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Chemical Kinetic Modeling of Hydrogen-Diesel Co-combustion in Compression Ignition Engines
Abstract:
The concurrent combustion of Diesel and hydrogen in a 1999 Cummins ISM 370
heavy-duty compression ignition engine was computationally investigated using a
three-dimensional (3-D) computational fluid dynamics (CFD) solver and compared
to literature reference data of the engine operated at 1200 rpm and 70% load.
Multiple computational combustion models and chemical mechanisms spanning from
global single-step kinetic reaction mechanisms, equilibrium reactions, to
detailed reduced mechanisms of 118 to 128 reversible kinetic reactions were
considered, along with several chemistry solvers of varying levels of
sophistication.
The results showed that all models based on single-step global kinetic Diesel
mechanisms failed to accurately predict ignition timing, leading to significant
overprediction of the maximum in-cylinder pressures. The detailed mechanisms
were able to predict ignition delay, maximum in-cylinder pressure within 2%, and
crank angle of maximum pressure within 0.5 ° CA.
Finally, the detailed mechanisms were artificially decoupled from the Diesel and
hydrogen reaction mechanisms by creating dummy species to prohibit any
communication between the hydrogen and Diesel mechanism. Decoupling of the
hydrogen and Diesel chemistry failed to fully ignite and combust the mixture.
Therefore, the results strongly suggest that there is a significant direct
chemical interaction between the Diesel and hydrogen radical pools and
intermediate species. These interactions are crucial for accurate modeling of
the combustion process.
Simplified 1-D constant volume combustion simulations suggested that the exchange
of hydrogen peroxide (H2O2) produced by the initial Diesel
reactions provides a shortened reaction path for the hydrogen combustion,
resulting in accelerated heat release, which, in turn, increases the Diesel
reaction rate.
Therefore, chemistry models for Diesel/hydrogen co-combustion simulations
consisting of independent global reactions for hydrogen and Diesel combustion,
respectively, without communication between these reactions should only be used
with great caution.