Page 32 - Understanding Automotive Electronics

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AUTOMOTIVE FUNDAMENTALS 1

within its optimal performance range regardless of the vehicle load or speed.
It provides a gear ratio between the engine speed and vehicle speed such that
the engine provides adequate power to drive the vehicle at any speed.
The transmission pro- To accomplish this with a manual transmission, the driver selects the
vides a match between correct gear ratio from a set of possible gear ratios (usually three to five for
engine speed and vehi- passenger cars). An automatic transmission selects this gear ratio by means of an
cle speed. automatic control system. Most automatic transmissions have three forward
gear ratios, although a few have two and some have four. A properly used
manual transmission normally has efficiency advantages over an automatic
transmission, but the automatic transmission is the most commonly used
transmission for passenger automobiles in the United States. In the past,
automatic transmissions have been controlled by a hydraulic and pneumatic
system, but the industry is moving toward electronic controls. The control
system must determine the correct gear ratio by sensing the driver-selected
command, accelerator pedal position, and engine load.
The proper gear ratio is actually computed in the electronic
transmission control system. Once again, as in the case of electronic engine
control, the electronic transmission control can optimize transmission
control. However, since the engine and transmission function together as a
power-producing unit, it is sensible to control both components in a single
electronic controller.

Drive Shaft
The drive shaft is used on front-engine, rear wheel drive vehicles to
couple the transmission output shaft to the differential input shaft. Flexible
couplings, called universal joints, allow the rear axle housing and wheels to
move up and down while the transmission remains stationary. In front
wheel drive automobiles, a pair of drive shafts couples the transmission to
the drive wheels through flexible joints known as constant velocity (CV)
joints.

Differential
The combination of The differential serves three purposes (see Figure 1.13). The most
drive shaft and differen- obvious is the right angle transfer of the rotary motion of the drive shaft to
tial completes the trans- the wheels. The second purpose is to allow each driven wheel to turn at a
fer of power from the different speed. This is necessary because the “outside” wheel must turn
engine to the rear faster than the “inside’’ wheel when the vehicle is turning a corner. The
wheels. third purpose is the torque increase provided by the gear ratio. This gear
ratio can be changed in a repair shop to allow different torque to be
delivered to the wheels while using the same engine and transmission. The
gear ratio also affects fuel economy. In front wheel drive cars, the
transmission differential and drive shafts are known collectively as the
transaxle assembly.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 19