2nd class Automobile Electricity chapter 4

Chapter four

Starting system
4.1 Introduction
An internal combustion engine requires the following criteria in order to start and continue
running.
1- Combustible mixture.
2- Compression stroke.
3- A form of ignition.
4- The minimum starting speed (about 100 rev/min).
In order to produce the first three of these, the minimum starting speed must be achieved. This is
where the electric starter comes in. The ability to reach this minimum speed is again dependent
on a number of factors.
* Rated voltage of the starting system.
* Lowest possible temperature at which it must still be possible to start the engine. This is known
as the starting limit temperature.
* Engine cranking resistance. In other words the torque required to crank the engine at its
starting limit temperature (including the initial stalled torque).
* Battery characteristics.
* Voltage drop between the battery and the starter.
*Starter-to-ring gear ratio.
* Characteristics of the starter.
*Minimum cranking speed of the engine at the starting limit temperature. It is not possible to
view the starter as an isolated component within the vehicle electrical system. The battery in
particular is of prime importance.
Another particularly important consideration in relation to engine starting requirements is the
starting limit temperature.

4.2 Choosing a starter motor

As a guide, the starter motor must meet all the criteria previously discussed. Manufacturers of
starter motors provide data in the form of characteristic curves. These are discussed in more
detail in the next section. The data will show the torque, speed, power and current consumption
of the starter at +20 ° C and -20 ° C.
The power rating of the motor is quoted as the maximum output at -20 ° C using the
recommended battery. A greater torque is required for engines with a lower number of cylinders
due to the greater piston displacement per cylinder. This will determine the peak torque values.
The other main factor is compression ratio.

1
 2nd class Automobile Electricity chapter 4

To illustrate the link between torque and power, we can assume that, under the worst conditions
(-20 ° C), a four-cylinder 2-litre engine requires 480 Nm to overcome static friction and 160 Nm
to maintain the minimum cranking speed of 100 rev/ min. With a starter pinion-to-ring gear ratio
of 10 : 1, the motor must therefore, be able to produce a maximum stalled torque of 48 Nm and a
driving torque of 16 Nm. This is working on the assumption that stalled torque is generally three
to four times the cranking torque, Figure 4.1.

Figure 4.1 Torque required for various engine sizes

4.3 Starting system circuits

In comparison with most other circuits on the modern vehicle, the starter circuit is very simple.
The problem to be overcome, however, is that of volt drop in the main supply wires. The starter
is usually operated by a spring-loaded key switch, and the same switch also controls the ignition
and accessories. The supply from the key switch, via a relay in many cases, causes the starter
solenoid to operate, and this in turn, by a set of contacts, controls the heavy current.

In some cases an extra terminal on the starter solenoid provides an output when cranking, which
is usually used to bypass a dropping resistor on the ignition or fuel pump circuits. The problem
of volt drop in the main supply circuit is due to the high current required by the starter,
particularly under adverse starting conditions such as very low temperatures.

A typical cranking current for a light vehicle engine is of the order of 150 A, but this may peak
in excess of 500 A to provide the initial stalled torque. It is generally accepted that a maximum
volt drop of only 0.5 V should be allowed between the battery and the starter when operating. An
Ohm’s law calculation indicates that the maximum allowed circuit resistance is 2.5m when
using a 12 V supply. This is a worst case situation and lower resistance values are used in most
applications. The choice of suitable conductors is therefore very important.

2
 2nd class Automobile Electricity chapter 4

4.4 Principle of operation

The simple definition of any motor is a machine to convert electrical energy into mechanical
energy. The starter motor is no exception. When current flows through a conductor placed in a
magnetic field, a force is created acting on the conductor relative to the field. The magnitude of
this force is proportional to the field strength, the length of the conductor in the field and the
current flowing in the conductor.
In any DC motor, the single conductor is of no practical use and so the conductor is shaped into a
loop or many loops to form the armature. A many segment commutator allows contact via
brushes to the supply current.
The force on the conductor is created due to the interaction of the main magnetic field and the
field created around the conductor. In a light vehicle starter motor, the main field was
traditionally created by heavy duty series windings wound around soft iron pole shoes.
Due to improvements in magnet technology, permanent magnet fields allowing a smaller and
lighter construction are replacing wire-wound fields. The strength of the magnetic field created
around the conductors in the armature is determined by the value of the current flowing.

