Indicator Diesel operators, being inexperienced in handling an indicator, do not understand the significance of an indicator card. For these, a few words of explanation will not be amiss.
It is obvious that, in any engine cylinder, we are confronted with the problem of a force or pressure acting on the piston for a certain distance. In Fig. 196, if the pressure exerted on the piston is represented by the height of the mark x from a base line yy', which is at zero gage, or atmospheric pressure, and the distance the piston moves, while pushed by the pressure x, indicated by the distance xx', then the line xy multiplied by the line xx' will represent the pressure times the distance moved; this value is the area of the figure xx'yy'. The area of this card then represents to some scale the work done on the piston. If the height xy is made 1 inch and represents 100 pounds, and the line xx' represents 1 foot of stroke, then the area obviously represents 100 pound-feet or 100 foot-pounds of work done. Since a horsepower is equal to 33,000 foot-pounds, this engine would have to make 330 of these strokes per minute to equal one horsepower.
With an actual engine the line xx' is not horizontal for its entire length. In Fig. 197 the fuel is injected and burned while the piston moves from x to a, the pressure being constant. At a the fuel valve closes, and the cylinder is filled with a charge of hot gases. The pressure exerted by these gases on the piston forces it to move outward, allowing the gas pressure to drop. The pressure line xx' then slopes downward toward the base or atmospheric line until the piston reaches the end of its stroke at x'. It will be noted that the gas pressure has dropped as the pis ton receded. The area of the figure still represents the work per formed on the piston. Since the line xx' is not horizontal, the area is no longer xx' times yy' but the average height of the line xx' times the actual stroke, which is yy'. This area can be deter mined by an instrument called a planimeter.
In an actual engine at the end of the stroke the exhaust valve opens, allowing the gas to blow out, and the line xx' drops sharply from x' to b, Fig. 198. The piston then moves forward, forcing all the gas out of the cylinder. This gas exerts a counter pressure on the piston which, while small, is indicated by the line bc. As the piston then reverses its travel, a partial vacuum
is formed in the cylinder, and the pressure drops below the atmos pheric line; this causes the outside air to rush in. Since the pressure is below atmospheric in the cylinder during this stroke, the pressure on the piston has a negative value as indicated by the line cd below the base line. At d the suction valve closes, and the piston compresses the charge of air. The pressure rises as the piston advances. This pressure works against the piston and is indicated by the line dx. At x the fuel valve opens, and the cycle is repeated. The area of the figure formed by these lines represents the actual work performed in the cylinder.
The cards taken from various Diesels all have a marked similarity. A few typical cards are shown.
McEwen Diesel Indicator 199 shows cards from a 14 X22 McEwen Diesel. All show a slight drop in the admis sion line, indicating that the resistance in the nozzle was too great for the injection-air pressure carried. Raising the air blast pressure would tend to bring the admission line to a hori zontal position. For the sake of clearness the exhaust and suction strokes are not indicated.
Indicator 200 shows typical cards from the Allis-Chalmers open-nozzle Diesel. The rising slope of the admission or combustion line indicates that the fuel, al though injection began early, did not ignite readily; at the be ginning of injection only the lighter portions ignited. This raised the temperature sufficiently to ignite the entire charge. The effect was accumulative, producing the rising line.
National Transit Indicator 201 depicts cards from a 153 X24 National Transit Diesel engine at 180 r.p.m.
The notable feature of these cards is the horizontal combustion line, produced, evidently, by a happy combination of efficient atomization and proper air-blast pressure. The quarter-load card shows a sharp peak at the end of the compression. Some attribute this to inertia in the indicator. It is very probable that this peak is a result of the inability of the air blast to pick up the fuel at the instant of valVe opening. The air had to attain a high velocity before the oil was swept into the cylinder.