Page 264 - Understanding Automotive Electronics

2735 | CH 7 Page 251 Tuesday, March 10, 1998 1:15 PM

DIGITAL ENGINE CONTROL SYSTEM 7

Figure 7.17
Secondary Air
Control System

FPO

collected by a charcoal filter in a canister. The collected fuel is released into the
intake through a solenoid valve controlled by the computer. This is done during
closed-loop operation to reduce fuel calculation complications in the open-loop
mode.

Torque Converter Lock-up Control
Automatic transmissions use a hydraulic or fluid coupling to transmit
engine power to the wheels. Because of slip, the fluid coupling is less efficient
than the nonslip coupling of a pressure-plate manual clutch used with a manual
transmission. Thus, fuel economy is usually lower with an automatic
transmission than with a standard transmission. This problem has been
partially remedied by placing a clutch functionally similar to a standard
pressure-plate clutch inside the torque converter of the automatic transmission
and engaging it during periods of steady cruise. This enables the automatic
transmission to provide fuel economy near that of a manual transmission and
still retain the automatic shifting convenience.
Here is a good example of the ease of adding a function to the electronic
engine control system. The torque converter locking clutch (TCC) is activated

UNDERSTANDING AUTOMOTIVE ELECTRONICS 251