Environmental Engineering Reference
In-Depth Information
complicated than in the case of either SI or diesel combustion. Combustion
timing in LTC is dominated by chemical kinetics, which depends on the in-cy-
linder concentrations of reactants and products, their temperature and the
amount of compression.
Cycle-to-Cycle Coupling
Subsequent engine cycles are coupled through the residual temperature.
Because the inducted reactant gas is heated by the residual, the residual tem-
perature from an engine cycle directly affects the chemical kinetic-dominated
combustion event on the subsequent cycle. The cyclic coupling plays a funda-
mental role in steady-state operation, operating point changes and mode transi-
tions. If care is not taken combustion timing becomes unstable, leading to
misfire, an unacceptable condition.
Dilution Limit
During LTC, the reactants (fuel and air) are diluted with either residual gas or
air. This dilution decreases the amount of work that can be extracted for a given
engine geometry. For these reasons, practical LTC will be accompanied with
either conventional SI or diesel strategies in a multi-mode engine. At high load
conditions the engine will run in the conventional mode. At low to moderate
load conditions the engine will run in LTC mode. A key issue is the transition
from the conventional mode to LTC. For residual-affected strategies, cyclic
coupling exists during transitions into the LTC mode. In fact, due to the higher
exhaust temperatures associated with the conventional modes, the dynamics of
a mode transition into LTC are even more dramatic than intra-LTC operating
point changes.
Cylinder-to-Cylinder Coupling
Coupling between engine cylinders also exists through the residual temperature,
as combustion gas exhausted from one cylinder may be reinducted by a neigh-
boring cylinder. Additionally, the presence of neighboring cylinder heat trans-
fer will become important. A strategy for controlling multi-cylinder LTC
engines will have to include these cylinder-coupling effects.
Achieving High Levels of Fuel-Air-Residual Mixing - Fuel Premixing
or Early Injection?
In order to achieve a well-mixed fuel-air-residual mixture, the fuel must be
either premixed with the air prior to induction through the intake valve or be
injected into the cylinder early. Both approaches present challenges. Premixing
in the intake can be achieved successfully with an atomizer [5]. Unfortunately,
this can add expense and complexity. Early injection using injectors designed
for conventional injection (i.e., when the piston is near top dead center) often
leads to wall wetting, which increases consumption and soot emissions.
