The cylinder head is bolted on top of the cylinder
block. It contains the valves that control flow into and out
of the cylinder. The camshaft operates the valves so that
they open and close to correspond to piston action, pro-
ducing the four-stroke cycle.
The intake stroke draws air-fuel mixture into the
engine. The compression stroke squeezes the mixture and
readies it for burning. The power stroke ignites the mixture
and the expanding gasses push the piston down with
tremendous force. The exhaust stroke purges the burned
gasses to prepare the cylinder for another intake stroke.
Every two crankshaft revolutions complete one four-
stroke cycle. Thus, one power stroke is produced every
two crankshaft revolutions. The flywheel helps keep the
crankshaft spinning on the nonpower-producing strokes
and smoothes out engine operation.
Various systems are needed to keep the engine run-
ning. The cooling system removes excess combustion heat
and prevents engine damage. The lubrication system also
prevents engine damage by reducing friction between
moving engine parts.
The ignition system is needed on a gasoline engine to
ignite the fuel and start it burning. A diesel engine uses
high-compression-stroke pressure to heat the air in the
cylinder enough to start combustion, instead of an electric
spark. Also, a diesel does not use a throttle valve to control
engine speed. The amount of fuel injected into the cylin-
der controls engine speed and power output.
The two types of fuel systems in use today are
gasoline injection and diesel injection. Modern fuel injec-
tion increases fuel economy over older fuel systems by
closer control of the fuel use by each cylinder. Carburetion
is not used today.
The starting system uses a powerful electric motor to
turn the engine flywheel until the engine can run on its own
power. The charging system recharges the battery and sup-
plies electricity to the vehicle while the engine is running.
A vehicle’s computer system is similar to the human
body’s central nervous system. The computer system
monitors various conditions by analyzing input signals
from sensors. Then, the control module (computer) deter-
mines what action should be taken to maintain maximum
efficiency. Based on this, the control module sends signals
to actuators that control the operation of the fuel system,
ignition system, transmission, and other devices.
Emission control systems reduce the amount of harm-
ful pollutants that enter the atmosphere. Some emission
control systems prevent fuel vapors from evaporating into
Chapter 1 Review of Engine Operation 25
B A
Engine
Transmission
Differential Front
drive axle
Dead axle Rear
drive axle
Differential
Drive shaft
Manual
transmission
Clutch
Engine
Clutch
Manual transaxle
Figure 1-30. The drive train uses engine power and crankshaft rotation to turn the vehicle’s drive wheels. A—A front-engine,
rear-wheel-drive vehicle. B—A front-engine, front-wheel-drive vehicle. Note the differential and transmission in one housing.