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Anybody got a explanation for this? (Onan)


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  #1  
Old 08-12-2014, 09:49:22 AM
Hummer Hummer is offline
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Default Anybody got a explanation for this? (Onan)

I am conducting a test series on an Onan AJ where I started off with HD30 and doing 20 minute run ups recording engine temps, Volts, Watts.

After initial run data. I then added a level tablespoon of moly powder and continued testing on following days.

On a previous run series I was about 8 feet from unit directly in line with the exhaust opening on muffler and I felt a puff of air like someone aimed a air nozzle at me and gave me a quick shot of air. There was no miss or change in engine sound at all.

I am between two open doors 10X10 and 14X14 to give plenty of room to vent exhaust gas.

Today on the run I just happened to be looking at the floor in front of the engine exhaust and a puff of high pressure exhaust came from the exhaust outlet with enough velocity to move welding slag on the floor exactly as a air hose would do with a quick shot of air.

Again the engine had no change in sound.

As the test progressed I have increased the load starting at 1608 Watts, 2021 Watts, and today 2264 Watts. Today at start up the cycles were at 57 but by the 5 minute interval were 57/58 and finally at 58/59.

Tomorrow I will increase load to 2366 watts.

Anybody got any theory for the shots of high pressure exhaust pressure?

The only theory/guess I can think of is the AJ is obviously old (65) and wasn't really cared for before I got it and I cleaned the fuel system, adjusted the linkage and it responded and went from 90Volts with 1412 watt load to the above and I am thinking it is just getting used to being run like it was designed to and is for lack of a better description loosening up and getting back in the groove.

The entire series has been run with 87 Octane NON ETHENOL

So does anyone have a similar experience or theory?
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Old 08-12-2014, 10:53:52 AM
Doug Reed Doug Reed is offline
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Default Re: Anybody got a explanation for this?

I have no explanation, but I'd wonder if it was just other air currents? Not sure what else it could be. If you want to play around and see what is happening in the room, I suppose you could blow up some balloons and hang them from the ceiling with string or thread or use light ribbons. But I expect that between the generator moving air and the outdoor wind moving air, that exhaust air variations should probably be expected. The exhaust isn't a focused blast so it probably moved fairly slow and would get blown around by other air movements. That would be my guess.
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Old 08-12-2014, 10:57:20 AM
Andrew Mackey Andrew Mackey is offline
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Default Re: Anybody got a explanation for this?

Exhaust does not flow at an even rate, it is expressed as pulses. These often do not flow out of the exhaust at an even velocity, nor direction. You just might be seeing or feeling a stray exhaust pulse, as it leaves the system. A few things to check, just to make sure everything else is OK: Remove all plugs and do a compression test. All cylinders should be within 10% . If this is a diesel, and you do not have a high pressure guage, crank the engine with the fuel off (no start) You want to hear an even cranking speed. A 'lope' in compresion indicates a weak cylinder, in which case, the following cylinder is making extra power to make up for speed loss in the weak cylinder. A gas engine, if it has a very weak compression on one cylinder, will also lope upon cranking. Does your exhaust sound muted? You might check for water in the muffler or vertical piping. Not often found, but it happens, especially if the engine is not run long enough to boil off any condensation in the system. you might have to make a slight adjustment to the throttle in order to get the cycles where they should be. No load, I would expect 61 to 62, loaded 59-61, heavy load, 59-60. 57 seems a bit low.
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Old 08-12-2014, 12:33:37 PM
Hummer Hummer is offline
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Default Re: Anybody got a explanation for this?

It is a single cylinder AJ and on the compression stroke cranking it feels like it has good compression. No smoking though I can see the exhaust particulates in the sunbeam coming in the open east door in the morning. Prior to opening the door I can see nothing in the dim light or in bight light if it is outside.

I test about 0700 and there is no wind stirring here outside. The temp on the back porch and the engine temp taken with digital infrared thermometer are within 1°F and the generator is stored in the shop with doors closed when not running. When the wind does crank up here it is constant from 4:00 to 10:00 or 10:00 to 4:00.

I have a 600 yard rifle range in back yard and shop and range face dead east so the directions indicated are from across my range. About 5:30 the wind dies.

I am wearing ear protectors during this series as well so I have to observe or feel though I can detect a change in engine pitch quickly. I have had pitch changes and look at multimeter and geneally the output voltage is not affected.
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Old 08-12-2014, 04:39:58 PM
Bob Willman Bob Willman is offline
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Default Re: Anybody got a explanation for this?

