THE SEMI-DIESEL OIL ENGINE The mechanism of the Diesel engine is rather complicated, and the manufacturing cost is too high for the small-powered units. To be economical in total expense, a Diesel must be at least of 100 h.p. capacity. Even in this size the working hours should extend over the major portion of the day. The chief objection to Diesels below 150 h.p. lies in the high attend ance charge per horsepower-hour. Regardless of the size of the unit, the operator must not only be intelligent but skilled as well. It is necessary to pay fairly high wages to secure the services of an experienced Diesel engineer.
To meet the demand for an engine that would be economical to the use of fuel and at the same time simple enough in design in permit an intelligent workman to operate it, the semi-Diesel engine was brought out.
As has been explained in a previous chapter, the basic principle of the present-day Diesel is the reception of the heat at constant pressure. While the term "semi-Diesel" has been applied to certain engines because their compression pressure was approxi mately half that occurring in a true Diesel, it so happens that all these true semi-Diesel designs embody the principle of re ceiving a part of the heat of combustion at constant pressure and a part at constant volume. A great many low-pressure engines are marketed under the trade name of "semi-Diesel." To estab lish the right of an engine to this name an indicator card from the engine should be studied, and the proper classification can be determined by the shape of the combustion line.
Indicator Cards of Semi-Diesel and Low-pressure Engines.— An indicator card taken from a two-stroke-cycle low-compression engine is shown in Fig. 215. The compression reaches a value of approximately 100 lbs. per sq. inch at the end A of- the stroke. At this point the fuel charge, which was gasified during the latter part of the compression stroke, is exploded instantaneously. The maximum explosion pressure reaches 400 lbs. per sq. inch at B.
In this type of engine all the heat is added along the vertical line AB, at constant volume.
Figure 216 is a card from a four-stroke-cycle semi-Diesel engine. In this engine a charge of fresh air is drawn into the cylinder from A to B and compressed from B to C. At the point C the atomizer explodes a primary charge, raising the pressure to about 600 lbs. per sq. inch at D, which causes the injection of the secondary or main fuel charge from E to F. This main charge is injected and burned at practically a constant pressure of 300 to 400 lbs. per sq. inch. The primary charge adds but a slight amount of power and may be ignored entirely when figuring the indicated horsepower. If the sharp peak from E to D is eliminated, the card will have a form which to all appearances is a typical Diesel engine card.
The card shown in Fig. 217 was taken from a two-stroke-cycle semi-Diesel engine and displays the same effects of the primary and secondary charges. Figure 218 is a 'card from a De La Vergne Type F.H., a semi-Diesel engine using a vaporizer and air in jection but without a primary Charge. This is quite similar to the true Diesel card, the chief differences being in the maxi mum compression pressure developed, which is lower than in the Diesel, and the slight increase in pressure during combustion.
Comparing these semi-Diesel cards with Fig. 215, taken from a low-pressure or hot-bulb engine, it is evident that the compres sion in the former is carried much higher and that the addition of heat is accomplished in a vastly different manner from that followed in the low-compression constant-volume engine.
Ignition Devices. Hvid or Brons Principle.—Several makes of semi-Diesel engines use the "cup" form of injection and atomiza tion of the fuel. With this device the combustion of the atomized fuel is accomplished by means of the heat of compression, exactly as in the true Diesel engine. The sectional view in Fig.