investigation presented in this paper concerns both pure hydrogen combustion under HCCI (homogeneous charge compression ignition) conditions and hydrogen–diesel co-combustion in a compression ignition (CI) engine. The investigation on the simultaneous combustion of hydrogen and diesel fuel was conducted with various hydrogen doses in the range from 0% to 17% with respect to energy percentage. With hydrogen of 17% the hydrogen–diesel–air mixture was stoichiometric and provided favorable conditions for generating combustion knock. Small amounts of hydrogen (about 5%) when added to a diesel engine shorten the diesel ignition lag and, in this way, decrease the rate of pressure rise. It provides better conditions for soft run of the engine and can increase engine durability. The final conclusions concerning hydrogen impact on combustion knock intensity, mass fraction burned (MFB) and heat release rate of the engine are detailed. [Copyright &y& Elsevier]