Is there an elbow near the end of the exhaust pipe? My Superior Type C 25hp gas engine is bolted to a foundation inside my blacksmith shop. The 6 inch diameter exhaust pipe is just over 20 ft. long. It goes down from under the cylinder into the dirt floor, then horizontally to just outside the building, then vertically outside about 6 ft. with two 45 degree segments back to horizontal with about an 8 to 10 in. length to the end.
Every once in a while when it fires, with no change in sound, there will be a swirling mass of air leave the pipe that slowly sinks towards the ground. There is a tree about 15 ft from the exhaust pipe and this swirling air will remove leaves from the tree. If one stands by the tree it feels like a blast of air hits your clothes. It will remove your hat immediately if it hits your head. Sometimes you can hear the slight whistling sound of the rotating air.
I don't know why this happens, but my guess is that the 90 degree bend at the end of the exhaust starts the air rotating inside the bend and if everything is just right the rotation continues after leaving the pipe.
It does make for an interesting conversation.

Bob
WB8NQW
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Old 08-12-2014, 04:46:23 PM
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KeithW KeithW is offline
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Default Re: Anybody got a explanation for this?

I suspect they are stable vortexes of exhaust gas. AKA smoke rings. Some engines will make them occasionally and others spit them out one after the other.
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Old 08-12-2014, 05:13:23 PM
ChrisinEstes ChrisinEstes is offline
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Default Re: Anybody got a explanation for this?

Quote:
Originally Posted by KeithW View Post
I suspect they are stable vortexes of exhaust gas. AKA smoke rings. Some engines will make them occasionally and others spit them out one after the other.
I'm with Keith. I used to have an air blaster cannon that was tuned so it would almost always shoot a puff of air like that, sometimes people could feel it 20-30 feet away. It was pretty amusing, actually... People looking around wondering what just bumped them...

Chris
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Old 08-12-2014, 05:34:55 PM
Thaumaturge Thaumaturge is offline
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Default Re: Anybody got a explanation for this?

Think of the phenomena as being similar to the "Seventh Wave" being the biggest on ocean shores. At times the reverberating waves in the exhaust system become additive in an outward direction...... poof! It would be interesting to plot the pressure waves. I smell a doctoral thesis for some young buck.
Doc
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Old 08-12-2014, 06:49:15 PM
Hummer Hummer is offline
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Default Re: Anybody got a explanation for this? (Onan)

I have a Casio Commando phone I kept that has recording capability. Should I become extremely bored I could rig up something and maybe cut up a sheet in 2" strips and staple them to a 2X2 about 10 feet away and aim the exhaust at them and start the recording. That could prove interesting.

Most of the times when it changes pitch nothing is reflected on multimeter and it makes one wonder just what is going on.

I had a friend who was maintenance chief in a mill and machinery was going all the time and he said if one changed tune it generally meant something bad was going down but that was on electric motors with pumps etc attached.
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Old 08-12-2014, 08:18:27 PM
Thaumaturge Thaumaturge is offline
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Default Re: Anybody got a explanation for this? (Onan)

Exhaust system design is not a simple science. I learned that the hard way while investigaging what I needed to build a custom replacement 4 to 1 manifold for a Honda CBR900RR engine I picked up for $100. I have all the charts... still pondering them.

Gets even weirder with 2 stroke expansion chambers. The go kart folks have a neat trick where they change a baffle in an expansion chamber to alter the torque of their engines at different RPM. Exhaust system subs for gearbox. Basically it is like impedance matching in electronics (itself not a simple science). Back pressure is like SWR.

But bottom line on original question: I wouldn't worry about it unless one particular cylinder showed noticably different burn characteristics (plug showing richer or leaner burn).
Doc
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Old 08-12-2014, 08:24:27 PM
Doug Reed Doug Reed is offline
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Default Re: Anybody got a explanation for this? (Onan)

Those suggestions do sound like a good bet. Essentially a pressure excursion caused by reflections within the exhaust system that beat against each other and sometimes combine at the far end of the pipe in an especially strong burp of air pressure. A kind of resonance effect similar to what you can demonstrate with a long rope tied to a solid object.

I think they had us do this back in grade school to show how reflections work. As you adjust the speed at which you wiggle the end of the rope (the frequency or RPM), you can see the patterns add and subtract along the rope. As you change the "wiggle rate" you eventually hit speeds where the return pulses become especially violent and jerk your hand around or even pull the rope out of your hand. But at the "resonant" rate you can hardly tell the rope is there.

