Why to start your engine with the charger OFF (a glow plug question)

[Bobg]

Ron, I only have 14 volts when the shore power is on, like I said the plugs are rated for 10.3v, stamped right on the plug, when I talked to the NGK service rep, he was adamant on not holding the plugs on for extended time, and that the 14 volts going to the plug was a definate no-no, maybe all those years I was simply lucky I didn't burn them out quicker.  I asked him "off the record" if he thought the plugs would take more than the 10.3V, he said maybe for a short period of time, few seconds, but would not recommend it, he told me to go to radio shack and get a resistor to limit voltage to 10.3 volts.   Anyway, I am going to contact Kabuto and find out why they put a 10.3volt plug in a 12 volt system without a resistor of some sort, unless there is one hidden in the line somewhere and I am not aware of it.  Going to the boat wed and see whats up.  will post my conversation with a kubota or universal tech rep.  Thanks Guys   Bob

[Ken Heyman]

I wonder if the "solenoid upgrade" could increase the likeliehood of burning out" the plugs if shore power is connected. Not being an electrical guru, I have no idea. I do know that the upgrade does increase the power (current and volts?) to the plugs. Perhaps this coupled with a jolt of shore power is the villain.

"shore" enough? ,

Ken

[karista]

I have started the engine with the shorepower connected for 18 years, never had a problem and still have the original glow plugs!!
Enough said! There is obviosly something wrong with the changes made!!

[Ron Hill]

Guys, Susan and D. Gill : It's not the fact that the shore power is ON, it's the fact that the AC battery charger is ON !!!! 
Many times I start the engine while plugged into shore power, but I turn the AC battery charger off first!! 
Bob try it with the AC battery charge OFF and I believe you'll be OK.  A thought

[Susan Ray]

Wouldn't just shutting off the AC Master switch shut down shore power to the glow plugs? I have also started mine with shore power on, with 2 Cat 30's over 20 years and my Cat 34 for the last 4+ years. The diver that cleans the bottom every month said to start the engine at least once a week and run in gear to clear the barnacles out of the cutlass bearing or I will have to replace it every haul. The water here is polluted, brackish and full of bad stuff that likes to hide in bearing...it's paradise man...

[sedelange]

In regards to why Kubota used 10.5 volt glow plugs, you have to understand how these engines were originally designed to be used.  The Universal Marine engines are derivatives of industrial engines that are started with minimal capacity batteries.  The voltage at the glow plugs will quickly drop to 10.5 when you engage the starter.  With a 12 volt battery, the glow plugs are only exposed to a 10% overvoltage condition, not a significantly excessive amount. A 14 volt supply would mean 33% overvoltage condition, which could possibly be significant at high duty cycle times.

Now in the marine environment, battery capacity can easily exceed 200Ah and with the addition of glow plug solenoids, voltages stay much closer to 12 volts during the  entire start cycle.

Assuming 10.5 volts at the glow plug and a 3 ohm resistance, the glow plug is putting out about 1.4 watt-hours if heat in a 15 second activation,  inceasing the voltage to 12 volts increases wattage to 1.8 watt-hours.  If you are applying 14 volts, wattage is increased to 2.45 watt-hours.

To reduce glow plug burnout under higher voltage conditions, you must adjust duty time accordingly.  If you assume 15 seconds is sufficient for preheating, then if you are putting 12 volts to the glow plugs the duty time should be reduced to 11.5 seconds and at 14 volts you only need ~8.5 seconds.

If an earlier comment of 40 second duty cycle was at 14 volts, the equivalent 10.5 volt cycle would have been over 70 seconds.

Adjustment of duty time would be preferable to installing a dropping resistor.  Assuming your battery and engine wiring is capable of delivering very near 12 volts and you want to keep glow plug voltages at 10.5 volts you would need a resistor 0.15 ohm 25 Watt resistor for a 3 cylinder engine and 0.11 ohm 25 watt resistor for a four cylinder engine.  The installation of a resistor would degrade performance when starting with a weak battery and would not drop the voltage properly should one or more glow plugs be inoperative or internally degraded.  Adjustment of glow plug timing to voltage conditions would make for more consistent staring and improved plug life.

[Ken Heyman]

Steve wrote:-----and with the addition of glow plug solenoids, voltages stay much closer to 12 volts during the  entire start cycle.------

Am I correct in concluding then (as i guessed at earlier) that if one insisted on starting their engine with their shore power  and  battery charger on that the chance of glow plug damage would be less if the solenoid upgrade had not been done?

lots of hypotheticals,

Ken

[sedelange]

The short answer is - of course.  The wattage inceases as a square of voltage so increases in voltage rapidly increase wear and aging. 

