Diesel Engine Basics for Boat Users 6 Things Electrical

This thread concerns the electrical niceties of our engines. For a further introduction please see the Intro thread under the same title.

Please note this thread will not cover Electronic Control Systems. I regret that these fall of the edge of my pitch.

This will cover …

12 V DC systems affecting diesel engines (similar rules apply for 24 V DC users)

Starter motors

Alternators

Key also to the electrical thread is Batteries, but I have decided to leave this out of this section, as otherwise it is becoming too long.

I have not covered ancillary circuits, such as Control systems, Gauges, Stop solenoids, Heater plugs, etc

12 V DC Systems

The electrical control systems on our engines are derived largely from automotive origins, and a good many of the components used are the same, or very similar with (hopefully !) improved materials for a Marine environment.

So let’s start here. Car based systems do not suffer the harsh salty environment, and so can use cheaper materials. Also the car market is a mass market so individual items can be from mass produced sources. Not so Marine engines. This explains some of the cost for items intended for Marine applications, and when dealing with the electrics it is with good reason that the manufacturers should not skimp. However, unfortunately some do. Indeed until the RCD came in there was no standard required at all for electrical installations on boats, and even with the RCD the standards are quite low. In the USA the ABYC standards, which are mandatory over there, set out a very good set of rules, and I would advise anyone wishing to learn more of this to purchase some of the excellent manuals available.

For instance … Boatowner’s Electrical & Mechanical Manual - Nigel Calder

Boatowner’s Wiring Manual – Charlie Wing

Powerboater’s Guide to Electrical - Ed Sherman

12 Volt Doctor

This thread is not the place for explaining basic electrical theory, but any of the foregoing will provide an excellent basis for this. Nigel Calder’s book covers both US and European applications with useful explanations of both the ABYC and ISO systems where differences arise, such as colour coding for AC systems.

I would suggest all boater’s should have available for their electrical mods …

Proper crimp terminals & ratchet crimper

Adhesive filled shrink wrap & heat gun – I use a gas fired combined heatgun and soldering iron

Self amalgamating tape

Time to work out properly what is needed, particularly correct cable size and quality

Proper regard for Fuses and / or Circuit Breakers

Starter Motors

Without this working you are not going anywhere. Most inconvenient if it won’t work on your home mooring. Damn well dangerous if out at sea and the engine has been stopped for any reason. Please note I have assumed that for virtually all readers of this thread their engine/s will be above a size that can be hand started.

Generally there are two broad types of starter motor – Pre-engaged and Bendix. The former uses a very large solenoid (an Electro-Magnetic Mechanical Switch) to pull the driver pinion into engagement with the Start gear ring normally mounted to the flywheel on the crankshaft. The Bendix uses a coarse thread which as the starter spins up allows the pinion to engage the starter ring by its momentum.

Nothing wrong with either type, but be aware occasionally they can jam. If this happens barring or turning the engine over by hand may free the thing off. If this happens to you then it suggests the mechanism is dirty, so time to remove the starter motor for a good clean of the engagement mechanism, including the starter ring. This can be a considerable job, and starter motors are not light.

You also need to work out how to bar the engine, and the best method is to use a suitable socket onto the nut securing the belt pulley onto the crankshaft free end. When doing this turn off the engine batteries, remove the start key/s, and keep them in your pocket. Always turn the crank in the normal direction of rotation – MARK IT with an indelible marker. Also avoid turning it backwards, as this can cause other issues with pump impellors, etc. All this is something to work out how to do as part of your familiarity checks on the engines, and make sure you have the right size tools for this job – a hefty ratchet or breaker bar and suitable large socket.

Electrical operation of the Starter

External to the starter will be a powerful Solenoid relay (additional to the pre-engagement solenoid on the motor itself). This takes a low current control supply from the start ‘Ignition’ switch, and in turn enables the massive current the starter motor requires to spin the engine. This solenoid is a common source of starter troubles. Repeated use leads to the internal contacts becoming dirty from arcing. Moisture affects electrical connections. So keep it clean and keep it dry, keep the electrical connections tight.

