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compression ratio and control combustion timing. Olsson et al. [38] modulated
the fuel amount to vary the work output while altering the mixture ratio of two
fuels to control combustion timing -a timing control strategy also adopted by
Bengtsson et al. [39]. Haraldsson et al. [37] took a similar approach but used
compression ratio instead of fuel mixture to shift combustion timing. These
studies indicate the usefulness of effective compression ratio and inducted gas
composition as control inputs for LTC.
4.2.3.2 Control Strategies Derived from Physics-Based Models
Each of these authors either used tuned controllers or synthesized a strategy
from a black-box model. Recent contributions [40, 41, 42, 43, 44] have also
demonstrated that LTC controllers can also be synthesized using physics-based
approaches. A physics-based control approach allows for a fundamental under-
standing of how control inputs affect the dynamics of the LTC process.
Furthermore, the approach is easily generalizable to other LTC engines because
model parameters are directly based on physical quantities, such as the cylinder
geometry, fuel used, cylinder/manifold orientation, etc. What follows is an
example of how ''simulation models'' and control design-amenable ''control
models'' can be developed, validated and utilized for generalizable physics-
based LTC control.
4.3 Examples of Physics-Based LTC Control
4.3.1 Status and Merits of Physics-Based LTC
Modeling and Control
While LTC is a complex physical process, the aspects most relevant for control -
in-cylinder pressure evolution, combustion timing, work output, cycle-to-cycle
dynamics, and cylinder-to-cylinder coupling - can be captured with accurate,
intuitive, physics-based simulation and control models [34, 45, 46]. Fromphysics-
based control models, a variety of novel control strategies are possible. For
example, Shaver et al. [41, 42] developed a controller that has been implemented
on a single-cylinder engine testbed to successfully track desired work output, in-
cylinder peak pressure and combustion timing during constant engine speed
operation via modulation of two control inputs, inducted gas composition and
effective compression ratio. These efforts will be outlined below. The focus of
future work must address the relaxation of the single-cylinder, constant speed
conditions, such that the physics-based approach applies more generally to the
following additional conditions and configurations:
multi-cylinder configuration
variable engine speed operation
