ICOMIA Duty Cycle

Postby **jimh** » Mon Oct 14, 2019 5:57 pm

References are often made in discussion of outboard engine performance to the ICOMIA duty cycle. The applicable standard is "ICOMIA Standard No. 36-88, Marine Engine Duty Cycle." Finding the text of this document can be difficult. The ICOMIA duty cycle has five modes of operation of the engine at five different levels of engine speed and torque. The duty cycle established in the standard allots various amounts of the engine operating time to various engine speed ranges. The duty cycle is believed to reflect the duty cycle an outboard engine will be operated by a typical boater.

The ICOMIA five mode cycle is as follows:

Mode Engine RPM    Time  Torque Fraction

1      Idle         40%  0.000
2      40% of max   25%  0.253
3      60% of max   15%  0.465
4      80% of max   14%  0.716
5      max speed     6%  1.000

The operation of the engine at idle speed for 40-percent of the total engine operating hours may come as a surprise to many boaters. Some modern engines record their engine speed history. These logged engine hours and engine speed data substantiate the ICOMIA duty cycle model.

My own outboard engine records its operating hour time and engine speed. I have a report from my engine (a 2010 Evinrude E-TEC 225-HP) taken after 478-hours of engine running time. The report shows a history of engine running time in various engine speed ranges as follows:

While I have not gone to the trouble of applying the ICOMIA engine speeds, the data from my engine as seen above shows rather clearly how much of the engine running time is spent at low engine speed ranges (65.8-percent) and how little is spent at full-throttle engine speeds (1.2-percent).

The ICOMIA duty cycle is often used to develop a figure for the average fuel consumption (gallons-per-hour) for an engine, based on measurement of the fuel flow at the five modes of operation and weighting of the flow volume by the specified percentage of operating time. Fuel consumption figures developed with this method allow comparison of the expected fuel consumption rates of engines of similar horsepower. Judging engine fuel consumption by this method isolates the fuel consumption rate to the engine. Comparisons of engines when mounted on boats and running particular boats at particular speeds with a miles-per-gallon figure for fuel economy does not really measure the engines directly. Many other factors such as hull design, propeller, weight, and environmental conditions affect boat fuel economy in miles-per-gallon. Engine fuel consumption with the ICOMIA duty cycle using controlled test conditions (and standard test fuels) give a much better indicator of the engine fuel efficiency.