Most starter designs use a four-pole four-brush system. Using four field poles concentrates the
magnetic field in four areas as shown in Figure 4.2. The magnetism is created in one of three
ways, permanent magnets, series field windings or series– parallel field windings. Figure 4.3
shows the circuits of the two methods where field windings are used. The series–parallel fields
can be constructed with a lower resistance, thereby increasing the current and hence torque of the
motor. Four brushes are used to carry the heavy current.

3
 2nd class Automobile Electricity chapter 4

The brushes are made of a mixture of copper and carbon, as is the case for most motor or
generator brushes. Starter brushes have a higher copper content to minimize electrical losses.
Figure 4.4 shows some typical field coils with brushes attached. The field windings on the right
are known as wave wound.
The armature consists of a segmented copper commutator and heavy duty copper windings. The
windings on a motor armature can, broadly speaking, be wound in two ways. These are known as
lap winding and wave winding. Starter motors tend to use wave winding as this technique gives
the most appropriate torque and speed characteristic for a four-pole system.

A starter must also have some method of engaging with, and release from, the vehicle’s flywheel
ring gear. In the case of light vehicle starters, this is achieved either by an inertia-type
engagement or a pre-engagement method.

4.5 DC motor characteristics

It is possible to design a motor with characteristics that are most suitable for a particular task.
For a comparison between the main types of DC motor, the speed–torque characteristics are
shown in Figure 4.5. The four main types of motor are referred to as shunt wound, series wound,
compound wound and permanent magnet excitation.
In shunt wound motors, the field winding is connected in parallel with the armature as shown in
Figure 4.6. Due to the constant excitation of the fields, the speed of this motor remains constant,
virtually independent of torque. Series wound motors have the field and armature connected in
series. Because of this method of connection, the armature current passes through the fields
making it necessary for the field windings to consist usually of only a few turns of heavy wire.
When this motor starts under load the high initial current, due to low resistance and no back
EMF, generates a very strong magnetic field and therefore high initial torque. This characteristic
makes the series wound motor ideal as a starter motor.

Figure 4.3 Starter internal circuits

4
 2nd class Automobile Electricity chapter 4

Figure 4.7 shows the circuit of a series wound motor. The compound wound motor, as shown in
Figure 4.8, is a combination of shunt and series wound motors. Depending on how the field
windings are connected, the characteristics can vary. The usual variation is where the shunt
winding is connected, which is either across the armature or across the armature and series
winding. Large starter motors are often compound wound and can be operated in two stages. The
first stage involves the shunt winding being connected in series with the armature.

Figure 4.4 Typical field coils and brushes

Figure 4.5 Speed and torque characteristics of DC motors

5
 2nd class Automobile Electricity chapter 4

Figure 4.6 Shunt wound motor (parallel wound) Figure 4.7 Series wound motor

Figure 4.8 Compound wound motor

This unusual connection allows for low meshing torque due to the resistance of the shunt
winding. When the pinion of the starter is fully in mesh with the ring gear, a set of contacts
causes the main supply to be passed through the series winding and armature giving full torque.
The shunt winding will now be connected in parallel and will act in such a way as to limit the
maximum speed of the motor.
Permanent magnet motors are smaller and simpler compared with the other three discussed.
Field excitation, as the name suggests, is by permanent magnet. This excitation will remain
constant under all operating conditions.

The characteristics of this type of motor are broadly similar to the shunt wound motors.
However, when one of these types is used as a starter motor, the drop in battery voltage tends to
cause the motor to behave in a similar way to a series wound machine. In some cases though, the
higher speed and lower torque characteristic are enhanced by using an intermediate transmission
gearbox inside the starter motor. Information on particular starters is provided in the form of
characteristic curves.

6
 2nd class Automobile Electricity chapter 4

Owing to the very high speeds developed under no load conditions, it is possible to damage this
type of motor. Running off load due to the high centrifugal forces on the armature may cause the
windings to be destroyed. Note that the maximum power of this motor is developed at midrange
speed but maximum torque is at zero speed.

4.6 Pre-engaged starters

Pre-engaged starters are fitted to the majority of vehicles in use today. They provide a positive
engagement with the ring gear, as full power is not applied until the pinion is fully in mesh. They
prevent premature ejection as the pinion is held into mesh by the action of a solenoid. A one-way
clutch is incorporated into the pinion to prevent the starter motor being driven by the engine. One
example of a pre-engaged starter in common use is shown in Figure 4.9, the Bosch EF starter.
Figure 4.10 shows the circuit associated with operating this type of pre-engaged starter. The
basic operation of the pre-engaged starter is as follows.

