Diesel World|January 2021
The oil engine was the lower-cost variant of the compression ignition engine. With compression ignition, Dr. Rudolph Diesel had a great idea but at around the turn of the century, the technology was not perfected. “True” diesels were large, expensive and had complex injection systems. They also required a license to manufacture them under the Diesel patent. The door was open, however, to other methods that skirted around the Diesel patent and delivered some of the benefits of a diesel. They were called oil engines.

Oil engines were on the diesel family tree but not technically diesels. There were two varieties, low compression (70 to 150 psi approximately 5:1 to 8:1 CR) or medium compression (between 150 and 250 psi - up to 13:1 CR, or so). In studying the old texts, the medium-compression oil engines were often called “semi-diesels” and in later years, oil engines were often lumped together under that term. Like the diesel, fuel was ignited by heat of compression but with the oil engine, the fuel had to be heated and vaporized first... sometimes known back in the day as “cracking.”

Typically that fuel heating was done by a hot spot in the combustion chamber. These hot spots came in many forms, from bulbs to tubes (so-called hot bulb and hot tube engines) and other types. On a cold, or even cool, start, external heat was applied to the hot spot, heating the combustion chamber so that when fuel was injected, it instantly vaporized so it could be ignited at the lower compression ratios.

When the engine was under load, the hot spot was designed to retain enough heat so that combustion could be maintained. The hotter it was, the better it ran... and vice versa. Typically, a kerosene torch was use to heat the hot spot and how long you had to do that depended upon the ambient temp, the condition of the engine and the lightness of the fuel (heavy oil took more heat).

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