Marine-engine electrical starting systems
THE ENGINE STARTER MOTOR, POWER BEHIND THE POWER
Most boats have a marine-engine. The starter motor is the essential component
to turning over and starting a diesel engine. It is a frequently cited along
with its associated circuitry as a cause of engine failure. The electrical systems
that make up an marine-engine starting system include the starting battery; the
engine control panel; the wiring loom; the preheating system and the starter
motor. It is important to understand the sequence of electrical functions that
occur to start an engine. When the key switch is turned to the on position, the
engine control circuit is energized, and when the key switch is turned to the start
position the start solenoid is energized and pulls in to supply current to the start
motor. The motor turns to bring the marine-engine up to a speed where ignition
occurs.
An electric starter consists of three main components, which are the DC motor,
the solenoid, and the pinion engaging drive. The DC motor must provide the
high initial torque required to overcome friction, inertia and cylinder compression
and accelerate the engine speed to a point where self-ignition temperatures and
combustion starts. This is typically in the range 60-200 rpm depending on
whether glow plugs are used. The starter motor torque is transmitted by the
pinion and ring gear onto the flywheel.
WHAT IS THE SOLENOID?
The solenoid is a high current relay that consists of coil and armature, moving
and fixed contacts. In most cases the solenoid is mounted directly to the starting
motor housing, which reduces cables and interconnections to a minimum,
although on some starters they are completely separate. When the solenoid coil
is energized by the starting circuit, the solenoid plunger is drawn into the
energized core and this closes the main contacts to supply current to the starter
motor. On many starters the solenoid also has a mechanical function. The
solenoid activates a shift or engaging lever to slide the overrunning clutch along
the shaft to mesh the pinion gear with the flywheel, and when engaged the
starter motor then turns the engine, so meshing occurs before starting.
WHAT ARE THE PARTS OF A STARTER MOTOR?
The starter motor consists of 4 poles shoes or magnets, and some starters use
permanent magnets. The poles are surrounded by the excitation winding which
creates the magnetic field when current is applied. The rotating part is called the
armature and also incorporates the commutator. The 4 carbon brushes provide
the positive and negative power supply via the commutator to the armature
windings. DC motor types are based on the connection configuration of the field
windings. The field windings are connected either in series or parallel with the
armature windings, and this includes the series wound, the shunt (parallel)
wound, the compound (series-compound) wound and the permanent magnet.
Shunt motors are not used in starters as they have a low starting torque. Series
wound motors are used as they have high starting torques, however they can run
at high speeds when under light loads. Compound wound starter motors are
also used as they have both high starting torques and some speed regulation
capability.
The pinion engaging drive is located within the end shield assembly of the starter
and consists of the pinion engaging drive and pinion, the overrunning clutch, the
engagement lever or linkage and spring. When the motor operates the drive
gear meshes with the ring gear or flywheel teeth to turn the engine, and then
disengages after starting. The overrunning clutch has two important functions,
the first is to transmit the power from the motor to the pinion, and the second
which is to stop the starter motor armature from over-speeding and being
damaged when the engine starts. Pre-engaged starters generally used a roller
type clutch, while larger multi-plate types are used in sliding gear starters.
The most common type of starter motor is the solenoid-operated direct drive unit
and the operating principles are the same for all solenoid-shifted starter motors.
Some manufacturers use a gear reduction starter to provide increased torque.
The gear reduction starter differs from most other designs in that the armature
does not drive the pinion directly. In this design, the armature drives a small gear
that is in constant mesh with a larger gear. Depending on the application, the
ratio between these two gears is between 2:1 and 3.5:1. The additional reduction
allows for a small motor to turn at higher speeds and produce more torque with
less current draw. The solenoid operation is similar to that of the solenoid-
shifted direct drive starter in that the solenoid moves the plunger, which engages
the starter drive. Sliding gear drives with electrical or mechanical pinion rotation
type starters are typified by the Bosch starters. Still seen on many boat engines
is the Bendix friction-clutch mechanism drive which was developed in the early
20th century. The starters use a drive friction clutch, which has a drive pinion
mounted on a spiral-threaded sleeve. The sleeve rotates within the pinion, and
moves the pinion outwards to mesh with the flywheel ring gear, and the impact of
this meshing action is absorbed by the friction clutch. The engine once started
then turns at a higher speed and drives the Bendix gear at a higher speed than
the starter motor. The pinion then rotates in the opposite direction to the spiral
shaft and then disengages.
RELIABILITY AND PERFORMANCE
The operation of starter motors and the performance depends on many of the
circuit elements. A starting system must be viewed as not simply a collection of
series connected components, but as a system. The typical marine-engine
starting system comprises the DC positive circuit, which includes the battery
connections, the isolator or changeover switch, the solenoid connection and
solenoid contacts, the starter motor and the various components such as
brushes, brush gear, commutator, bearings, windings. The DC negative circuit,
which also includes the battery connections, the engine block which is often part
of the return path, the cable back to the battery, and the meter shunt if fitted.
Last is the engine control system from the panel which includes the key switch,
stop and start buttons, wiring harness, connectors and fuses.