[karista]

Ron
To clarify my message, when I said the shore power was on, I also meant that my Battery Charger was on, as I always have it on.!!!

[Stu Jackson]

... I also meant that my Battery Charger was on, as I always have it on.!!!

If that was the case (i.e., charger on all the time and batteries are fully charged) then the charger output voltage should be about 13.4 V in float mode.   If it's higher than that, either you just turned it on and it's going through it's cycle, OR your charger should be considered with suspicion for quality battery treatment.  With Steve's excellent explanation, you can do the math.

Ken, longer wires mean more voltage drop.  Period.

"and  battery charger on that the chance of glow plug damage would be less if the solenoid upgrade had not  been done?"

That's pretty much it.  Could be higher voltage but for a LOT less run time.  I have the solenoid in and have shortened glow plug time significantly and originally from 20 - 30 seconds before to 12 - 15 seconds with, and have improved that by minimizing it sometime down to 5 seconds, and the engine starts right up.  Don't even use them if the engine's been run, and sometime 2 to 3 hours later is still warm enough to start right up without the glow plugs.

Give it a try, and just be careful.

Very interesting topic.  Thanks to all.

[Craig Illman]

Everyone - I have a PbCa automotive-type sealed battery as my "start battery". Their acceptance voltage in Bulk mode is over 15 volts. When I engage the glow plugs, they draw 20 amps and my Xantrex XC30 thinks it can go into Bulk mode from Float mode and rises to +15V. Now, I can't actually see the Xantrex display or Link display, but that's my theory on why I fried my new-ish glow plugs while starting the engine at the dock. (This was after having the glow plugs out to do a compression test and the engine wasn't starting because I managed to contaminate the fuel injectors because I had to remove the lines from the injection pump to the injectors to put the compression tester in glow plug hole for cylinder #1, but just before my foot slipped on the Ulimate Sole in the aft cabin and I rolled my full weight on my other ankle breaking it. All in December, no one around the marina, 700 feet down the dock from my car).

Some things you realize should be left for professionals.....but where's the fun in that?

Craig

Footnote, the ankle injury was December 07.

[Ken Heyman]

Craig--law of unintended consequences---sorry to hear about your ankle.

Ken

[Ron Hill]

Guys and Ken : All the addition of the solenoid did was to take the "available" voltage/amperage and not let it deteriorate drastically before it reaches the glow plugs where it is needed. 
The only way you can get more than battery voltage (about 12.75 + Volts) is to get it from another source like a battery charger!!  A thought.   

[Ken Heyman]

Ron (and guys)

I am convinced that the solenoid upgrade is a good thing. Nevertheless, two things seem clear from this thread. If Westerbeke (Kabota)  installed 10.5 volt plugs for a "reason",  maybe it was to  help guys like me who had let their batteries run down (the pre solar panel days) and were starting the engine on a particularly chilly autumn day. I would have a better chance of starting with these plugs. Secondly a 12.5 volt plug will have more tolerance for excessive voltage (current) at start-up--whatever the reason. It's clear that his is not a good idea so your point, Ron is well taken It seems the solenoid only puts one at risk of plug burnout if the redundancy of a battery charger(or other charging source) is in use when starting. It may also be the case that the 10.5 volt plugs would have less tolerance for keeping the glow plug button depressed for more than the suggested time.. We are dealing with a double edged sword.

Ken

[Ron Hill]

Ken : Let me try again.  The batteries at 100% charge are at 12.75volts!!

The whole idea of the solenoid was to reroute the voltage/amperage from the devious wiring that the factory installed - from the battery/s back 15feet to the key switch and then back another 15feet to the glow plugs.  This was all done with a #12 gage wire.  That's why it took the glow plugs atleast 30 seconds to warm up which also put a real voltage/amperage drain on the battery/s!!
The solenoid shortened that 30ft routing to about 5 feet with a #10 gage wire shortening the switch engagement time to 10/15 seconds.  This also was a much lower drain on the battery/s so more voltage is avialable to the starter.
 
When you engage the glow plug switch there is about a 2 to 3 volt drop on your Vmeter, so the glow plugs are only really getting about 10.5 volts or less.

Enter the shore power charger and it sees a drop in battery/s voltage and it kicks up the voltage to cover the Voltage drop.  Now you are getting more volts than the glow plugs are rated for.  Even with out the solenoid the higher voltage will still reach the glow plugs if they stay engaged for 30 seconds.