Eldorado.Railroad, on 24 October 2017 - 04:28 PM, said:
For a continuous stream of burnt fuel you would have to have continuous combustion pouring out of EACH cylinder. Alas this is not so. Large engines do not rotate at 8000 rpm, where to the naked eye it might seem like a continuous stream because we cannot see the pauses. A diesel engine @ 200 rpm, and with multiple cylinders, you can not only hear each exhaust pulse, but see that pulse at the stack. A max rotation of 800 rpm for such a large engine is the upper limit.
What I mean by "continuous stream" is an individual pulse and puff when each cylinder exhausts. In my observation, this isn't the case, the puff (and audible pulse) are tied precisely to rotation frequency, occurring half as often in a four-stroke engine. I noticed this when I was analyzing my recordings and videos of EMD 567-powered GP7 and 9 rebuilds. When I bandpassed the high frequency noise, leaving only the low-frequency exhaust pulses, I noticed that each individual audible pulse occurred about every .2 seconds at idle and about every .07 seconds at maximum RPM. This corresponds precisely to the duration of each engine rotation. I then examined my videos of EMD 567-powered locomotives, and, sure enough, the individual exhaust puffs coincided precisely with the audible exhaust pulses - one per rotation. I used this information to very precisely set the start and end points of the final audio files. I simply ensured that the idle clips were some multiple of .20 seconds in length, and the maximum RPM clips were some multiple of .07 seconds in length. If there were an individual pulse for each cylinder exhausting, I would need to use numbers 1/16th as large. I can actually use this technique, in reverse, to tell you precisely what RPM the engine is running at.
In
this video. one of the locomotives is in less-than-ideal mechanical condition and it's easy to hear how the exhaust pulses synchronize precisely with engine rotation. When you've trained yourself to hear this rhythm, you can go back about 30 seconds, and note that you can hear the exact same synchronization in the deep bass note of the other locomotive's exhaust. When I analyzed a GE engine running at about 900 RPM, it became apparent that it was making exhaust puffs and audible pulses at about half the rate as the two-stroke EMD 567 at 835 RPM. It can't be coincidence that the 7FDL has half as many power strokes per rotation as the 567. I have found a similar correlation with ALCo 251 and 244 engines, although the water gets very muddy at low RPM when the engine is in "glass bottles crashing together" mode.
I have attached one of my recordings of an idling EMD 567 at 275 RPM, bandpassed to remove all information above 180Hz to make the exhaust pulses more audible, with cues every .21818 seconds (60/275, the time between each cycle of the engine). You will notice the audible pulses synchronize almost precisely with the cue points (I think this engine was running ever-so-slightly above 275 RPM, so the alignment isn't absolutely precise, the clip ends about .059 seconds before where I would have placed another cue point).
idle demo with cues.zip (1.19MB)
Number of downloads: 245
For the fun of it, I created another clip where there is a pulse every 0.01364 seconds - this can be expressed as a fraction: (60/275)/16. This is what an EMD 567's exhaust would sound like at idle if you could hear each individual cylinder exhausting:
sixteenth.zip (32.52K)
Number of downloads: 224
I can't possibly be the first person to have noticed this.
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