With a lean or slow burning gas, that is, a gas slower than used on the diagram, combustion would not be complete at the end of the stroke if the same point of ignition were used. This effect is shown by Fig. (11), in which the full line diagram BCDE represents the ideal diagram (Y), and BCFG represents the slow burning mixture with the same point of ignition (X). The compression curves of both diagrams are coincident as far as C, the ideal diagram shooting straight up at this point and the weak mixture diagram staying at the same level. When under the influence of the mixture (X) the piston starts from left to right and reaches the point F before the slow burning gas reaches its maximum pressure. During this part of the stroke there has been very little pressure on the piston and it will be noticed that the maximum pressure is far below that of the ideal diagram. This low maximum is due principally to the reduced compression under which the gas has been burning, from C to F.
Figs. 13–14. The First Diagram (13) Shows a Two Port Two Stroke Diagram, the Second Shows a Typical Diesel Card.
As the gas has but a small part of the stroke left in which to expand, the pressure at the point of release is much higher than the release pressure of the ideal diagram, which means that a considerable amount of heat and pressure have been wasted through the exhaust pipe. Besides the heat loss, the high temperature of the escaping gas has a bad effect on the exhaust valve and passage. The great volume of gas passing through the exhaust valve also increases the back pressure on the scavenging stroke.
Delayed or retarded ignition will cause a low combustion pressure and slow combustion with any type of fuel or compression pressure as will be seen from Fig. 12. In this case the compression pressures of the ideal diagram Y and the diagram X showing the retarded spark are of course the same, the compression line extending from B to C in the direction of the arrows. At C the ignition occurs for curve Y, and the pressure immediately rises to D. In the case of curve X, ignition does not occur until the point I is reached, the compression falling on the line CI with the forward movement of the piston as far as the point I. At this point the compression pressure is very low which results in the slow combustion indicated by the slant of the combustion line IF. The point of maximum pressure F is much below D of the ideal curve, and as there is no opportunity for complete expansion during the rest of the stroke, the release pressure is high causing a great heat loss. If running on a LATE or RETARDED spark is continued for any length of time the excessive heat that passes out of the exhaust will destroy the valves.
It is apparent that for the best results, the spark should occur slightly before ignition in order to gain the effects of the compression, and a high working pressure on the piston. It is also evident that the point of ignition should be varied for different mixtures that have different rates of burning. With engines that govern their speeds by throttling or by changing the quality of the mixture it is necessary for the best results, to vary the point of ignition with each quality of fuel that is admitted by the governor. The retard and advance of the ignition is very necessary on an automobile engine because of the throttling control and constant variation of the load and speed. All automobilists know of the heating troubles caused by running on a retarded spark.
(38) Two Stroke Cycle Diagram.
In the two stroke cycle diagram, the lines showing the suction and scavenging strokes are missing if the indicator is applied only to the working cylinder.
Starting at the beginning of the working stroke as at A in Fig. 13, the gas expands during the working stroke until the piston uncovers the exhaust port at B where the pressure drops to C. A slight travel uncovers the inlet port with the pressure still above atmosphere due to the pressure in the crank case filling the cylinder. The crank case pressure continues from C to D or to the end of the stroke, the pressure dropping slightly at the latter point.
The compression stroke now takes place with the piston moving from right to left, the compression pressure reaching a maximum at F. Ignition occurs slightly before the point of greatest compression, at I, and the expanded gas increases in pressure until the point A is reached. From this point the same cycle of events is repeated. Because of the dilution of the charge by the burnt gases of the preceding combustion, the mixture burns slowly as will be seen from the inclined combustion line FA. Due to this delayed combustion, the piston travels the distance S on the working stroke before the pressure reaches a maximum. This diagram is typical of the small marine type of two stroke cycle engine which has no further scavenging than that performed by the rush of the entering mixture. The diagram of the pressures and vacuums in the crank case are similar to those of suction and compression in the four stroke cycle type.