In a dire emergency (i.e. at sea with no engine running or otherwise being able to start, and after you have called for external assistance) the solenoid can be manually bypassed with a chunky piece of cable bridging the terminals, but manually starting an engine in this way really is a last resort. Expect heavy sparks to fly as the terminals are bridged, so do not ever do this on a petrol engine. If you have two engines then live with it until you return to a safe mooring. If you only have one engine, use a short piece of heavy cable with one end at least lugged, so it can be physically attached to one of the main terminals, will reduce the sparking. Also do not allow your hand to touch any bare metal on the bridge cable of anywhere else, as the current drawn will be massive and potentially dangerous.

In a similar vein keep all associated electrical feeds from the battery terminals to and from the starter clean, dry and tight. Regular maintenance should be checking for loose terminals, and any found loose should be dismantled and cleaned. If verdigris (greenish corrosion products) is building up on any terminals clean this and apply a cost of Vaseline. If any are rusting they are steel and should be changed or at least greased well.

Time limits for operating starter motors

The current flowing into the starter during its operation is huge. A side effect of this is massive heating of the electrical coils within the starter motor. If the motor is repeatedly operated for more than a few seconds these will get hotter and hotter, and after say 30 seconds of operation could be attaining internal temperatures sufficient to melt the insulation coating on the individual wires.

Therefore avoid excessive starting attempts. If you have used up say 15 seconds in a short burst or series of bursts of starting then wait 5 minutes for the next session, then if this is still fruitless wait 10 minutes. On many engines excessive starter use may be required to bleed the engine filters, so be careful, and after doing this feel the casings of the starter motor to see just how hot they get.

Alternators

As the title suggests, alternators produce an alternating current AC, but our boat’s system are DC – direct current. Generally this is a three phase current, and this is immediately rectified by a grouping of six individual diodes. From each phase a pair of diodes (electrical version of a non-return or check valve) are attached one with the anode terminal to the alternator output – other end to Negative –VE, and the other with the cathode terminal to alternator – the other end to +VE That is with the output within the alternator connected to the central ‘o’in this …

-VE ->| -o- >|- +VE

The three pairs provide continuous output of the individual alternating currents either out to the +VE whilst the phase current is positive, or back in to the stator whilst the current is –VE phase. For the circuit to work as much current must flow out as flows back in. I regret this is not the place to explain further how AC currents, especially 3 phase work. The output from the combine dsix dioes will be a DC voltage, and the design will set this to nominal 12 v or 24 v. The actual voltage will be higher – generally around 13.5 volts for 12 v systems and 27 volts for 24 v systems.

All the while an alternator is producing current is must be drained off somewhere through the diodes to a load circuit, e.g. a battery. The loading this creates must not be broken whilst the alternator is operating. If this does happen the circuit still wishes to be continued, i.e. the power has to go somewhere. What happens in practice if the circuit is broken is the circuit voltage rises uncontrollably, and massively, until eventually the voltage breaks down the diode’s non-conductive one way barrier, and this wrecks the diodes, and shorts out the alternator fields. So DO NOT DISCONNECT THE ENGINE FROM THE BATTERY WHILST THE ENGINE IS RUNNING. This means do not turn off the battery isolator switches, or you risk a destroyed alternator.

Excitation

Like a good (or indeed bad) woman, alternators require excitement to get going !

The reason alternators work is caused by a magnetic field rotating within a static group of coils or windings – the Stator. The stronger this magnetic field the higher the output current will become. However, the control of the current is not the direct control, instead the voltage is regulated. This is because as the load and hence current increases, the voltage of the output tends to fall, so by

introducing a control device that can regulate the output voltage to a constant level, the current produced becomes self governing.

