Page 106 - Automotive Engineering

Digital engine control systems CHAPTER 4.1

and RPM as well as transmission output shaft RPM. These differential is a necessary component of the drivetrain
RPM measurements are made using noncontacting angu- because the left and right drive wheels turn at different
lar speed sensors (usually magnetic in nature). Engine load speeds whenever the car moves along a curve (e.g.,
can be measured directly from MAP or from MAF and turning a corner). Unfortunately, wherever there is
from a somewhat complicated algorithm relating these a large difference between the tire/road friction from left
measurements to desired gear ratio. Once this desired to right, the differential will tend to spin the low friction
gear ratio is determined, the set of clutches to be activated wheel. An extreme example of this occurs whenever one
is uniquely determined, and control signals are sent to the drive wheel is on ice and the other is on dry road. In this
appropriate clutches. case, the tire on the ice side will spin and the wheel on
Normally, the highest gear ratio (i.e., ratio of input the dry side will not. Typically, the vehicle will not move
shaft speed to output shaft speed) is desired when the in such circumstances.
vehicle is at low speed such as in accelerating from a stop. Certain cars are equipped with so-called traction
As vehicle speed increases from a stop, a switching level control devices that can overcome this disadvantage of
will be reached at which the next lowest gear ratio is the differential. Such cars have differentials that in-
selected. This switching (gear-changing) threshold is an corporate solenoid-activated clutches that can ‘‘lock’’ the
increasing function of load (i.e., MAP). differential, permitting power to be delivered to both
At times (particularly under steady vehicle speed drive wheels. It is only desirable to activate these
conditions), the driver demands increasing engine power clutches in certain conditions and to disable them during
(e.g., for heavy acceleration). In this case, the controller normal driving, permitting the differential to perform its
shifts to a higher gear ratio, resulting in higher accelera- intended task.
tion than would be possible in the previous gear setting. A traction control system incorporates sensors for
The functional relationship between gear ratio and op- measuring wheel speed and a controller that determines
erating condition is often termed the ‘‘shift schedule.’’ the wheel-slip condition based on these relative speeds.
Wherever a wheel-spin condition is detected, the con-
troller sends electrical signals to the solenoids, thereby
4.1.10.5 Torque converter lock-up activating the clutches to eliminate the wheel slip.
control

Automatic transmissions use a hydraulic or fluid coupling 4.1.10.7 HV powertrain control
to transmit engine power to the wheels. Because of slip,
the fluid coupling is less efficient than the nonslip cou- The propulsive power coming from an ICE and an EM, is
pling of a pressure-plate manual clutch used with the basic concept of an HV. The HV combines the low
a manual transmission. Thus, fuel economy is usually (ideally zero) emissions of an electric vehicle with the
lower with an automatic transmission than with a stan- range and performance capabilities of ICE-powered cars.
dard transmission. This problem has been partially rem- However, optimization of emissions performance and/or
edied by placing a clutch functionally similar to a standard fuel economy is a complex control problem.
pressure-plate clutch inside the torque converter of the There are numerous issues and considerations in-
automatic transmission and engaging it during periods of volved in HV powertrain control, including the efficien-
steady cruise. This enables the automatic transmission to cies of the ICE and EM as a function of operating
provide fuel economy near that of a manual transmission condition; the size of the vehicle and the power capacity
and still retain the automatic shifting convenience. of the ICE and EM; the storage capacity and state of
Here is a good example of the ease of adding a function charge (SOC) of the battery pack; accessory load char-
to the electronic engine control system. The torque con- acteristics of the vehicle; and finally, the driving charac-
verter locking clutch (TCC) is activated by a lock-up so- teristics. With respect to this latter issue, it would be
lenoid controlled by the engine control system computer. possible to optimize vehicle emissions and performance
The computer determines when a period of steady cruise if the exact route, including vehicle speed, acceleration,
exists from throttle position and vehicle speed changes. It deceleration, road inclination, and wind characteristics,
pulls in the locking clutch and keeps it engaged until it could be programmed into the control memory before
senses conditions that call for disengagement. any trip were to begin. It is highly impractical to do such
pre-programming. However, by monitoring in-
stantaneous vehicle operation, it is possible to achieve
4.1.10.6 Traction control good, though suboptimal, vehicle performance and
emissions.
It was earlier explained that the transmission output Depending on operating conditions, the controller can
shaft is coupled to the drive axles via the differential. The command pure electric vehicle operation, pure ICE

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