Type of Service

The type of engine service is determined by the load-intermittent or continuous. An intermittent load on an engine is a load that varies in torque and speed; for example, tractors in tillage operations are subject to intermittent loads. Continuous loads provide little variation in the torque and speed demands placed on the engine; irrigation pumps are an example of a continuous load. Some manufacturers may indicate the type of service for the rated power. If not, the engine should be derated for the type of service. The available power must be reduced by 10% for intermittent loads and 20% for continuous loads.

Problem: What is the usable power for an engine that has been derated for accessories, temperature, and altitude to 115.3 hp if it will be used for continuous duty?

Solution: Derating for continuous duty: 100% - 20%

From this discussion it is evident that an important difference may exist between rated power and usable power for stationary engines. Also note that if a tractor is being used for stationary power for an extended period of time, it should be derated. When derating a tractor, remember that the PTO power will be equal to that of a fully equipped engine.

The previous section explained the procedure for derating an engine for individual factors. One question that remains is how is an engine derated when more than one factors is involved. This is accomplished by summing up the reduction for each individual factor and multiplying this number by the appropriate power rating.

Problem: Determine the usable power for a fully equipped, non-turbocharged, diesel engine rated at 115.0 power when it is operated on continuous duty, at 3,350 ft elevation and with an air temperature of 98.5 degrees.

Solution: For this example the engine must be derated for temperature, altitude and duty. The example was worked using the decimal form of percentages. Temperature:

Duty: Continuous duty requires a derating of 20%. Total adjustment:

Total = temperature + altitude + duty = 0.027 + 0.0855 + 0.20 = 0.3125 EHp = 115.0 hp x (1 - 0.3125) = 79.0625 or 79.1 hp

A 115.0 horsepower, non-turbocharged diesel engine operating under these conditions will have 79.1 useable power.

Early tractors were designed to deliver power in three ways: by belt pulley, drawbar, and power take-off. On newer tractors the belt pulley has been eliminated. One problem the owner/manager has faced since the first tractor was designed is lack of information on the power ratings of tractors. As a rule, tractor manufacturers do not advertise the drawbar or PTO ratings of their tractors; instead many use brake power or engine power. If engine power is used, it could be a theoretical power rating for the engine.

In 1918, the Nebraska legislature passed a law that established the Nebraska Tractor Test Station. This law mandated that a typical model of tractor must be tested before it could be sold within the state of Nebraska, and that the manufacturer must maintain a part supply depot within the state. This testing station has provided the only independent evaluation of tractors in the United States.

As time passed and a large percentage of tractor manufacturing moved outside the United States, a change was needed. Two changes have occurred: the Nebraska


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