[Vwdiesel] glow plugs low voltage

Sandy Cameron scameron at compmore.net
Sun Feb 13 22:23:40 EST 2005

Musings from the near north

I set up a haywire test set on the bench tonight to try some things.

Heavy duty batt charger
Metered Variac (variable transformer to adjust primary voltage)
Digital volt meter
DC ammeter

2 different glow plugs, one Beru (made in Germany) straight tube with blunt end,
The other, a Bosch with the tapered tube (made in France) part # are on the
plug, but I didn't bother reading that just yet (very tiny print vs. old
eyes, Need the illuminated magnifier)

The following figures are not absolute, but practical. Perhaps later I will
do some more accurate measurements.

Reasoning that extended time operation would likely be best done at a
voltage/current resulting in  just barely visible redness, adjusted the
variac to get a faint glow.

3 to 4 amps, about 4 volts = 16 watts, 4 of them would be about 64 watts,
about  that of a 60 watt light bulb. Not much compared to the common block

Typical frost plug block heater for VW is about 300 watts.
With a thoroughly cold soaked engine to -20C, a half hour of 300 watts will
bring the engine to a good starting temperature (start guaranteed).

There was an interesting difference between the Beru and the Bosch.

The Beru draws more current as the voltage is raised, more or less linearly,
while the bosch, once the voltage gets to 6 or so, any increase in voltage
is met by a decrease in current, keeping the wattage more or less constant
above 6 or 7 volts. As I increase the voltage, the current goes up for a
moment, then drops back to slightly less than before. This is obviously the
ballast trick we have recently read about on the bosch web site.

At this level, from 7 volts up, the color remains about the same, quite
bright. I really need to mount it in a large heat sink like a chunk of
engine head to do a fair assessment, as quite a bit of the heat goes out
through the threads into the head.

The 115 volt block heater is about 300 watts, and is very effective, Able to
warm a COLD engine to easy starting temperature in about half an hour, and
if left on all night, the engine will be at summer temperature in the morning.

The glow plugs, in normal use,  produce slightly more than 100 watts each,
x4=400 watts

Use of the glow plugs at lowered voltage (4 to 6 volts) as a head heater
would probably be practical, if used in conjunction with synth oil to ensure
fast cranking.

6 volts would probably be the max for the plugs, to avoid running them at a
temperature high enough to cause deterioration of the iconel sheath, or
internal resistive element

The argument for 6 volts would probably be good for plugs mounted in a sink
like the head, to prevent overheating

This would result in about 100 watts total for the head,  1/3rd that of the
block heater, and only heating the upper third of the engine (the head)
where it will do the most good

Now the how?

I opened a glow plug relay to ensure there is no connection of any kind from
the glow plug rail back to the circuitry. There is not, as long as the relay
is not operated.

This would permit the use of a 6 volt,  20 amp transformer (120 watts) to
power the plugs with A/C.
These may be found by dumpster diving (a la Hagar), or ham radio flea
markets, but it occurs to me there are tons of 5 volt, 20 amp and up
computer power supplies lying around everywhere. I must have half a dozen.
Bonus, there's usually a 12 volt section , that if adjustable, could be set
to 13.6 volts for the most perfect regulated battery float charger you could
ever ask for.  I would not suggest the power supply from a PC clone, as they
are not usually adjustable, but supplies from larger computing equipment
that are a glut even at junkyards, would do the job.

Now, what to do in the middle of a 40-acre parking lot.

1. The large 6 volt battery on the glow plug rail for 3 or 4 hours, total
16-20 amps, needs to be about 100 ampere-hours, and in addition, needs to be
a deep cycle, as an ordinary cranking battery will tolerate about 6 total
discharge, recharge cycles before it lies down and dies....permanently
2. There is a better way, a 12 volt, 120 ampere hour, deep cycle RV battery,
with a 400 watt inverter to get 115 volts A/C, applied to the electric block
heater for 1 hour max, will get you a toasty start, and you can recharge it
on the way home from your alternator. Because of entropy (water runs
downhill, but you have to push it back up, and sweat at the same time), You
need a battery that can maintain its terminal voltage above the inverter's
auto cut-off voltage, for the time required. This means you need battery
capacity about twice as much as the calculated draw of the block heater for
the required time. The fully charged RV battery will crank out the 28 amps
needed for well in excess of the hour you need.

Furthermore since batteries cannot be forced to take a complete charge in a
short time at 13.6 volts (your alternator output, if you are lucky), you
will have to charge the battery at home all night at a few amps to bring it
up to full health for the next day. 
But it can (and has ) been done.

This is my choice until I can get my oil burning wintermiser heater installed.

Some of us must park many yards, or even miles, from the nearest electrical
outlet, (like my wilderness camp) and these tricks can help.

And, of course, there is the Coleman stove, the pan full of briquettes, the
propane lance on the block, etc. All these have been done and work.


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