On an IC engine’s torque-speed map, the locus of
maximum efficiency does not necessarily correspond to
the loci of optimum emissions. In some cases, there are
even two sections in the map of optimum performance.
For a compression-ignition engine, the four regulated
emissions are Hydrocarbons (HC), Carbon Monoxide
(CO), Nitrous Oxides (NOx), and Particulate Matter
(PM). In particular, there is a definite tradeoff between
NOx emissions and energy use. The challenge for the
control strategy is to simultaneously balance the goals of
lower energy usage and lower emissions. Using the
ADVISOR vehicle simulation tool [1,4-5], the RTCS was
developed to better optimize a vehicle’s performance in
both areas.
PROBLEM DEFINITION
A driver typically controls vehicle speed by depressing
the accelerator pedal to request positive torque or
depressing the brake pedal to request negative torque.
In a conventional vehicle, positive torque is supplied only
by the combustion engine, and negative torque is
supplied only by the brakes (with the exception of closed
throttle engine braking energy, ignored here). Control is
therefore straightforward: the engine supplies all positive
torque, and the brakes supply all negative torque.
In a parallel hybrid vehicle, there is an additional source
of torque available; the motor can draw electric energy
from the battery to apply positive torque that accelerates
the vehicle, and it can supply electric energy to the
battery by applying negative torque that decelerates the
vehicle. These two functions represent torque assist
and regeneration, respectively.
The parallel hybrid vehicle controller must determine
how to distribute the driver’s single torque request into
separate torque requests for the engine, motor, and
brakes. For negative torque requests, the sum of the
motor and brake torques must equal the driver’s request.
brakemotornegrequest,
TTT +=
Eqn. 1
For positive torque requests, the sum of the engine and
motor torques must equal the driver’s request.
motorengineposrequest,
TTT +=
Eqn. 2
Typically, negative torque requests can be handled with
a relatively simple strategy: The motor recovers the
maximum possible regeneration energy within
constraints imposed by the motor, the battery, the
brakes and vehicle stability considerations. The brakes
only supply whatever is left over. In this way, the
maximum amount of “free” braking energy is captured.
For a positive torque request, the choice is not as
straightforward. For each driver’s torque request, there
is a range of combined motor torques and corresponding
engine torques that will add up to the request. The goal
of the strategy is to choose an operating point
(distribution of torque requests) that minimizes the
engine’s fuel consumption and emissions. The net
energy consumed by the motor (i.e. the energy drawn
from the battery) must be negligible over the course of
driving. If the vehicle increases or depletes the battery
energy indefinitely, the battery will be damaged or its
usable life will be shortened. Also, proposed Federal
fuel economy tests for hybrid vehicles, such as SAE
J1711 [6], will require that no net battery energy is
consumed over the course of a test; any net
consumption of battery energy would artificially inflate
the vehicle’s reported fuel economy.
Simple approaches to this problem of maximizing
efficiency are not optimal. Applying as much electric
motor torque as possible will temporarily minimize
combustion engine fuel consumption, but that would
eventually deplete the battery. Other approaches would
predetermine the desired engine torque based on torque
request, without regard to the vehicle’s operating history.
For instance, the engine can exclusively and completely
fulfill all torque requests below its maximum for the
current speed. Another such strategy would require that
the engine torque request be at its most efficient or
cleanest possible operating point. These strategies
would not necessarily result in a balance of net charge in
the battery; the battery would probably be charged or
discharged over time, and the controller would
eventually have to switch to an alternate strategy to
restore the battery charge. That restoring strategy could
compromise overall system efficiency. In addition, this
purely efficiency-based approach does not consider
emissions generated by the engine.
The RTCS distributes torque between the motor and the
engine in order to both maintain SOC and optimize fuel
economy and emissions.
BASELINE STATIC CONTROL STRATEGY
The baseline control strategy (BCS) currently used by
ADVISOR for a parallel HEV is described here. Like the
RTCS, the BCS attempts to minimize fuel use and
balance SOC. Unlike the RTCS, the BCS does not
consider recent vehicle operation, it does not account for
battery energy, and it does not optimize emissions.
Simulations comparing this strategy with the RTCS are
presented in the Results section.
This baseline strategy uses the engine as a primary
source of torque, and it uses the motor for supplemental
power. When the battery SOC is low, the BCS switches
to a charge mode in order to replenish the battery. The
BCS attempts to minimize engine energy usage without
regard to emissions or the effect of the motor or batteries
during operation. Its operation is defined by six
independent input parameters (see Table 1).
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