74 Auto Engine Performance and Driveability
Engine problems can have a major effect on vehicle
driveability. For example, without good compression or
proper valve timing, the engine will not properly operate.
This chapter covers the major engine components and
reviews engine operation.
Four-Stroke Cycle Engine Operation
One power cycle in an automotive engine is a four-stroke
process, which is why these engines are known as four-stroke
cycle engines. The piston moves up twice in the cylinder
and down twice in the cylinder for each power-producing
cycle. This requires two revolutions of the crankshaft. Refer to
Figure 5-1 as you read the following paragraphs. Some diesel
engines, however, use only two strokes rather than four.
The downward movement of the piston on the intake
stroke draws the air-fuel mixture into the cylinder through
the intake valve(s). When the piston reaches the bottom of
the intake stroke, the intake valves are closed. The piston
then moves up in the cylinder, compressing the air-fuel
mixture. This is called the compression stroke. When the
air-fuel mixture in the cylinder is ignited, the burning fuel
and oxygen produce heat. The heat causes the gases in the
cylinder to expand, forcing the piston down under pres-
sure. This is called the power stroke.
The downward motion of the piston is transferred to
the connecting rod, which in turn pushes on the crankshaft.
The connecting rod and crankshaft arrangement changes
the linear piston movement into rotation of the crankshaft.
This rotational movement is used to move the vehicle.
When the piston reaches the bottom of the power
stroke, the exhaust valve is open. The exhaust stroke then
begins as the piston moves up the cylinder, pushing the
exhaust gases out of the open exhaust valve. One cycle is
now complete. The cycle then repeats.
Engine Components
This section describes the major components of the
engine compression system. The compression system con-
tains the parts that develop engine compression and allow
the heat energy in the burning air-fuel mixture to become
mechanical energy.
p Engine block.
p Pistons.
p Rings.
p Connecting rods.
p Crankshaft.
p Cylinder heads.
p Valves and related components.
p Valve train.
p Harmonic balancers.
p Balance shafts.
Although the exact design and number of engine compo-
nents varies with the type of engine and number of cyl-
inders, the basic components and their functions are the
same as described here.
Engine Block
The engine block is the basic support and attaching
point for all other engine parts. Engine blocks are made by
pouring molten cast iron, steel, or aluminum into molds,
Figure 5-2A. After the metal cools, the molding sand is
washed out and the block is machined to allow other parts
to be installed or attached, Figure 5-2B. The major parts
installed in or on the block are the pistons, crankshaft, cam-
shaft, cylinder heads, and manifolds.
Figure 5-1. The cycle shown here is typical of all four-stroke cycle engines: intake, compression, power, and exhaust.
Intake
Compression
Power
Exhaust
3 1
2 4