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Engine Order Classification
Engine First Order Classification
1. Convert the engine speed in revolutions per minute (RPM), recorded during duplication of the disturbance into
Hertz, revolutions per second (RPS), by dividing the RPM by 60 seconds. Refer to the following example:
1,200 RPM divided by 60 = 20 Hz (or RPS)
2. Compare the dominant frequency in Hz, recorded during duplication of the disturbance with the engine speed just
converted into Hz, to determine if they are related.
3. If the dominant frequency in Hz, recorded during duplication of the disturbance and the engine speed, converted
into Hz, ARE related, then an engine FIRST ORDER related disturbance is present. Engine first order
disturbances are usually related to an imbalanced component. Refer to the Engine Order Related Disturbances
table.
4. If the dominant frequency in Hz, recorded during duplication of the disturbance and the engine speed, converted
into Hz, are NOT related, then determine if the disturbance is related to the engine's firing frequency. Proceed to
Engine Firing Frequency Classification.
Engine Firing Frequency Classification
Engine firing frequency is a term used to describe the number of firing pulses (one firing pulse = one cylinder firing)
that occur during ONE complete revolution of the crankshaft, multiplied by the number of crankshaft revolutions per
second, Hz.
1. Calculate the engine firing frequency.
– To determine the firing frequency of a 4-stroke engine during ONE complete revolution of the crankshaft,
multiply the engine speed, converted into Hz, by HALF of the total number of cylinders in the engine.
– For example: The engine speed, converted into Hz, was 20 Hz; if the vehicle was equipped with a V8 engine, 4
of the 8 cylinders would actually fire during ONE complete revolution of the crankshaft.
– Multiply the converted engine speed (20 Hz) by 4 cylinders firing.
20 Hz X 4 = 80 Hz
– The engine firing frequency for a V8 engine at the original engine speed of 1,200 RPM, recorded during
duplication of the disturbance, would be 80 Hz.
– In like manner, a 6-cylinder engine would have a firing frequency of 60 Hz at the same engine speed of 1,200
RPM.
20 Hz X 3 = 60 Hz
2. Compare the dominant frequency in Hz, recorded during duplication of the disturbance with the engine firing
frequency in Hz, just calculated, to determine if they are related.
3. If the dominant frequency in Hz, recorded during duplication of the disturbance and the engine firing frequency in
Hz, just calculated ARE related, then an engine FIRING FREQUENCY related disturbance is present. Engine
firing frequency disturbances are usually related to improper isolation of a component. Refer to the Engine Order
Related Disturbances table.
4. If the dominant frequency in Hz, recorded during duplication of the disturbance and the engine firing frequency in
Hz, just calculated are NOT related, then determine if the disturbance is related to another engine order
classification. Proceed to Other Engine Order Classification.
Other Engine Order Classification
1. Multiply the engine speed, converted into Hz, recorded during duplication of the disturbance by different possible
order-numbers, other than 1 (first order) or the number used to determine the firing frequency of the engine.
2. Compare the dominant frequency in Hz, recorded during duplication of the disturbance with the other possible
engine orders just calculated, to determine if they are related.
3. If the dominant frequency in Hz, recorded during duplication of the disturbance and one of the other engine order
frequencies in Hz, just calculated ARE related, then an engine related disturbance of that order is present. If an
engine related disturbance is present that is NOT related to first order or firing frequency, then it could be related to
an engine driven accessory system. Proceed to Engine Driven Accessories Related to Engine Order.
Engine Mechanical:
1D-55
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