Learn the ins and outs of diesel ignition system basics.
This circa 1929 50 HP Otto diesel engine was reported to be the last or nearly last engine to leave Otto when the company closed in 1929. The engine follows German design and employs the complete diesel principle, having no additional heat except compression to start ignition.
Editor’s note: This is the fifth in a series of articles by Andrew K. Mackey examining antique engine fuel and ignition system basics.
Old diesel engines follow three basic designs. All use high compression and the heat generated by that high pressure to affect ignition of the fuel used. There is no throttle on the air intake on a true diesel. On all diesel engines, the basic principle is the same, but different means are used to ignite and supply fuel to the combustion chamber.
First up is the HVID system, named after its inventor, Rasmus Martin Hvid. This system allows a fuel supply to be injected at low pressure into a pre-combustion cup within the combustion chamber. Then the engine is turned over, and high-compression pressure is introduced into it. The fuel is controlled by the governor and a mechanical linkage. As pressure rises, the lower temperature volatile materials in the fuel vaporize, and ignite, thus blowing the main body of fuel out of the cup into the superheated compressed air in the combustion chamber. The rest of the fuel ignites, thus providing thrust against the piston. Early engines of this type were very prone to mechanical failure, as linkages wore and fine adjustments changed with engine usage. They also tended to knock pretty hard, which led to mechanical failure.
Next is the direct injection system. This system uses high pressure pumps and injectors to introduce fuel at the correct time, under high pressure, directly into the combustion chamber and the super-heated air within. The time and length of fuel injection determined speed and power generated. Typically, for best power and economy of operation, a true diesel runs at a compression ratio of approximately 16-22:1.
A third system had a different approach. It used a second set of valves to lower the engine compression in order to allow the engine to start on gasoline, with lower compression and spark plug ignition. As the engine warmed, the second set of valves closed, the gasoline system was isolated from the intake, and the diesel system began operating as in a direct injection system. This was a complicated system, but allowed for easier starts and better diesel operations, as the engine was warmed by the operation of the gasoline system. These engines tended to be of lower compression for the operation (about 16:1 ratio).
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