Franzis PE01 Instruction Manual Download Page 20

Page: 20 / 20

HOW AN ENGINE WORKS

Basic Principles

To explain how a full-size car engine works, we’ll use a simplified version of a real engine -
our model engine in fact. You can switch on your finished model as you read this
explanation and it will help you to understand how a real engine works.

An engine crankshaft is described as such because it is a ‘cranked’ shaft – the shaft has
offset sections, to which the six piston/connecting rod assemblies are connected. In the
model, as the crankshaft turns (driven by the electric motor), the cranked sections move
around the centreline of the crankshaft, and convert the rotation to the reciprocating (up
and down) movement of the pistons. In a real engine, it is the movement of the pistons
(driven by the burning of fuel in the cylinders) that causes the crankshaft to rotate.

The engine’s crankshaft drives the transmission, which includes various different
components connected together whose job it is to drive the car’s wheels, making the
car move.

Now let’s look at how the engine works in more detail. We’ll look at just one cylinder of the
engine. A cylinder is the hollow cylinder inside the engine in which one piston moves up and
down. The piston has seals, called piston rings, around its edge, which aren’t shown on our
model and these seals stop gases passing round the sides of the piston. Our model has
six cylinders.

When you turn the engine to start the car, the starter (a powerful electric motor, powered by
the car’s battery) turns the crankshaft, moving the pistons up and down.

As a piston moves down from its highest position inside the cylinder (called ‘Top Dead
Centre’ or ‘TDC’), a mixture of fuel and air is sucked into the space inside the cylinder
above the piston. This is called the intake stroke.

The starter continues to turn the crankshaft and, as the piston moves back up towards the
top of its cylinder, the fuel/air mixture is compressed (squashed) in the space above the
piston. This is called the compression stroke.

As the piston reaches its highest point, a spark plug creates a spark above the piston and
this spark ignites the fuel/air mixture, causing a small controlled explosion above the piston.
The explosion pushes the piston downwards and this is called the power stroke.

Once the piston has reached its lowest point, it starts to move back up its cylinder, pushing
the burnt gases out through the top of the cylinder. This is called the exhaust stroke.

So, the engine has four strokes; intake, compression, power and exhaust, or ‘suck’,
‘squeeze’, ‘bang’ and ‘blow’ to make it simpler. These four strokes make up the
4-stoke cycle.

The fuel/air mixture flows into the space above the piston, called the combustion chamber.
The mixture flows in through a small hole which is opened and closed by the inlet valve.
The burnt gas flows out of the cylinder through another hole which is opened and closed by
the exhaust valve. The valves are normally held closed by springs, but as the engine turns the
valves are pushed open in the correct order by the rocker arms, which are moved by the
camshaft. The rocker arm pushes the valve down against its spring and, as the rocker moves
up, the spring pushes the valve closed.

The 1st stroke (intake)

The piston starts off at the top of the
cylinder. The exhaust valve is closed and
the inlet valve is open. As the crankshaft
turns and the piston moves down inside the
cylinder, fuel/air mixture is sucked in
through the inlet valve. When the piston
reaches its lowest point inside the cylinder,
the cylinder is filled with fuel/air mixture and
the inlet valve closes. This is the end of the
intake stroke.

The 2nd stroke (Compression)

At the start of the compression stroke, the
piston is at its lowest point inside the
cylinder and the inlet and exhaust valves are
closed. The crankshaft continues turning
and the piston moves upwards. As the
piston moves upwards it squeezes
(compresses) the fuel/air mixture and this
increases the temperature of the mixture
very quickly. When the piston reaches its
highest point the mixture is fully compressed
and this is the end of the compression
stroke.

4-stroke cycle

Inlet valve open

Valves closed

Piston rises
compressing
fuel/air
mixture

Exhaust

valve

closed

Inlet port

Piston moves
down sucking
in fuel/air
mixture

Connecting

rod

Crankshaft
rotation

The 4th stroke (Exhaust)

As the piston starts to move up inside the
cylinder again, the exhaust valve opens to
allow the burnt (exhaust) gas to escape
from the cylinder and as the piston moves
up the cylinder it pushes the gas out
through the exhaust valve. As the piston
reaches the top of the cylinder the exhaust
valve closes, the exhaust stroke ends and
the 4-stroke cycle begins again with another
intake stroke.

The 3rd stroke (Power)

The very high pressure and temperature
inside the combustion chamber cause the
fuel/air mixture to break up into very fine
particles, like a mist. These are perfect
conditions for burning a gas. All that’s
needed to start the burning (or ‘combustion’ –
a kind of controlled explosion) is a spark. A
very high electrical voltage supplied to the
spark plug causes a spark to jump across the
gap at the end of the spark plug. This ignites
the fuel/air mixture and starts the combustion.
The force of the controlled explosion and the
expanding gases forces the piston
downwards, which pushes the crankshaft
round. During the power stroke, three things
happen to the fuel/air mixture – it ignites,
combusts (burns) and expands. The
expansion of the gas pushes the piston
down, which transfers power to the
crankshaft. When the piston reaches its
lowest point inside the cylinder this is the end
of the power stroke.