How to detect when the ignition on a diesel engine it turned on?

[towlerg]

The engine in question is a very basic small 2 cylinder engine. I guess the only power used is for the heater plugs and relay the drives the starter.

I thought perhaps measure the current or put a relay coil in-line (that sounds dangerous or attach to a potential divider and the adc on uC. The las is obviously simplest but is there some drawback?

[sokoloff]

There’s no “ignition” per-se.

You might tap into the fuel valve or fuel pump.

[cs.dk]

Can you be more specific about what your goal is?

[Ian.M]

A diesel engine can be entirely mechanical with absolutely no electrical system.  Unless it uses electrically actuated injectors or an electric fuel pump, there is no guaranteed equivalent to the switched supply to the ignition system of a petrol engine.

The easy way to detect if the engine is running is to fit an extra oil pressure switch - if it hasn't got oil pressure, its either stopped or if running will rapidly stop due to bearing failure.  You may also be able to tap into an existing oil pressure switch and sense the voltage.

If its got an alternator, you can sense the voltage on the warning light terminal as an alternative indication its running, but that can be prone to disturbances due to belt slip or other charging system faults that don't prevent the engine running otherwise normally.

[HighVoltage]

The ignition on the diesel engine, may turn on the glow plugs.

If the engine has glow plugs (heater plugs) make sure they get their voltage and a current flows.

On some older engines, the glow plugs are in series and if one is broken, the other one will not get any current.
On newer engines they are in parallel, if one is broken, at least the other one will get power to glow.

Under warm weather, the glow plugs may not be needed to start the engine.
Under cold winter conditions, it is nearly impossible to start without the glow plugs / other help.

[Tom45]

As Ian says, a diesel engine needs no electricity to run. An electric starter cranks it to get it running. A fuel shutoff switch is used to stop it.

Older diesel engines have an oil pressure switch that turns on the electrical system to allow gauges to function. The tachometer is done mechanically.

[Seekonk]

I'd get a knock sensor or magnetic sensor for a keyway in the shaft.

[tautech]

Tap into the alternator field supply.
You'll get full battery voltage with the engine not running and less when it's charging and the charge rate's being regulated but this may suit your needs.
Some alternators have a charge light output which goes either high or low when charging (depending on type) and you can use this also. Just invert it or not for whatever you needs are.

Old generators with mechanical regulators used the D connection from the dynamo as ground for the charge light when the motor wasn't running and when it was the bulb wouldn't light as it had 12V on either terminal.

[cs.dk]

Some alternators would also feature a W-terminal for tacho-output.

I recall that some older diesels had a sensor clamped on one of the injector pipes, usually cylinder 1. I don't know the purpose of it, but it may detect a pulse when an injection is happening. Don't remember if it was VW or PSA though.

[GeorgeOfTheJungle]

The engine in question is a very basic small 2 cylinder engine. I guess the only power used is for the heater plugs and relay the drives the starter.

The glowplugs are only turned on briefly before cranking and are off when running.

[HighVoltage]

I recall that some older diesels had a sensor clamped on one of the injector pipes, usually cylinder 1. I don't know the purpose of it, but it may detect a pulse when an injection is happening. Don't remember if it was VW or PSA though.

That was a piezo pulse generator, reaction on the tiny expansion of the injector tube.
It was used for some service to detect RPM and injection timing.

[mikerj]

The engine in question is a very basic small 2 cylinder engine. I guess the only power used is for the heater plugs and relay the drives the starter.

The glowplugs are only turned on briefly before cranking and are off when running.

True for basic diesel engines like the OP's but not true for modern common rail engines, many of then run the glowplugs during normal operation.

[johnkenyon]

The engine in question is a very basic small 2 cylinder engine. I guess the only power used is for the heater plugs and relay the drives the starter.

The glowplugs are only turned on briefly before cranking and are off when running.

On PSA (aka Peugeot/Citroen) vehicles fitted with the XUD9T engine (aka mechanical/in-direct injection 1.9D Turbo), the glow plugs remained on for a period after the engine started.
Anyone who fitted replacement glow plugs designed for the normally aspirated version would find themselves having to fork out for the correct set after a few weeks, as they weren't tough enough.

By the way, on those engines the only way to confirm the ignition was turned on would have been the stop solenoid feed (12v = ignition on).
Engine running could be confirmed using the alternator lamp.
Good luck with the separate rev counter sensor - it had a knack of getting knackere, leaving you with a randomly flapping meter needle...

[GeorgeOfTheJungle]

The glowplugs are only turned on briefly before cranking and are off when running.

True for basic diesel engines like the OP's but not true for modern common rail engines, many of then run the glowplugs during normal operation.

Jeez, I'm getting old.

[ejeffrey]

Jeez, I'm getting old.

Username checks out.

[max_torque]

As mentioned above, if the engine in question can be started and stopped from a key, then there must be a "stop solenoid" somewhere on the injection pump (Assuming it's an old mechanically pumped unit and not a modern ecu control common rail unit).
  As diesel engines don't have spark plugs, the only way to stop them is to interrupt the flow of fuel, so an electromagnetic stop solenoid is fitted, which when powered  acts to allow the pump to build fuel pressure, but when unpowered, dumps that pressure, so nothing is injected and the engine stops firing.   

