Figure 347 shows a card from a Mietz and Weiss engine. This engine carries a much lower compression pressure, about 90 pounds, and the card here shown is a fairly representative one when such pressures are used. The fact that an oil engine does operate satisfactorily with such a low compression pressure proves that the time element, as well as the temperature range, influences the ignition of the fuel charge. In the Mietz and Weiss engine the fuel is injected very—early in the compression stroke; this allows ample time for the oil to distil or vaporize at a compara tively low temperature. The objection to this low pressure is the liability of the fuel not igniting in case there is any leak around the piston, which would produce a reduction of the already low compression pressure.
Figures 348 and 349 are cards taken from the air compressor of the Primm oil engine. The rear of the cylinder was used as the air compressor. Figure 348 was taken from the engine when it was of the three-ported design. In this construction the piston uncovered the air-intake port. This card shows that the piston on the power stroke created a suction in the air compartment until about 25 degrees from dead-center. The suction pressure was around 8 pounds absolute or 7 pounds below atmospheric, while the maximum compression reached 8 pounds gage. Figure
349 is from the engine after being equipped with an automatic air intake valve. It is apparent that the suction pressure is slightly below atmospheric save at the point "a." The cause of this por tion of the suction line being above the atmospheric line is doubt less the blowing back of the engine exhaust gases through the air discharge ports. From these cards it would appear that the power requirements of the air compressor on a three-ported engine are practically twice that demanded by an engine using an auto matic air-intake valve.
General.—In the operation of a low-compression oil engine the operator must put aside any and all prejudices he may enter tain as to the suitability of an internal-combustion engine. He must bear in mind that thousands are daily producing power at a total overhead cost that makes the oil engine a successful competitor of the Central Station Service. If he is possessed with the belief that he is a "natural-born" engineer and that the oil engine is an open book to him for his own good, this belief had best be thrust aside. On the other hand, the oil engine is not an unanswerable puzzle by any means. Each effect has its cause, and it only requires level-headedness• and clear thinking to enable the operator to understand the machine intrusted to him.