When the key switch is operated, a supply is made to terminal 50 on the solenoid. This causes
two windings to be energized, the hold-on winding and the pull-in winding. Note that the pull-in
winding is of very low resistance and hence a high current flows. This winding is connected in
series with the motor circuit and the current flowing will allow the motor to rotate slowly to
facilitate engagement.

Figure 4.9 Pre-engaged starter

At the same time, the magnetism created in the solenoid attracts the plunger and, via an
operating lever, pushes the pinion into mesh with the flywheel ring gear. When the pinion is
fully in mesh the plunger, at the end of its travel, causes a heavy-duty set of copper contacts to
close. These contacts now supply full battery power to the main circuit of the starter motor.

7
 2nd class Automobile Electricity chapter 4

When the main contacts are closed, the pull-in winding is effectively switched off due to equal
voltage supply on both ends.

Figure 4.10 Starter circuit

The hold-on winding holds the plunger in position as long as the solenoid is supplied from the
key switch. When the engine starts and the key is released, the main supply is removed and the
plunger and pinion return to their rest positions under spring tension. A lost motion spring
located on the plunger ensures that the main contacts open before the pinion is retracted from
mesh.
During engagement, if the teeth of the pinion hit the teeth of the flywheel (tooth to tooth
abutment), the main contacts are allowed to close due to the engagement spring being
compressed. This allows the motor to rotate under power and the pinion will slip into mesh.

Figure 4.11 shows a sectioned view of a one-way clutch assembly. The torque developed by the
starter is passed through the clutch to the ring gear. The purpose of this free-wheeling device is
to prevent the starter being driven at an excessively high speed if the pinion is held in mesh after
the engine has started. The clutch consists of a driving and driven member with several rollers
between the two.

The rollers are spring loaded and either wedge-lock the two members together by being
compressed against the springs, or free-wheel in the opposite direction. Many variations of the
pre-engaged starter are in common use, but all work on similar lines to the above description.
The wound field type of motor has now largely been replaced by the permanent magnet version.

8
 2nd class Automobile Electricity chapter 4

Figure 4.11 One-way roller clutch drive pinion

Figure 4.12 Starter motor intermediate transmission

4.7 Integrated starters

A device called a ‘dynastart’ was used on a number of vehicles from the 1930s through to the
1960s. This device was a combination of the starter and a dynamo. The device, directly mounted
on the crankshaft, was a compromise and hence not very efficient. The method is now known as
an Integrated Starter Alternator Damper (ISAD). It consists of an electric motor, which functions
as a control element between the engine and the transmission, and can also be used to start the
engine and deliver electrical power to the batteries and the rest of the vehicle systems.

The electric motor replaces the mass of the flywheel. The motor transfers the drive from the
engine and is also able to act as a damper/vibration absorber unit. The damping effect is achieved

9
 2nd class Automobile Electricity chapter 4

by a rotation capacitor. A change in relative speed between the rotor and the engine due to the
vibration, causes one pole of the capacitor to be charged. The effect of this is to take the energy
from the vibration.

Using ISAD to start the engine is virtually noiseless, and cranking speeds of 700 rev/min are
possible. Even at -25 ° C it is still possible to crank at about 400 rev/min. A good feature of this
is that a stop/start function is possible as an economy and emissions improvement technique.
Because of the high speed cranking, the engine will fire up in about 0.1–0.5 seconds. The motor
can also be used to aid with acceleration of the vehicle.

This feature could be used to allow a smaller engine to be used or to enhance the performance of
a standard engine. When used in alternator mode, the ISAD can produce up to 2 kW at idle
speed. It can supply power at different voltages as both AC and DC. Through the application of
intelligent control electronics, the ISAD can be up to 80% efficient. Citroën have used the ISAD
system in a Xsara model prototype. The car can produce 150 Nm for up to 30 seconds, which is
significantly more than the 135 Nm peak torque of the 1580 cc, 65 kW fuel injected version.
Citroën call the system ‘Dynalto’. A 220 V outlet is even provided inside the car to power
domestic electrical appliances!

Figure 4.13 Integrated starter alternator damper

10