|
R C Glowplugs |
R. W. Stuart One of the most active LCRC flyers has asked me to write about glow plugs so we will pass on what info I can find. The two sources that responded were FOX (manufacturer) and SIG (distributor for ROSSI). A glowplug is that little feed through device that screws into the cylinder head of your engine and allows you to put heat from electrical energy into the compression area to start and then maintains heat from the burning fuel to allow engine operation without further electrical input. The element is platinum or a platinum-rhodium alloy designed to survive the combined electrical- fuel burning heat, yet be able to stay hot after the battery is removed. Electrical insulation with pressure sealing is obvious. Platinum, an atomic element and a metal, is very dense (1335 lbs/cu ft), melts at near 3190 degrees F, and if memory serves is quite malleable. Chemically is very inert, but probably is catalytic with respect to methanol at temperature. Rhodium, is an atomic element, has medium density and 3542 degree F melting point. Two voltage ratings are provided; 1.5 volts for dry cells or Nicads and 2 volts for lead-acid cells. A lead-acid cell will burn up a 1.5 volt plug and a Nicad will not light a 2 volt plug. There is no way to tell about a plug after it is out of the package so a hand full of used plugs may be either 1.5 or 2 volts. Test the whole bunch on Nicads and those that do not light well are either burned out or 2 volters and the remainder can be tested with a lead-acid cell or be practical and throw out the whole mess. A visual test of a plug can provide you with more info than yes or no. The color of the radiation reveals the approximate temperature of the element- dull red 1100 degrees F, straw yellow 1800 degrees F, and bright yellow being close to burn out ( pure white is 3250 degrees F ). Methanol vapor lights somewhere around 725 degrees F, but a good bright plug can boil off more fluid fuel faster than one that just barely glows. The idle bars milled in or welded onto some plugs are called heat savers, but they do not directly save heat. Instead they act to shield or shadow the coils from direct fuel spray as the engine slows, cools, and becomes richer. Idle bars are generally not used on 4 cycle engines. Once the glow driver is removed from an engine the combustion keeps the coil radiant. Flame temperature- methanol-in the engines is 2400 to 2600 degrees F and varies with excess air and % oil. Engine wear may cause ions or particles of metal to stick or plate out on the surface of the element resulting in loss of heat transfer and loss of catalytic action between the surface of the element and the fuel. Aluminum particles carry a tenacious aluminum oxide layer which is a thermal insulator and prevents intimate contact between the methanol and the platinum. This could stop an engine or it might also cause the element to burn out. One of our flyers read about a material for cleaning glowplugs but could not remember the name of the cleaner. Any info you have will be printed if you let me know. Cranking a badly flooded engine might distort or mechanically break the element ( or the rod!). Fluid spray from near hydraulic ram conditions can be very abrasive; all of the force you put into the prop could be transferred directly to the element. Once the element moves near the barrel it may electrically short or may become thermally "grounded"- either of which results in failure. Any increase in compression ratio such as head carbon buildup or even early ignition, produces near sonic noise in the space between the top of the piston and the head and has potential for element destruction. Any excessive engine vibration may knock out the rather delicate little element coil. Most plugs, except the button types (COX), are threaded 1/4- 32NF. The short plugs have 5/32" long threads and are generally for engines 0.15 cu in or smaller and the long plugs with 7/32" long threads are for the larger engines. Examine a new plug carefully for smooth threading as a bad thread will act like a tap and redo the top of your engine for you. In general it is sound mechanical design to have thread engagement equal to the bolt diameter for max hold. Coil wire gauge, coil turn diameter, number of coil turns, alloys of platinum and rhodium, and weldments relate to high nitro, high temp operation, general quality and cost- things that are not directly measurable, but which will eventually influence your opinion about a certain plug. Costs range from $2.00 through $7.00. We have a bit of "snake oil" out there: "gold plated parts, miracle metals, special assembly methods, secrete materials, super seals, weather proof, etc". At temperatures below 43 degrees F (methanol flash point) no vapor exists and the fuel will not light. A good bright plug, however, will vaporize the fuel droplets as they spray in allowing a start. Hand warming the cylinder might help. .. BE SAFE .. |
Return to Stuart
Return to Home Page