Lobo Pup Twin 1.6 cc diesel

[GailInNM]

I received a PM today from an HMEM member who is starting to build a glow plug version of the Lobo Pup Twin. He asked for recommendation on the glow head and cylinder modifications.

I have sketched a glow head and revised cylinder of what should work OK. I have not tried this, but I have built successful side port induction glow engines following this general layout before. Unless I have overlooked something this should work OK, but no rash promises. Fins were added to the head as glow engines normally run hotter than compression ignition and the additional fins will help make up for the loss of two cooling fins on the cylinder. Everything else should remain the same between the two versions. The drawing is attached as a PDF file.

I forgot to dimension the counter bore for the head bolts on the drawing. It is 0.156.

Gail in NM

View attachment GLOWHEAD.pdf

 

[ebrathole]

I ended up completing the glow version of the Lobo and it runs, sort of. It's very tough to get started, then will run for about 30 seconds and not restart. After waiting a while (maybe until it cools) it will restart. It will also only start with a finger flick to the propeller (8x4 plastic prop). Cylinders are 1144 and pistons are cast iron. Fuel's tried have been Patriot R/C supreme 10% nitro 18% oil, and Traxxass top fuel 20% Nitro. Either fuel seems very hit or miss wanting to fire or run. My two theories are either a) the 1144 expands faster than the cast iron and causes a compression loss, or b) starting and not wanting to restart is a fluke and my real problem is a fuel delivery problem. Any tests, ideas, or parts to remake? I think the fact that i've got it to run at all is a success for a first attempt at an IC, but I'd love to have a reliable runner.


Thanks,

Eric

 

[ShedBoy]

Hi Eric, how about posting some pictures of your engine in a thread in the finished engine section. I am interested in making the glow version of this.
Brock

 

[GailInNM]

Eric,
Sorry to be slow in responding about your build. With lots of life getting in the way about the same time you posted things kind of slipped through the cracks.

My GUESS is that the difference in thermal expansion is not what is giving you problems. 1144 and cast iron are both close to each other with 1144 being slightly greater. This is offset by the cylinder running cooler than the piston anyway so I don't think that should be a problem.

I would guess that the fit and finish between the piston and cylinder is the problem. When hot the lubricant in the fuel is much thinner and will not make as good a compression seal and when cold.

I have seen this behavior on early 1/2 A class engines as the piston/cylinder became worn and even on some new commercial engines where the piston and cylinder were not properly fitted at the factory. On my engines I make the piston/cylinder initial fit slightly tight and and most often put a slight taper in the cylinder to provide a squeez point at the last 10 percent of the piston travel as it approaches top dead center. In engines of this size it is essential that both the piston and cylinder be lapped with a very fine lapping compound using internal and external laps and very fine lapping compound. I typically use either 5 or 10 micron diamond lapping compound for the final fit. 5 micron is about 3000 mesh and 10 micron is about 1800 mesh.

Never try to lap the piston and cylinder to each other by putting any kind of polishing or lapping compound on the parts and then fitting them together. This will produce a fit that looks nice but will be too loose for good operation. Larger engines, say 3/4 inch bore, can get away with it but not small bore engines.

Gail in NM