Reflections occur all over in electronics and radio and in the physical world. A pipe organ is one more example of reflections and resonance. Each pipe is a different length for a different tone. You could probably design your exhaust system to resonate with the engine running at 1800RPM. The right length would improve scavenging of exhaust gasses while the wrong length would impede the exhaust and result in less power being delivered. This is the principle behind "tuned exhaust" such as those little Subaru(?) sports cars with the annoyingly loud mufflers....

Yes, I could see this puff or air occurring because every few seconds the speed of the engine and length of the pipe, all the bends, restrictions, and everything, just happens to combine a little bit higher pressure at the output of the muffler and some leaves or dirt blow around.... Not that I can prove it. But it is an interesting observation.
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Old 08-13-2014, 01:07:56 AM
ros ros is offline
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Default Re: Anybody got a explanation for this? (Onan)

Suppose, that since this engine has been sitting for a long time and maybe has never had the carbon blown out of it and it is running very rich. Now, a vapor charge rapidly builds up and ignites in the muffler/exhaust system where it won't affect engine speed or meter readings but the ignition of that vapor by red hot particulates being blown out has created an air cannon.

Ros
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Old 08-13-2014, 02:42:29 AM
SoTexRattler SoTexRattler is offline
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Default Re: Anybody got a explanation for this? (Onan)

You guys probably remember the Mattell Sonic Blaster toy from the '60's don't you?
Big low pressure air rings could be launched at friends, houses of cards, lit candles, pets, little sisters, etc, from a pretty good distance away.
I had one like this... Powerful! https://www.youtube.com/watch?v=El6t8lcz0V4
This was a smaller handheld air blaster... less power, more rugged, simpler design. http://www.timewarptoys.com/ablaster1.jpg

My 4stroke Honda mower exhaust launches tight little air rings like the toys above and they too can travel an amazing distance.
I can see their effect when I turn the mower towards tall grass ahead and carefully watch invisible puffs of air make the grass jolt. They aren't well aimed but are sort of randomly launched in the general direction of the forward facing exhaust pipe tip.
I pointthe mower at the dog and you can see him flinch slightly when one of the air do-nuts hits him in the face.
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Old 08-13-2014, 04:11:51 AM
YellowLister YellowLister is offline
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Default Re: Anybody got a explanation for this? (Onan)

sometimes my walk behind ransoms mower with a Kawasaki engine will do that.. randomly shoots a puff of exhaust on grass when normally if goes straight out.. about 2 feet above ground.. ill see blades of grass move when they shouldn't.. so one day I standing there for a min and it did it.. really blew my mind.. lol not really.. also my snapper will do that to.. has a Honda engine and the exhaust comes out from the front of the engine.. and will randomly shoot it down to the right or left.. pretty strange..
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Old 08-13-2014, 12:57:34 PM
len k len k is offline
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Default Re: Anybody got a explanation for this? (Onan)

If you pour a little oil down carb while it's running you can see the white smoke rings, if they are there.

Last edited by len k; 08-13-2014 at 06:16:31 PM. Reason: spelling
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Old 08-13-2014, 05:29:04 PM
Hummer Hummer is offline
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Default Re: Anybody got a explanation for this? (Onan)

Did another run this morning and ambient temp was much lower. Ever since I started ambient outside and in my shop have been the same but when I walked out this morning it felt much cooler and thermometer on porch said 70F and I opened shop doors and it was warmer inside and engine was 75.

I stacked up two carboard boxes six feet from the muffler and aimed it right at it and cranked it off. I increased the load and it had to run 15 minutes before the cycles came into range.

I watched a paper towel on the floor that was oil soaked and maybe every 30 seconds or so it would raise up and flap and drop with no detectable change in the engine sound. Boxes did not fall over but they would shake a bit and then stop shaking and then shake some more but I saw no evidence of the big blast.
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Old 08-13-2014, 06:03:59 PM
gootsch gootsch is offline
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Default Re: Anybody got a explanation for this? (Onan)

My Kubota rototolller and my Graham/Paige/Fraser Rototiller would both do the same thing while I was tilling gardens. Just an occasional puff of dusty soil would blow up in to the air, the rest of the time nothing.
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Old 08-13-2014, 06:18:58 PM
len k len k is offline
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Default Re: Anybody got a explanation for this? (Onan)

Used to have a 5hp engine that burned oil on my rototiller, occationally you would see white smoke ring blow off into the distance. Amazing how far it went before it disapated.
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