Hydrogen Admixture Strategies for Small-Scale Cogeneration Gas Engines: A Comparison between different Engine Combustion Processes
Authors: Beltaifa Youssef , Kettner Maurice , Salim Naqib , Berlet Peter , Pöhlmann Klaus , Züfle Michael 
Published: June 2023

<div class="section abstract"><div class="htmlview paragraph">Cogeneration represents a key element within the energy transition by enabling a balancing of the long-term fluctuations of regeneratives. Regarding the expected increase of hydrogen share in natural gas pipelines in Germany, this work deals with investigations of hydrogen-associated advantages for the lean and stoichiometric operations of natural gas cogeneration engines, in relation to numerous challenges, such as the efficiency-NO<sub>x</sub> trade-off. Charge dilution is commonly regarded as one of the most effective ways for improving thermal efficiency of spark-ignition gas engines. While excess air serves as a diluent in the lean combustion process, stoichiometric combustion dilution may be obtained by exhaust gas recirculation (EGR). Combining hydrogen addition with mixture dilution is an appealing approach for a better handling of the efficiency-emissions trade-off.</div><div class="htmlview paragraph">The lean and the diluted stoichiometric combustion processes with hydrogen blending were investigated beforehand numerically and compared within 0D/1D combustion simulations, which enabled a deep insight into the expected impact of different mixture strategies on the flame temperature, combustion speed, reachable engine efficiency and emissions behavior. To put the engine concepts under investigation into practice, an engine test bench with extensive metrology and hydrogen supply trail was built. Engine trials for the lean-burn process show that H<sub>2</sub> reduces the burning delay, HC emissions and enables higher thermal efficiency, by allowing a stable (low COV) and leaner combustion. Considering the stoichiometric combustion process with EGR, H<sub>2</sub> enables a significant reduction of the real and imperfect combustion losses without significantly increasing the wall heat losses (starting from a certain EGR rate), resulting in an overall higher thermal efficiency. A major advantage of the stoichiometric combustion process is the possibility of using a three-way-catalyst, which enables ultra-low emissions and a decoupling of the efficiency-NO<sub>x</sub> dependency. The admixture of H<sub>2</sub> leads to a shorter combustion duration and an increase of in-cylinder peak pressures. Therefore, the effects of operation with H<sub>2</sub> on the connecting rod bearing were investigated. Online oil consumption measurements using tracer methods were also conducted to ensure that there was no unacceptably high oil consumption due to the hydrogen combustion, which features a smaller quenching gap and thus comes closer to the cylinder walls. In addition to the aforementioned internal engine effects of H<sub>2</sub> operation, a comprehensive safety concept was developed. As part of this, the hydrogen content in the engine blow-by was measured.</div><div class="htmlview paragraph">To combine the benefits of both dilution types, an engine arrangement including both the stoichiometric and lean burning processes was developed. Indeed, three (out of four) cylinders were operated stoichiometrically with CO<sub>2</sub>-free and partially dry EGR, coming entirely from a 100 % hydrogen lean combustion, which takes place in the fourth cylinder. Given the associated gas dynamic challenges, a methodological approach using 1D engine process simulation and design of experiments (DoE) was employed for this particular engine gas-path design, to ensure the even cylinder-to-cylinder EGR distribution. Here, the CO<sub>2</sub>-free and partially dry EGR increases the isentropic exponent of the stoichiometric mixture and improves the dilution tolerance in the three natural gas cylinders, resulting in increased thermal efficiency. As long as its exhaust gas is completely recirculated, the hydrogen-operated cylinder provides due to an optimized combustion phasing, lean-burn and fast combustion, an elevated indicated efficiency level without emissions constraints.</div></div>