Environmental Engineering Reference
In-Depth Information
4.2.2.3 Segregated, Sequential Fluid Mechanical - Thermo-Kinetic
Multi-Zone Approaches
These approaches [26, 27] attempt to more tightly couple the mixing process
prior to combustion and the chemical kinetics of the autoignition process. The
distribution of reactant and diluting gas is modeled with a fluid mechanics
solver. Prior to the combustion event the gases are sequestered into a number
of zones. The combustion process is then carried out by using a multi-zone
combustion approach like that used in the quasi-dimensional models. This
approach allows the mixing process during induction to be modeled, so that
the effects of inhomogeneity on the autoignition process can be explicitly
captured.
4.2.2.4 Multi-Dimensional Fluid Mechanics with Coupled Kinetics
In this approach [28, 29, 30, 31, 32, 33], by far the most complex and compu-
tationally intensive, an attempt is made to completely couple the fluid
mechanics and chemical kinetics in three dimensions. In this case, the fluid
mechanics and chemical kinetics solvers are run in parallel so that the effect of
the combustion process on the fluid motion, and vice versa, can be explicitly
captured. This approach allows more accurate representation of composition
and temperature inhomogeneities, in some cases leading to more accurate
predictions of NO
x
and soot formation.
As outlined previously, the last 10 years has seen substantial progress in LTC
modeling. With a variety of approaches a large number of important engine
characteristics have been captured, including combustion timing, peak in-
cylinder pressure, work output, maximum rate of pressure rise, exhaust gas
temperature, emissions and extent of combustion. The dynamic cycle-to-cycle
coupling via exhaust gas temperature that exists with residual-affected LTC
strategies has also been recently captured for the first time in a modeling
strategy [34, 35]. For residual-affected LTC this coupling plays a fundamental
role in steady-state operation, during operating point changes, and across SI-
to-LTC (or diesel-to-LTC) mode transitions. The dynamic nature of the cycle-
to-cycle coupling also has critical implications for controlling the process,
because the control inputs depend not only on the desired engine behavior for
the current engine cycle, but also on what occurred during the previous cycle.
4.2.3 Approaches to Control of LTC
4.2.3.1 Control Strategies Derived from ''Black-Box'' or ''Data-Driven'' Models
In a number of experimentally validated studies, closed-loop control has been
utilized to fix combustion timing. Several approaches have been demonstrated
[36, 37, 38, 39]. Agrell et al. [36] used valve timings to effectively alter the
