Semi-diesel or Hot Head Engines

The final in this six-part series explores the basics of hot head engines.

| February/March 2015

  • 4 HP Charter-Mietz
    This 4 HP Charter-Mietz hot-bulb engine was featured in the July 2006 issue of Gas Engine Magazine in the first of a three-part series by Andrew Mackey.
    Photo by Andrew K. Mackey

  • 4 HP Charter-Mietz

Editor’s note: This is the last of a six-part series of articles by Andrew K. Mackey examining antique engine fuel and ignition system basics.

The semi-diesel or hot-head engine is not a true diesel, but retains the operating functions of the diesel with the exception of high compression. On these engines, the head or a bulb in the combustion chamber is heated to near red heat, usually with a kerosene torch. The engine usually has a manual pump that will force an amount of fuel that will be sprayed through a nozzle in the combustion chamber against the heated area. As the engine is rolled against compression, the manual pump is activated and the sprayed fuel ignites. This in turn creates pressure in the combustion chamber against the piston, thus starting the engine.

Once the engine starts, fuel addition, under pressure, continues automatically and is controlled by the governor. As in a true diesel, the engine speed is controlled by the timing and length of fuel addition, during the power stroke. Also, as in the true diesel engine, this has no throttle on the air intake. Most semi-diesel engines are 2-stroke, but there are 4-stroke types, as well.

In a semi-diesel, once the engine starts the heat source for the hot-head or bulb can be removed; the heat is generated by the combustion itself to continue the ignition process. Typically, semi-diesel engines have from 5:1 to 7:1 compression ratios. They can use a variety of liquid heavy fuels, from kerosene to heavy bunker oil, and almost anything in between. Their ignition does not come from the pressure generated by the engine as a true diesel does.



The only way to stop a semi-diesel is to either cut off the air supply or cut off the fuel supply. As in a true diesel, excess oil or fuel in the crankcase can cause a runaway condition. For the first start on either type of engine, a means of blocking the intake and stopping fuel introduction are a must! A good compression release might slow a semi- or true diesel, but do not count on it for stopping a flooded engine of this type.

More Gas Engine Basics

Hot Tube Engine Basics
Low-Tension Ignition Basics
High-Tension Ignition Basics
Flame Ignition Basics
Diesel Ignition Basics



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