“It is often asserted without hesitation by the laity even in scientific circles that the important point of the Diesel process is the self-ignition of the fuel; and that the object of the high compression is that the fuel, injected at dead centre, shall ignite itself, the high degree of compression being demanded by this self ignition.
Nothing is more incorrect than this superficial view, which is directly contrary to the facts and especially to the historical development (of the idea).
Motors with self-ignition of the fuel had existed before. Neither in my patents or my writings did I mention self-ignition as a goal to be sought. I was seeking a process with the highest heat efficiency, as it turned out; self-ignition was naturally involved in this. If the air is heated by compression to a point far above the ignition point of the fuel, the ignition of the fuel in contact with this air follows automatically; but it is not the object of this high compression. Self-ignition of every liquid or vaporised fuel within an engine already heated by work takes place at quite low pressure, of 5-10 or at most 15 Atmospheres. It would thus be much simpler to construct lighter and cheaper engines for these degrees of compression, and overcome the difficulty of ignition in a cold engine by employing artificial ignition temporarily. It would be absurd merely for the sake of ignition when the machine is cold to construct such heavy and unhandy engines to withstand a pressure of 30-40 Atmospheres, since, once the engines are warmed by work, they run quite as satisfactorily under lower pressures, as has often been shown by experiment.
The object of the system, sought through many long years and realised with such difficulty, is however quite different, namely the attainment of the highest possible fuel efficiency; this object necessitates highly compressed air. Since, however, this produces self-ignition much too soon in the fuel mixed with the air, therefore self-ignition by compression as it was known in the gas engines of those days was an obstacle to the process and had to be avoided in that form. It was necessary that the air alone should be so highly compressed by mechanical means as to give the desired heat efficiency.
The degree of this compression is determined not by the ignition of the fuel but, corresponding to the original object, by the maximum utilization of the fuel.”
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