Start Your Engines!

Part three of three on restoring a 4 HP Charter-Mietz oil engine


| October 2006


I knew how a hot bulb engine worked, so I figured it wouldn't be too hard to start it on my own. The more I thought about it though, I figured I would read the section in Dusty Erickson's book (Mietz & Weiss, New York, America's First Successful Oil Engine) to be sure I got it right the first time.

A hot bulb or oil engine is not the same as a hot tube engine. Although a hot bulb engine also uses heat to light the charge, it is a totally different system. On a hot bulb engine, the engine head has a bulb or protrusion on the head of the engine that is heated red hot with a torch. On the intake stroke, only air is drawn into the engine cylinder. At the proper time, during the compression stroke, fuel is introduced into the bulb via the fuel injector where it vaporizes, mixes with the air in the combustion chamber and flashes into flame, providing thrust against the piston.

Like a diesel engine, the timing and the duration of the injection of fuel determine the power and speed of the engine. Oil engines are sometimes called semi-diesels. The major difference between the true diesel and the hot bulb engine is the compression ratio. A true diesel engine operates by compressing the air in the cylinder at fantastic pressures. This pressure generates heat, which in turn ignites the fuel as it is injected into the combustion chamber. Some of the lower compression diesel engines need a glow plug to provide enough heat for starting in cold conditions, but once started, the glow plug cools off and the engine runs on compression alone. Diesels run most efficiently between 18-to-1 and 22-to-1 compression ratio. The typical oil engine is nowhere close to that. My Charter-Mietz, for instance, has a compression ration of only 5.5-to-1. Once a hot bulb engine starts, the preheating flame can be removed as the heat supplied by engine combustion provides enough heat to maintain ignition.

I was told that the small Bunsen burner I was given was used to start the engine, so I hooked it up to a new 14 ounce propane bottle and prepared to heat the hot bulb. I had read in Dusty's book that you had to open the shutter in front of the hot bulb chamber in order to allow the flame to travel around the bulb. I tried to open the door in front of the head by gently tapping on it, but it was frozen fast. I then used a large pair of channel lock water pump pliers and the door finally moved. It took the mounting bolt with it! I put the bolt in my vice, poured a lot of Liquid Wrench on it, heated it bright red and then allowed it to cool until it was no longer red. The bolt then came loose easily.



Next, I decided to inspect the heating chamber to see if there were any unwanted guests. I found some! There were two mud dauber nests, a yellow jacket's nest (with bees to boot) and a lot of spiders. A load of Berkbile 2+2 carburetor cleaner and the air compressor took care of the critters.

I reinstalled the cleaned door assembly onto the hot bulb chamber and then opened the door. I lit the Bunsen burner and let it burn under the hot bulb for about 10 minutes. I then tried to roll the engine over to start it, but I could not get it to go over the compression stroke. I opened the compression release and tried again. This time the engine rolled over, but still gave no hint of trying to start. I let the burner burn another five minutes until it almost went out and tried again. Still no luck. I had totally used up the bottle of propane. I decided that more heat was needed.












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