Mazda Training manual — part 230

2 – BASIC OPERATION

5

Piston Engine Fundamentals

TC010-05-01S

In a car or truck, the engine provides rotating power to drive the wheels. This power is
transferred to the wheels through the transmission and driving axle. The source of this
rotating power is the energy released when fuel burns in the engine’s cylinders.

This section provides an overview of how the engine converts
energy from burning fuel into power that drives the vehicle’s
wheels.



OBJECTIVES

After completing this section, you will be able to describe how:

•

The cylinders and pistons convert energy from burning fuel into power.

•

The crankshaft converts up-and-down motion into rotational (turning) motion.

•

The flywheel stores energy for a smooth transfer of power.

•

The four-stroke cycle operates.

•

Valves control intake and exhaust in a cylinder.

•

Engines are classified by their design characteristics, including:

- Cylinder

configuration

-

Valve train type

-

Bore, stroke, and displacement

-

Compression ratio

2 – BASIC OPERATION

6

Piston Engine Fundamentals

TC010-05-01S

FIGURE 1.
Power is
developed by
burning fuel in a
cylinder.

Burning
mixture of air
and fuel

Piston

Cylinder

Crankshaft

HOW POWER IS DEVELOPED

Figure 1 shows how the energy from burning fuel is converted into rotating
power. In an engine, a piston is closely fitted into a hollow cylinder. The
cylinder has free space at the top, where a mixture of air and fuel is
inserted.

The air-fuel mixture is ignited, and the burning gases from the mixture
expand, creating very high pressure. This pressure pushes down on the
piston, causing it to move down in the cylinder.

The process shown in Figure 1 is how the engine produces power.
Everything else on the engine is designed to control this process, harness
the power, and transmit it to the wheels.



Harnessing Power

To harness this power, the up-and-down movement of the piston is
changed into a turning (rotary) motion by connecting the piston to a
crankshaft,

as shown in Figure 1.

2 – BASIC OPERATION

7

Piston Engine Fundamentals

TC010-05-01S

The engine’s cylinders are arranged to fire one after the other. In this way,
some pistons are always supplying power to the crankshaft while other
pistons are moving up into their cylinders. To keep the crankshaft rotating
smoothly, it is connected to a flywheel, which is a heavy round plate that
turns with the crankshaft. See Figure 2.























Because of its weight, the flywheel tends to keep turning smoothly even
though power is applied in spurts by the pistons. The end of the crankshaft
is also connected to the vehicle’s transmission to continue the power flow
through the drive train to the wheels.

Controlling Combustion

The process of burning the air-fuel mixture in the cylinder is called
combustion

. The combustion process in an engine requires four steps:

1. Admit the proper mixture of air and fuel into the cylinder.

2. Squeeze (compress) the mixture so it will burn better and deliver

more power.

FIGURE 2. The
flywheel Keeps the
crank- shaft
turning smoothly.

Piston

Crankshaft

Flywheel

To transmission

2 – BASIC OPERATION

8

Piston Engine Fundamentals

TC010-05-01S

FIGURE 3.
Valves and a
spark plug help
control
combustion in
the cylinder.

Intake valve
Spark plug
Exhaust
valve

3. Burn the air-fuel mixture.

4. Remove the burned gases from the cylinder so intake, compression,

and combustion can be repeated.

As Figure 3 shows, each cylinder has several parts that help control the
combustion process:

• An

intake valve

lets the air-fuel mixture into the cylinder.

• A

spark plug

ignites the air-fuel mixture.

• An

exhaust valve

lets the burned gases out of the cylinder.