I HAVE made model engines for many years at my home on the Danish island of Bornholm, starting with steam engines, then four-stroke gas engines with high tension or hot tube ignition, and my latest models are four-stroke, lamp start, hot-bulb engines, with a bore 20mm. and a stroke 30-36mm.
None of my engines are scale models, but so-called ‘freelance’ engines of my own design. However, they are all inspired by engines built in England around the century shift 1890-1920 such as, Crossley, Hornsby, Blackstone, etc.
I make the drawings, and all of the parts for the engines myself.
Fabrications
Some of the parts that could have been cast in a foundry are fabricated such as the base, flywheel, cylinder, etc. The crankshafts I make, are not constructed from several parts, but are cut from a solid round or a section steel bar.
There are some difficulties associated with these four-stroke - lamp start - hot-bulb - engines, with injected fuel. It is difficult to scale a hot-bulb system from the ‘big engine’ to a small engine, if you want the engine to run without a continuous flame from a blowlamp. The heat from the hot-bulb will soon escape to the cylinder head and water-cooled injector, because of the short distance between these. So I had to make my own hot-bulb system.
Small scale difficulties
After making several different hot-bulbs, and spending many hours experimenting, I ended up with a useful one. For the heat from the hot-bulb escaping to the cylinder head, I gave the hot-bulb a
‘long’ thin walled neck. To prevent heat escape from the hot-bulb to the injector, I moved the injector from the hot-bulb to the cylinder head, thereby having an indirect injection, not into the hot-bulb, but on a vaporiser tube that is connected to the inside end of the hot-bulb. So when the hot-bulb is heated the vaporiser tube is heated, too.
Then I had to make a govenor system, an injection pump, and a useful injector. The governor system could be a hit and miss system, but I rather like a fly ball governor controlled injection pump.
So I made a system, where a fly ball governor moves a conical bar in and out between the injection pump cam and the injection pump plunger, thereby changing the stroke of the pump plunger.
The injection pump is an ordinary type with one inlet, and one outlet ball valve, and a plunger. The valve balls are 1mm. and the plunger is 2mm. thick. I found, that when the engine runs at idle speed, about 360 rpm with no load, the plunger only moves 0,03mm.
Tiny injectors
The injector has a spring-loaded injector valve, with a valve head angle of 14 degrees and a 0.9mm valve stem. The valve stem is threaded so that the spring can be adjusted to the right fuel pressure with one nut, and locked with another nut. The blowlamp works with lighter gas, and the gas is filled into the bottom of the blowlamp, and the flame is controlled by a needle valve.
The problem with such a blowlamp is that it must deliver enough heat to warm the hot bulb, via a short thin flame tube. Turning up the gas needle valve too much, and the flame will blow out. So I cross-bored the flame tube so that a small bar could be placed there, thereby spreading the fast gas flow, to a slower gas flow, and that worked.
Latest model
My latest model, illustrated here, is a double-opposed 2-cylinder hot-bulb engine.
It was inspired by a Blackstone double opposed 2-cylinder hot bulb engine made 1904. I built this 2-cylinder horizontal four-stroke lamp-start hot-bulb engine using the same hot-bulb, governor system as on my previous build engines.
For more of Find’s engines, see: http://www.findsminimodelhotbulbengines.dk/
