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