Environmental Engineering Reference
In-Depth Information
gases, level of mixedness and heat transfer must be well understood. The
development and validation of models facilitates this understanding. A wide
variety of modeling approaches have been considered, including from least to
most complex:
1. zero-dimensional thermo-kinetic
2. quasi-dimensional thermo-kinetic
3. segregated, sequential fluid mechanics - thermo-kinetic multi-zone approaches
4. multi-dimensional fluid mechanics with coupled kinetics
Each approach presents its own set of advantages and disadvantages. The
implementation of one approach over another depends on the aim of the
user.
4.2.2.1 Zero-Dimensional Thermo-Kinetic Models
Models of this type utilize a single-zone approach to modeling the in-cylinder
gases. There are a large number of efforts in this area [10, 11, 12, 13, 14, 15,
16, 17, 18, 19, 20, 21]. The first law of thermodynamics is applied to a
homogeneous mixture of in-cylinder gases. The effects of the fluid mechanics
are not directly considered except, in some cases, when deriving relevant heat
transfer coefficients. The composition of the gases are either determined with
standard valve flow relations or through approximation from steady-state
experiment. In this approach, the largest computational cost is associated
with the chemical kinetics model, which can vary from a very simple tem-
perature threshold to detailed kinetics utilizing thousands of rate equations.
The benefits of the zero-dimensional approach follow from the homogeneity
assumption of the in-cylinder gases, resulting in a dramatic reduction in
complexity and computational costs relative to more complex modeling
approaches. Zero-dimensional thermo-kinetic models can capture LTC
engine outputs, including work output, combustion timing and peak in-
cylinder pressures. However, due to the homogeneity assumption, near
wall/piston quenching effects are difficult to capture, leading to inaccuracies
predicting emissions and completeness of combustion. Additionally, these
approaches cannot capture the mixing process during induction.
4.2.2.2 Quasi-Dimensional Thermo-Kinetic Models
This modeling approach [22, 23, 24, 25] builds on the zero-dimensional model-
ing approach by considering multiple zones in the cylinder. By using this
methodology the effects of temperature stratification and near wall/crevice
quenching can be considered. This leads to improvements in emissions predic-
tion compared to the zero-dimensional approaches, albeit at the expense of
added modeling complexity and computation.
