Effects of Ambient Air Humidity on Emissions and Efficiency of Large-Bore Lean-Burn Otto Gas Engines in Development and Application
Authors: Bartkowski Tomas , Eicheldinger Stefan , Prager Maximilian , Wachtmeister Georg 
Published: October 2020

Abstract
The use of large-bore Otto gas engines is currently spreading widely considering the growing share of Power-To-Gas (P2G) solutions using renewable energies. P2G with a Combined Heat and Power (CHP) plant offers a promising way of utilizing chemical energy storage to provide buffering for volatile energy sources such as wind and solar power all over the world. Therefore, ambient conditions like air temperature, humidity and pressure can differ greatly between the location and time of engine operation, influencing its performance. Especially lean-burn Otto processes are sensitive to changes in ambient conditions. Besides, targeted use of humidity variation (e.g. through water injection in the charge air or combustion chamber) can help to reduce NOx emissions at the cost of a slightly lower efficiency in gas engines, being an alternative to selective catalytic reduction (SCR) exhaust gas aftertreatment. The ambient air condition boundaries have to be considered already in the early stages of combustion development, as they can also have a significant effect on generated measurement data in combustion research.
To investigate the behavior, a test bench with a natural gas (CNG) powered single-cylinder research engine (piston displacement 4.77 1) at the Institute of Internal Combustion Engines (LVK) of the Technical University of Munich (TUM) was equipped with a sophisticated charge air conditioning system. This includes an air compressor and refrigeration dryer, followed by temperature and pressure control, as well as a controlled injection system for saturated steam and homogenizing containers, enabling the test bench to precisely emulate a widespread area of charge air parameters in terms of pressure, temperature and humidity.
With this setup, different engine tests were conducted, monitoring and evaluating the engine’s emission and efficiency behavior regarding charge air humidity.
In a first approach, the engine was operated maintaining a steady air-fuel equivalence ratio λ, fuel energy input (Q̇fuel = const.) and center of combustion (MFB 50%) while the relative ambient humidity was varied in steps between 21% and 97% (at 22 °C and 1013.25 hPa). Results show a significant decrease in nitrogen oxides (NOx) emissions (−39.5%) and a slight decrease in indicated efficiency (−1,9%) while hydrocarbon (THC) emissions increased by around 60%.
The generated data shows the high significance of considering charge air conditioning already in the development stage at the engine test bench. The comparability of measurement data depends greatly on ambient air humidity.
In a second approach, the engine was operated at a constant load and constant NOx emissions, while again varying the charge air humidity. This situation rather reflects an actual engine behavior at a CHP plant, where today often NOx–driven engine control is used, maintaining constant NOx emissions. The decrease in indicated efficiency was comparable to the prior measurements, while the THC emissions showed only a mild increase (5%).
From the generated data it is, for instance, possible to derive operational strategies to compensate for changes in ambient conditions while maintaining emission regulations as well as high-efficiency output. Furthermore, the results suggest possibilities, but also challenges of utilizing artificial humidification (e.g. through water injection) considering the effects on THC emissions and efficiency. A possible shift of the knocking limit to earlier centers of combustion with higher humidity is to be investigated.
The main goal is the further decrease of NOx emissions, increase of efficiency, while still maintaining hydrocarbon emissions.