Environmental Engineering Reference
In-Depth Information
utilization of cylinder-independent, cycle-to-cycle modulation of reactant
gas equivalence ratio (i.e., the amount of fuel), a key next step in the practical
implementation of LTC.
4.3.2 Leveraging LTC ''Simulation Models'' for Control Design
In [34, 35, 47, 48], the author and colleagues formulated a single-cylinder
10-state simulation model which predicts the effects of the VVA system on
the LTC combustion process during constant engine speed. A single-zone
model of the in-cylinder gases captures the compression, combustion initia-
tion, energy release and expansion processes. An integrated Arrhenius rate
describes the dependence of combustion timing on reactant concentration,
temperature, and amount of compression. The in-cylinder dynamics are
coupled with a single-zone model of the exhaust manifold gases to predict
the cycle-to-cycle coupling through the exhaust gas temperature resulting in
the first model of LTC capable of capturing the cyclic coupling. The resulting
model agrees with experimental values of inlet reactant flow rate, combus-
tion timing, in-cylinder pressure evolution, work output and exhaust gas
temperature during steady-state operation. The dynamics of operating point
change and mode-transition dynamics are also captured - another first in
LTC research. The simulation model provides valuable insights for the
formulation of control strategies - inducted gas composition can be varied
via modulation of the valves, residual-affected LTC exhibits a self-stabilizing
behavior due to the competing influences of mixture temperature and reac-
tant concentration, and cyclic coupling is inherent to the process and must be
included. Furthermore, given its physically oriented formulation it should be
extendable to other conditions, including dynamic wall temperature condi-
tions [49] and multi-cylinder, variable engine speed operation. Additionally,
the simulation model has provided an excellent virtual testbed for analyzing
feedback control strategies.
4.3.3 Physics-Based ''Control Models'' for LTC
Using insights gained from the simulation modeling effort, the author and
colleagues developed a reduced-order nonlinear control-oriented model [40,
41, 42, 50] with inducted gas composition and effective compression ratio as
inputs and peak pressure and combustion timing as outputs. This was achieved
by discretizing, as shown in Fig. 4.4, the LTC process into six distinct stages:
induction, compression, combustion, expansion, exhaust, and residence in the
exhaust manifold. Pictorially, the inputs and outputs of the control model are
shown in Fig. 4.5.
