Need Help!- designing simple 12vdc temp controller

[mike_belben]

hello gents

im hoping some of you fine folks can steer me in the right direction here. i need to make a self adjusting temperature regulating device to control the power output of a single 12 volt DC glowplug, as in diesel engine automotive heating element. the application is for a vegetable oil conversion on my dodge diesel. my grease is getting safely up to 160*F at the heat exchanger, but there is still a length of tubing with cool grease between exchanger and injection pump, that has to be inducted during initial switch over that may eventually cause issues.

the heating element will also serve as a redundant last chance temp booster in the event that my coolant system just isnt up to the task on extremely cold massachusetts days. i will be machining a simple aluminum block with a provision for threading the glowplug into it, that has separate passages for fuel. the block will be directly grounded and the glowplug should probably be powered by an ignition switch activated relay so that cheap, low power switches can be used, and also to prevent drained batteries from forgetting to turn the unit off. i also have to be able to manually switch it off when diesel fuel is flowing through, heating that reduces lubricity for the injection pump.

i will be using an autometer electronic temp sender and gauge to monitor temperature of the device so no fancy readout or anything is required. the circuit must have the ability to self regulate, because the vegetable oil temperature will increase as the heat exchanger takes effect and raises the oil temp.

please assume that my electronics knowledge doesnt go far beyond E/I/R

thanks for any help.
mike

[vegipete]

I would be very cautious glow plugs this way. By your description, it sounds like the plug will not be in direct contact with the oil, which is good, but failures could result in fire. I would also want some idea of what kind of duty cycle a glow plug can survive. As an alternative, wire 2 glow plugs in series and see how hot that gets - maybe no electronic controls would even be necessary. Full on temperature controllers are not trivial.

[Al_The_Man]

I am not sure if it is the right product for your application, but have you looked at simple self-regulating heat trace cable?
Al.

[RomanLini]

You can use a temperature operated switch at about 50'C (etc) and hook it up to a resistive heating element. Both the switch and resistor elements can be bought with metal sink tabs attached so they can be bolted directly to a metal bar (say 1" x 1/2" aluminium bar?) then drill a hole through the bar and run the oil through it.

Once it tests ok you wrap it in heat insulation to reduce the power wasted. It will only use power as needed to heat th eoil to 50'C.

Glowplugs produce a high heat in a very small area, they are totally unsuitable and will burn out.

[Linghunt]

Interesting Problem. The glow plug might not be the best choice for a heater element. Don't have a specification for which plug and it's rating that you are looking for.

There are lots of cartridge heaters out there on the Market.

One Example, just for ideas

http://www.watlow.com/downloads/en/b.../stlfr0706.pdf

There are also Thermal switches available, that will make it work in a closed loop. You might want to have a kill switch so this heater system is not be "on" unless you need it.

If you don't have good heat transfer out of the Block, the Heater could fail, where you mount the thermal switch will impact it as well.

Doesn't sound like you need to fancy of a system. you could have a plug in to 120VAC as well, if the cold is impacting you in driveway, vs out in the hills.

-John