The piston, in the meantime, had retreated; the combustion of the fuel was insufficient to keep the pressure constant.
Standard Fuel Oil Indicator Cards.—Figure 202 shows cards for the Standard Fuel Oil two-stroke-cycle Diesel engine, and they are typical cards from an engine working under this cycle.
Faulty Indicator Cards.—Figure 203 is a card from a two-stroke cycle Diesel. The compression was carried to the end of the stroke at c. On the power stroke the combustion did not occur until the pressure had dropped to the point a. Since the oil was light, the combustion was in the form of an explosion, the pressure line rising to b. This was due to faulty setting of the fuel valve, causing it to open late. The injection pressure was high, and this forced the entire fuel charge into the cylinder as soon as the valve opened. Figure 204 is a card from the same engine with an advanced timing of the fuel valve. The combus tion was initiated when the compression line reached the point c. Figure 205 was taken from a four-stroke-cycle Diesel. The compression was carried to a. Due to the heavy character of the oil, ignition did not take place until b, where the entire charge ignited, raising the combustion line to the point c. Figure 206 may be considered a perfect card and is from the same engine. The shape of the combustion ab in Fig. 207 discloses a high injection velocity, with a consequent rapid rate of combus tion. With a lower injection pressure the combustion would be along the broken line ac. The peculiar shape of Fig. 208 is traceable to early timing of the injection valve when using an untopped crude oil. The valve opened at a; the gasolene and kerosene content ignited, raising the pressure from a to b. The heavier particles did not ignite until the piston started on the return stroke, as evidenced by the point c.
Figure 209 shows an early opening of the exhaust valve.
Figure 210 betrays a late exhaust opening. The piston reaches the end of the stroke before the exhaust valve opens. Figure 211 reveals a late closure of the suction valve; this valve closed at b, producing a low terminal compression.
Distorted Cards.—The process of combustion occurring in the engine cylinder is of more vital interest to the operator than anything else. The timing of the valves is easily checked while the actual events within the cylinder must be deduced from the indicator card. With the usual card the line is of small length, making impossible any determination of the combustion processes. If the indicator is connected in such a way as to have it set 90 degrees ahead of the engine crank, the combustion line is exaggerated in length. This can be accom plished with the indicator rigging in Fig. 214 by shifting the eccentric 90 degrees. A card secured with this setting appears in Fig. 212. It is apparent that the lengthened line ab gives an increased insight into the cylinder pressure change during combustion.
Indicator Rigging.—For a vertical box-frame engine a rigging along the lines of Fig. 213 can be easily installed. The link to the piston is hinged to a lug which is cap-screwed to the inside of the piston. If the engine stroke is 24 inches, the leverage can be set at a ratio of 8 to 1, giving the indicator a 3-inch travel.
Figure 214 covers a form of indicator rigging that can be quickly applied to any type of oil engine. The eccentric c is bored to fit the engine shaft D, to which it is fastened by set screws. The stand B is of cast iron and supports the lever A. This lever is held against the eccentric by the spring and rotates the indicator drum as it moves up and down. By setting the eccentric so that its throw DD' is in line with the crank throw, the indicator receives a true motion as the engine revolves.