This study demonstrates that hydrogen enrichment in lean-burn spark-ignition engines can simultaneously improve three key performance metrics, thermal efficiency, combustion stability, and nitrogen oxide emissions, without requiring modifications to the engine hardware or ignition timing. This finding offers a novel control approach to a well-documented trade-off in existing research, where typically only two of these factors are improved at the expense of the third. Unlike previous studies, the present work achieves simultaneous improvement of all three metrics without hardware modification or ignition timing adjustment, relying solely on the optimization of the air–fuel equivalence ratio 𝜆. Experiments were conducted on a six-cylinder engine for combined heat and power application, fueled with hydrogen–natural gas blends containing up to 30% hydrogen by volume. By optimizing only the air–fuel equivalence ratio, it was possible to extend the lean-burn limit from 𝜆≈1.6 to 𝜆>1.9, reduce nitrogen oxide emissions by up to 70%, enhance thermal efficiency by up to 2.2 percentage points, and significantly improve combustion stability, reducing cycle-by-cycle variationsfrom 2.1% to 0.7%. A defined 𝜆 window was identified in which all three key performance indicators simultaneously meet or exceed the natural gas baseline. Within this window, balanced improvements in nitrogen oxide emissions, efficiency, and stability are achievable, although the individual maxima occur at different operating points. Cylinder pressure analysis confirmed that combustion dynamics can be realigned with original equipment manufacturer characteristics via mixture leaning alone, mitigating hydrogen-induced pressure increases to just 11% above the natural gas baseline. These results position hydrogen as a performance booster for natural gas engines in stationary applications, enabling cleaner, more efficient, and smoother operation without added system complexity. The key result is the identification of a 𝜆 window that enables simultaneous optimization of nitrogen oxide emissions, efficiency, and combustion stability using only mixture control.

This study explores the unique operating behavior of an alkaline water electrolysis cell equipped with an ion-solvating membrane, operated with a diluted alkaline electrolyte, specifically 1-M potassium hydroxide (1M KOH), in anode feed mode. Our investigations reveal several key insights. Charge transport: In an ion-solvating membrane, charge transport occurs through both the cations and anions of the electrolyte. Due to electro-osmosis, cation transport to the cathode results in a combined hydrogen-electrolyte discharge from the cathode compartment of the electrolysis cell. The discharged electrolyte is more concentrated than the electrolyte supplied to the anode. The concentration and flow rate of the electrolyte increase with current density and electrolyte temperature. Current density dependence: Since only a fraction of the total charge is transferred by hydroxide ions within the membrane, current density strongly depends on the electrolyte flow through the anode compartment. Membrane stability and performance: The membrane’s mechanical and chemical stability enables operation at high temperatures, up to 80 ◦C. This stability enables increased current density at a given cell voltage. Effects of catalyst use: Using cathode catalysts with high surface areas, such as Raney-Ni, enhances current density because highly concentrated liquid potassium hydroxide forms at the cathode during operation. Anode catalysts with high surface areas increase current density, but only if the flow of hydroxide ions is not impeded. Otherwise, the jV-curve exhibits transport-limited behavior.

This study demonstrates the need for novel gas engine control systems for com-
bined heat and power plants, also known as cogeneration power plants, connected to natural
gas grids. Hydrogen addition to natural gas grids in a range of up to 5% by volume is already
permitted throughout Europe. This offers the possibility to reduce carbon dioxide emissions
of end consumers connected to public natural gas grids and contributes to climate protec-
tion. However, conventional engine controls are not designed for natural gas/hydrogen mixture
operation. We tested fuels with up to 30% hydrogen by volume using a commercial six-cylinder
spark ignition engine, designed for natural gas or biogas operation in power plants. With engine
settings according to usual cogeneration operation, nitrogen oxide emissions increased expo-
nentially with increasing hydrogen amounts. We demonstrate that the usual approach of using
the lower heating value of the fuel mixture to regulate the engine is unable to accommodate the
hydrogen induced changes. For this reason, we developed a mathematical model to determine
the nitrogen oxide emissions based on boost pressure and power output. The idea behind this
novel approach is to regulate the engine based on emissions, regardless of the fuel gas. In this
work the approach for this virtual sensor is described and its performance demonstrated.

This study investigates the potential benefits and drawbacks of adding hydrogen to natural
gas grids on stationary cogeneration plants. Fuel blended with up to 30% hydrogen by
volume was tested using a commercial six-cylinder spark ignition engine designed for pure
natural gas operation without modifications to the engine. In line with normal practice for
cogeneration plant engines, the power output, the lower heating value of the air/fuel
mixture, the ignition timing and the engine speed were held constant. Results show that
increasing hydrogen concentration led to an earlier peak cylinder pressure, indicating
significantly accelerated combustion. As a result, peak pressures were up to 39% higher
than with natural gas and up to 10% of fuel burned before top dead center. Despite this,
thermal efficiency improved up to 6%. Cycle-by-cycle variation decreased by half, indi-
cating reduced misfires on account of hydrogen. However, nitrogen oxide emissions
increased exponentially with increasing hydrogen amounts. Our findings suggest that
hydrogen-enriched natural gas is a promising fuel for stationary cogeneration plants, but
modifications to engine control settings are necessary to ensure optimal performance and
compliance with nitrogen oxide emission regulations. These modifications might include
adjustments to the mixture control system and ignition timing.