Hot Bulb Engine

[cfellows]

I've been noodling around with the Hot Bulb engine project although my efforts have been elsewhere.  Today I had some time and decided to fabricate a hot bulb and try it out on my Plumbing Parts Engine.  I made it out of a 1/2" diameter piece of mystery steel.  It is about 1 3/8" long, the hot part on the outside is 9/32" and the other end is turned and threaded to a metric thread 10mm - 1mm to replace the CM-6 spark plug.  There is a 3/16" hole drilled most of the way through the inside.

I took out the spark plug and replaced it with my hot bulb and heated it cherry red with my Mapp gas torch.  Everything else was the same, including the vapor fuel tank, coleman fuel, etc.  I gave it a couple of spins and it started.  It ran good and kept running as long as I kept the torch on the hot bulb.  When I removed the torch, it would fire another half dozen times then quit.  Unfortunately, one side affect is that the cylinder head and cylinder get very hot from heat conducted from the hot bulb.  But, I'm encouraged by the results.  I think the hot bulb wall needs to be much thinner, maybe .010" or .015" thick to make it easier to heat and to slow heat conductance into the cylinder head.  And, I may next give Find Hansen's hot bulb design a try.  But so far, haven't had to mess with injector pumps or injectors.  Of course that will be required if I'm to switch to heavier fuels like turpentine or kerosene.

Chuck

[NickG]

Chuck, that's interesting  . Do glow plug engines (similar concept?) have a higher compression ratio to keep the heat  too?

Nick

[John Hill]

Hi Chuck

If I recall correctly from what I have read there are two basic types of hot bulb ignition.

There were 'hot tube' ignitions on some of the early gasoline engines, cars etc,  this was a long(ish) thin wall tube that had a continuous external heat source.  As the piston came up the mixture was compressed into the tube until it reached a point hot enough for ignition.


Then there were the 'hot bulb' ignition engines which used injectors.  In those engines the bulb was a very heavy (for example cast iron) bulb which was initially heated with an external source.  The fuel was injected into the hot bulb where it vaporised but the mixture in there was too rich for ignition.  When the piston came up in pushed air into the bulb until the mixture was weak enough to ignite. 

Precis:  The hot tube gas engines required continous external heat whereas the hot bulb oil injection engines required external heat for starting only.

I have never seen a hot tube ignition but I have some experience of hot bulb oil engines on old Lanz tractors.

John

[John Hill]

[thinking]

Maybe the exhaust could be used to slow the cooling of the hot tube?

[cfellows]

Thanks, John.  I think there are a number of things that I could do.  Making the neck on the hot tube smaller with a thinner wall would be a good start.  I also like Find Hansen's approach where he uses a tube inside of a tube for extra insulation.  My first concern with this test was just to see if a heated tube would provide ignition at the proper timing.  Seems it will.

Chuck

[John Hill]

I took a peep at what Wiki had to say about hot tube ignition and see that they were typically 6 to 12 inches long and had a red hot spot that was kept hot by the external heat source.

[madjackghengis]

Hi Chuck, you might find more success if you turn the bulb inside out, so to speak, with the bulb being where the gap would be with the plug it replaced, as you say, about ten to fifteen thousandths thick, definitely rounded, and with the worst heat path possible to the threads, and perhaps a small plug that can be inserted once the engine runs, to close the opening of the bulb you use to heat it, so it won't lose heat to air convection.  The hot bulb setup is essentially the forerunner to the glow plug idea, before tungsten wire was available, which will glow long, without burning away soon.  I would consider turning a bulb of stainless, as it holds heat well, and doesn't conduct well, a bit smaller than the shell of a cm-6 plug, then turn the rolled shoulder of a plug which holds the ceramic into the plug, allowing you to remove it, and have a "shell" with threads, but with some air clearance between the outside of the bulb, and the inside of the part of the plug which is threaded, with the bulb protruding as far as is reasonable, into the combustion chamber, and with the connection to the plug "shell" at the far outside so heat has the longest to travel to get to the shell by conduction.  I would either tig weld it at that juncture, or absent tig, I would braze it using silicon bronze rod, with the tig being preferable as it would allow using stainless rod which is again, a poor heat conductor.  The bulb would only need be of similar size to the center electrode of the former plug, but sticking past the end of the threads would help it get enough heat to stay hot enough.  Ultimately, the timing issue is best controlled by the compression, as it's effect on the temperature of the mixture is all you have control of, with regard to the ignition, once the bulb is hot.  The earliest iterations used continuous heat on the tube or bulb, but it was figured out soon enough that with the proper placement, the heat of combustion would keep it hot, but requiring a firing on every stroke, and no "hit or miss", as that would allow the bulb to cool completely.  Having a "tit" on the very inner tip would do no harm either, and leave a point for the heat to remain concentrated. ttfn, Jack

[John Hill]

MJH, I think it would be quite a challenge to design a hot plug that could be externally heated without resorting to the electric principles of the 'glow plug'.