All parts offer common points that prevent starter operation, and bad connections
are the most common cause of failure. The most common faults are high
resistances and excess voltage drops in both the positive and negative circuits.
Battery changeover switches are also a major culprit here and I have frequently
encountered this, and it pays to have high quality switches. The main start
cables should be rated to have minimal voltage drop at full rated current. In
addition the cables should be kept as short as possible and as large as possible
to also minimize losses and maximize power availability to the starter. As an
indication of start current levels, the instantaneous stalled condition short circuit
currents can be up to 3500 amps in large engine starters before the load drops to
a few hundred amps. Even small values of resistance and voltage drop can have
significant effects and reduce or prevent starting.
CLICK HERE FOR MORE ESSENTIAL INFORMATION ON MARINE-ENGINE BATTERY CHARGING
THE MARINE-ENGINE STARTER INSTALLATION
The marine-engine starter installation is generally part of the marine-engine manufacturers specification and
boat user factors are limited to a few factors only. The first is being mechanically
secure, and the second is that the attached cables are of the correct rating, and
that the terminal nuts are properly torqued up so that they do not work loose.
Things that are within a persons ability to control are the connection of the
negative cable which should be attached as close as practicable to the starter. In
most cases it is fastened at the closest engine point, which generally makes a
poor contact and inserts resistance into the circuit, as the marine-engine block becomes
part of the circuit. Starter motor design is generally robust as it must withstand
the shocks of meshing, engine vibration, salt and moisture laden air, water, oil,
temperature extremes, and high levels of overload.
Marine-engine starters on boats are by default located low down towards engine
bilges, subject to leakages from seawater cooling systems, seawater injection
points into exhaust elbows, as well as the unexpected high bilge level. The
position generally is out of sight and therefore general inspections are often
cursory and inadequate. Check your marine-engine and check the starter, if its
leaking at the top end then it's probably leaking over the starter also. Corrosion
can be rapid on a starter. Another area to check is whether you can physically
remove the starter, in many installations this is extraordinarily difficult, with tool
access to remove holding bolts nearly impossible. One thing a marine-engine manufacturer really considers is access to starters. A little practice can pay
dividends when things go wrong. In addition remove each bolt and apply some
thread lubricant such as copperslip or similar, so that bolts don’t seize in
STARTER MAINTENANCE
Preventive maintenance is essential to ensuring reliability. A common problem
especially on little used vessels is the formation of surface corrosion, or
accumulations of dirt on the shaft and pinion gear assembly, and lack of
operation and lubrication causes seizure or a failure to engage. It is good
practice to operate the starter as often as possible, the heat dries the windings
out and also remove the starter every 12-18 months or prior to an extended
cruise to clean and lightly oil the components according to manufacturers
recommendations. Alternatively drop the starter into your local automotive
electrician who can run a quick test and perform the basic maintenance.
The most common problems occur with seized brushes, and this is due primarily
to a lack of use. Where the starter design allows access, manually check that
brushes are moving freely in the brush-holders, and that the commutator is also
clean. Remove all dust and particles using a vacuum cleaner. If the commutator
is dirty wash it and the brush gear out with a quality spray electrical cleaner if
badly soiled. Under no circumstances clean and polish the commutator with any
abrasive materials as this will quickly ruin it. Commutators build up a surface
patina that should not be removed, as sparking brushes will rapidly degrade the
surface and edges of the slots.
SO IT WILL NOT START!
There are few of us who haven't heard the deafening silence of that ominous
click, and a failure to start, or a chattering solenoid or a failure to get the marine-engine turning over fast enough to start. The following are typical faults to look at
when troubleshooting.
LOW VOLTAGE
Low Voltage is the most common cause, and the battery and battery connections
are the first place to look. The main starter motor and solenoid terminals are the
next common failure area and often work loose or make inadequate contact.
Warm or hot terminals are a good sign of problems and I have seen smoke
curling up on some occasions from battery and starter connections. Also check
the engine negative connection, as loose bolts are common.
OVERHEATING
Winding overheating on marine-engine starter motors is a direct result of attempting too many
starts or excessively long start attempts. The marine-engine cannot be turned
over at the required speed either due to lower voltages, or excessive loads
especially in cold conditions where high oil viscosity is a factor. Partial or
complete failure of the preheating glow plugs is also a reason for slow starts and
this should be looked at.
NOISE and VIBRATION
Excessive starter motor noise and vibration may indicate that bolts are loose.
Bearing failures are comparatively few in marine-engine boat starters as the number of starts is relatively few. Another cause of noise can be attributable to the pinion drive and the overrunning clutch having defects or jamming when disengaging.
SUMMARY
Without the starter the marine-engine is just another piece of cold iron. Starter
reliability and performance comes down to ensuring the cables and power supply
are sound, and the connections secure. In addition the minimization of seawater
exposure and maximization of starts will reduce increase reliability and reduce
failures. All these tasks are within the scope of any boat owner, and only require
simple vigilance and routine visual tasks.
Make starter motor inspection an essential part of your marine-engine maintenance regime, it will pay you dividends in safety and reliability
