Glow Plug Electrical Characteristics

Hey Natural Philosopher,

Did you have your spell checker turned off, Or was it just late at night?.

(i)Netscrape doesn't have a spell checker

(ii) It was VERY late at night.

Phil,

Interesting project. I guess you want to attack this because you want the satisfaction of doing it yourself, or you see some shortcomings with the existing off-the-shelf designs.

I played with the idea in the 60's before the commercial control units were available. ICs were just starting to be applied to reduce the size of some "hybrid circuits". The "power" transistors were expensive and stained my project budget. Rather than prattle along about design details, I'll tell you about my initial experimental set-up.

As a first step, I mounted the same brand and number plug that I intended to use in an engine head that I had removed from the engine cylindar. The head was set into a small jig so that it was inverted. The jig was mounted on a base large enough to hold the bread-board circuit and the battery pack. The battery pack was set so that I could select the number of cells to be used to "step" the approximate input voltage between 1.5 (single carbon zinc cell) and 6.0V (4 cells). Today, with advanced chemisty cells, the voltage will be slightly different.

Because I was flying during the winter "up north" at the time, I had the entire assembly mounted so that I could slip it into my freezer. That way I could cold-soak the entire head, plug, and cells. This was done only because of the design specification for starting a cold engine at "0" F. I wound resistance wire around the head (salvaged from a hot-plate element) so that I could raise the head temperature to different ambient temperatures and to quickly jack it up to simulate (badly) the head getting hot with combustion.

I went with descrete electronic components because ICs at the time were not available to the casual experimenter. I used a mechanical switch to sense the throttle servo position. One circuit design required me to wind a current "choke" to limit in-rush current. I told you this was back in the "Stone age". However, the basic test jig would work just as well today.

I used analog voltage meters to monitor the voltage drop across the plug right at the plug, the output voltage at the board, the input voltage to the board (battery voltage) and current meters to measure the plug current and the input current (draw from the cells) to the board. This permitted me to have real-time indications of both I & E. The initial cold resistance of the plug circuit (from the selected "ground" point on the engine through the plug connector and element) was measured using an ohm meter.

The inverted position of the head permitted me to use an atomizer and small glass dropper to "wet" the plug with fuel. As a side note - putting the fuel in a tightly sealed jar in the freezer sure showed up any extra water. That taught me not to open the fuel can in the house after I had it outside during the winter. The old metal cans were great vater vapor condensation generators.

The rig permitted me to develop the initial current curve for the plug and the current required to keep the voltage drop across the plug at a safe level for the element once it came up to temperature. I would have liked to measure the element temperature, but IR temperature sensors were available only in high-end labs back then and the cost of the instrument and the liquid nitrogen were well out of my budget. I ended up constructing a "dark box" around the assembly with a small magnifier mounted directly over the head and glow plug so that I could see the element's color.

I conducted the experiments at initial head temperatures between 0 F. and

250 F. (high temps to simulates "hot" starts") with the glow element "dry" and with various levels of fuel wetting from misting it lightly (my wife probably still wonders what happened to her expensive cologne atomizer) to being completely flooded. The circuit and battery environmental temperatures used ranged from 0 F. to 100 F.

Once I had the glow circuit designed and fabricated, I repeated the experiments to tune the circuit performance and determine the crude point at which it could be turned "off" by the throttle servo arm. Final tuning was conducted by mounting the assembled engine and circuit on an engine test stand. My wife put her foot down about my running the engine in her kitchen and freezer - so that part had to wait until winter was cold enough.

The end result was a circuit assembly and three cells that just barely fit into the cabin of a big Robin-Hood like design. I originally had a circuit design that was "on" all of the time and would monitor the glow plug current at a very reduced voltage. If the current increased beyond a pre-set value, it indicated that the plug had cooled (down-side of maneuver flooding at chopped throttle). It would switch "Heat" regardless of the position of the servo arm switch. I had some problems with the phyical size of the assembly and eliminated this feature for the final flying version.

The one thing that I added that really increased the advantage of the device was an external power jack and a "pre-set" switch that permitted me to power the circuit from a "ground power unit" (Three huge 1.5V surplus cells in series to form a 4.5 V battery). This added the advantage of having a large Amp/hour source for pre-heating using a lower controlled current to the glow plug. If I put an insulated oven mit over the engine and let the plug run at a low current for 15 minutes for pre-heat, the engine would start instantly on even the coldest days. Try flying at 18 degrees below zero in a breeze while wearing artic gloves and a parka closed over your face so that only your eyes are exposed. I had to keep the fuel in a small "COX" fuel can inside the parka to keep it warm. And, I had to insulate the GPU battery so that it would not cool too quickly once it was taken out of a warm car.

Just for reference and part of an "old fart story", the initial engine used was a control line engine fitted with an exhaust restrictor type throttle. It didn't work worth a hoot. I lived hundreds of miles from a decent hobby shop. An airline pilot friend who was flying the California to Tokyo route took pity on me and started to bring me back engines with a real RC carbs. This was before any of the local hobby shops could get Enya and the exchange rate was 360 Yen to the Dollar! He also brought me my first "Futaba proportional" radio. I was hesitant to switch to "Japanees Junk" from my Kraft units. If I had known then what I know now---- I still have the last engine he brought me - a early 1960's Enya "60 RC"

This is also off-topic, but might be interesting. Circumstances put me in Japan in 1969 and I visited a RC club flying from recovered land near Yokohama. It was definately a "rich man's hobby" in Japan back then. This club flew only WWII scale of both Japaneese and U.S. Pacific Theater aircraft. There was a small salt-water "bay" on the land and they even had scale ships cruising around. One of the battle ships was bigger than the Chris-Craft "recovery" boat they used. That was the first time I had ever seen a RC "sub". I was astounded at the quality of the models. Many of these guys hired whole teams of craftsmen to build and paint their ships and aircraft. Most of the guys were high-level management with the Japanese industries. Again, if I had known then --- and had been older and wiser - I sure would have bought some Pacific Rim stocks.

Anyway, have fun with your project.

J

I built a simple glow plug driver that I used for many years. It consisted of an LM317 voltage regulator operating in a constant current mode. If I remember correctly, I set it up to output 3 amps. The power source was the same 12volt battery used for my electric starter. There are many different versions of the LM317 voltage regulator. Many of them will only handle 1.5amps. You need to get one that is packaged in a "TO-3" case that will handle 5 amps.

Greg

Phil,

An NE555 can be used as a variable duty cycle trigger with variable frequency as well. Look up the astable mode here:

Run the output through a suitable MOSFet and yo have no heat losses.

I'm running Netscape 7.1. It DOES have a spell checker. It also has a nifty feature to block pop-ups. Try it, you'll like it.

Jim - AMA 501383