Look on the injector pump, for a small solenoid with a single wire attached to it.  This will be powered from the ignition switch when it is in the "run" position, so you can easily monitor that voltage.  (make sure your input circuitry to your micro has sufficient protection to mean the inductive kick of that solenoid doesn't kill your micro btw!)

[bob225]

what make and model of engine is it - is it in a bob cat or quadracycle ?

[Ian.M]

Its not uncommon for non-common-rail small industrial and marine diesels to have a manual stop lever or pull-knob, connected by bowden cable to the injection pump where it overrides the governor to move the fuel control rack to a cutoff position.   Such an engine may be started by hand cranking even if the 'ignition' switch isn't on.  The 'ignition' switch if present, only controls power to the gauges,  alternator field supply terminal, via the start button to the starter solemoid coil, and possibly to a relay controlled by a preheat button if the engine has glow-plugs fitted.   

At its simplest for an engine with electric start and glow-plugs, if a PM alternator is fitted, there may be no switched 'ignition' supply, just a three position momentary keyswitch: Off/Run, Preheat, Start, (spring return to Off/Run) with the (minimal) gauges and/or 'idiot lights' fed from the alternator before the charge controller so they are off with the engine stopped.

Therefore unless the O.P. posts the make and model of his engine and either we can find the wiring diagram from the service manual online, or he has a copy, there's not much we can do to suggest where to pick up an 'ignition' on signal or even an engine running signal.
 

[towlerg]

Thanks to all who posted.

My purpose is to disconnect PV monitoring when engine (with all its spike and general nastiness on the 12v line) is about to start. Maybe ignition was the wrong word, the engine is controlled by a n way switch from furthest acw, cutout, off, on, heater, crank. The heaters are only on while the switch is held in that position. Both crank and cutoff a springloaded to prevent the switch staying in that position when it is released.

There is a solenoid valve in the fuel line which "clicks" when the switch is turned to on and again when turned to cutoff. BTW its a Mitsubishi dumper engine on a small boat.

If I go with detected the fuel cutoff solenoid, what sort of protection do I need from the coil in the solenoid?

[sokoloff]

Does the boat have a load-shedding relay already? Or, said differently, are there accessories that are not powered while the switch is in crank?

It's common on cars to cut out some circuits while cranking.

[towlerg]

@sokoloff no, there is no automatic system to control exteral loads.

[tautech]

Thanks to all who posted.

My purpose is to disconnect PV monitoring when engine (with all its spike and general nastiness on the 12v line) is about to start. Maybe ignition was the wrong word, the engine is controlled by a n way switch from furthest acw, cutout, off, on, heater, crank. The heaters are only on while the switch is held in that position. Both crank and cutoff a springloaded to prevent the switch staying in that position when it is released.

There is a solenoid valve in the fuel line which "clicks" when the switch is turned to on and again when turned to cutoff. BTW its a Mitsubishi dumper engine on a small boat.

If I go with detected the fuel cutoff solenoid, what sort of protection do I need from the coil in the solenoid?

Easy, reverse bias diode across the fuel solenoid.

This WILL be your best and simplest 'engine ON' indicator by far and if you'd said it had an electric fuel solenoid we'd have all jumped on that one from the start. 

[towlerg]

if you'd said it had an electric fuel solenoid we'd have all jumped on that one from the start.

Sorry about that, I thought all diesel engines had cutoffs. How could you stop it without one?

I don't see the difference between "looking" at the terminal of the cutoff or "looking" at the start switch.

[CJay]

Easy, reverse bias diode across the fuel solenoid.

This WILL be your best and simplest 'engine ON' indicator by far and if you'd said it had an electric fuel solenoid we'd have all jumped on that one from the start. 

It'd be more unusual to find an old diesel that doesn't have a fuel cutoff solenoid though, so it'd be the first suggestion I'd have made.

They're there to stop runaway engines (even when the diesel from the tank is cut off it's not a guarantee the engine will stop) but aren't as necessary on newer fuel systems.

[Ian.M]

Easy, reverse bias diode across the fuel solenoid.

This WILL be your best and simplest 'engine ON' indicator by far and if you'd said it had an electric fuel solenoid we'd have all jumped on that one from the start. 

It'd be more unusual to find an old diesel that doesn't have a fuel cutoff solenoid though, so it'd be the first suggestion I'd have made.

They're there to stop runaway engines (even when the diesel from the tank is cut off it's not a guarantee the engine will stop) but aren't as necessary on newer fuel systems.

Shutting off the fuel or opening a 'dump' valve on the injection pump only helps if the runaway is due to a stuck injector rack.

2-stroke Detroit Diesels are notorious for running away, and continuing to runaway even with the fuel shut off, though it can happen with just about any poorly maintained turbo diesel, or even with a normally aspirated one with grossly excessive blowby and a high crankcase oil level.   Basically if oil can get into the intake and you don't have some means of totally shutting off the air supply, you risk an uncontrolled runaway with the engine consuming its own lube oil as fuel.

In the absence of an emergency air shutoff valve, you need something that will seal the air intake effectively.   If you are mildly paranoid and have ever been near a runaway diesel, you may choose to keep a pingpong bat clipped to the inside of the engine box lid.  Otherwise, grab a closed cell foam cushion or the Nautical Almanac and shove it up against the intake.   Don't try to use anything thin and flexible or the palm of your hand as the intake can pull a fairly high vacuum before the engine dies.