Mazda Training manual — part 233

2 – BASIC OPERATION

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Piston Engine Fundamentals

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Bore, Stroke, and Displacement

Bore, stroke,

and displacement are measurements of the cylinders and

pistons in an engine. These three measurements are shown in Figure 10.

Bore

is the diameter of a cylinder, usually expressed in millimeters (mm).

Stroke

is the length of piston travel between TDC and BDC, usually

expressed in millimeters.

The displacement of a cylinder is the volume of the cylinder between the
TDC and BDC positions of the piston. The displacement of an engine is the
total displacement of all the cylinders in the engine. Displacement is
normally measured in cubic centimeters (cc) or liters (L).

Generally speaking, engine displacement is a rough indicator of power
output. For example, a 2000 cc (2.0 L) engine is usually more powerful
than a 1500 cc (1.5 L) engine. Displacement can be increased by opening
the bore to a larger diameter, or by increasing the length of the stroke.

FIGURE 10. Bore
and stroke
determine the
displacement of
a cylinder.

Bore (mm)

TDC

BDC

Stroke (mm)

Displacement

(cc or L)

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FIGURE 11. The
compression
ratio compares
the cylinder
volume at BDC
to the volume at
TDC.

BDC

Volume
before
compression

TDC

Volume after
compression

Compression Ratio

The compression ratio measures how much the air-fuel mixture is
squeezed during the compression stroke. The compression ratio is the
ratio of the cylinder volume at BDC to the volume at TDC. For example,
Figure 11 shows a compression ratio of 8 to 1, where the volume at BDC
is eight times larger than the volume when the piston is at TDC.

























Generally, a higher compression ratio means greater power output. For
example, a compression ratio of 8 to 1 will probably produce more power
than a ratio of 7 to 1. However, higher compression engines may require
premium fuel. Most engines have compression ratios of 9.5 to 1 or less so
they can take advantage of regular unleaded fuel.

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ENGINE COMPONENTS AND SYSTEMS

In this section, you have learned how the four-stroke cycle produces power in an engine.
The rest of this guide covers the following components and systems, which are important
to the engine’s overall operation:

• Short block — A term used in the automotive repair business to describe the cylinder

block, crankshaft, bearings, connecting rods, and pistons as a unit.

• Valve train — Opens and closes the intake and exhaust valves so that events are

properly timed in the four-stroke cycle.

• Lubrication system — Lubricates all moving parts to reduce heat and wear.

• Cooling system — Maintains ideal operating temperatures in the engine.

Each of these components and systems is described in detail in the following sections of
this guide.


REVIEW EXERCISE 2

Fill in the words that correctly complete these sentences. Check your answers with the
answer key on page 20.

1. A V-6 engine usually has six cylinders arranged in _____________________ banks of

________________________ cylinders each.

2. An engine that uses separate intake and exhaust camshafts mounted above the

cylinders is called a ________________________ design.

3. The diameter of a cylinder is called its __________________________.

4. The _______________________ of an engine measures how much the air-fuel

mixture is squeezed before it is ignited.

5. The ______________________ of an engine is the total volume of all the cylinders

between the TDC and BDC positions.

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Answers to
Review
Exercise 2

1. 2 banks; 3

cylinders

2. DOHC

3. bore

4. compression

ratio

5. displacement