Tools and methodologies for analysis and development of zero impact energy conversion based on hydrogen combustion

To limit global warming, a reduction of anthropogenic CO2 emissions is essential. This also requires a climate-neutral
mobility sector and efficient drive solutions for the different mobility requirements. Energy conversion on the
basis of hydrogen combustion offers great potential for future concepts with high power and range requirements
to demonstrate sustainable drive systems. The holistic understanding of engine energy conversion is a key element
for the development of energy conversion systems with high performance, efficiency and lifetime. Hydrogen
as a fuel is inherently CO2-free and offers advantageous combustion properties due to wide ignition limits
and high flame velocities. High excess air rates allow low NOx raw emissions with MFB50 favourable to efficiency.
However, less lean mixtures, as can occur, for example, during load steps or due to inhomogeneities, tend to exhibit
pronounced combustion anomalies. For the analysis of relevant processes, suitable analysis tools and methods
with exemplary key results are presented, which enable a consistent understanding of the process. These include,
for example, methods for the analysis of mixture formation as well as tools for the identification of
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preignition and knocking locations as well as triggering mechanisms. However, H2 combustion also poses additional
challenges in thermodynamic experiments on the engine. For example, a CO2-based EGR rate determination
cannot be used, the increased condensate accumulation must be considered as well as the hydrogen specifics
for cylinder pressure indication.
One method for visualising gas injection is the „Background Oriented Schlieren“ method. This makes it possible
to analyse the spray image of the injected hydrogen of an injector, to design it with corresponding knowledge and
to validate associated simulations. Through an optically accessible pressure chamber, the results can be recorded
under real pressure conditions and transferred to the engine. By means of numerical methods, the mixture preparation
can be analysed and represented as a homogenisation index.
The standard for EGR-rate determination is the measurement of CO2 in the intake manifold and exhaust. However,
this measurement method is not possible for hydrogen engines. Therefore, two alternative methods are established.
Instead of CO2, O2 can be measured in the intake manifold and exhaust, and on the other hand the respective
gas humidity can be use for EGR-rate calculation.
Hydrogen rich mixtures near the cylinder pressure sensor place increased demands on the measuring principle as
well as the robustness of cylinder pressure sensors. Thermodynamic parameters that are relevant for the pressure
indication of hydrogen combustion (e.g. polytropic exponent, offset determination) require corresponding calculation
methods.
In order to identify causes of combustion anomalies such as knocking or pre-ignition, optical analysis tools are an
effective means of localising flame propagation and local self-ignition phenomena. For this purpose, endoscopic
or fibre-optic visualisation techniques are available to detect the corresponding emission spectra.
The high water content of the exhaust gas poses special challenges for the exhaust gas measurement technology.
Sector field mass spectrometers are used to determine the concentration of unburnt hydrogen within the exhaust
gas, and conventional methods are used for NOx. Both emissions offer themselves as indirect indicators for
locally lean mixtures with partially increased H2 emissions or for locally richer mixtures with increased NOx emissions
for the assessment of mixture homogenisation.
These methods and tools provide a holistic picture of the processes of combustion engine hydrogen energy conversion
and thus support the development process of future zero impact propulsion solutions.