This is the job of the voltage regulator – more of this device shortly. Before this I will try to explain what the rotor field is and does. In addition to the main output stationary Stator windings, there is an additional winding within the Rotor device. This Rotor has its magnetic field produced by a DC current, at quite low level, being passed through it. This is where the brushes and generator warning light come in.

When you commence the start sequence the Generator Light will come on, then as you start the engine this light extinguishes as the engine first revs up. So why does this happen ?

Embedded within the alternator, normally adjacent to the main rectifier diodes will be three smaller diodes one from each phase of the AC stator main coils ((cathode side), and the output from these (anode side) connects to the same wire feeding one of the brushes to the field coil (Pos or Neg – I will come to this but also further on). When the alternator is static and then just starting its rotation there is no output, and without a current to the rotor there would be no rotating magnetic field and hence no current output. So a secondary feed is provided via a small indicator light – the Generator Warning Light you see on the dash panel. This is normally a standard 12 v bulb of around 1 – 2 watts output, so you can see the current flowing to this field is very small (Ohms law Power = IV=Watts or I = Watts/Volts so I (current) = 1 or 2 / 12 so approx 0.1 – 0.15 amps.

However, this small DC current flowing around the rotor coil is sufficient to get the alternator ‘excited’ (I wish women were this low cost to excite), or producing output current, and the Stator output current will be many times higher than the DC Rotor field current. By installing three small Field current diodes this taps some of the generated power to feed the Field coil of the rotor, once the stator is providing an output. So as the Stator output voltage rises the feed to the field comes from this source, and as this voltage is the same as the feed voltage to the Warning light (both nominal voltage of the system, e.g. 12 v, current stops flowing through the warning light and the light extinguishes. This small wattage load is crucial, so never replace the field light with a very low power LED, the alternator simply will not excite.

Field control

Above I mentioned the field supply could connect to either Neg or Pos feed to the field. Essentially the field circuit is a simple coil connected to a 12 v DC supply. Where the load (warning lamp) between the Onboard 12 v source Positive +VE appears relative to the Rotor field doesn’t matter, unless you wish to use an alternator controller (more of this later).

Also in direct series with the Field rotor but from the other brush is found the voltage regulator.

The voltage regulator monitors the output voltage and adjusts the Rotor field strength to maintain a constant output voltage. So as the Alternator load increases as a result of the external voltage falling slightly, the output current tends to rise, but as it does so the terminal output voltage tends to fall, and upon sensing this fall the VR increases the relatively small current flow to the rotor field coil, this increases the strength of the rotating magnetic field, forcing the terminal voltage back up. Now and due to the external load increase the output will be a higher current. It will keep increasing for the load falling until is reaches its maximum output capacity.

Conversely if the Load current falls, the terminal voltage will tend to rise, so the VR senses this and reduces the current to the field rotor, which reduces the terminal voltage and hence the current output will fall back.

The VR may be either a fast acting mechanical relay, or a transistor device.

External Alternator control

A primary problem with this mode of control concerns the sensed voltage with standard batteries. When the engine is started a mass surge of power is drawn from the batteries, and this causes the terminal voltage to fall, so as soon as the alternator first excites its load will immediately be high. However, due to their internal nature the battery voltage will recover long before its charge state has optimised, and as soon as this does recover the alternator output will fall, which means alternators are not ideal for re-charging partially flat batteries. The purpose of an External Voltage Controller is to take over control of the alternator, whereby it ignores the battery terminal voltage, and will instead apply a higher demand to the Field Rotor coil, increasing the alternator output voltage, and this provides for more effective battery charging.

If more on external Alternator Control is required I can provide details of Sterling’s devices which explain this very well.

Rudimentary forms of manual external Alternator control are also provided in the 12 Volt Doctor.

Good reading I have found for Alternators are the books mentioned above, but also the following, which are freely available for download from certain websites …

12 Volt Doctor

Alternator Handbook

I believe these are also out of print, so I can provide a direct links here if required

Trevor