Pre-engaged starters the teeth of the flywheel (tooth to tooth abutment),
7.26.3 Pre-engaged starters the teeth of the flywheel (tooth to tooth abutment),
the main contacts are allowed to close due to the Pre-engaged starters are fitted to the majority of
engagement spring being compressed. This allows vehicles in use today. They provide a positive
the motor to rotate under power and the pinion
Engine systems 161
Figure 7.61 Supply link wire from solenoid to starter motor
Starter switch
7.26.4 Permanent magnet (PM) starters
Permanent magnet starters began to appear on production vehicles in the late eighties. The two
Solenoid
advantages of these motors, compared to conven-
Hold-on
winding
tional types, are less weight and smaller size. This
makes the PM starter a popular choice by vehicle
Battery winding
manufacturers as, due to the lower lines of today’s cars, less space is now available for engine electri- cal systems. The reduction in weight provides a
Motor
contribution towards reducing fuel consumption. The principle of operation is similar in most respects to the conventional pre-engaged starter motor, the main difference being the replacement
Figure 7.62 Starter circuit (Pre-engaged)
of field windings and pole shoes with high qual- ity permanent magnets. The reduction in weight is in the region of 15% and the diameter of the
The torque developed by the starter is passed yoke can be reduced by a similar factor. through a one-way clutch to the ring gear. The
Permanent magnets provide constant excitation purpose of this free wheeling device is to prevent
and it would be reasonable to expect the speed and the starter being driven at excessively high speed
torque characteristic to be constant. However, due if the pinion is held in mesh after the engine has
to the fall in battery voltage under load and the low started. The clutch consists of a driving and driven
resistance of the armature windings, the character- member with several rollers in between the two.
istic is comparable to series wound motors. The rollers are spring loaded and either wedge-lock
Development by some manufacturers has also the two members together by being compressed
taken place in the construction of the brushes. A against the springs, or free wheel in the opposite
copper and graphite mix is used but the brushes direction.
are made in two parts allowing a higher copper Many variations of pre-engaged starter are in
content in the power zone and a higher graphite common use but all work on similar lines to the
content in the commutation zone. This results in above description. The wound field type of motor
increased service life and a reduction in volt drop has now largely been replaced by the permanent
giving improved starter power. Figure 7.63 shows
162 Advanced automotive fault diagnosis
Figure 7.63 Modern permanent magnet starters
1 Pinion 2 Ring gear 3 Roller type overrunning clutch 4 Engaging lever 5 Planetary gear 6 Permanent magnet
7 Armature 8 Commutator with carbon brushes 9 Solenoid switch with pull-in and hold-on windings
10 Starter switch 11 Battery
1 Drive end shield 2 Pinion 3 Solenoid switch 4 Terminal 5 Commutator end shield 6 Brush plate with carbon brushes 7 Commutator 8 Armature 9 Permanent magnet
10 Field frame 11 Planetary gear (intermediate
transmission) 12 Engaging lever 13 Pinion-engaging drive
Figure 7.64 PM starter with intermediate transmission (Source: Bosch)
For applications with a higher power require- engines up to about 1.6 litres. This form of PM ment PM motors with intermediate transmission
motor can give a weight saving of up to 40%. The have been developed. This allows the armature to
principle of operation is again similar to the con- rotate at a higher and more efficient speed whilst
ventional pre-engaged starter. still providing the torque, due to the gear reduc-
The sun gear is on the armature shaft and tion. Permanent magnet starters with intermedi-
the planet carrier drives the pinion. The ring gear or ate transmission are available with power outputs
annulus remains stationary and also acts as an of about 1.7 kW, suitable for spark ignition engines
intermediate bearing. This arrangement of gears up to about five litres or compression ignition
gives a reduction ratio of about 5:1. Figure 7.64
Engine systems 163
shows a PM starter with intermediate transmission,
Solenoid contact
together with its circuit and operating mechanism.
supply terminal
Starter switch