homework encyclopedia

198 Science and Technology

A machine that converts energy from a fuel to do work

is an engine. Steam engines were the first engines for

transport and industry. Internal combustion engines

power road vehicles and many trains. Jet engines

power aircraft, and. TURBINES drive ships.

WHAT IS A HEAT ENGINE?

Most engines convert heat energy into motion. The

heat comes from burning a fuel such as coal, petrol,

or hydrogen gas. The heat makes a gas, such as air,

expand rapidly. In a piston engine, the expanding gas

pushes a piston down a cylinder. The piston moves

down on the power stroke, which drives the machine.

The amount of fuel an engine uses to run for a given

time is called its. FUEL CONSUMPTION.

4 STEAM ENGINE

The first steam engines pumped water from deep mines. Scottish

engineer James Watt (1736-1819) introduced many improvements

to the steam engine. His ideas led to efficient steam engines that

could power factories and drive heavy locomotives, such

as this one from Harbin, Manchuria, in China.

WHAT IS AN INTERNAL COMBUSTION ENGINE?

In this engine, fuel is burnt in a cylinder. The cylinder

draws in air and fuel through a valve as the piston

moves down. As the piston moves up, it compresses

the air and fuel, causing them to heat up. The fuel

combusts (explodes), and the expanding gases from

the explosion push the piston down, producing power.

HOW DOES A STEAM ENGINE WORK?

In a steam engine, fuel is burned outside the cylinder

• coal heats water in a boiler, which makes steam.
  Steam is fed into the cylinder, where it expands and
  pushes the piston. The piston pushes a rod connected
  to a crank to turn a wheel.

Petrol engines are internal

combustion engines. Most

modern petrol engines operate

on a four-stroke cycle. Fuel and

air are drawn into the cylinder,

the mixture is compressed, and

ignited. The expanding gases

push down the piston. As the

piston descends, power is

applied to the crankshaft, and

finally the exhaust gases are

forced out. Each cylinder

operates out of step with the

others, so the four work in

sequence. This produces a

continuous output of power

so that the car runs smoothly.

Engines

INTAKE STROKE

The inlet valve opens. Fuel and

air are drawn into the cylinder

as the piston descends.

Air intake

draws in air needed for

fuel to burn. A filter

traps dust and dirt

COMPRESSION STROKE

The mixture of fuel and air is

compressed as the piston rises.

Then, the spark plug ignites the

mixture and it explodes.

Piston

drives the crankshaft as

it is forced down by

expanding hot gases

Valves

open to let

fuel and air

mixture into

cylinder and

to let exhaust

gases out

Spark plug

ignites the fuel

and air mixture

PETROL ENGINE

Camshaft controls

the opening and

closing of the valves

Crankshaft

turns the wheels

Cylinder

where the

fuel is

ignited

Ignition

system

causes a

spark

Inlet

valve

Spark plug

Crankshaft

Piston