TOOL GP 1773934 - Caterpillar

Illustration 1. Typical single power "dog leg" torque curve. Engine speed in rpm is on the horizontal axis. Torque is on the vertical axis.
Illustration 2. Typical dual power "dog leg" torque curve. Engine speed in rpm is on the horizontal axis. Torque is on the vertical axis.2. The purpose of the "DOG LEG" portion of the HIGH POWER lug curve (between POINTS 1 and 3 in Illustration 1) is to improve the ease of shifting of the transmission during high gear operation when high torque rise is a plus. The result is smoother machine operation.3. The "dog leg" shape in the High Power lug curve is the result of the Full Load (FL) power screw compressing a very soft torque spring (actually two torque springs sandwiched together) from between Points 1 and 3 in Illustration 1. At Point 3 the torque spring is completely collapsed, and further torque rise below 1900 rpm is called NATURAL TORQUE RISE.4. The purpose of the LOW POWER lug curve, which has NATURAL TORQUE RISE only, is to provide protection of the drive train components from excessive torque during low gear operation of the machine.5. The purpose of the LOW POWER lug curve, which has NATURAL TORQUE RISE only, is to provide protection of the drive train components from excessive torque during low gear operation of the machine.Segment 1: The OVERRUN CURVE from the no-load high idle speed to POINT 1, when the FL power screw begins to compress the torque spring.POINT 1 also corresponds to the CORRECTED FULL LOAD (FL) POWER AND SPEED shown on the HIGH POWER TEST SPECIFICATION. A continuity light is used to check this point (actually the SET POINT speed is about 50 rpm higher for repeatability) and calibrate the Full Load Static Fuel Setting (FL ST F SET) in the high power test specification.
Segment 2: The "DOG LEG" CURVE for the motor graders has a HIGH TORQUE RISE that begins at POINT 1 (Illustration 1) of the high power lug curve at 2100 rpm. As the engine lugs down from 2100 rpm, the FL power screw compresses the torque spring. This spring has a very soft spring rate for more fuel rack per drop in engine speed rpm, resulting in a large torque rise. At the 1900 rpm RATED SPEED, the torque spring is completely compressed. The high torque rise along Segment 2 also results in an increase in engine power from 2100 rpm to 1900 rpm. The GROSS ENGINE POWER of POINT 3 (Illustration 1) is greater than the gross engine power for POINT 1, and does in fact correspond to the MAXIMUM POWER point of the engine. Note the difference between GROSS POWER and NET or ADVERTISED POWER is the parasitic loads from the fan, alternator, muffler, air cleaner, and air conditioner compressor. The advertised power rating of any Caterpillar machine usually corresponds to the point of maximum power, and thus the 1900 rpm speed is used for the advertised rating instead of the test