Environmental Engineering Reference
In-Depth Information
physics-based LTC control results. A benefit of a physics-based approach to
modeling and control of LTC is that previous work may be extendable to
additional operating conditions (e.g., variable engine speed), configurations
(e.g., multiple cylinders), and control inputs (e.g., cylinder-independent, cycle-
to-cycle modulation of fuel).
Consider the control model equations (Eqs. 4.1 and 4.2)
under the formulation only the two ''system outputs'' (peak pressure P and
combustion timing
23
) and the two ''system inputs'' (inducted gas composi-
tion,
and effective compression ratio) are assumed to vary on a cycle-to-
cycle basis
while the control model depends on the equivalence ratio
and engine speed,
these parameters are assumed constant, with no change on a cycle-to-cycle
basis
the temperature of the reinducted residual gas is only related to the gas
exhausted by the same cylinder on the previous engine cycle, with no depen-
dence on the temperature of exhaust from neighboring cylinders.
To accommodate practical multi-cylinder, variable speed operation the con-
trol model, and the controller synthesized from it, must reflect variation of the
engine speed and dependence of residual temperature on the exhaust from
neighboring cylinders. Furthermore, a cycle-to-cycle variation of the equiva-
lence ratio could be used to reflect the modulation of injected fuel as another
control input. The reformulation and validation of the control models with
cycle-to-cycle modulation in equivalence ratio and engine speed variation
should be key tasks in future efforts.
As done in previous work, the discretization and simplification of the induc-
tion, compression, combustion, expansion, and exhaust processes, as shown in
Fig. 4.4 might be useful and valid assumptions in future efforts. Furthermore,
integrating a physics-based description of the cylinder-coupling process in a
control model framework will be necessary to develop generalizable control
strategies for multi-cylinder residual-affected LTC.
4.4 Conclusion
A wonderful opportunity exists to capitalize on recent improvements in on-
engine technologies in the pursuit of cleaner and more efficient automobiles.
While a considerable amount of attention has been given to hybrid and FC
approaches, significant improvements can be made, and are required, in the
area of advanced combustion strategies. One such strategy, LTC combines
benefits of both diesel and SI methodologies to produce a strategy that has
NO
x
and soot emissions significantly lower than either approach. Residual-
affected LTC uses VVA to reinduct or trap hot combustion gases, enabling
dilute, stable autoignition. As a result, residual-affected LTC has an efficiency
