18 Section 1 Introduction to Automotive Technology
Clutch
The clutch allows the driver to engage or disengage
the engine and manual transmission or transaxle.
When the clutch pedal is in the released position, the
clutch locks the engine fl ywheel and the transmis-
sion input shaft together. This causes engine power
to rotate the transmission gears and other parts of
the drive train to propel the vehicle. When the driver
presses the clutch pedal, the clutch disengages power
fl ow and the engine no longer turns the transmission
input shaft and gears. Refer to Figure 1-18.
Transmission
The transmission uses various gear combinations,
or ratios, to multiply engine speed and torque to
accommodate driving conditions. Low gear ratios
allow the vehicle to accelerate quickly. High gear
ratios permit lower engine speed, providing good
gas mileage.
A manual transmission lets the driver change
gear ratios to better accommodate driving condi-
tions, Figure 1-19A. A clutch is used to engage and
disengage the gearsets when changing gear ratios
and vehicle speeds.
A semi-automatic transmission often uses pad-
dle shift buttons on the steering wheel instead of
a console shift lever. The computer control system
reacts to signals from the paddle shifter switches
to change gear locations semi-automatically using
electric actuators.
An automatic transmission, on the other hand,
does not have to be shifted by the driver. It uses an
internal hydraulic system and electronic controls to
shift gears. The input shaft of an automatic transmis-
sion is connected to the engine crankshaft through
an oil-fi lled torque converter (fl uid coupling) instead
of a dry friction clutch. The elementary parts of an
automatic transmission are pictured in Figure 1-19B.
Tech Tip
A hybrid transmission is usually an
automatic transmission that also contains
electric drive motor-generator(s) for electric
drive propulsion, high-voltage (HV) battery
recharging, and regenerative braking.
Drive Train Systems
The drive train transfers turning force from the
engine crankshaft to the drive wheels. Drive train con-
fi gurations vary, depending on vehicle design. The
most common drive train confi gurations are front-
wheel drive, rear-wheel drive, and all-wheel drive.
Front-wheel drive is commonly found on pas-
senger cars. A front-wheel drive vehicle has supe-
rior traction because the weight of the engine and
transaxle is directly above the drive wheels. Note
the lightweight “dead axle” in the rear of the car.
See Figure 1-18A.
The front-engine, rear-wheel drive confi guration
shown in Figure 1-18B is popular because the heavy
engine is still in the front for safety. Because of the
weight distribution, vehicles with this drive train
confi guration have more precise steering than front-
engine, front-wheel drive vehicles. However, they
have less traction on slippery surfaces than front-
wheel drive vehicles because less of the vehicle’s
weight is over the drive wheels.
Some rear-wheel drive vehicles have a mid-
engine drive train. In this confi guration, the engine
is located right behind the driver’s seat with a trans-
axle mounted behind it. This provides for almost
equal weight distribution to each tire for high cor-
nering capabilities. See Figure 1-18C.
An all-wheel drive vehicle often has the engine in
the front and twin differentials to distribute engine/
electric power to all four wheels and tires. It has the
most traction on wet or snow-covered roads. See
Figure 1-18D.
Drive Train Parts
The drive train parts commonly found on a front-
engine, rear-wheel-drive vehicle include the clutch,
transmission, drive shaft, and rear axle assembly.
The drive train parts used on most front-engine,
front-wheel-drive vehicles include the clutch, trans-
axle, and drive axles. Refer to Figure 1-19 as these
components and assemblies are discussed.
Discussed briefl y, the parts included in the drive
train can vary with vehicle design and whether the
vehicle is front-wheel drive or rear-wheel drive.