1983/11/23 Caterpillar


Explanation Of Engine High Idle RPM And Correct Adjustment Of Engine Setpoint

Usage:


All Caterpillar-Built Diesel Engines

The purpose of this article is to explain high idle rpm, full load rpm and set (balance) point rpm. The article also describes the use of the high idle adjustment screw. It has recommendations on when the high idle screw can or can not be adjusted. It also tells why there can be differences between the high idle rpm setting on the engine information plate and the actual measured rpm of the engine.

HIGH IDLE RPM

The high idle rpm of an engine is not an adjusted specification. It is the result of three factors:

1. Engine set point rpm (which is adjusted with the high idle screw).
2. The differences of governor springs and weights.
3. Parasitic (accessory) loads on the engine (fan, alternator, compressors, pumps, BrakeSaver, etc

The high idle rpm is shown on the engine information plate or listed in the fuel setting charts. The engine information plate must be used for all measurements if it is available.

FULL LOAD RPM

Full load rpm is the point at which the engine will develop the rated horsepower. If the engine is equipped with a torque spring, this may or may not be the rpm at which the engine develops maximum horsepower.

SET POINT

The engine set point is an adjusted specification and must be maintained. High idle rpm is not an adjusted specification. Set point (formerly balance point) is full load rpm plus an additional 20 rpm. Set point is the rpm at which the fuel rack just starts to make contact with the stop bar or the first torque spring. The rack at this rpm still moves back and forth a small amount. When additional load is put on the engine, the rack will advance and become stable against the torque spring or stop bar. Set point is controlled by the fuel setting in the high idle adjustment screw. Measurements have shown that the former balance point was not a point, but a general area.

The governor springs and weights control the rpm of the engine. Because of the normal production variations in springs and weights of the same part numbers, it is possible to have engines with the same set point but with a different high idle rpm. This is why set point must be adjusted and not high idle rpm.

The parasitic loads on an engine will also cause a change in the measured high idle rpm. These loads will decrease the high idle rpm according to the amount of power used by them. Normally, the parasitic loads on an engine will cause the measured high idle rpm to be 15 to 20 rpm less than the original bare engine high idle rpm with no parasitic loads. The decrease in the high idle rpm caused by the parasitic loads does not change the set point of the engine.

Because set point, governor springs and weights and parasitic loads all have an effect on the measured high idle rpm on an engine, it is not possible to accurately adjust the governor by only measuring the high idle. The correct way to adjust the governor is to apply a load and use a 6V2100 Digital Tachometer with either the 6V4060 Engine Set Point Indicator or a continuity light. The 6V4060 Indicator is the more accurate tool for adjusting the governor and is recommended by Caterpillar. Also, another possible method is to check the engine on a correctly calibrated dynamometer. Special Instruction, Form SEHS7931, has information for the use of the 6V4060 Engine Set Point Indicator.

NOTE: An engine set point which is not adjusted correctly, can greatly affect the fuel consumption and the actual performance for that engine.

The performance of the governor can be checked by a measurement of the engine high idle rpm. Make sure an accurate tachometer is used to check the rpm. If the high idle rpm of a bare engine is within the tolerance shown in the fuel setting charts and its set point is correct, it is an indication that the governor spring and weights are correct and in good condition. No adjustment is necessary. If the engine has parasitic loads on it, 15 to 20 rpm must be added to the measured rpm to allow for their horsepower usage before a comparison is made between it and the high idle rpm specification. If the measured high idle rpm is not within the tolerances of the specification, test the engine with a full rated load applied to see if the set point is correct, before an adjustment is made.

If the set point is correct and the high idle rpm is within the specification, no adjustment is necessary. If the set point is not correct, the high idle screw must be adjusted to get the correct set point rpm. If the measured high idle rpm plus the parasitic rpm allowance of 15 to 20 rpm is higher than the specification, but the set point rpm is correct, then the governor spring and weights can be checked. If the measured high idle rpm plus the parasitic rpm allowance is lower than the specification, but the set point rpm is correct, then the amount of the parasitic loads must be checked before the governor spring and weights are checked.

Remember that the measured high idle rpm of an engine is only a result of the set point rpm. After the set point rpm has been adjusted, the high idle rpm of the engine is controlled by the governor spring and weights and the parasitic loads on the engine.

The illustrations which follow show a typical engine power curve. The letters on the curve have these meanings:

Point R is the engine rated full load rpm.
Point S is the engine set point (Point R plus 20 rpm).
Point B is the bare engine high idle rpm specification.
Points B1 and B2 are the acceptable tolerance range of Point B.
Point H is a typical engine high idle rpm measurement.
Point P is the parasitic engine high idle rpm with a parasitic load.
Points P1 and P2 are the range of Point P.

When a governor spring and weight assembly has been put in an engine, that one engine will have a straight line of governor operation. This is shown in Illustration 1 by the line from Point B to Point S. The range in high idle rpm from B1 to B2 would be acceptable for many engines with the same set point. These engines will also each have a straight line of governor operation that starts somewhere at or between Points B1 and B2 and goes to Point S. The difference in the slope between lines BS, B1S and B2S is due to the tolerance range for governor springs and weights of the same part number.


Illustration 1.

Illustration 2 shows the effect of a parasitic load on an engine and how the measured engine high idle rpm is lowered from Point B to Point P. A difference of approximately 15 to 20 rpm.

NOTE: If an adjustment is made to change high idle rpm of an engine with parasitic loads (Point P) to the bare engine specification (Point H), the set point will not be correct. An adjustment to high idle rpm will affect the set point.


Illustration 2.

Illustration 3 shows how the parasitic loads lower the overall tolerance range. The rpm range between Points B1 and B2 is approximately 15 to 20 rpm higher than P1 to P2.


Illustration 3.

During production of an engine arrangement there will be small changes in the engine components due to changes in machine tools, materials, etc. The changes in engine components can cause a change in the average engine high idle rpm that is measured at the factory. This average high idle rpm number is put on the engine information plate. If changes in engine components cause the average engine high idle rpm to be outside the acceptable tolerance, then the rpm number which is put on the plate at the factory is changed to a new number. If there is a difference between the high idle rpm specification on the engine information plate and the fuel setting microfiche, use the specification on the engine information plate.

Remember that the high idle rpm is not an adjusted specification and is only to be used to check governor operation.

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